ipfw

IPFW(8)                    BSD System Manager's Manual                   IPFW(8)

NAME
     ipfw — User interface for firewall, traffic shaper, packet scheduler, in-
     kernel NAT.

SYNOPSIS
   FIREWALL CONFIGURATION
     ipfw [-cq] add rule
     ipfw [-acdefnNStT] [set N] {list | show} [rule | first-last ...]
     ipfw [-f | -q] [set N] flush
     ipfw [-q] [set N] {delete | zero | resetlog} [number ...]

     ipfw set [disable number ...] [enable number ...]
     ipfw set move [rule] number to number
     ipfw set swap number number
     ipfw set show

        SYSCTL SHORTCUTS
     ipfw enable {firewall | altq | one_pass | debug | verbose | dyn_keepalive}
     ipfw disable {firewall | altq | one_pass | debug | verbose | dyn_keepalive}

        LOOKUP TABLES
     ipfw [set N] table name create create-options
     ipfw [set N] table {name | all} destroy
     ipfw [set N] table name modify modify-options
     ipfw [set N] table name swap name
     ipfw [set N] table name add table-key [value]
     ipfw [set N] table name add [table-key value ...]
     ipfw [set N] table name atomic add [table-key value ...]
     ipfw [set N] table name delete [table-key ...]
     ipfw [set N] table name lookup addr
     ipfw [set N] table name lock
     ipfw [set N] table name unlock
     ipfw [set N] table {name | all} list
     ipfw [set N] table {name | all} info
     ipfw [set N] table {name | all} detail
     ipfw [set N] table {name | all} flush

        DUMMYNET CONFIGURATION (TRAFFIC SHAPER AND PACKET SCHEDULER)
     ipfw {pipe | queue | sched} number config config-options
     ipfw [-s [field]] {pipe | queue | sched} {delete | list | show}
          [number ...]

        IN-KERNEL NAT
     ipfw [-q] nat number config config-options

        STATEFUL IPv6/IPv4 NETWORK ADDRESS AND PROTOCOL TRANSLATION
     ipfw [set N] nat64lsn name create create-options
     ipfw [set N] nat64lsn name config config-options
     ipfw [set N] nat64lsn {name | all} {list | show} [states]
     ipfw [set N] nat64lsn {name | all} destroy
     ipfw [set N] nat64lsn name stats [reset]

        STATELESS IPv6/IPv4 NETWORK ADDRESS AND PROTOCOL TRANSLATION
     ipfw [set N] nat64stl name create create-options
     ipfw [set N] nat64stl name config config-options
     ipfw [set N] nat64stl {name | all} {list | show}
     ipfw [set N] nat64stl {name | all} destroy
     ipfw [set N] nat64stl name stats [reset]

        XLAT464 CLAT IPv6/IPv4 NETWORK ADDRESS AND PROTOCOL TRANSLATION
     ipfw [set N] nat64clat name create create-options
     ipfw [set N] nat64clat name config config-options
     ipfw [set N] nat64clat {name | all} {list | show}
     ipfw [set N] nat64clat {name | all} destroy
     ipfw [set N] nat64clat name stats [reset]

        IPv6-to-IPv6 NETWORK PREFIX TRANSLATION
     ipfw [set N] nptv6 name create create-options
     ipfw [set N] nptv6 {name | all} {list | show}
     ipfw [set N] nptv6 {name | all} destroy
     ipfw [set N] nptv6 name stats [reset]

        INTERNAL DIAGNOSTICS
     ipfw internal iflist
     ipfw internal talist
     ipfw internal vlist

        LIST OF RULES AND PREPROCESSING
     ipfw [-cfnNqS] [-p preproc [preproc-flags]] pathname

DESCRIPTION
     The ipfw utility is the user interface for controlling the ipfw(4)
     firewall, the dummynet(4) traffic shaper/packet scheduler, and the in-
     kernel NAT services.

     A firewall configuration, or ruleset, is made of a list of rules numbered
     from 1 to 65535.  Packets are passed to the firewall from a number of
     different places in the protocol stack (depending on the source and
     destination of the packet, it is possible for the firewall to be invoked
     multiple times on the same packet).  The packet passed to the firewall is
     compared against each of the rules in the ruleset, in rule-number order
     (multiple rules with the same number are permitted, in which case they are
     processed in order of insertion).  When a match is found, the action
     corresponding to the matching rule is performed.

