nft

NFT(8)                                                                  NFT(8)



NAME
       nft - Administration tool of the nftables framework for packet
       filtering and classification

SYNOPSIS
       nft [ -nNscaeSupyjt ] [ -I directory ] [ -f filename | -i | cmd ...]
       nft -h
       nft -v

DESCRIPTION
       nft is the command line tool used to set up, maintain and inspect
       packet filtering and classification rules in the Linux kernel, in the
       nftables framework. The Linux kernel subsystem is known as nf_tables,
       and ‘nf’ stands for Netfilter.

OPTIONS
       For a full summary of options, run nft --help.

       -h, --help
           Show help message and all options.

       -v, --version
           Show version.

       -n, --numeric
           Print fully numerical output.

       -s, --stateless
           Omit stateful information of rules and stateful objects.

       -N, --reversedns
           Translate IP address to names via reverse DNS lookup. This may slow
           down your listing since it generates network traffic.

       -S, --service
           Translate ports to service names as defined by /etc/services.

       -u, --guid
           Translate numeric UID/GID to names as defined by /etc/passwd and
           /etc/group.

       -p, --numeric-protocol
           Display layer 4 protocol numerically.

       -y, --numeric-priority
           Display base chain priority numerically.

       -c, --check
           Check commands validity without actually applying the changes.

       -a, --handle
           Show object handles in output.

       -e, --echo
           When inserting items into the ruleset using add, insert or replace
           commands, print notifications just like nft monitor.

       -j, --json
           Format output in JSON. See libnftables-json(5) for a schema
           description.

       -I, --includepath directory
           Add the directory directory to the list of directories to be
           searched for included files. This option may be specified multiple
           times.

       -f, --file filename
           Read input from filename. If filename is -, read from stdin.

       -i, --interactive
           Read input from an interactive readline CLI. You can use quit to
           exit, or use the EOF marker, normally this is CTRL-D.

       -T, --numeric-time
           Show time, day and hour values in numeric format.

       -t, --terse
           Omit contents of sets from output.

INPUT FILE FORMATS
   LEXICAL CONVENTIONS
       Input is parsed line-wise. When the last character of a line, just
       before the newline character, is a non-quoted backslash (\), the next
       line is treated as a continuation. Multiple commands on the same line
       can be separated using a semicolon (;).

       A hash sign (#) begins a comment. All following characters on the same
       line are ignored.

       Identifiers begin with an alphabetic character (a-z,A-Z), followed zero
       or more alphanumeric characters (a-z,A-Z,0-9) and the characters slash
       (/), backslash (\), underscore (_) and dot (.). Identifiers using
       different characters or clashing with a keyword need to be enclosed in
       double quotes (").

   INCLUDE FILES
           include filename

       Other files can be included by using the include statement. The
       directories to be searched for include files can be specified using the
       -I/--includepath option. You can override this behaviour either by
       prepending ‘./’ to your path to force inclusion of files located in the
       current working directory (i.e. relative path) or / for file location
       expressed as an absolute path.

       If -I/--includepath is not specified, then nft relies on the default
       directory that is specified at compile time. You can retrieve this
       default directory via -h/--help option.

       Include statements support the usual shell wildcard symbols (\*,?,[]).
       Having no matches for an include statement is not an error, if wildcard
       symbols are used in the include statement. This allows having
       potentially empty include directories for statements like include
       "/etc/firewall/rules/". The wildcard matches are loaded in alphabetical
       order. Files beginning with dot (.) are not matched by include
       statements.

   SYMBOLIC VARIABLES
           define variable = expr
           $variable

       Symbolic variables can be defined using the define statement. Variable
       references are expressions and can be used initialize other variables.
       The scope of a definition is the current block and all blocks contained
       within.

       Using symbolic variables.

           define int_if1 = eth0
           define int_if2 = eth1
           define int_ifs = { $int_if1, $int_if2 }

           filter input iif $int_ifs accept


ADDRESS FAMILIES
       Address families determine the type of packets which are processed. For
       each address family, the kernel contains so called hooks at specific
       stages of the packet processing paths, which invoke nftables if rules
       for these hooks exist.


       ip       IPv4 address family.

       ip6      IPv6 address family.

       inet     Internet (IPv4/IPv6)
                address family.

       arp      ARP address family,
                handling IPv4 ARP packets.

       bridge   Bridge address family,
                handling packets which
                traverse a bridge device.

       netdev   Netdev address family,
                handling packets from
                ingress.


       All nftables objects exist in address family specific namespaces,
       therefore all identifiers include an address family. If an identifier
       is specified without an address family, the ip family is used by
       default.

   IPV4/IPV6/INET ADDRESS FAMILIES
       The IPv4/IPv6/Inet address families handle IPv4, IPv6 or both types of
       packets. They contain five hooks at different packet processing stages
       in the network stack.

       Table 1. IPv4/IPv6/Inet address family hooks
       ┌────────────┬────────────────────────────┐
       │Hook        Description                │
       ├────────────┼────────────────────────────┤
       │            │                            │
       │prerouting  │ All packets entering the   │
       │            │ system are processed by    │
       │            │ the prerouting hook. It is │
       │            │ invoked before the routing │
       │            │ process and is used for    │
       │            │ early filtering or         │
       │            │ changing packet attributes │
       │            │ that affect routing.       │
       ├────────────┼────────────────────────────┤
       │            │                            │
       │input       │ Packets delivered to the   │
       │            │ local system are processed │
       │            │ by the input hook.         │
       ├────────────┼────────────────────────────┤
       │            │                            │
       │forward     │ Packets forwarded to a     │
       │            │ different host are         │
       │            │ processed by the forward   │
       │            │ hook.                      │
       ├────────────┼────────────────────────────┤
       │            │                            │
       │output      │ Packets sent by local      │
       │            │ processes are processed by │
       │            │ the output hook.           │
       ├────────────┼────────────────────────────┤
       │            │                            │
       │postrouting │ All packets leaving the    │
       │            │ system are processed by    │
       │            │ the postrouting hook.      │
       └────────────┴────────────────────────────┘

   ARP ADDRESS FAMILY
       The ARP address family handles ARP packets received and sent by the
       system. It is commonly used to mangle ARP packets for clustering.

       Table 2. ARP address family hooks
       ┌───────┬────────────────────────────┐
       │Hook   Description                │
       ├───────┼────────────────────────────┤
       │       │                            │
       │input  │ Packets delivered to the   │
       │       │ local system are processed │
       │       │ by the input hook.         │
       ├───────┼────────────────────────────┤
       │       │                            │
       │output │ Packets send by the local  │
       │       │ system are processed by    │
       │       │ the output hook.           │
       └───────┴────────────────────────────┘

   BRIDGE ADDRESS FAMILY
       The bridge address family handles Ethernet packets traversing bridge
       devices.

       The list of supported hooks is identical to IPv4/IPv6/Inet address
       families above.

   NETDEV ADDRESS FAMILY
       The Netdev address family handles packets from ingress.

       Table 3. Netdev address family hooks
       ┌────────┬──────────────────────────┐
       │Hook    Description              │
       ├────────┼──────────────────────────┤
       │        │                          │
       │ingress │ All packets entering the │
       │        │ system are processed by  │
       │        │ this hook. It is invoked │
       │        │ before layer 3 protocol  │
       │        │ handlers and it can be   │
       │        │ used for early filtering │
       │        │ and policing.            │
       └────────┴──────────────────────────┘

RULESET
           {list | flush} ruleset [family]

       The ruleset keyword is used to identify the whole set of tables,
       chains, etc. currently in place in kernel. The following ruleset
       commands exist:


       list    Print the ruleset in
               human-readable format.

       flush   Clear the whole ruleset.
               Note that, unlike
               iptables, this will remove
               all tables and whatever
               they contain, effectively
               leading to an empty
               ruleset - no packet
               filtering will happen
               anymore, so the kernel
               accepts any valid packet
               it receives.


       It is possible to limit list and flush to a specific address family
       only. For a list of valid family names, see the section called “ADDRESS
       FAMILIES” above.

       By design, list ruleset command output may be used as input to nft -f.
       Effectively, this is the nft-equivalent of iptables-save and
       iptables-restore.

TABLES
           {add | create} table [family] table [{ flags flags ; }]
           {delete | list | flush} table [family] table
           list tables [family]
           delete table [family] handle handle

       Tables are containers for chains, sets and stateful objects. They are
       identified by their address family and their name. The address family
       must be one of ip, ip6, inet, arp, bridge, netdev. The inet address
       family is a dummy family which is used to create hybrid IPv4/IPv6
       tables. The meta expression nfproto keyword can be used to test which
       family (ipv4 or ipv6) context the packet is being processed in. When no
       address family is specified, ip is used by default. The only difference
       between add and create is that the former will not return an error if
       the specified table already exists while create will return an error.

       Table 4. Table flags
       ┌────────┬────────────────────────────┐
       │Flag    Description                │
       ├────────┼────────────────────────────┤
       │        │                            │
       │dormant │ table is not evaluated any │
       │        │ more (base chains are      │
       │        │ unregistered).             │
       └────────┴────────────────────────────┘

       Add, change, delete a table.

           # start nft in interactive mode
           nft --interactive

           # create a new table.
           create table inet mytable

           # add a new base chain: get input packets
           add chain inet mytable myin { type filter hook input priority 0; }

           # add a single counter to the chain
           add rule inet mytable myin counter

           # disable the table temporarily -- rules are not evaluated anymore
           add table inet mytable { flags dormant; }

           # make table active again:
           add table inet mytable



       add      Add a new table for the
                given family with the
                given name.

       delete   Delete the specified
                table.

       list     List all chains and rules
                of the specified table.

       flush    Flush all chains and rules
                of the specified table.


CHAINS
           {add | create} chain [family] table chain [{ type type hook hook [device device] priority priority ; [policy policy ;] }]
           {delete | list | flush} chain [family] table chain
           list chains [family]
           delete chain [family] table handle handle
           rename chain [family] table chain newname

       Chains are containers for rules. They exist in two kinds, base chains
       and regular chains. A base chain is an entry point for packets from the
       networking stack, a regular chain may be used as jump target and is
       used for better rule organization.


       add      Add a new chain in the
                specified table. When a
                hook and priority value
                are specified, the chain
                is created as a base chain
                and hooked up to the
                networking stack.


       create   Similar to the add
                command, but returns an
                error if the chain already
                exists.

       delete   Delete the specified
                chain. The chain must not
                contain any rules or be
                used as jump target.

       rename   Rename the specified
                chain.

       list     List all rules of the
                specified chain.

       flush    Flush all rules of the
                specified chain.


       For base chains, type, hook and priority parameters are mandatory.

       Table 5. Supported chain types
       ┌───────┬───────────────┬────────────────┬──────────────────┐
       │Type   Families      Hooks          Description      │
       ├───────┼───────────────┼────────────────┼──────────────────┤
       │       │               │                │                  │
       │filter │ all           │ all            │ Standard chain   │
       │       │               │                │ type to use in   │
       │       │               │                │ doubt.           │
       ├───────┼───────────────┼────────────────┼──────────────────┤
       │       │               │                │                  │
       │nat    │ ip, ip6, inet │ prerouting,    │ Chains of this   │
       │       │               │ input, output, │ type perform     │
       │       │               │ postrouting    │ Native Address   │
       │       │               │                │ Translation      │
       │       │               │                │ based on         │
       │       │               │                │ conntrack        │
       │       │               │                │ entries. Only    │
       │       │               │                │ the first packet │
       │       │               │                │ of a connection  │
       │       │               │                │ actually         │
       │       │               │                │ traverses this   │
       │       │               │                │ chain - its      │
       │       │               │                │ rules usually    │
       │       │               │                │ define details   │
       │       │               │                │ of the created   │
       │       │               │                │ conntrack entry  │
       │       │               │                │ (NAT statements  │
       │       │               │                │ for instance).   │
       ├───────┼───────────────┼────────────────┼──────────────────┤
       │       │               │                │                  │
       │route  │ ip, ip6       │ output         │ If a packet has  │
       │       │               │                │ traversed a      │
       │       │               │                │ chain of this    │
       │       │               │                │ type and is      │
       │       │               │                │ about to be      │
       │       │               │                │ accepted, a new  │
       │       │               │                │ route lookup is  │
       │       │               │                │ performed if     │
       │       │               │                │ relevant parts   │
       │       │               │                │ of the IP header │
       │       │               │                │ have changed.    │
       │       │               │                │ This allows to   │
       │       │               │                │ e.g. implement   │
       │       │               │                │ policy routing   │
       │       │               │                │ selectors in     │
       │       │               │                │ nftables.        │
       └───────┴───────────────┴────────────────┴──────────────────┘

       Apart from the special cases illustrated above (e.g. nat type not
       supporting forward hook or route type only supporting output hook),
       there are two further quirks worth noticing:

       •   The netdev family supports merely a single combination, namely
           filter type and ingress hook. Base chains in this family also
           require the device parameter to be present since they exist per
           incoming interface only.

       •   The arp family supports only the input and output hooks, both in
           chains of type filter.

       The priority parameter accepts a signed integer value or a standard
       priority name which specifies the order in which chains with same hook
       value are traversed. The ordering is ascending, i.e. lower priority
       values have precedence over higher ones.

       Standard priority values can be replaced with easily memorizable names.
       Not all names make sense in every family with every hook (see the
       compatibility matrices below) but their numerical value can still be
       used for prioritizing chains.

       These names and values are defined and made available based on what
       priorities are used by xtables when registering their default chains.

       Most of the families use the same values, but bridge uses different
       ones from the others. See the following tables that describe the values
       and compatibility.

       Table 6. Standard priority names, family and hook compatibility matrix
       ┌─────────┬───────┬────────────────┬─────────────┐
       │Name     Value Families       Hooks       │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │raw      │ -300  │ ip, ip6, inet  │ all         │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │mangle   │ -150  │ ip, ip6, inet  │ all         │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │dstnat   │ -100  │ ip, ip6, inet  │ prerouting  │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │filter   │ 0     │ ip, ip6, inet, │ all         │
       │         │       │ arp, netdev    │             │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │security │ 50    │ ip, ip6, inet  │ all         │
       ├─────────┼───────┼────────────────┼─────────────┤
       │         │       │                │             │
       │srcnat   │ 100   │ ip, ip6, inet  │ postrouting │
       └─────────┴───────┴────────────────┴─────────────┘

       Table 7. Standard priority names and hook compatibility for the bridge
       family
       ┌───────┬───────┬─────────────┐
       │       │       │             │
       │Name   │ Value │ Hooks       │
       ├───────┼───────┼─────────────┤
       │       │       │             │
       │dstnat │ -300  │ prerouting  │
       ├───────┼───────┼─────────────┤
       │       │       │             │
       │filter │ -200  │ all         │
       ├───────┼───────┼─────────────┤
       │       │       │             │
       │out    │ 100   │ output      │
       ├───────┼───────┼─────────────┤
       │       │       │             │
       │srcnat │ 300   │ postrouting │
       └───────┴───────┴─────────────┘

       Basic arithmetic expressions (addition and subtraction) can also be
       achieved with these standard names to ease relative prioritizing, e.g.
       mangle - 5 stands for -155. Values will also be printed like this until
       the value is not further than 10 form the standard value.

       Base chains also allow to set the chain’s policy, i.e. what happens to
       packets not explicitly accepted or refused in contained rules.
       Supported policy values are accept (which is the default) or drop.