     Depending on the action and certain system settings, packets can be
     reinjected into the firewall at some rule after the matching one for
     further processing.

     A ruleset always includes a default rule (numbered 65535) which cannot be
     modified or deleted, and matches all packets.  The action associated with
     the default rule can be either deny or allow depending on how the kernel is
     configured.

     If the ruleset includes one or more rules with the keep-state,
     record-state, limit or set-limit option, the firewall will have a stateful
     behaviour, i.e., upon a match it will create dynamic rules, i.e., rules
     that match packets with the same 5-tuple (protocol, source and destination
     addresses and ports) as the packet which caused their creation.  Dynamic
     rules, which have a limited lifetime, are checked at the first occurrence
     of a check-state, keep-state or limit rule, and are typically used to open
     the firewall on-demand to legitimate traffic only.  Please note, that
     keep-state and limit imply implicit check-state for all packets (not only
     these matched by the rule) but record-state and set-limit have no implicit
     check-state.  See the STATEFUL FIREWALL and EXAMPLES Sections below for
     more information on the stateful behaviour of ipfw.

     All rules (including dynamic ones) have a few associated counters: a packet
     count, a byte count, a log count and a timestamp indicating the time of the
     last match.  Counters can be displayed or reset with ipfw commands.

     Each rule belongs to one of 32 different sets , and there are ipfw commands
     to atomically manipulate sets, such as enable, disable, swap sets, move all
     rules in a set to another one, delete all rules in a set.  These can be
     useful to install temporary configurations, or to test them.  See Section
     SETS OF RULES for more information on sets.

     Rules can be added with the add command; deleted individually or in groups
     with the delete command, and globally (except those in set 31) with the
     flush command; displayed, optionally with the content of the counters,
     using the show and list commands.  Finally, counters can be reset with the
     zero and resetlog commands.

   COMMAND OPTIONS
     The following general options are available when invoking ipfw:

     -a      Show counter values when listing rules.  The show command implies
             this option.

     -b      Only show the action and the comment, not the body of a rule.
             Implies -c.

     -c      When entering or showing rules, print them in compact form, i.e.,
             omitting the "ip from any to any" string when this does not carry
             any additional information.

     -d      When listing, show dynamic rules in addition to static ones.

     -D      When listing, show only dynamic states.  When deleting, delete only
             dynamic states.

     -f      Run without prompting for confirmation for commands that can cause
             problems if misused, i.e., flush.  If there is no tty associated
             with the process, this is implied.  The delete command with this
             flag ignores possible errors, i.e., nonexistent rule number.  And
             for batched commands execution continues with the next command.

     -i      When listing a table (see the LOOKUP TABLES section below for more
             information on lookup tables), format values as IP addresses.  By
             default, values are shown as integers.

     -n      Only check syntax of the command strings, without actually passing
             them to the kernel.

     -N      Try to resolve addresses and service names in output.

     -q      Be quiet when executing the add, nat, zero, resetlog or flush
             commands; (implies -f).  This is useful when updating rulesets by
             executing multiple ipfw commands in a script (e.g.,
             ‘sh /etc/rc.firewall’), or by processing a file with many ipfw
             rules across a remote login session.  It also stops a table add or
             delete from failing if the entry already exists or is not present.

             The reason why this option may be important is that for some of
             these actions, ipfw may print a message; if the action results in
             blocking the traffic to the remote client, the remote login session
             will be closed and the rest of the ruleset will not be processed.
             Access to the console would then be required to recover.

     -S      When listing rules, show the set each rule belongs to.  If this
             flag is not specified, disabled rules will not be listed.

     -s [field]
             When listing pipes, sort according to one of the four counters
             (total or current packets or bytes).

     -t      When listing, show last match timestamp converted with ctime().

     -T      When listing, show last match timestamp as seconds from the epoch.
             This form can be more convenient for postprocessing by scripts.

   LIST OF RULES AND PREPROCESSING
     To ease configuration, rules can be put into a file which is processed
     using ipfw as shown in the last synopsis line.  An absolute pathname must
     be used.  The file will be read line by line and applied as arguments to
     the ipfw utility.