RULES
           {add | insert} rule [family] table chain [handle handle | index index] statement ... [comment comment]
           replace rule [family] table chain handle handle statement ... [comment comment]
           delete rule [family] table chain handle handle

       Rules are added to chains in the given table. If the family is not
       specified, the ip family is used. Rules are constructed from two kinds
       of components according to a set of grammatical rules: expressions and
       statements.

       The add and insert commands support an optional location specifier,
       which is either a handle or the index (starting at zero) of an existing
       rule. Internally, rule locations are always identified by handle and
       the translation from index happens in userspace. This has two potential
       implications in case a concurrent ruleset change happens after the
       translation was done: The effective rule index might change if a rule
       was inserted or deleted before the referred one. If the referred rule
       was deleted, the command is rejected by the kernel just as if an
       invalid handle was given.

       A comment is a single word or a double-quoted (") multi-word string
       which can be used to make notes regarding the actual rule. Note: If you
       use bash for adding rules, you have to escape the quotation marks, e.g.
       \"enable ssh for servers\".


       add       Add a new rule described
                 by the list of statements.
                 The rule is appended to
                 the given chain unless a
                 location is specified, in
                 which case the rule is
                 inserted after the
                 specified rule.

       insert    Same as add except the
                 rule is inserted at the
                 beginning of the chain or
                 before the specified rule.

       replace   Similar to add, but the
                 rule replaces the
                 specified rule.

       delete    Delete the specified rule.


       add a rule to ip table input chain.

           nft add rule filter output ip daddr 192.168.0.0/24 accept # 'ip filter' is assumed
           # same command, slightly more verbose
           nft add rule ip filter output ip daddr 192.168.0.0/24 accept

       delete rule from inet table.

           # nft -a list ruleset
           table inet filter {
                   chain input {
                           type filter hook input priority 0; policy accept;
                           ct state established,related accept # handle 4
                           ip saddr 10.1.1.1 tcp dport ssh accept # handle 5
                     ...
           # delete the rule with handle 5
           # nft delete rule inet filter input handle 5


SETS
       nftables offers two kinds of set concepts. Anonymous sets are sets that
       have no specific name. The set members are enclosed in curly braces,
       with commas to separate elements when creating the rule the set is used
       in. Once that rule is removed, the set is removed as well. They cannot
       be updated, i.e. once an anonymous set is declared it cannot be changed
       anymore except by removing/altering the rule that uses the anonymous
       set.

       Using anonymous sets to accept particular subnets and ports.

           nft add rule filter input ip saddr { 10.0.0.0/8, 192.168.0.0/16 } tcp dport { 22, 443 } accept

       Named sets are sets that need to be defined first before they can be
       referenced in rules. Unlike anonymous sets, elements can be added to or
       removed from a named set at any time. Sets are referenced from rules
       using an @ prefixed to the sets name.

       Using named sets to accept addresses and ports.

           nft add rule filter input ip saddr @allowed_hosts tcp dport @allowed_ports accept

       The sets allowed_hosts and allowed_ports need to be created first. The
       next section describes nft set syntax in more detail.

           add set [family] table set { type type | typeof expression ; [flags flags ;] [timeout timeout ;] [gc-interval gc-interval ;] [elements = { element[, ...] } ;] [size size ;] [policy policy ;] [auto-merge ;] }
           {delete | list | flush} set [family] table set
           list sets [family]
           delete set [family] table handle handle
           {add | delete} element [family] table set { element[, ...] }

       Sets are element containers of a user-defined data type, they are
       uniquely identified by a user-defined name and attached to tables.
       Their behaviour can be tuned with the flags that can be specified at
       set creation time.


       add      Add a new set in the
                specified table. See the
                Set specification table
                below for more information
                about how to specify a
                sets properties.

       delete   Delete the specified set.

       list     Display the elements in
                the specified set.

       flush    Remove all elements from
                the specified set.


       Table 8. Set specifications
       ┌────────────┬──────────────────────┬─────────────────────┐
       │Keyword     Description          Type                │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │type        │ data type of set     │ string: ipv4_addr,  │
       │            │ elements             │ ipv6_addr,          │
       │            │                      │ ether_addr,         │
       │            │                      │ inet_proto,         │
       │            │                      │ inet_service, mark  │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │typeof      │ data type of set     │ expression to       │
       │            │ element              │ derive the data     │
       │            │                      │ type from           │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │flags       │ set flags            │ string: constant,   │
       │            │                      │ dynamic, interval,  │
       │            │                      │ timeout             │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │timeout     │ time an element      │ string, decimal     │
       │            │ stays in the set,    │ followed by unit.   │
       │            │ mandatory if set is  │ Units are: d, h, m, │
       │            │ added to from the    │ s                   │
       │            │ packet path          │                     │
       │            │ (ruleset).           │                     │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │gc-interval │ garbage collection   │ string, decimal     │
       │            │ interval, only       │ followed by unit.   │
       │            │ available when       │ Units are: d, h, m, │
       │            │ timeout or flag      │ s                   │
       │            │ timeout are active   │                     │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │elements    │ elements contained   │ set data type       │
       │            │ by the set           │                     │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │size        │ maximum number of    │ unsigned integer    │
       │            │ elements in the      │ (64 bit)            │
       │            │ set, mandatory if    │                     │
       │            │ set is added to      │                     │
       │            │ from the packet      │                     │
       │            │ path (ruleset).      │                     │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │policy      │ set policy           │ string: performance │
       │            │                      │ [default], memory   │
       ├────────────┼──────────────────────┼─────────────────────┤
       │            │                      │                     │
       │auto-merge  │ automatic merge of   │                     │
       │            │ adjacent/overlapping │                     │
       │            │ set elements (only   │                     │
       │            │ for interval sets)   │                     │
       └────────────┴──────────────────────┴─────────────────────┘

MAPS
           add map [family] table map { type type | typeof expression [flags flags ;] [elements = { element[, ...] } ;] [size size ;] [policy policy ;] }
           {delete | list | flush} map [family] table map
           list maps [family]

       Maps store data based on some specific key used as input. They are
       uniquely identified by a user-defined name and attached to tables.


       add              Add a new map in the
                        specified table.

       delete           Delete the specified map.

       list             Display the elements in
                        the specified map.

       flush            Remove all elements from
                        the specified map.

       add element      Comma-separated list of
                        elements to add into the
                        specified map.

       delete element   Comma-separated list of
                        element keys to delete
                        from the specified map.


       Table 9. Map specifications
       ┌─────────┬─────────────────────┬─────────────────────┐
       │Keyword  Description         Type                │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │type     │ data type of map    │ string: ipv4_addr,  │
       │         │ elements            │ ipv6_addr,          │
       │         │                     │ ether_addr,         │
       │         │                     │ inet_proto,         │
       │         │                     │ inet_service, mark, │
       │         │                     │ counter, quota.     │
       │         │                     │ Counter and quota   │
       │         │                     │ can’t be used as    │
       │         │                     │ keys                │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │typeof   │ data type of set    │ expression to       │
       │         │ element             │ derive the data     │
       │         │                     │ type from           │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │flags    │ map flags           │ string: constant,   │
       │         │                     │ interval            │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │elements │ elements contained  │ map data type       │
       │         │ by the map          │                     │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │size     │ maximum number of   │ unsigned integer    │
       │         │ elements in the map │ (64 bit)            │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │policy   │ map policy          │ string: performance │
       │         │                     │ [default], memory   │
       └─────────┴─────────────────────┴─────────────────────┘

ELEMENTS
           {add | create | delete | get } element [family] table set { ELEMENT[, ...] }

           ELEMENT := key_expression OPTIONS [: value_expression]
           OPTIONS := [timeout TIMESPEC] [expires TIMESPEC] [comment string]
           TIMESPEC := [numd][numh][numm][num[s]]

       Element-related commands allow to change contents of named sets and
       maps. key_expression is typically a value matching the set type.
       value_expression is not allowed in sets but mandatory when adding to
       maps, where it matches the data part in it’s type definition. When
       deleting from maps, it may be specified but is optional as
       key_expression uniquely identifies the element.

       create command is similar to add with the exception that none of the
       listed elements may already exist.

       get command is useful to check if an element is contained in a set
       which may be non-trivial in very large and/or interval sets. In the
       latter case, the containing interval is returned instead of just the
       element itself.

       Table 10. Element options
       ┌────────┬───────────────────────────┐
       │Option  Description               │
       ├────────┼───────────────────────────┤
       │        │                           │
       │timeout │ timeout value for         │
       │        │ sets/maps with flag       │
       │        │ timeout                   │
       ├────────┼───────────────────────────┤
       │        │                           │
       │expires │ the time until given      │
       │        │ element expires, useful   │
       │        │ for ruleset replication   │
       │        │ only                      │
       ├────────┼───────────────────────────┤
       │        │                           │
       │comment │ per element comment field │
       └────────┴───────────────────────────┘

FLOWTABLES
           {add | create} flowtable [family] table flowtable { hook hook priority priority ; devices = { device[, ...] } ; }
           list flowtables [family]
           {delete | list} flowtable [family] table flowtable
           delete flowtable [family] table handle handle

       Flowtables allow you to accelerate packet forwarding in software.
       Flowtables entries are represented through a tuple that is composed of
       the input interface, source and destination address, source and
       destination port; and layer 3/4 protocols. Each entry also caches the
       destination interface and the gateway address - to update the
       destination link-layer address - to forward packets. The ttl and
       hoplimit fields are also decremented. Hence, flowtables provides an
       alternative path that allow packets to bypass the classic forwarding
       path. Flowtables reside in the ingress hook that is located before the
       prerouting hook. You can select which flows you want to offload through
       the flow expression from the forward chain. Flowtables are identified
       by their address family and their name. The address family must be one
       of ip, ip6, or inet. The inet address family is a dummy family which is
       used to create hybrid IPv4/IPv6 tables. When no address family is
       specified, ip is used by default.

       The priority can be a signed integer or filter which stands for 0.
       Addition and subtraction can be used to set relative priority, e.g.
       filter + 5 equals to 5.


       add      Add a new flowtable for
                the given family with the
                given name.

       delete   Delete the specified
                flowtable.

       list     List all flowtables.


STATEFUL OBJECTS
           {add | delete | list | reset} type [family] table object
           delete type [family] table handle handle
           list counters [family]
           list quotas [family]

       Stateful objects are attached to tables and are identified by an unique
       name. They group stateful information from rules, to reference them in
       rules the keywords "type name" are used e.g. "counter name".


       add      Add a new stateful object
                in the specified table.

       delete   Delete the specified
                object.

       list     Display stateful
                information the object
                holds.

       reset    List-and-reset stateful
                object.


   CT HELPER
           ct helper helper { type type protocol protocol ; [l3proto family ;] }

       Ct helper is used to define connection tracking helpers that can then
       be used in combination with the ct helper set statement. type and
       protocol are mandatory, l3proto is derived from the table family by
       default, i.e. in the inet table the kernel will try to load both the
       ipv4 and ipv6 helper backends, if they are supported by the kernel.

       Table 11. conntrack helper specifications
       ┌─────────┬─────────────────────┬─────────────────────┐
       │Keyword  Description         Type                │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │type     │ name of helper type │ quoted string (e.g. │
       │         │                     │ "ftp")              │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │protocol │ layer 4 protocol of │ string (e.g. ip)    │
       │         │ the helper          │                     │
       ├─────────┼─────────────────────┼─────────────────────┤
       │         │                     │                     │
       │l3proto  │ layer 3 protocol of │ address family      │
       │         │ the helper          │ (e.g. ip)           │
       └─────────┴─────────────────────┴─────────────────────┘

       defining and assigning ftp helper.

           Unlike iptables, helper assignment needs to be performed after the conntrack
           lookup has completed, for example with the default 0 hook priority.

           table inet myhelpers {
             ct helper ftp-standard {
                type "ftp" protocol tcp
             }
             chain prerouting {
                 type filter hook prerouting priority 0;
                 tcp dport 21 ct helper set "ftp-standard"
             }
           }


   CT TIMEOUT
           ct timeout name { protocol protocol ; policy = { state: value [, ...] } ; [l3proto family ;] }

       Ct timeout is used to update connection tracking timeout values.Timeout
       policies are assigned with the ct timeout set statement. protocol and
       policy are mandatory, l3proto is derived from the table family by
       default.

       Table 12. conntrack timeout specifications
       ┌─────────┬─────────────────────┬──────────────────┐
       │Keyword  Description         Type             │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │protocol │ layer 4 protocol of │ string (e.g. ip) │
       │         │ the timeout object  │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │state    │ connection state    │ string (e.g.     │
       │         │ name                │ "established")   │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │value    │ timeout value for   │ unsigned integer │
       │         │ connection state    │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │l3proto  │ layer 3 protocol of │ address family   │
       │         │ the timeout object  │ (e.g. ip)        │
       └─────────┴─────────────────────┴──────────────────┘

       defining and assigning ct timeout policy.

           table ip filter {
                   ct timeout customtimeout {
                           protocol tcp;
                           l3proto ip
                           policy = { established: 120, close: 20 }
                   }

                   chain output {
                           type filter hook output priority filter; policy accept;
                           ct timeout set "customtimeout"
                   }
           }

       testing the updated timeout policy.

           % conntrack -E

           It should display:

           [UPDATE] tcp      6 120 ESTABLISHED src=172.16.19.128 dst=172.16.19.1
           sport=22 dport=41360 [UNREPLIED] src=172.16.19.1 dst=172.16.19.128
           sport=41360 dport=22


   CT EXPECTATION
           ct expectation name { protocol protocol ; dport dport ; timeout timeout ; size size ; [*l3proto family ;] }

       Ct expectation is used to create connection expectations. Expectations
       are assigned with the ct expectation set statement. protocol, dport,
       timeout and size are mandatory, l3proto is derived from the table
       family by default.

       Table 13. conntrack expectation specifications
       ┌─────────┬─────────────────────┬──────────────────┐
       │Keyword  Description         Type             │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │protocol │ layer 4 protocol of │ string (e.g. ip) │
       │         │ the expectation     │                  │
       │         │ object              │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │dport    │ destination port of │ unsigned integer │
       │         │ expected connection │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │timeout  │ timeout value for   │ unsigned integer │
       │         │ expectation         │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │size     │ size value for      │ unsigned integer │
       │         │ expectation         │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │l3proto  │ layer 3 protocol of │ address family   │
       │         │ the expectation     │ (e.g. ip)        │
       │         │ object              │                  │
       └─────────┴─────────────────────┴──────────────────┘

       defining and assigning ct expectation policy.

           table ip filter {
                   ct expectation expect {
                           protocol udp
                           dport 9876
                           timeout 2m
                           size 8
                           l3proto ip
                   }

                   chain input {
                           type filter hook input priority filter; policy accept;
                           ct expectation set "expect"
                   }
           }


   COUNTER
           counter [packets bytes]

       Table 14. Counter specifications
       ┌────────┬──────────────────┬──────────────────┐
       │Keyword Description      Type             │
       ├────────┼──────────────────┼──────────────────┤
       │        │                  │                  │
       │packets │ initial count of │ unsigned integer │
       │        │ packets          │ (64 bit)         │
       ├────────┼──────────────────┼──────────────────┤
       │        │                  │                  │
       │bytes   │ initial count of │ unsigned integer │
       │        │ bytes            │ (64 bit)         │
       └────────┴──────────────────┴──────────────────┘

   QUOTA
           quota [over | until] [used]

       Table 15. Quota specifications
       ┌────────┬───────────────────┬────────────────────┐
       │Keyword Description       Type               │
       ├────────┼───────────────────┼────────────────────┤
       │        │                   │                    │
       │quota   │ quota limit, used │ Two arguments,     │
       │        │ as the quota name │ unsigned integer   │
       │        │                   │ (64 bit) and       │
       │        │                   │ string: bytes,     │
       │        │                   │ kbytes, mbytes.    │
       │        │                   │ "over" and "until" │
       │        │                   │ go before these    │
       │        │                   │ arguments          │
       ├────────┼───────────────────┼────────────────────┤
       │        │                   │                    │
       │used    │ initial value of  │ Two arguments,     │
       │        │ used quota        │ unsigned integer   │
       │        │                   │ (64 bit) and       │
       │        │                   │ string: bytes,     │
       │        │                   │ kbytes, mbytes     │
       └────────┴───────────────────┴────────────────────┘

EXPRESSIONS
       Expressions represent values, either constants like network addresses,
       port numbers, etc., or data gathered from the packet during ruleset
       evaluation. Expressions can be combined using binary, logical,
       relational and other types of expressions to form complex or relational
       (match) expressions. They are also used as arguments to certain types
       of operations, like NAT, packet marking etc.

       Each expression has a data type, which determines the size, parsing and
       representation of symbolic values and type compatibility with other
       expressions.

   DESCRIBE COMMAND
           describe expression | data type

       The describe command shows information about the type of an expression
       and its data type. A data type may also be given, in which nft will
       display more information about the type.

       The describe command.

           $ nft describe tcp flags
           payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits

           predefined symbolic constants:
           fin                           0x01
           syn                           0x02
           rst                           0x04
           psh                           0x08
           ack                           0x10
           urg                           0x20
           ecn                           0x40
           cwr                           0x80


DATA TYPES
       Data types determine the size, parsing and representation of symbolic
       values and type compatibility of expressions. A number of global data
       types exist, in addition some expression types define further data
       types specific to the expression type. Most data types have a fixed
       size, some however may have a dynamic size, f.i. the string type. Some
       types also have predefined symbolic constants. Those can be listed
       using the nft describe command:

           $ nft describe ct_state
           datatype ct_state (conntrack state) (basetype bitmask, integer), 32 bits

           pre-defined symbolic constants (in hexadecimal):
           invalid                         0x00000001
           new ...

       Types may be derived from lower order types, f.i. the IPv4 address type
       is derived from the integer type, meaning an IPv4 address can also be
       specified as an integer value.

       In certain contexts (set and map definitions), it is necessary to
       explicitly specify a data type. Each type has a name which is used for
       this.

   INTEGER TYPE
       ┌────────┬─────────┬──────────┬───────────┐
       │Name    Keyword Size     Base type │
       ├────────┼─────────┼──────────┼───────────┤
       │        │         │          │           │
       │Integer │ integer │ variable │ -         │
       └────────┴─────────┴──────────┴───────────┘

       The integer type is used for numeric values. It may be specified as a
       decimal, hexadecimal or octal number. The integer type does not have a
       fixed size, its size is determined by the expression for which it is
       used.

   BITMASK TYPE
       ┌────────┬─────────┬──────────┬───────────┐
       │Name    Keyword Size     Base type │
       ├────────┼─────────┼──────────┼───────────┤
       │        │         │          │           │
       │Bitmask │ bitmask │ variable │ integer   │
       └────────┴─────────┴──────────┴───────────┘

       The bitmask type (bitmask) is used for bitmasks.

   STRING TYPE
       ┌───────┬─────────┬──────────┬───────────┐
       │Name   Keyword Size     Base type │
       ├───────┼─────────┼──────────┼───────────┤
       │       │         │          │           │
       │String │ string  │ variable │ -         │
       └───────┴─────────┴──────────┴───────────┘

       The string type is used for character strings. A string begins with an
       alphabetic character (a-zA-Z) followed by zero or more alphanumeric
       characters or the characters /, -, _ and .. In addition, anything
       enclosed in double quotes (") is recognized as a string.

       String specification.

           # Interface name
           filter input iifname eth0

           # Weird interface name
           filter input iifname "(eth0)"


   LINK LAYER ADDRESS TYPE
       ┌───────────┬─────────┬──────────┬───────────┐
       │Name       Keyword Size     Base type │
       ├───────────┼─────────┼──────────┼───────────┤
       │           │         │          │           │
       │Link layer │ lladdr  │ variable │ integer   │
       │address    │         │          │           │
       └───────────┴─────────┴──────────┴───────────┘

       The link layer address type is used for link layer addresses. Link
       layer addresses are specified as a variable amount of groups of two
       hexadecimal digits separated using colons (:).

       Link layer address specification.

           # Ethernet destination MAC address
           filter input ether daddr 20:c9:d0:43:12:d9


   IPV4 ADDRESS TYPE
       ┌─────────────┬───────────┬────────┬───────────┐
       │Name         Keyword   Size   Base type │
       ├─────────────┼───────────┼────────┼───────────┤
       │             │           │        │           │
       │IPV4 address │ ipv4_addr │ 32 bit │ integer   │
       └─────────────┴───────────┴────────┴───────────┘

       The IPv4 address type is used for IPv4 addresses. Addresses are
       specified in either dotted decimal, dotted hexadecimal, dotted octal,
       decimal, hexadecimal, octal notation or as a host name. A host name
       will be resolved using the standard system resolver.

       IPv4 address specification.

           # dotted decimal notation
           filter output ip daddr 127.0.0.1

           # host name
           filter output ip daddr localhost


   IPV6 ADDRESS TYPE
       ┌─────────────┬───────────┬─────────┬───────────┐
       │Name         Keyword   Size    Base type │
       ├─────────────┼───────────┼─────────┼───────────┤
       │             │           │         │           │
       │IPv6 address │ ipv6_addr │ 128 bit │ integer   │
       └─────────────┴───────────┴─────────┴───────────┘

       The IPv6 address type is used for IPv6 addresses. Addresses are
       specified as a host name or as hexadecimal halfwords separated by
       colons. Addresses might be enclosed in square brackets ("[]") to
       differentiate them from port numbers.

       IPv6 address specification.

           # abbreviated loopback address
           filter output ip6 daddr ::1

       IPv6 address specification with bracket notation.

           # without [] the port number (22) would be parsed as part of the
           # ipv6 address
           ip6 nat prerouting tcp dport 2222 dnat to [1ce::d0]:22


   BOOLEAN TYPE
       ┌────────┬─────────┬───────┬───────────┐
       │Name    Keyword Size  Base type │
       ├────────┼─────────┼───────┼───────────┤
       │        │         │       │           │
       │Boolean │ boolean │ 1 bit │ integer   │
       └────────┴─────────┴───────┴───────────┘

       The boolean type is a syntactical helper type in userspace. Its use is
       in the right-hand side of a (typically implicit) relational expression
       to change the expression on the left-hand side into a boolean check
       (usually for existence).

       Table 16. The following keywords will automatically resolve into a
       boolean type with given value
       ┌────────┬───────┐
       │Keyword Value │
       ├────────┼───────┤
       │        │       │
       │exists  │ 1     │
       ├────────┼───────┤
       │        │       │
       │missing │ 0     │
       └────────┴───────┘

       Table 17. expressions support a boolean comparison
       ┌───────────┬─────────────────────────┐
       │Expression Behaviour               │
       ├───────────┼─────────────────────────┤
       │           │                         │
       │fib        │ Check route existence.  │
       ├───────────┼─────────────────────────┤
       │           │                         │
       │exthdr     │ Check IPv6 extension    │
       │           │ header existence.       │
       ├───────────┼─────────────────────────┤
       │           │                         │
       │tcp option │ Check TCP option header │
       │           │ existence.              │
       └───────────┴─────────────────────────┘

       Boolean specification.

           # match if route exists
           filter input fib daddr . iif oif exists

           # match only non-fragmented packets in IPv6 traffic
           filter input exthdr frag missing

           # match if TCP timestamp option is present
           filter input tcp option timestamp exists


   ICMP TYPE TYPE
       ┌──────────┬───────────┬───────┬───────────┐
       │Name      Keyword   Size  Base type │
       ├──────────┼───────────┼───────┼───────────┤
       │          │           │       │           │
       │ICMP Type │ icmp_type │ 8 bit │ integer   │
       └──────────┴───────────┴───────┴───────────┘

       The ICMP Type type is used to conveniently specify the ICMP header’s
       type field.

       Table 18. Keywords may be used when specifying the ICMP type
       ┌────────────────────────┬───────┐
       │Keyword                 Value │
       ├────────────────────────┼───────┤
       │                        │       │
       │echo-reply              │ 0     │
       ├────────────────────────┼───────┤
       │                        │       │
       │destination-unreachable │ 3     │
       ├────────────────────────┼───────┤
       │                        │       │
       │source-quench           │ 4     │
       ├────────────────────────┼───────┤
       │                        │       │
       │redirect                │ 5     │
       ├────────────────────────┼───────┤
       │                        │       │
       │echo-request            │ 8     │
       ├────────────────────────┼───────┤
       │                        │       │
       │router-advertisement    │ 9     │
       ├────────────────────────┼───────┤
       │                        │       │
       │router-solicitation     │ 10    │
       ├────────────────────────┼───────┤
       │                        │       │
       │time-exceeded           │ 11    │
       ├────────────────────────┼───────┤
       │                        │       │
       │parameter-problem       │ 12    │
       ├────────────────────────┼───────┤
       │                        │       │
       │timestamp-request       │ 13    │
       ├────────────────────────┼───────┤
       │                        │       │
       │timestamp-reply         │ 14    │
       ├────────────────────────┼───────┤
       │                        │       │
       │info-request            │ 15    │
       ├────────────────────────┼───────┤
       │                        │       │
       │info-reply              │ 16    │
       ├────────────────────────┼───────┤
       │                        │       │
       │address-mask-request    │ 17    │
       ├────────────────────────┼───────┤
       │                        │       │
       │address-mask-reply      │ 18    │
       └────────────────────────┴───────┘

       ICMP Type specification.

           # match ping packets
           filter output icmp type { echo-request, echo-reply }


   ICMP CODE TYPE
       ┌──────────┬───────────┬───────┬───────────┐
       │Name      Keyword   Size  Base type │
       ├──────────┼───────────┼───────┼───────────┤
       │          │           │       │           │
       │ICMP Code │ icmp_code │ 8 bit │ integer   │
       └──────────┴───────────┴───────┴───────────┘

       The ICMP Code type is used to conveniently specify the ICMP header’s
       code field.

       Table 19. Keywords may be used when specifying the ICMP code
       ┌─────────────────┬───────┐
       │Keyword          Value │
       ├─────────────────┼───────┤
       │                 │       │
       │net-unreachable  │ 0     │
       ├─────────────────┼───────┤
       │                 │       │
       │host-unreachable │ 1     │
       ├─────────────────┼───────┤
       │                 │       │
       │prot-unreachable │ 2     │
       ├─────────────────┼───────┤
       │                 │       │
       │port-unreachable │ 3     │
       ├─────────────────┼───────┤
       │                 │       │
       │frag-needed      │ 4     │
       ├─────────────────┼───────┤
       │                 │       │
       │net-prohibited   │ 9     │
       ├─────────────────┼───────┤
       │                 │       │
       │host-prohibited  │ 10    │
       ├─────────────────┼───────┤
       │                 │       │
       │admin-prohibited │ 13    │
       └─────────────────┴───────┘

   ICMPV6 TYPE TYPE
       ┌────────────┬────────────┬───────┬───────────┐
       │Name        Keyword    Size  Base type │
       ├────────────┼────────────┼───────┼───────────┤
       │            │            │       │           │
       │ICMPv6 Type │ icmpx_code │ 8 bit │ integer   │
       └────────────┴────────────┴───────┴───────────┘

       The ICMPv6 Type type is used to conveniently specify the ICMPv6
       header’s type field.

       Table 20. keywords may be used when specifying the ICMPv6 type:
       ┌────────────────────────┬───────┐
       │Keyword                 Value │
       ├────────────────────────┼───────┤
       │                        │       │
       │destination-unreachable │ 1     │
       ├────────────────────────┼───────┤
       │                        │       │
       │packet-too-big          │ 2     │
       ├────────────────────────┼───────┤
       │                        │       │
       │time-exceeded           │ 3     │
       ├────────────────────────┼───────┤
       │                        │       │
       │parameter-problem       │ 4     │
       ├────────────────────────┼───────┤
       │                        │       │
       │echo-request            │ 128   │
       ├────────────────────────┼───────┤
       │                        │       │
       │echo-reply              │ 129   │
       ├────────────────────────┼───────┤
       │                        │       │
       │mld-listener-query      │ 130   │
       ├────────────────────────┼───────┤
       │                        │       │
       │mld-listener-report     │ 131   │
       ├────────────────────────┼───────┤
       │                        │       │
       │mld-listener-done       │ 132   │
       ├────────────────────────┼───────┤
       │                        │       │
       │mld-listener-reduction  │ 132   │
       ├────────────────────────┼───────┤
       │                        │       │
       │nd-router-solicit       │ 133   │
       ├────────────────────────┼───────┤
       │                        │       │
       │nd-router-advert        │ 134   │
       ├────────────────────────┼───────┤
       │                        │       │
       │nd-neighbor-solicit     │ 135   │
       ├────────────────────────┼───────┤
       │                        │       │
       │nd-neighbor-advert      │ 136   │
       ├────────────────────────┼───────┤
       │                        │       │
       │nd-redirect             │ 137   │
       ├────────────────────────┼───────┤
       │                        │       │
       │router-renumbering      │ 138   │
       ├────────────────────────┼───────┤
       │                        │       │
       │ind-neighbor-solicit    │ 141   │
       ├────────────────────────┼───────┤
       │                        │       │
       │ind-neighbor-advert     │ 142   │
       ├────────────────────────┼───────┤
       │                        │       │
       │mld2-listener-report    │ 143   │
       └────────────────────────┴───────┘

       ICMPv6 Type specification.

           # match ICMPv6 ping packets
           filter output icmpv6 type { echo-request, echo-reply }


   ICMPV6 CODE TYPE
       ┌────────────┬─────────────┬───────┬───────────┐
       │Name        Keyword     Size  Base type │
       ├────────────┼─────────────┼───────┼───────────┤
       │            │             │       │           │
       │ICMPv6 Code │ icmpv6_code │ 8 bit │ integer   │
       └────────────┴─────────────┴───────┴───────────┘

       The ICMPv6 Code type is used to conveniently specify the ICMPv6
       header’s code field.

       Table 21. keywords may be used when specifying the ICMPv6 code
       ┌─────────────────┬───────┐
       │Keyword          Value │
       ├─────────────────┼───────┤
       │                 │       │
       │no-route         │ 0     │
       ├─────────────────┼───────┤
       │                 │       │
       │admin-prohibited │ 1     │
       ├─────────────────┼───────┤
       │                 │       │
       │addr-unreachable │ 3     │
       ├─────────────────┼───────┤
       │                 │       │
       │port-unreachable │ 4     │
       ├─────────────────┼───────┤
       │                 │       │
       │policy-fail      │ 5     │
       ├─────────────────┼───────┤
       │                 │       │
       │reject-route     │ 6     │
       └─────────────────┴───────┘

   ICMPVX CODE TYPE
       ┌────────────┬─────────────┬───────┬───────────┐
       │Name        Keyword     Size  Base type │
       ├────────────┼─────────────┼───────┼───────────┤
       │            │             │       │           │
       │ICMPvX Code │ icmpv6_type │ 8 bit │ integer   │
       └────────────┴─────────────┴───────┴───────────┘

       The ICMPvX Code type abstraction is a set of values which overlap
       between ICMP and ICMPv6 Code types to be used from the inet family.

       Table 22. keywords may be used when specifying the ICMPvX code
       ┌─────────────────┬───────┐
       │Keyword          Value │
       ├─────────────────┼───────┤
       │                 │       │
       │no-route         │ 0     │
       ├─────────────────┼───────┤
       │                 │       │
       │port-unreachable │ 1     │
       ├─────────────────┼───────┤
       │                 │       │
       │host-unreachable │ 2     │
       ├─────────────────┼───────┤
       │                 │       │
       │admin-prohibited │ 3     │
       └─────────────────┴───────┘

   CONNTRACK TYPES
       Table 23. overview of types used in ct expression and statement
       ┌─────────────────┬───────────┬─────────┬───────────┐
       │Name             Keyword   Size    Base type │
       ├─────────────────┼───────────┼─────────┼───────────┤
       │                 │           │         │           │
       │conntrack state  │ ct_state  │ 4 byte  │ bitmask   │
       ├─────────────────┼───────────┼─────────┼───────────┤
       │                 │           │         │           │
       │conntrack        │ ct_dir    │ 8 bit   │ integer   │
       │direction        │           │         │           │
       ├─────────────────┼───────────┼─────────┼───────────┤
       │                 │           │         │           │
       │conntrack status │ ct_status │ 4 byte  │ bitmask   │
       ├─────────────────┼───────────┼─────────┼───────────┤
       │                 │           │         │           │
       │conntrack event  │ ct_event  │ 4 byte  │ bitmask   │
       │bits             │           │         │           │
       ├─────────────────┼───────────┼─────────┼───────────┤
       │                 │           │         │           │
       │conntrack label  │ ct_label  │ 128 bit │ bitmask   │
       └─────────────────┴───────────┴─────────┴───────────┘

       For each of the types above, keywords are available for convenience:

       Table 24. conntrack state (ct_state)
       ┌────────────┬───────┐
       │Keyword     Value │
       ├────────────┼───────┤
       │            │       │
       │invalid     │ 1     │
       ├────────────┼───────┤
       │            │       │
       │established │ 2     │
       ├────────────┼───────┤
       │            │       │
       │related     │ 4     │
       ├────────────┼───────┤
       │            │       │
       │new         │ 8     │
       ├────────────┼───────┤
       │            │       │
       │untracked   │ 64    │
       └────────────┴───────┘

       Table 25. conntrack direction (ct_dir)
       ┌─────────┬───────┐
       │Keyword  Value │
       ├─────────┼───────┤
       │         │       │
       │original │ 0     │
       ├─────────┼───────┤
       │         │       │
       │reply    │ 1     │
       └─────────┴───────┘

       Table 26. conntrack status (ct_status)
       ┌───────────┬───────┐
       │Keyword    Value │
       ├───────────┼───────┤
       │           │       │
       │expected   │ 1     │
       ├───────────┼───────┤
       │           │       │
       │seen-reply │ 2     │
       ├───────────┼───────┤
       │           │       │
       │assured    │ 4     │
       ├───────────┼───────┤
       │           │       │
       │confirmed  │ 8     │
       ├───────────┼───────┤
       │           │       │
       │snat       │ 16    │
       ├───────────┼───────┤
       │           │       │
       │dnat       │ 32    │
       ├───────────┼───────┤
       │           │       │
       │dying      │ 512   │
       └───────────┴───────┘

       Table 27. conntrack event bits (ct_event)
       ┌──────────┬───────┐
       │Keyword   Value │
       ├──────────┼───────┤
       │          │       │
       │new       │ 1     │
       ├──────────┼───────┤
       │          │       │
       │related   │ 2     │
       ├──────────┼───────┤
       │          │       │
       │destroy   │ 4     │
       ├──────────┼───────┤
       │          │       │
       │reply     │ 8     │
       ├──────────┼───────┤
       │          │       │
       │assured   │ 16    │
       ├──────────┼───────┤
       │          │       │
       │protoinfo │ 32    │
       ├──────────┼───────┤
       │          │       │
       │helper    │ 64    │
       ├──────────┼───────┤
       │          │       │
       │mark      │ 128   │
       ├──────────┼───────┤
       │          │       │
       │seqadj    │ 256   │
       ├──────────┼───────┤
       │          │       │
       │secmark   │ 512   │
       ├──────────┼───────┤
       │          │       │
       │label     │ 1024  │
       └──────────┴───────┘

       Possible keywords for conntrack label type (ct_label) are read at
       runtime from /etc/connlabel.conf.

PRIMARY EXPRESSIONS
       The lowest order expression is a primary expression, representing
       either a constant or a single datum from a packet’s payload, meta data
       or a stateful module.

   META EXPRESSIONS
           meta {length | nfproto | l4proto | protocol | priority}
           [meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype | skuid | skgid | nftrace | rtclassid | ibrname | obrname | pkttype | cpu | iifgroup | oifgroup | cgroup | random | ipsec | iifkind | oifkind | time | hour | day }

       A meta expression refers to meta data associated with a packet.

       There are two types of meta expressions: unqualified and qualified meta
       expressions. Qualified meta expressions require the meta keyword before
       the meta key, unqualified meta expressions can be specified by using
       the meta key directly or as qualified meta expressions. Meta l4proto is
       useful to match a particular transport protocol that is part of either
       an IPv4 or IPv6 packet. It will also skip any IPv6 extension headers
       present in an IPv6 packet.

       meta iif, oif, iifname and oifname are used to match the interface a
       packet arrived on or is about to be sent out on.

       iif and oif are used to match on the interface index, whereas iifname
       and oifname are used to match on the interface name. This is not the
       same — assuming the rule

           filter input meta iif "foo"

       Then this rule can only be added if the interface "foo" exists. Also,
       the rule will continue to match even if the interface "foo" is renamed
       to "bar".

       This is because internally the interface index is used. In case of
       dynamically created interfaces, such as tun/tap or dialup interfaces
       (ppp for example), it might be better to use iifname or oifname
       instead.

       In these cases, the name is used so the interface doesn’t have to exist
       to add such a rule, it will stop matching if the interface gets renamed
       and it will match again in case interface gets deleted and later a new
       interface with the same name is created.

       Like with iptables, wildcard matching on interface name prefixes is
       available for iifname and oifname matches by appending an asterisk (*)
       character. Note however that unlike iptables, nftables does not accept
       interface names consisting of the wildcard character only - users are
       supposed to just skip those always matching expressions. In order to
       match on literal asterisk character, one may escape it using backslash
       (\).

       Table 28. Meta expression types
       ┌──────────┬─────────────────────┬─────────────────────┐
       │Keyword   Description         Type                │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │length    │ Length of the       │ integer (32-bit)    │
       │          │ packet in bytes     │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │nfproto   │ real hook protocol  │ integer (32 bit)    │
       │          │ family, useful only │                     │
       │          │ in inet table       │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │l4proto   │ layer 4 protocol,   │ integer (8 bit)     │
       │          │ skips ipv6          │                     │
       │          │ extension headers   │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │protocol  │ EtherType protocol  │ ether_type          │
       │          │ value               │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │priority  │ TC packet priority  │ tc_handle           │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │mark      │ Packet mark         │ mark                │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │iif       │ Input interface     │ iface_index         │
       │          │ index               │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │iifname   │ Input interface     │ ifname              │
       │          │ name                │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │iiftype   │ Input interface     │ iface_type          │
       │          │ type                │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │oif       │ Output interface    │ iface_index         │
       │          │ index               │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │oifname   │ Output interface    │ ifname              │
       │          │ name                │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │oiftype   │ Output interface    │ iface_type          │
       │          │ hardware type       │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │sdif      │ Slave device input  │ iface_index         │
       │          │ interface index     │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │sdifname  │ Slave device        │ ifname              │
       │          │ interface name      │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │skuid     │ UID associated with │ uid                 │
       │          │ originating socket  │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │skgid     │ GID associated with │ gid                 │
       │          │ originating socket  │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │rtclassid │ Routing realm       │ realm               │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │ibrname   │ Input bridge        │ ifname              │
       │          │ interface name      │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │obrname   │ Output bridge       │ ifname              │
       │          │ interface name      │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │pkttype   │ packet type         │ pkt_type            │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │cpu       │ cpu number          │ integer (32 bit)    │
       │          │ processing the      │                     │
       │          │ packet              │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │iifgroup  │ incoming device     │ devgroup            │
       │          │ group               │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │oifgroup  │ outgoing device     │ devgroup            │
       │          │ group               │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │cgroup    │ control group id    │ integer (32 bit)    │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │random    │ pseudo-random       │ integer (32 bit)    │
       │          │ number              │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │ipsec     │ boolean             │ boolean (1 bit)     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │iifkind   │ Input interface     │                     │
       │          │ kind                │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │oifkind   │ Output interface    │                     │
       │          │ kind                │                     │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │time      │ Absolute time of    │ Integer (32 bit) or │
       │          │ packet reception    │ string              │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │day       │ Day of week         │ Integer (8 bit) or  │
       │          │                     │ string              │
       ├──────────┼─────────────────────┼─────────────────────┤
       │          │                     │                     │
       │hour      │ Hour of day         │ String              │
       └──────────┴─────────────────────┴─────────────────────┘

       Table 29. Meta expression specific types
       ┌──────────────┬────────────────────────────┐
       │Type          Description                │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │iface_index   │ Interface index (32 bit    │
       │              │ number). Can be specified  │
       │              │ numerically or as name of  │
       │              │ an existing interface.     │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │ifname        │ Interface name (16 byte    │
       │              │ string). Does not have to  │
       │              │ exist.                     │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │iface_type    │ Interface type (16 bit     │
       │              │ number).                   │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │uid           │ User ID (32 bit number).   │
       │              │ Can be specified           │
       │              │ numerically or as user     │
       │              │ name.                      │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │gid           │ Group ID (32 bit number).  │
       │              │ Can be specified           │
       │              │ numerically or as group    │
       │              │ name.                      │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │realm         │ Routing Realm (32 bit      │
       │              │ number). Can be specified  │
       │              │ numerically or as symbolic │
       │              │ name defined in            │
       │              │ /etc/iproute2/rt_realms.   │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │devgroup_type │ Device group (32 bit       │
       │              │ number). Can be specified  │
       │              │ numerically or as symbolic │
       │              │ name defined in            │
       │              │ /etc/iproute2/group.       │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │pkt_type      │ Packet type: host          │
       │              │ (addressed to local host), │
       │              │ broadcast (to all),        │
       │              │ multicast (to group),      │
       │              │ other (addressed to        │
       │              │ another host).             │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │ifkind        │ Interface kind (16 byte    │
       │              │ string). Does not have to  │
       │              │ exist.                     │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │time          │ Either an integer or a     │
       │              │ date in ISO format. For    │
       │              │ example: "2019-06-06       │
       │              │ 17:00". Hour and seconds   │
       │              │ are optional and can be    │
       │              │ omitted if desired. If     │
       │              │ omitted, midnight will be  │
       │              │ assumed. The following     │
       │              │ three would be equivalent: │
       │              │ "2019-06-06", "2019-06-06  │
       │              │ 00:00" and "2019-06-06     │
       │              │ 00:00:00". When an integer │
       │              │ is given, it is assumed to │
       │              │ be a UNIX timestamp.       │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │day           │ Either a day of week       │
       │              │ ("Monday", "Tuesday",      │
       │              │ etc.), or an integer       │
       │              │ between 0 and 6. Strings   │
       │              │ are matched                │
       │              │ case-insensitively, and a  │
       │              │ full match is not expected │
       │              │ (e.g. "Mon" would match    │
       │              │ "Monday"). When an integer │
       │              │ is given, 0 is Sunday and  │
       │              │ 6 is Saturday.             │
       ├──────────────┼────────────────────────────┤
       │              │                            │
       │hour          │ A string representing an   │
       │              │ hour in 24-hour format.    │
       │              │ Seconds can optionally be  │
       │              │ specified. For example,    │
       │              │ 17:00 and 17:00:00 would   │
       │              │ be equivalent.             │
       └──────────────┴────────────────────────────┘

       Using meta expressions.

           # qualified meta expression
           filter output meta oif eth0

           # unqualified meta expression
           filter output oif eth0

           # packet was subject to ipsec processing
           raw prerouting meta ipsec exists accept


   SOCKET EXPRESSION
           socket {transparent | mark}

       Socket expression can be used to search for an existing open TCP/UDP
       socket and its attributes that can be associated with a packet. It
       looks for an established or non-zero bound listening socket (possibly
       with a non-local address).

       Table 30. Available socket attributes
       ┌────────────┬─────────────────────┬─────────────────┐
       │Name        Description         Type            │
       ├────────────┼─────────────────────┼─────────────────┤
       │            │                     │                 │
       │transparent │ Value of the        │ boolean (1 bit) │
       │            │ IP_TRANSPARENT      │                 │
       │            │ socket option in    │                 │
       │            │ the found socket.   │                 │
       │            │ It can be 0 or 1.   │                 │
       ├────────────┼─────────────────────┼─────────────────┤
       │            │                     │                 │
       │mark        │ Value of the socket │ mark            │
       │            │ mark (SOL_SOCKET,   │                 │
       │            │ SO_MARK).           │                 │
       └────────────┴─────────────────────┴─────────────────┘

       Using socket expression.

           # Mark packets that correspond to a transparent socket
           table inet x {
               chain y {
                   type filter hook prerouting priority -150; policy accept;
                   socket transparent 1 mark set 0x00000001 accept
               }
           }

           # Trace packets that corresponds to a socket with a mark value of 15
           table inet x {
               chain y {
                   type filter hook prerouting priority -150; policy accept;
                   socket mark 0x0000000f nftrace set 1
               }
           }

           # Set packet mark to socket mark
           table inet x {
               chain y {
                   type filter hook prerouting priority -150; policy accept;
                   tcp dport 8080 mark set socket mark
               }
           }


   OSF EXPRESSION
           osf [ttl {loose | skip}] {name | version}

       The osf expression does passive operating system fingerprinting. This
       expression compares some data (Window Size, MSS, options and their
       order, DF, and others) from packets with the SYN bit set.

       Table 31. Available osf attributes
       ┌────────┬─────────────────────┬────────┐
       │Name    Description         Type   │
       ├────────┼─────────────────────┼────────┤
       │        │                     │        │
       │ttl     │ Do TTL checks on    │ string │
       │        │ the packet to       │        │
       │        │ determine the       │        │
       │        │ operating system.   │        │
       ├────────┼─────────────────────┼────────┤
       │        │                     │        │
       │version │ Do OS version       │        │
       │        │ checks on the       │        │
       │        │ packet.             │        │
       ├────────┼─────────────────────┼────────┤
       │        │                     │        │
       │name    │ Name of the OS      │ string │
       │        │ signature to match. │        │
       │        │ All signatures can  │        │
       │        │ be found at pf.os   │        │
       │        │ file. Use "unknown" │        │
       │        │ for OS signatures   │        │
       │        │ that the expression │        │
       │        │ could not detect.   │        │
       └────────┴─────────────────────┴────────┘

       Available ttl values.

           If no TTL attribute is passed, make a true IP header and fingerprint TTL true comparison. This generally works for LANs.

           * loose: Check if the IP header's TTL is less than the fingerprint one. Works for globally-routable addresses.
           * skip: Do not compare the TTL at all.

       Using osf expression.

           # Accept packets that match the "Linux" OS genre signature without comparing TTL.
           table inet x {
               chain y {
                   type filter hook input priority 0; policy accept;
                   osf ttl skip name "Linux"
               }
           }


   FIB EXPRESSIONS
           fib {saddr | daddr | mark | iif | oif} [. ...] {oif | oifname | type}

       A fib expression queries the fib (forwarding information base) to
       obtain information such as the output interface index a particular
       address would use. The input is a tuple of elements that is used as
       input to the fib lookup functions.

       Table 32. fib expression specific types
       ┌────────┬──────────────────┬──────────────────┐
       │Keyword Description      Type             │
       ├────────┼──────────────────┼──────────────────┤
       │        │                  │                  │
       │oif     │ Output interface │ integer (32 bit) │
       │        │ index            │                  │
       ├────────┼──────────────────┼──────────────────┤
       │        │                  │                  │
       │oifname │ Output interface │ string           │
       │        │ name             │                  │
       ├────────┼──────────────────┼──────────────────┤
       │        │                  │                  │
       │type    │ Address type     │ fib_addrtype     │
       └────────┴──────────────────┴──────────────────┘

       Use nft describe fib_addrtype to get a list of all address types.

       Using fib expressions.

           # drop packets without a reverse path
           filter prerouting fib saddr . iif oif missing drop

           In this example, 'saddr . iif' looks up routing information based on the source address and the input interface.
           oif picks the output interface index from the routing information.
           If no route was found for the source address/input interface combination, the output interface index is zero.
           In case the input interface is specified as part of the input key, the output interface index is always the same as the input interface index or zero.
           If only 'saddr oif' is given, then oif can be any interface index or zero.

           # drop packets to address not configured on incoming interface
           filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop

           # perform lookup in a specific 'blackhole' table (0xdead, needs ip appropriate ip rule)
           filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }


   ROUTING EXPRESSIONS
           rt [ip | ip6] {classid | nexthop | mtu | ipsec}

       A routing expression refers to routing data associated with a packet.

       Table 33. Routing expression types
       ┌────────┬─────────────────────┬─────────────────────┐
       │Keyword Description         Type                │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │classid │ Routing realm       │ realm               │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │nexthop │ Routing nexthop     │ ipv4_addr/ipv6_addr │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │mtu     │ TCP maximum segment │ integer (16 bit)    │
       │        │ size of route       │                     │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │ipsec   │ route via ipsec     │ boolean             │
       │        │ tunnel or transport │                     │
       └────────┴─────────────────────┴─────────────────────┘

       Table 34. Routing expression specific types
       ┌──────┬────────────────────────────┐
       │Type  Description                │
       ├──────┼────────────────────────────┤
       │      │                            │
       │realm │ Routing Realm (32 bit      │
       │      │ number). Can be specified  │
       │      │ numerically or as symbolic │
       │      │ name defined in            │
       │      │ /etc/iproute2/rt_realms.   │
       └──────┴────────────────────────────┘

       Using routing expressions.

           # IP family independent rt expression
           filter output rt classid 10
           filter output rt ipsec missing

           # IP family dependent rt expressions
           ip filter output rt nexthop 192.168.0.1
           ip6 filter output rt nexthop fd00::1
           inet filter output rt ip nexthop 192.168.0.1
           inet filter output rt ip6 nexthop fd00::1


   IPSEC EXPRESSIONS
           ipsec {in | out} [ spnum NUM ]  {reqid | spi}
           ipsec {in | out} [ spnum NUM ]  {ip | ip6} {saddr | daddr}

       An ipsec expression refers to ipsec data associated with a packet.

       The in or out keyword needs to be used to specify if the expression
       should examine inbound or outbound policies. The in keyword can be used
       in the prerouting, input and forward hooks. The out keyword applies to
       forward, output and postrouting hooks. The optional keyword spnum can
       be used to match a specific state in a chain, it defaults to 0.

       Table 35. Ipsec expression types
       ┌────────┬─────────────────────┬─────────────────────┐
       │Keyword Description         Type                │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │reqid   │ Request ID          │ integer (32 bit)    │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │spi     │ Security Parameter  │ integer (32 bit)    │
       │        │ Index               │                     │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │saddr   │ Source address of   │ ipv4_addr/ipv6_addr │
       │        │ the tunnel          │                     │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │daddr   │ Destination address │ ipv4_addr/ipv6_addr │
       │        │ of the tunnel       │                     │
       └────────┴─────────────────────┴─────────────────────┘

   NUMGEN EXPRESSION
           numgen {inc | random} mod NUM [ offset NUM ]

       Create a number generator. The inc or random keywords control its
       operation mode: In inc mode, the last returned value is simply
       incremented. In random mode, a new random number is returned. The value
       after mod keyword specifies an upper boundary (read: modulus) which is
       not reached by returned numbers. The optional offset allows to
       increment the returned value by a fixed offset.

       A typical use-case for numgen is load-balancing:

       Using numgen expression.

           # round-robin between 192.168.10.100 and 192.168.20.200:
           add rule nat prerouting dnat to numgen inc mod 2 map \
                   { 0 : 192.168.10.100, 1 : 192.168.20.200 }

           # probability-based with odd bias using intervals:
           add rule nat prerouting dnat to numgen random mod 10 map \
                   { 0-2 : 192.168.10.100, 3-9 : 192.168.20.200 }


   HASH EXPRESSIONS
           jhash {ip saddr | ip6 daddr | tcp dport | udp sport | ether saddr} [. ...] mod NUM [ seed NUM ] [ offset NUM ]
           symhash mod NUM [ offset NUM ]

       Use a hashing function to generate a number. The functions available
       are jhash, known as Jenkins Hash, and symhash, for Symmetric Hash. The
       jhash requires an expression to determine the parameters of the packet
       header to apply the hashing, concatenations are possible as well. The
       value after mod keyword specifies an upper boundary (read: modulus)
       which is not reached by returned numbers. The optional seed is used to
       specify an init value used as seed in the hashing function. The
       optional offset allows to increment the returned value by a fixed
       offset.

       A typical use-case for jhash and symhash is load-balancing:

       Using hash expressions.

           # load balance based on source ip between 2 ip addresses:
           add rule nat prerouting dnat to jhash ip saddr mod 2 map \
                   { 0 : 192.168.10.100, 1 : 192.168.20.200 }

           # symmetric load balancing between 2 ip addresses:
           add rule nat prerouting dnat to symhash mod 2 map \
                   { 0 : 192.168.10.100, 1 : 192.168.20.200 }


PAYLOAD EXPRESSIONS
       Payload expressions refer to data from the packet’s payload.

   ETHERNET HEADER EXPRESSION
           ether {daddr | saddr | type}

       Table 36. Ethernet header expression types
       ┌────────┬────────────────────┬────────────┐
       │Keyword Description        Type       │
       ├────────┼────────────────────┼────────────┤
       │        │                    │            │
       │daddr   │ Destination MAC    │ ether_addr │
       │        │ address            │            │
       ├────────┼────────────────────┼────────────┤
       │        │                    │            │
       │saddr   │ Source MAC address │ ether_addr │
       ├────────┼────────────────────┼────────────┤
       │        │                    │            │
       │type    │ EtherType          │ ether_type │
       └────────┴────────────────────┴────────────┘

   VLAN HEADER EXPRESSION
           vlan {id | cfi | pcp | type}

       Table 37. VLAN header expression
       ┌────────┬─────────────────────┬──────────────────┐
       │Keyword Description         Type             │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │id      │ VLAN ID (VID)       │ integer (12 bit) │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │cfi     │ Canonical Format    │ integer (1 bit)  │
       │        │ Indicator           │                  │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │pcp     │ Priority code point │ integer (3 bit)  │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │type    │ EtherType           │ ether_type       │
       └────────┴─────────────────────┴──────────────────┘

   ARP HEADER EXPRESSION
           arp {htype | ptype | hlen | plen | operation | saddr { ip | ether } | daddr { ip | ether }

       Table 38. ARP header expression
       ┌────────────┬─────────────────────┬──────────────────┐
       │Keyword     Description         Type             │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │htype       │ ARP hardware type   │ integer (16 bit) │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │ptype       │ EtherType           │ ether_type       │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │hlen        │ Hardware address    │ integer (8 bit)  │
       │            │ len                 │                  │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │plen        │ Protocol address    │ integer (8 bit)  │
       │            │ len                 │                  │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │operation   │ Operation           │ arp_op           │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │saddr ether │ Ethernet sender     │ ether_addr       │
       │            │ address             │                  │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │daddr ether │ Ethernet target     │ ether_addr       │
       │            │ address             │                  │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │saddr ip    │ IPv4 sender address │ ipv4_addr        │
       ├────────────┼─────────────────────┼──────────────────┤
       │            │                     │                  │
       │daddr ip    │ IPv4 target address │ ipv4_addr        │
       └────────────┴─────────────────────┴──────────────────┘

   IPV4 HEADER EXPRESSION
           ip {version | hdrlength | dscp | ecn | length | id | frag-off | ttl | protocol | checksum | saddr | daddr }

       Table 39. IPv4 header expression
       ┌──────────┬─────────────────────┬──────────────────┐
       │Keyword   Description         Type             │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │version   │ IP header version   │ integer (4 bit)  │
       │          │ (4)                 │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │hdrlength │ IP header length    │ integer (4 bit)  │
       │          │ including options   │ FIXME scaling    │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │dscp      │ Differentiated      │ dscp             │
       │          │ Services Code Point │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │ecn       │ Explicit Congestion │ ecn              │
       │          │ Notification        │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │length    │ Total packet length │ integer (16 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │id        │ IP ID               │ integer (16 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │frag-off  │ Fragment offset     │ integer (16 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │ttl       │ Time to live        │ integer (8 bit)  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │protocol  │ Upper layer         │ inet_proto       │
       │          │ protocol            │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │checksum  │ IP header checksum  │ integer (16 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │saddr     │ Source address      │ ipv4_addr        │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │daddr     │ Destination address │ ipv4_addr        │
       └──────────┴─────────────────────┴──────────────────┘

   ICMP HEADER EXPRESSION
           icmp {type | code | checksum | id | sequence | gateway | mtu}

       This expression refers to ICMP header fields. When using it in inet,
       bridge or netdev families, it will cause an implicit dependency on IPv4
       to be created. To match on unusual cases like ICMP over IPv6, one has
       to add an explicit meta protocol ip6 match to the rule.

       Table 40. ICMP header expression
       ┌─────────┬─────────────────────┬──────────────────┐
       │Keyword  Description         Type             │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │type     │ ICMP type field     │ icmp_type        │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │code     │ ICMP code field     │ integer (8 bit)  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │checksum │ ICMP checksum field │ integer (16 bit) │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │id       │ ID of echo          │ integer (16 bit) │
       │         │ request/response    │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │sequence │ sequence number of  │ integer (16 bit) │
       │         │ echo                │                  │
       │         │ request/response    │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │gateway  │ gateway of          │ integer (32 bit) │
       │         │ redirects           │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │mtu      │ MTU of path MTU     │ integer (16 bit) │
       │         │ discovery           │                  │
       └─────────┴─────────────────────┴──────────────────┘

   IGMP HEADER EXPRESSION
           igmp {type | mrt | checksum | group}

       This expression refers to IGMP header fields. When using it in inet,
       bridge or netdev families, it will cause an implicit dependency on IPv4
       to be created. To match on unusual cases like IGMP over IPv6, one has
       to add an explicit meta protocol ip6 match to the rule.

       Table 41. IGMP header expression
       ┌─────────┬─────────────────────┬──────────────────┐
       │Keyword  Description         Type             │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │type     │ IGMP type field     │ igmp_type        │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │mrt      │ IGMP maximum        │ integer (8 bit)  │
       │         │ response time field │                  │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │checksum │ IGMP checksum field │ integer (16 bit) │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │group    │ Group address       │ integer (32 bit) │
       └─────────┴─────────────────────┴──────────────────┘

   IPV6 HEADER EXPRESSION
           ip6 {version | dscp | ecn | flowlabel | length | nexthdr | hoplimit | saddr | daddr}

       This expression refers to the ipv6 header fields. Caution when using
       ip6 nexthdr, the value only refers to the next header, i.e. ip6 nexthdr
       tcp will only match if the ipv6 packet does not contain any extension
       headers. Packets that are fragmented or e.g. contain a routing
       extension headers will not be matched. Please use meta l4proto if you
       wish to match the real transport header and ignore any additional
       extension headers instead.

       Table 42. IPv6 header expression
       ┌──────────┬─────────────────────┬──────────────────┐
       │Keyword   Description         Type             │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │version   │ IP header version   │ integer (4 bit)  │
       │          │ (6)                 │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │dscp      │ Differentiated      │ dscp             │
       │          │ Services Code Point │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │ecn       │ Explicit Congestion │ ecn              │
       │          │ Notification        │                  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │flowlabel │ Flow label          │ integer (20 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │length    │ Payload length      │ integer (16 bit) │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │nexthdr   │ Nexthdr protocol    │ inet_proto       │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │hoplimit  │ Hop limit           │ integer (8 bit)  │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │saddr     │ Source address      │ ipv6_addr        │
       ├──────────┼─────────────────────┼──────────────────┤
       │          │                     │                  │
       │daddr     │ Destination address │ ipv6_addr        │
       └──────────┴─────────────────────┴──────────────────┘

       Using ip6 header expressions.

           # matching if first extension header indicates a fragment
           ip6 nexthdr ipv6-frag


   ICMPV6 HEADER EXPRESSION
           icmpv6 {type | code | checksum | parameter-problem | packet-too-big | id | sequence | max-delay}

       This expression refers to ICMPv6 header fields. When using it in inet,
       bridge or netdev families, it will cause an implicit dependency on IPv6
       to be created. To match on unusual cases like ICMPv6 over IPv4, one has
       to add an explicit meta protocol ip match to the rule.

       Table 43. ICMPv6 header expression
       ┌──────────────────┬────────────────────┬──────────────────┐
       │Keyword           Description        Type             │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │type              │ ICMPv6 type field  │ icmpv6_type      │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │code              │ ICMPv6 code field  │ integer (8 bit)  │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │checksum          │ ICMPv6 checksum    │ integer (16 bit) │
       │                  │ field              │                  │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │parameter-problem │ pointer to problem │ integer (32 bit) │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │packet-too-big    │ oversized MTU      │ integer (32 bit) │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │id                │ ID of echo         │ integer (16 bit) │
       │                  │ request/response   │                  │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │sequence          │ sequence number of │ integer (16 bit) │
       │                  │ echo               │                  │
       │                  │ request/response   │                  │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │max-delay         │ maximum response   │ integer (16 bit) │
       │                  │ delay of MLD       │                  │
       │                  │ queries            │                  │
       └──────────────────┴────────────────────┴──────────────────┘

   TCP HEADER EXPRESSION
           tcp {sport | dport | sequence | ackseq | doff | reserved | flags | window | checksum | urgptr}

       Table 44. TCP header expression
       ┌─────────┬──────────────────┬──────────────────┐
       │Keyword  Description      Type             │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │sport    │ Source port      │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │dport    │ Destination port │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │sequence │ Sequence number  │ integer (32 bit) │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │ackseq   │ Acknowledgement  │ integer (32 bit) │
       │         │ number           │                  │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │doff     │ Data offset      │ integer (4 bit)  │
       │         │                  │ FIXME scaling    │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │reserved │ Reserved area    │ integer (4 bit)  │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │flags    │ TCP flags        │ tcp_flag         │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │window   │ Window           │ integer (16 bit) │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │checksum │ Checksum         │ integer (16 bit) │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │urgptr   │ Urgent pointer   │ integer (16 bit) │
       └─────────┴──────────────────┴──────────────────┘

   UDP HEADER EXPRESSION
           udp {sport | dport | length | checksum}

       Table 45. UDP header expression
       ┌─────────┬─────────────────────┬──────────────────┐
       │Keyword  Description         Type             │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │sport    │ Source port         │ inet_service     │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │dport    │ Destination port    │ inet_service     │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │length   │ Total packet length │ integer (16 bit) │
       ├─────────┼─────────────────────┼──────────────────┤
       │         │                     │                  │
       │checksum │ Checksum            │ integer (16 bit) │
       └─────────┴─────────────────────┴──────────────────┘

   UDP-LITE HEADER EXPRESSION
           udplite {sport | dport | checksum}

       Table 46. UDP-Lite header expression
       ┌─────────┬──────────────────┬──────────────────┐
       │Keyword  Description      Type             │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │sport    │ Source port      │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │dport    │ Destination port │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │checksum │ Checksum         │ integer (16 bit) │
       └─────────┴──────────────────┴──────────────────┘

   SCTP HEADER EXPRESSION
           sctp {sport | dport | vtag | checksum}

       Table 47. SCTP header expression
       ┌─────────┬──────────────────┬──────────────────┐
       │Keyword  Description      Type             │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │sport    │ Source port      │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │dport    │ Destination port │ inet_service     │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │vtag     │ Verification Tag │ integer (32 bit) │
       ├─────────┼──────────────────┼──────────────────┤
       │         │                  │                  │
       │checksum │ Checksum         │ integer (32 bit) │
       └─────────┴──────────────────┴──────────────────┘

   DCCP HEADER EXPRESSION
           dccp {sport | dport}

       Table 48. DCCP header expression
       ┌────────┬──────────────────┬──────────────┐
       │Keyword Description      Type         │
       ├────────┼──────────────────┼──────────────┤
       │        │                  │              │
       │sport   │ Source port      │ inet_service │
       ├────────┼──────────────────┼──────────────┤
       │        │                  │              │
       │dport   │ Destination port │ inet_service │
       └────────┴──────────────────┴──────────────┘

   AUTHENTICATION HEADER EXPRESSION
           ah {nexthdr | hdrlength | reserved | spi | sequence}

       Table 49. AH header expression
       ┌──────────┬────────────────────┬──────────────────┐
       │Keyword   Description        Type             │
       ├──────────┼────────────────────┼──────────────────┤
       │          │                    │                  │
       │nexthdr   │ Next header        │ inet_proto       │
       │          │ protocol           │                  │
       ├──────────┼────────────────────┼──────────────────┤
       │          │                    │                  │
       │hdrlength │ AH Header length   │ integer (8 bit)  │
       ├──────────┼────────────────────┼──────────────────┤
       │          │                    │                  │
       │reserved  │ Reserved area      │ integer (16 bit) │
       ├──────────┼────────────────────┼──────────────────┤
       │          │                    │                  │
       │spi       │ Security Parameter │ integer (32 bit) │
       │          │ Index              │                  │
       ├──────────┼────────────────────┼──────────────────┤
       │          │                    │                  │
       │sequence  │ Sequence number    │ integer (32 bit) │
       └──────────┴────────────────────┴──────────────────┘

   ENCRYPTED SECURITY PAYLOAD HEADER EXPRESSION
           esp {spi | sequence}

       Table 50. ESP header expression
       ┌─────────┬────────────────────┬──────────────────┐
       │Keyword  Description        Type             │
       ├─────────┼────────────────────┼──────────────────┤
       │         │                    │                  │
       │spi      │ Security Parameter │ integer (32 bit) │
       │         │ Index              │                  │
       ├─────────┼────────────────────┼──────────────────┤
       │         │                    │                  │
       │sequence │ Sequence number    │ integer (32 bit) │
       └─────────┴────────────────────┴──────────────────┘

   IPCOMP HEADER EXPRESSION
       comp {nexthdr | flags | cpi}

       Table 51. IPComp header expression
       ┌────────┬─────────────────┬──────────────────┐
       │Keyword Description     Type             │
       ├────────┼─────────────────┼──────────────────┤
       │        │                 │                  │
       │nexthdr │ Next header     │ inet_proto       │
       │        │ protocol        │                  │
       ├────────┼─────────────────┼──────────────────┤
       │        │                 │                  │
       │flags   │ Flags           │ bitmask          │
       ├────────┼─────────────────┼──────────────────┤
       │        │                 │                  │
       │cpi     │ compression     │ integer (16 bit) │
       │        │ Parameter Index │                  │
       └────────┴─────────────────┴──────────────────┘

   RAW PAYLOAD EXPRESSION
           @base,offset,length

       The raw payload expression instructs to load length bits starting at
       offset bits. Bit 0 refers to the very first bit — in the C programming
       language, this corresponds to the topmost bit, i.e. 0x80 in case of an
       octet. They are useful to match headers that do not have a
       human-readable template expression yet. Note that nft will not add
       dependencies for Raw payload expressions. If you e.g. want to match
       protocol fields of a transport header with protocol number 5, you need
       to manually exclude packets that have a different transport header, for
       instance by using meta l4proto 5 before the raw expression.

       Table 52. Supported payload protocol bases
       ┌─────┬─────────────────────────┐
       │Base Description             │
       ├─────┼─────────────────────────┤
       │     │                         │
       │ll   │ Link layer, for example │
       │     │ the Ethernet header     │
       ├─────┼─────────────────────────┤
       │     │                         │
       │nh   │ Network header, for     │
       │     │ example IPv4 or IPv6    │
       ├─────┼─────────────────────────┤
       │     │                         │
       │th   │ Transport Header, for   │
       │     │ example TCP             │
       └─────┴─────────────────────────┘

       Matching destination port of both UDP and TCP.

           inet filter input meta l4proto {tcp, udp} @th,16,16 { 53, 80 }

       The above can also be written as

           inet filter input meta l4proto {tcp, udp} th dport { 53, 80 }

       it is more convenient, but like the raw expression notation no
       dependencies are created or checked. It is the users responsibility to
       restrict matching to those header types that have a notion of ports.
       Otherwise, rules using raw expressions will errnously match unrelated
       packets, e.g. mis-interpreting ESP packets SPI field as a port.

       Rewrite arp packet target hardware address if target protocol address
       matches a given address.

           input meta iifname enp2s0 arp ptype 0x0800 arp htype 1 arp hlen 6 arp plen 4 @nh,192,32 0xc0a88f10 @nh,144,48 set 0x112233445566 accept


   EXTENSION HEADER EXPRESSIONS
       Extension header expressions refer to data from variable-sized protocol
       headers, such as IPv6 extension headers, TCP options and IPv4 options.

       nftables currently supports matching (finding) a given ipv6 extension
       header, TCP option or IPv4 option.

           hbh {nexthdr | hdrlength}
           frag {nexthdr | frag-off | more-fragments | id}
           rt {nexthdr | hdrlength | type | seg-left}
           dst {nexthdr | hdrlength}
           mh {nexthdr | hdrlength | checksum | type}
           srh {flags | tag | sid | seg-left}
           tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp} tcp_option_field
           ip option { lsrr | ra | rr | ssrr } ip_option_field

       The following syntaxes are valid only in a relational expression with
       boolean type on right-hand side for checking header existence only:

           exthdr {hbh | frag | rt | dst | mh}
           tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp}
           ip option { lsrr | ra | rr | ssrr }

       Table 53. IPv6 extension headers
       ┌────────┬────────────────────────┐
       │Keyword Description            │
       ├────────┼────────────────────────┤
       │        │                        │
       │hbh     │ Hop by Hop             │
       ├────────┼────────────────────────┤
       │        │                        │
       │rt      │ Routing Header         │
       ├────────┼────────────────────────┤
       │        │                        │
       │frag    │ Fragmentation header   │
       ├────────┼────────────────────────┤
       │        │                        │
       │dst     │ dst options            │
       ├────────┼────────────────────────┤
       │        │                        │
       │mh      │ Mobility Header        │
       ├────────┼────────────────────────┤
       │        │                        │
       │srh     │ Segment Routing Header │
       └────────┴────────────────────────┘

       Table 54. TCP Options
       ┌───────────────┬─────────────────────┬─────────────────────┐
       │Keyword        Description         TCP option fields   │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │eol            │ End if option list  │ kind                │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │noop           │ 1 Byte TCP No-op    │ kind                │
       │               │ options             │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │maxseg         │ TCP Maximum Segment │ kind, length, size  │
       │               │ Size                │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │window         │ TCP Window Scaling  │ kind, length, count │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack-permitted │ TCP SACK permitted  │ kind, length        │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack           │ TCP Selective       │ kind, length, left, │
       │               │ Acknowledgement     │ right               │
       │               │ (alias of block 0)  │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack0          │ TCP Selective       │ kind, length, left, │
       │               │ Acknowledgement     │ right               │
       │               │ (block 0)           │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack1          │ TCP Selective       │ kind, length, left, │
       │               │ Acknowledgement     │ right               │
       │               │ (block 1)           │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack2          │ TCP Selective       │ kind, length, left, │
       │               │ Acknowledgement     │ right               │
       │               │ (block 2)           │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │sack3          │ TCP Selective       │ kind, length, left, │
       │               │ Acknowledgement     │ right               │
       │               │ (block 3)           │                     │
       ├───────────────┼─────────────────────┼─────────────────────┤
       │               │                     │                     │
       │timestamp      │ TCP Timestamps      │ kind, length,       │
       │               │                     │ tsval, tsecr        │
       └───────────────┴─────────────────────┴─────────────────────┘

       Table 55. IP Options
       ┌────────┬─────────────────────┬─────────────────────┐
       │Keyword Description         IP option fields    │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │lsrr    │ Loose Source Route  │ type, length, ptr,  │
       │        │                     │ addr                │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │ra      │ Router Alert        │ type, length, value │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │rr      │ Record Route        │ type, length, ptr,  │
       │        │                     │ addr                │
       ├────────┼─────────────────────┼─────────────────────┤
       │        │                     │                     │
       │ssrr    │ Strict Source Route │ type, length, ptr,  │
       │        │                     │ addr                │
       └────────┴─────────────────────┴─────────────────────┘

       finding TCP options.

           filter input tcp option sack-permitted kind 1 counter

       matching IPv6 exthdr.

           ip6 filter input frag more-fragments 1 counter

       finding IP option.

           filter input ip option lsrr exists counter


   CONNTRACK EXPRESSIONS
       Conntrack expressions refer to meta data of the connection tracking
       entry associated with a packet.

       There are three types of conntrack expressions. Some conntrack
       expressions require the flow direction before the conntrack key, others
       must be used directly because they are direction agnostic. The packets,
       bytes and avgpkt keywords can be used with or without a direction. If
       the direction is omitted, the sum of the original and the reply
       direction is returned. The same is true for the zone, if a direction is
       given, the zone is only matched if the zone id is tied to the given
       direction.

           ct {state | direction | status | mark | expiration | helper | label}
           ct [original | reply] {l3proto | protocol | bytes | packets | avgpkt | zone | id}
           ct {original | reply} {proto-src | proto-dst}
           ct {original | reply} {ip | ip6} {saddr | daddr}

       Table 56. Conntrack expressions
       ┌───────────┬─────────────────────┬─────────────────────┐
       │Keyword    Description         Type                │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │state      │ State of the        │ ct_state            │
       │           │ connection          │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │direction  │ Direction of the    │ ct_dir              │
       │           │ packet relative to  │                     │
       │           │ the connection      │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │status     │ Status of the       │ ct_status           │
       │           │ connection          │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │mark       │ Connection mark     │ mark                │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │expiration │ Connection          │ time                │
       │           │ expiration time     │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │helper     │ Helper associated   │ string              │
       │           │ with the connection │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │label      │ Connection tracking │ ct_label            │
       │           │ label bit or        │                     │
       │           │ symbolic name       │                     │
       │           │ defined in          │                     │
       │           │ connlabel.conf in   │                     │
       │           │ the nftables        │                     │
       │           │ include path        │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │l3proto    │ Layer 3 protocol of │ nf_proto            │
       │           │ the connection      │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │saddr      │ Source address of   │ ipv4_addr/ipv6_addr │
       │           │ the connection for  │                     │
       │           │ the given direction │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │daddr      │ Destination address │ ipv4_addr/ipv6_addr │
       │           │ of the connection   │                     │
       │           │ for the given       │                     │
       │           │ direction           │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │protocol   │ Layer 4 protocol of │ inet_proto          │
       │           │ the connection for  │                     │
       │           │ the given direction │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │proto-src  │ Layer 4 protocol    │ integer (16 bit)    │
       │           │ source for the      │                     │
       │           │ given direction     │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │proto-dst  │ Layer 4 protocol    │ integer (16 bit)    │
       │           │ destination for the │                     │
       │           │ given direction     │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │packets    │ packet count seen   │ integer (64 bit)    │
       │           │ in the given        │                     │
       │           │ direction or sum of │                     │
       │           │ original and reply  │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │bytes      │ byte count seen,    │ integer (64 bit)    │
       │           │ see description for │                     │
       │           │ packets keyword     │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │avgpkt     │ average bytes per   │ integer (64 bit)    │
       │           │ packet, see         │                     │
       │           │ description for     │                     │
       │           │ packets keyword     │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │zone       │ conntrack zone      │ integer (16 bit)    │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │count      │ count number of     │ id                  │
       │           │ connections integer │                     │
       │           │ (32 bit)            │                     │
       └───────────┴─────────────────────┴─────────────────────┘

       A description of conntrack-specific types listed above can be found
       sub-section CONNTRACK TYPES above.

       restrict the number of parallel connections to a server.

           filter input tcp dport 22 meter test { ip saddr ct count over 2 } reject


STATEMENTS
       Statements represent actions to be performed. They can alter control
       flow (return, jump to a different chain, accept or drop the packet) or
       can perform actions, such as logging, rejecting a packet, etc.

       Statements exist in two kinds. Terminal statements unconditionally
       terminate evaluation of the current rule, non-terminal statements
       either only conditionally or never terminate evaluation of the current
       rule, in other words, they are passive from the ruleset evaluation
       perspective. There can be an arbitrary amount of non-terminal
       statements in a rule, but only a single terminal statement as the final
       statement.

   VERDICT STATEMENT
       The verdict statement alters control flow in the ruleset and issues
       policy decisions for packets.

           {accept | drop | queue | continue | return}
           {jump | goto} chain

       accept and drop are absolute verdicts — they terminate ruleset
       evaluation immediately.


       accept       Terminate ruleset
                    evaluation and accept the
                    packet. The packet can
                    still be dropped later by
                    another hook, for instance
                    accept in the forward hook
                    still allows to drop the
                    packet later in the
                    postrouting hook, or
                    another forward base chain
                    that has a higher priority
                    number and is evaluated
                    afterwards in the
                    processing pipeline.

       drop         Terminate ruleset
                    evaluation and drop the
                    packet. The drop occurs
                    instantly, no further
                    chains or hooks are
                    evaluated. It is not
                    possible to accept the
                    packet in a later chain
                    again, as those are not
                    evaluated anymore for the
                    packet.

       queue        Terminate ruleset
                    evaluation and queue the
                    packet to userspace.
                    Userspace must provide a
                    drop or accept verdict. In
                    case of accept, processing
                    resumes with the next base
                    chain hook, not the rule
                    following the queue
                    verdict.

       continue     Continue ruleset
                    evaluation with the next
                    rule. This is the default
                    behaviour in case a rule
                    issues no verdict.

       return       Return from the current
                    chain and continue
                    evaluation at the next
                    rule in the last chain. If
                    issued in a base chain, it
                    is equivalent to the base
                    chain policy.

       jump chain   Continue evaluation at the
                    first rule in chain. The
                    current position in the
                    ruleset is pushed to a
                    call stack and evaluation
                    will continue there when
                    the new chain is entirely
                    evaluated or a return
                    verdict is issued. In case
                    an absolute verdict is
                    issued by a rule in the
                    chain, ruleset evaluation
                    terminates immediately and
                    the specific action is
                    taken.

       goto chain   Similar to jump, but the
                    current position is not
                    pushed to the call stack,
                    meaning that after the new
                    chain evaluation will
                    continue at the last chain
                    instead of the one
                    containing the goto
                    statement.


       Using verdict statements.

           # process packets from eth0 and the internal network in from_lan
           # chain, drop all packets from eth0 with different source addresses.

           filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
           filter input iif eth0 drop


   PAYLOAD STATEMENT
           payload_expression set value

       The payload statement alters packet content. It can be used for example
       to set ip DSCP (diffserv) header field or ipv6 flow labels.

       route some packets instead of bridging.

           # redirect tcp:http from 192.160.0.0/16 to local machine for routing instead of bridging
           # assumes 00:11:22:33:44:55 is local MAC address.
           bridge input meta iif eth0 ip saddr 192.168.0.0/16 tcp dport 80 meta pkttype set unicast ether daddr set 00:11:22:33:44:55

       Set IPv4 DSCP header field.

           ip forward ip dscp set 42


   EXTENSION HEADER STATEMENT
           extension_header_expression set value

       The extension header statement alters packet content in variable-sized
       headers. This can currently be used to alter the TCP Maximum segment
       size of packets, similar to TCPMSS.

       change tcp mss.

           tcp flags syn tcp option maxseg size set 1360
           # set a size based on route information:
           tcp flags syn tcp option maxseg size set rt mtu


   LOG STATEMENT
           log [prefix quoted_string] [level syslog-level] [flags log-flags]
           log group nflog_group [prefix quoted_string] [queue-threshold value] [snaplen size]
           log level audit

       The log statement enables logging of matching packets. When this
       statement is used from a rule, the Linux kernel will print some
       information on all matching packets, such as header fields, via the
       kernel log (where it can be read with dmesg(1) or read in the syslog).

       In the second form of invocation (if nflog_group is specified), the
       Linux kernel will pass the packet to nfnetlink_log which will multicast
       the packet through a netlink socket to the specified multicast group.
       One or more userspace processes may subscribe to the group to receive
       the packets, see libnetfilter_queue documentation for details.

       In the third form of invocation (if level audit is specified), the
       Linux kernel writes a message into the audit buffer suitably formatted
       for reading with auditd. Therefore no further formatting options (such
       as prefix or flags) are allowed in this mode.

       This is a non-terminating statement, so the rule evaluation continues
       after the packet is logged.

       Table 57. log statement options
       ┌────────────────┬─────────────────────┬───────────────────┐
       │Keyword         Description         Type              │
       ├────────────────┼─────────────────────┼───────────────────┤
       │                │                     │                   │
       │prefix          │ Log message prefix  │ quoted string     │
       ├────────────────┼─────────────────────┼───────────────────┤
       │                │                     │                   │
       │level           │ Syslog level of     │ string: emerg,    │
       │                │ logging             │ alert, crit, err, │
       │                │                     │ warn [default],   │
       │                │                     │ notice, info,     │
       │                │                     │ debug, audit      │
       ├────────────────┼─────────────────────┼───────────────────┤
       │                │                     │                   │
       │group           │ NFLOG group to send │ unsigned integer  │
       │                │ messages to         │ (16 bit)          │
       ├────────────────┼─────────────────────┼───────────────────┤
       │                │                     │                   │
       │snaplen         │ Length of packet    │ unsigned integer  │
       │                │ payload to include  │ (32 bit)          │
       │                │ in netlink message  │                   │
       ├────────────────┼─────────────────────┼───────────────────┤
       │                │                     │                   │
       │queue-threshold │ Number of packets   │ unsigned integer  │
       │                │ to queue inside the │ (32 bit)          │
       │                │ kernel before       │                   │
       │                │ sending them to     │                   │
       │                │ userspace           │                   │
       └────────────────┴─────────────────────┴───────────────────┘

       Table 58. log-flags
       ┌─────────────┬───────────────────────────┐
       │Flag         Description               │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │tcp sequence │ Log TCP sequence numbers. │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │tcp options  │ Log options from the TCP  │
       │             │ packet header.            │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │ip options   │ Log options from the      │
       │             │ IP/IPv6 packet header.    │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │skuid        │ Log the userid of the     │
       │             │ process which generated   │
       │             │ the packet.               │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │ether        │ Decode MAC addresses and  │
       │             │ protocol.                 │
       ├─────────────┼───────────────────────────┤
       │             │                           │
       │all          │ Enable all log flags      │
       │             │ listed above.             │
       └─────────────┴───────────────────────────┘

       Using log statement.

           # log the UID which generated the packet and ip options
           ip filter output log flags skuid flags ip options

           # log the tcp sequence numbers and tcp options from the TCP packet
           ip filter output log flags tcp sequence,options

           # enable all supported log flags
           ip6 filter output log flags all


   REJECT STATEMENT
           reject [ with REJECT_WITH ]

           REJECT_WITH := icmp type icmp_code |
                            icmpv6 type icmpv6_code |
                            icmpx type icmpx_code |
                            tcp reset

       A reject statement is used to send back an error packet in response to
       the matched packet otherwise it is equivalent to drop so it is a
       terminating statement, ending rule traversal. This statement is only
       valid in base chains using the input, forward or output hooks, and
       user-defined chains which are only called from those chains.

       Table 59. different ICMP reject variants are meant for use in different
       table families
       ┌────────┬────────┬─────────────┐
       │Variant Family Type        │
       ├────────┼────────┼─────────────┤
       │        │        │             │
       │icmp    │ ip     │ icmp_code   │
       ├────────┼────────┼─────────────┤
       │        │        │             │
       │icmpv6  │ ip6    │ icmpv6_code │
       ├────────┼────────┼─────────────┤
       │        │        │             │
       │icmpx   │ inet   │ icmpx_code  │
       └────────┴────────┴─────────────┘

       For a description of the different types and a list of supported
       keywords refer to DATA TYPES section above. The common default reject
       value is port-unreachable.

       Note that in bridge family, reject statement is only allowed in base
       chains which hook into input or prerouting.

   COUNTER STATEMENT
       A counter statement sets the hit count of packets along with the number
       of bytes.

           counter packets number bytes number
           counter { packets number | bytes number }

   CONNTRACK STATEMENT
       The conntrack statement can be used to set the conntrack mark and
       conntrack labels.

           ct {mark | event | label | zone} set value

       The ct statement sets meta data associated with a connection. The zone
       id has to be assigned before a conntrack lookup takes place, i.e. this
       has to be done in prerouting and possibly output (if locally generated
       packets need to be placed in a distinct zone), with a hook priority of
       -300.

       Table 60. Conntrack statement types
       ┌────────┬─────────────────────┬──────────────────┐
       │Keyword Description         Value            │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │event   │ conntrack event     │ bitmask, integer │
       │        │ bits                │ (32 bit)         │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │helper  │ name of ct helper   │ quoted string    │
       │        │ object to assign to │                  │
       │        │ the connection      │                  │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │mark    │ Connection tracking │ mark             │
       │        │ mark                │                  │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │label   │ Connection tracking │ label            │
       │        │ label               │                  │
       ├────────┼─────────────────────┼──────────────────┤
       │        │                     │                  │
       │zone    │ conntrack zone      │ integer (16 bit) │
       └────────┴─────────────────────┴──────────────────┘

       save packet nfmark in conntrack.

           ct mark set meta mark

       set zone mapped via interface.

           table inet raw {
             chain prerouting {
                 type filter hook prerouting priority -300;
                 ct zone set iif map { "eth1" : 1, "veth1" : 2 }
             }
             chain output {
                 type filter hook output priority -300;
                 ct zone set oif map { "eth1" : 1, "veth1" : 2 }
             }
           }

       restrict events reported by ctnetlink.

           ct event set new,related,destroy


   META STATEMENT
       A meta statement sets the value of a meta expression. The existing meta
       fields are: priority, mark, pkttype, nftrace.

           meta {mark | priority | pkttype | nftrace} set value

       A meta statement sets meta data associated with a packet.

       Table 61. Meta statement types
       ┌─────────┬─────────────────────┬───────────┐
       │Keyword  Description         Value     │
       ├─────────┼─────────────────────┼───────────┤
       │         │                     │           │
       │priority │ TC packet priority  │ tc_handle │
       ├─────────┼─────────────────────┼───────────┤
       │         │                     │           │
       │mark     │ Packet mark         │ mark      │
       ├─────────┼─────────────────────┼───────────┤
       │         │                     │           │
       │pkttype  │ packet type         │ pkt_type  │
       ├─────────┼─────────────────────┼───────────┤
       │         │                     │           │
       │nftrace  │ ruleset packet      │ 0, 1      │
       │         │ tracing on/off. Use │           │
       │         │ monitor trace       │           │
       │         │ command to watch    │           │
       │         │ traces              │           │
       └─────────┴─────────────────────┴───────────┘

   LIMIT STATEMENT
           limit rate [over] packet_number / TIME_UNIT [burst packet_number packets]
           limit rate [over] byte_number BYTE_UNIT / TIME_UNIT [burst byte_number BYTE_UNIT]

           TIME_UNIT := second | minute | hour | day
           BYTE_UNIT := bytes | kbytes | mbytes

       A limit statement matches at a limited rate using a token bucket
       filter. A rule using this statement will match until this limit is
       reached. It can be used in combination with the log statement to give
       limited logging. The optional over keyword makes it match over the
       specified rate.

       Table 62. limit statement values
       ┌──────────────┬───────────────────┬──────────────────┐
       │Value         Description       Type             │
       ├──────────────┼───────────────────┼──────────────────┤
       │              │                   │                  │
       │packet_number │ Number of packets │ unsigned integer │
       │              │                   │ (32 bit)         │
       ├──────────────┼───────────────────┼──────────────────┤
       │              │                   │                  │
       │byte_number   │ Number of bytes   │ unsigned integer │
       │              │                   │ (32 bit)         │
       └──────────────┴───────────────────┴──────────────────┘

   NAT STATEMENTS
           snat to address [:port] [PRF_FLAGS]
           snat to address - address [:port - port] [PRF_FLAGS]
           snat { ip | ip6 } to address - address [:port - port] [PR_FLAGS]
           dnat to address [:port] [PRF_FLAGS]
           dnat to address [:port - port] [PR_FLAGS]
           dnat { ip | ip6 } to address [:port - port] [PR_FLAGS]
           masquerade to [:port] [PRF_FLAGS]
           masquerade to [:port - port] [PRF_FLAGS]
           redirect to [:port] [PRF_FLAGS]
           redirect to [:port - port] [PRF_FLAGS]

           PRF_FLAGS := PRF_FLAG [, PRF_FLAGS]
           PR_FLAGS  := PR_FLAG [, PR_FLAGS]
           PRF_FLAG  := PR_FLAG | fully-random
           PR_FLAG   := persistent | random

       The nat statements are only valid from nat chain types.

       The snat and masquerade statements specify that the source address of
       the packet should be modified. While snat is only valid in the
       postrouting and input chains, masquerade makes sense only in
       postrouting. The dnat and redirect statements are only valid in the
       prerouting and output chains, they specify that the destination address
       of the packet should be modified. You can use non-base chains which are
       called from base chains of nat chain type too. All future packets in
       this connection will also be mangled, and rules should cease being
       examined.

       The masquerade statement is a special form of snat which always uses
       the outgoing interface’s IP address to translate to. It is particularly
       useful on gateways with dynamic (public) IP addresses.

       The redirect statement is a special form of dnat which always
       translates the destination address to the local host’s one. It comes in
       handy if one only wants to alter the destination port of incoming
       traffic on different interfaces.

       When used in the inet family (available with kernel 5.2), the dnat and
       snat statements require the use of the ip and ip6 keyword in case an
       address is provided, see the examples below.

       Before kernel 4.18 nat statements require both prerouting and
       postrouting base chains to be present since otherwise packets on the
       return path won’t be seen by netfilter and therefore no reverse
       translation will take place.

       Table 63. NAT statement values
       ┌───────────┬─────────────────────┬─────────────────────┐
       │Expression Description         Type                │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │address    │ Specifies that the  │ ipv4_addr,          │
       │           │ source/destination  │ ipv6_addr, e.g.     │
       │           │ address of the      │ abcd::1234, or you  │
       │           │ packet should be    │ can use a mapping,  │
       │           │ modified. You may   │ e.g. meta mark map  │
       │           │ specify a mapping   │ { 10 : 192.168.1.2, │
       │           │ to relate a list of │ 20 : 192.168.1.3 }  │
       │           │ tuples composed of  │                     │
       │           │ arbitrary           │                     │
       │           │ expression key with │                     │
       │           │ address value.      │                     │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │port       │ Specifies that the  │ port number (16     │
       │           │ source/destination  │ bit)                │
       │           │ address of the      │                     │
       │           │ packet should be    │                     │
       │           │ modified.           │                     │
       └───────────┴─────────────────────┴─────────────────────┘

       Table 64. NAT statement flags
       ┌─────────────┬─────────────────────────────┐
       │Flag         Description                 │
       ├─────────────┼─────────────────────────────┤
       │             │                             │
       │persistent   │ Gives a client the same     │
       │             │ source-/destination-address │
       │             │ for each connection.        │
       ├─────────────┼─────────────────────────────┤
       │             │                             │
       │random       │ In kernel 5.0 and newer     │
       │             │ this is the same as         │
       │             │ fully-random. In earlier    │
       │             │ kernels the port mapping    │
       │             │ will be randomized using a  │
       │             │ seeded MD5 hash mix using   │
       │             │ source and destination      │
       │             │ address and destination     │
       │             │ port.                       │
       ├─────────────┼─────────────────────────────┤
       │             │                             │
       │fully-random │ If used then port mapping   │
       │             │ is generated based on a     │
       │             │ 32-bit pseudo-random        │
       │             │ algorithm.                  │
       └─────────────┴─────────────────────────────┘

       Using NAT statements.

           # create a suitable table/chain setup for all further examples
           add table nat
           add chain nat prerouting { type nat hook prerouting priority 0; }
           add chain nat postrouting { type nat hook postrouting priority 100; }

           # translate source addresses of all packets leaving via eth0 to address 1.2.3.4
           add rule nat postrouting oif eth0 snat to 1.2.3.4

           # redirect all traffic entering via eth0 to destination address 192.168.1.120
           add rule nat prerouting iif eth0 dnat to 192.168.1.120

           # translate source addresses of all packets leaving via eth0 to whatever
           # locally generated packets would use as source to reach the same destination
           add rule nat postrouting oif eth0 masquerade

           # redirect incoming TCP traffic for port 22 to port 2222
           add rule nat prerouting tcp dport 22 redirect to :2222

           # inet family:
           # handle ip dnat:
           add rule inet nat prerouting dnat ip to 10.0.2.99
           # handle ip6 dnat:
           add rule inet nat prerouting dnat ip6 to fe80::dead
           # this masquerades both ipv4 and ipv6:
           add rule inet nat postrouting meta oif ppp0 masquerade


   TPROXY STATEMENT
       Tproxy redirects the packet to a local socket without changing the
       packet header in any way. If any of the arguments is missing the data
       of the incoming packet is used as parameter. Tproxy matching requires
       another rule that ensures the presence of transport protocol header is
       specified.

           tproxy to address:port
           tproxy to {address | :port}

       This syntax can be used in ip/ip6 tables where network layer protocol
       is obvious. Either IP address or port can be specified, but at least
       one of them is necessary.

           tproxy {ip | ip6} to address[:port]
           tproxy to :port

       This syntax can be used in inet tables. The ip/ip6 parameter defines
       the family the rule will match. The address parameter must be of this
       family. When only port is defined, the address family should not be
       specified. In this case the rule will match for both families.

       Table 65. tproxy attributes
       ┌────────┬────────────────────────────┐
       │Name    Description                │
       ├────────┼────────────────────────────┤
       │        │                            │
       │address │ IP address the listening   │
       │        │ socket with IP_TRANSPARENT │
       │        │ option is bound to.        │
       ├────────┼────────────────────────────┤
       │        │                            │
       │port    │ Port the listening socket  │
       │        │ with IP_TRANSPARENT option │
       │        │ is bound to.               │
       └────────┴────────────────────────────┘

       Example ruleset for tproxy statement.

           table ip x {
               chain y {
                   type filter hook prerouting priority -150; policy accept;
                   tcp dport ntp tproxy to 1.1.1.1
                   udp dport ssh tproxy to :2222
               }
           }
           table ip6 x {
               chain y {
                  type filter hook prerouting priority -150; policy accept;
                  tcp dport ntp tproxy to [dead::beef]
                  udp dport ssh tproxy to :2222
               }
           }
           table inet x {
               chain y {
                   type filter hook prerouting priority -150; policy accept;
                   tcp dport 321 tproxy to :ssh
                   tcp dport 99 tproxy ip to 1.1.1.1:999
                   udp dport 155 tproxy ip6 to [dead::beef]:smux
               }
           }


   SYNPROXY STATEMENT
       This statement will process TCP three-way-handshake parallel in
       netfilter context to protect either local or backend system. This
       statement requires connection tracking because sequence numbers need to
       be translated.

           synproxy [mss mss_value] [wscale wscale_value] [SYNPROXY_FLAGS]

       Table 66. synproxy statement attributes
       ┌───────┬────────────────────────────┐
       │Name   Description                │
       ├───────┼────────────────────────────┤
       │       │                            │
       │mss    │ Maximum segment size       │
       │       │ announced to clients. This │
       │       │ must match the backend.    │
       ├───────┼────────────────────────────┤
       │       │                            │
       │wscale │ Window scale announced to  │
       │       │ clients. This must match   │
       │       │ the backend.               │
       └───────┴────────────────────────────┘

       Table 67. synproxy statement flags
       ┌──────────┬────────────────────────────┐
       │Flag      Description                │
       ├──────────┼────────────────────────────┤
       │          │                            │
       │sack-perm │ Pass client selective      │
       │          │ acknowledgement option to  │
       │          │ backend (will be disabled  │
       │          │ if not present).           │
       ├──────────┼────────────────────────────┤
       │          │                            │
       │timestamp │ Pass client timestamp      │
       │          │ option to backend (will be │
       │          │ disabled if not present,   │
       │          │ also needed for selective  │
       │          │ acknowledgement and window │
       │          │ scaling).                  │
       └──────────┴────────────────────────────┘

       Example ruleset for synproxy statement.

           Determine tcp options used by backend, from an external system

                         tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
                             port 80 &
                         telnet 192.0.2.42 80
                         18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
                             Flags [S.], seq 360414582, ack 788841994, win 14480,
                             options [mss 1460,sackOK,
                             TS val 1409056151 ecr 9690221,
                             nop,wscale 9],
                             length 0

           Switch tcp_loose mode off, so conntrack will mark out-of-flow packets as state INVALID.

                         echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose

           Make SYN packets untracked.

                   table ip x {
                           chain y {
                                   type filter hook prerouting priority raw; policy accept;
                                   tcp flags syn notrack
                           }
                   }

           Catch UNTRACKED (SYN  packets) and INVALID (3WHS ACK packets) states and send
           them to SYNPROXY. This rule will respond to SYN packets with SYN+ACK
           syncookies, create ESTABLISHED for valid client response (3WHS ACK packets) and
           drop incorrect cookies. Flags combinations not expected during  3WHS will not
           match and continue (e.g. SYN+FIN, SYN+ACK). Finally, drop invalid packets, this
           will be out-of-flow packets that were not matched by SYNPROXY.

               table ip foo {
                       chain z {
                               type filter hook input priority filter; policy accept;
                               ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
                               ct state invalid drop
                       }
               }

           The outcome ruleset of the steps above should be similar to the one below.

                   table ip x {
                           chain y {
                                   type filter hook prerouting priority raw; policy accept;
                                   tcp flags syn notrack
                           }

                           chain z {
                                   type filter hook input priority filter; policy accept;
                                   ct state { invalid, untracked } synproxy mss 1460 wscale 9 timestamp sack-perm
                                   ct state invalid drop
                           }
                   }


   FLOW STATEMENT
       A flow statement allows us to select what flows you want to accelerate
       forwarding through layer 3 network stack bypass. You have to specify
       the flowtable name where you want to offload this flow.

       flow add @flowtable

   QUEUE STATEMENT
       This statement passes the packet to userspace using the nfnetlink_queue
       handler. The packet is put into the queue identified by its 16-bit
       queue number. Userspace can inspect and modify the packet if desired.
       Userspace must then drop or re-inject the packet into the kernel. See
       libnetfilter_queue documentation for details.

           queue [num queue_number] [bypass]
           queue [num queue_number_from - queue_number_to] [QUEUE_FLAGS]

           QUEUE_FLAGS := QUEUE_FLAG [, QUEUE_FLAGS]
           QUEUE_FLAG  := bypass | fanout

       Table 68. queue statement values
       ┌──────────────────┬────────────────────┬──────────────────┐
       │Value             Description        Type             │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │queue_number      │ Sets queue number, │ unsigned integer │
       │                  │ default is 0.      │ (16 bit)         │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │queue_number_from │ Sets initial queue │ unsigned integer │
       │                  │ in the range, if   │ (16 bit)         │
       │                  │ fanout is used.    │                  │
       ├──────────────────┼────────────────────┼──────────────────┤
       │                  │                    │                  │
       │queue_number_to   │ Sets closing queue │ unsigned integer │
       │                  │ in the range, if   │ (16 bit)         │
       │                  │ fanout is used.    │                  │
       └──────────────────┴────────────────────┴──────────────────┘

       Table 69. queue statement flags
       ┌───────┬────────────────────────────┐
       │Flag   Description                │
       ├───────┼────────────────────────────┤
       │       │                            │
       │bypass │ Let packets go through if  │
       │       │ userspace application      │
       │       │ cannot back off. Before    │
       │       │ using this flag, read      │
       │       │ libnetfilter_queue         │
       │       │ documentation for          │
       │       │ performance tuning         │
       │       │ recommendations.           │
       ├───────┼────────────────────────────┤
       │       │                            │
       │fanout │ Distribute packets between │
       │       │ several queues.            │
       └───────┴────────────────────────────┘

   DUP STATEMENT
       The dup statement is used to duplicate a packet and send the copy to a
       different destination.

           dup to device
           dup to address device device

       Table 70. Dup statement values
       ┌───────────┬─────────────────────┬─────────────────────┐
       │Expression Description         Type                │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │address    │ Specifies that the  │ ipv4_addr,          │
       │           │ copy of the packet  │ ipv6_addr, e.g.     │
       │           │ should be sent to a │ abcd::1234, or you  │
       │           │ new gateway.        │ can use a mapping,  │
       │           │                     │ e.g. ip saddr map { │
       │           │                     │ 192.168.1.2 :       │
       │           │                     │ 10.1.1.1 }          │
       ├───────────┼─────────────────────┼─────────────────────┤
       │           │                     │                     │
       │device     │ Specifies that the  │ string              │
       │           │ copy should be      │                     │
       │           │ transmitted via     │                     │
       │           │ device.             │                     │
       └───────────┴─────────────────────┴─────────────────────┘

       Using the dup statement.

           # send to machine with ip address 10.2.3.4 on eth0
           ip filter forward dup to 10.2.3.4 device "eth0"

           # copy raw frame to another interface
           netdetv ingress dup to "eth0"
           dup to "eth0"

           # combine with map dst addr to gateways
           dup to ip daddr map { 192.168.7.1 : "eth0", 192.168.7.2 : "eth1" }


   FWD STATEMENT
       The fwd statement is used to redirect a raw packet to another
       interface. It is only available in the netdev family ingress hook. It
       is similar to the dup statement except that no copy is made.

       fwd to device

   SET STATEMENT
       The set statement is used to dynamically add or update elements in a
       set from the packet path. The set setname must already exist in the
       given table and must have been created with one or both of the dynamic
       and the timeout flags. The dynamic flag is required if the set
       statement expression includes a stateful object. The timeout flag is
       implied if the set is created with a timeout, and is required if the
       set statement updates elements, rather than adding them. Furthermore,
       these sets should specify both a maximum set size (to prevent memory
       exhaustion), and their elements should have a timeout (so their number
       will not grow indefinitely) either from the set definition or from the
       statement that adds or updates them. The set statement can be used to
       e.g. create dynamic blacklists.

           {add | update} @setname { expression [timeout timeout] [comment string] }

       Example for simple blacklist.

           # declare a set, bound to table "filter", in family "ip". Timeout and size are mandatory because we will add elements from packet path.
           nft add set ip filter blackhole "{ type ipv4_addr; flags timeout; size 65536; }"

           # whitelist internal interface.
           nft add rule ip filter input meta iifname "internal" accept

           # drop packets coming from blacklisted ip addresses.
           nft add rule ip filter input ip saddr @blackhole counter drop

           # add source ip addresses to the blacklist if more than 10 tcp connection requests occurred per second and ip address.
           # entries will timeout after one minute, after which they might be re-added if limit condition persists.
           nft add rule ip filter input tcp flags syn tcp dport ssh meter flood size 128000 { ip saddr timeout 10s limit rate over 10/second} add @blackhole { ip saddr timeout 1m } drop

           # inspect state of the rate limit meter:
           nft list meter ip filter flood

           # inspect content of blackhole:
           nft list set ip filter blackhole

           # manually add two addresses to the set:
           nft add element filter blackhole { 10.2.3.4, 10.23.1.42 }


   MAP STATEMENT
       The map statement is used to lookup data based on some specific input
       key.

           expression map { MAP_ELEMENTS }

           MAP_ELEMENTS := MAP_ELEMENT [, MAP_ELEMENTS]
           MAP_ELEMENT  := key : value

       The key is a value returned by expression.

       Using the map statement.

           # select DNAT target based on TCP dport:
           # connections to port 80 are redirected to 192.168.1.100,
           # connections to port 8888 are redirected to 192.168.1.101
           nft add rule ip nat prerouting dnat tcp dport map { 80 : 192.168.1.100, 8888 : 192.168.1.101 }

           # source address based SNAT:
           # packets from net 192.168.1.0/24 will appear as originating from 10.0.0.1,
           # packets from net 192.168.2.0/24 will appear as originating from 10.0.0.2
           nft add rule ip nat postrouting snat to ip saddr map { 192.168.1.0/24 : 10.0.0.1, 192.168.2.0/24 : 10.0.0.2 }


   VMAP STATEMENT
       The verdict map (vmap) statement works analogous to the map statement,
       but contains verdicts as values.

           expression vmap { VMAP_ELEMENTS }

           VMAP_ELEMENTS := VMAP_ELEMENT [, VMAP_ELEMENTS]
           VMAP_ELEMENT  := key : verdict

       Using the vmap statement.

           # jump to different chains depending on layer 4 protocol type:
           nft add rule ip filter input ip protocol vmap { tcp : jump tcp-chain, udp : jump udp-chain , icmp : jump icmp-chain }


ADDITIONAL COMMANDS
       These are some additional commands included in nft.

   MONITOR
       The monitor command allows you to listen to Netlink events produced by
       the nf_tables subsystem, related to creation and deletion of objects.
       When they occur, nft will print to stdout the monitored events in
       either JSON or native nft format.

       To filter events related to a concrete object, use one of the keywords
       tables, chains, sets, rules, elements, ruleset.

       To filter events related to a concrete action, use keyword new or
       destroy.

       Hit ^C to finish the monitor operation.

       Listen to all events, report in native nft format.

           % nft monitor

       Listen to deleted rules, report in JSON format.

           % nft -j monitor destroy rules

       Listen to both new and destroyed chains, in native nft format.

           % nft monitor chains

       Listen to ruleset events such as table, chain, rule, set, counters and
       quotas, in native nft format.

           % nft monitor ruleset


ERROR REPORTING
       When an error is detected, nft shows the line(s) containing the error,
       the position of the erroneous parts in the input stream and marks up
       the erroneous parts using carets (^). If the error results from the
       combination of two expressions or statements, the part imposing the
       constraints which are violated is marked using tildes (~).

       For errors returned by the kernel, nft cannot detect which parts of the
       input caused the error and the entire command is marked.

       Error caused by single incorrect expression.

           <cmdline>:1:19-22: Error: Interface does not exist
           filter output oif eth0
                             ^^^^

       Error caused by invalid combination of two expressions.

           <cmdline>:1:28-36: Error: Right hand side of relational expression (==) must be constant
           filter output tcp dport == tcp dport
                                   ~~ ^^^^^^^^^

       Error returned by the kernel.

           <cmdline>:0:0-23: Error: Could not process rule: Operation not permitted
           filter output oif wlan0
           ^^^^^^^^^^^^^^^^^^^^^^^


EXIT STATUS
       On success, nft exits with a status of 0. Unspecified errors cause it
       to exit with a status of 1, memory allocation errors with a status of
       2, unable to open Netlink socket with 3.

SEE ALSO
           libnftables(3), libnftables-json(5), iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)

       There is an official wiki at: https://wiki.nftables.org

AUTHORS
       nftables was written by Patrick McHardy and Pablo Neira Ayuso, among
       many other contributors from the Netfilter community.

COPYRIGHT
       Copyright © 2008-2014 Patrick McHardy <kaber@trash.net> Copyright ©
       2013-2018 Pablo Neira Ayuso <pablo@netfilter.org>

       nftables is free software; you can redistribute it and/or modify it
       under the terms of the GNU General Public License version 2 as
       published by the Free Software Foundation.

       This documentation is licensed under the terms of the Creative Commons
       Attribution-ShareAlike 4.0 license, CC BY-SA 4.0
       http://creativecommons.org/licenses/by-sa/4.0/.



                                  06/15/2020                            NFT(8)