     Optionally, a preprocessor can be specified using -p preproc where pathname
     is to be piped through.  Useful preprocessors include cpp(1) and m4(1).  If
     preproc does not start with a slash (‘/’) as its first character, the usual
     PATH name search is performed.  Care should be taken with this in
     environments where not all file systems are mounted (yet) by the time ipfw
     is being run (e.g. when they are mounted over NFS).  Once -p has been
     specified, any additional arguments are passed on to the preprocessor for
     interpretation.  This allows for flexible configuration files (like
     conditionalizing them on the local hostname) and the use of macros to
     centralize frequently required arguments like IP addresses.

   TRAFFIC SHAPER CONFIGURATION
     The ipfw pipe, queue and sched commands are used to configure the traffic
     shaper and packet scheduler.  See the TRAFFIC SHAPER (DUMMYNET)
     CONFIGURATION Section below for details.

     If the world and the kernel get out of sync the ipfw ABI may break,
     preventing you from being able to add any rules.  This can adversely affect
     the booting process.  You can use ipfw disable firewall to temporarily
     disable the firewall to regain access to the network, allowing you to fix
     the problem.

PACKET FLOW
     A packet is checked against the active ruleset in multiple places in the
     protocol stack, under control of several sysctl variables.  These places
     and variables are shown below, and it is important to have this picture in
     mind in order to design a correct ruleset.

                  ^    to upper layers    V
                  |                       |
                  +----------->-----------+
                  ^                       V
            [ip(6)_input]           [ip(6)_output]     net.inet(6).ip(6).fw.enable=1
                  |                       |
                  ^                       V
            [ether_demux]        [ether_output_frame]  net.link.ether.ipfw=1
                  |                       |
                  +-->--[bdg_forward]-->--+            net.link.bridge.ipfw=1
                  ^                       V
                  |      to devices       |

     The number of times the same packet goes through the firewall can vary
     between 0 and 4 depending on packet source and destination, and system
     configuration.

     Note that as packets flow through the stack, headers can be stripped or
     added to it, and so they may or may not be available for inspection.  E.g.,
     incoming packets will include the MAC header when ipfw is invoked from
     ether_demux(), but the same packets will have the MAC header stripped off
     when ipfw is invoked from ip_input() or ip6_input().

     Also note that each packet is always checked against the complete ruleset,
     irrespective of the place where the check occurs, or the source of the
     packet.  If a rule contains some match patterns or actions which are not
     valid for the place of invocation (e.g. trying to match a MAC header within
     ip_input or ip6_input ), the match pattern will not match, but a not
     operator in front of such patterns will cause the pattern to always match
     on those packets.  It is thus the responsibility of the programmer, if
     necessary, to write a suitable ruleset to differentiate among the possible
     places.  skipto rules can be useful here, as an example:

           # packets from ether_demux or bdg_forward
           ipfw add 10 skipto 1000 all from any to any layer2 in
           # packets from ip_input
           ipfw add 10 skipto 2000 all from any to any not layer2 in
           # packets from ip_output
           ipfw add 10 skipto 3000 all from any to any not layer2 out
           # packets from ether_output_frame
           ipfw add 10 skipto 4000 all from any to any layer2 out

     (yes, at the moment there is no way to differentiate between ether_demux
     and bdg_forward).

     Also note that only actions allow, deny, netgraph, ngtee and related to
     dummynet are processed for layer2 frames and all other actions act as if
     they were allow for such frames.  Full set of actions is supported for IP
     packets without layer2 headers only.  For example, divert action does not
     divert layer2 frames.

SYNTAX
     In general, each keyword or argument must be provided as a separate command
     line argument, with no leading or trailing spaces.  Keywords are case-
     sensitive, whereas arguments may or may not be case-sensitive depending on
     their nature (e.g. uid's are, hostnames are not).

     Some arguments (e.g., port or address lists) are comma-separated lists of
     values.  In this case, spaces after commas ',' are allowed to make the line
     more readable.  You can also put the entire command (including flags) into
     a single argument.  E.g., the following forms are equivalent:

           ipfw -q add deny src-ip 10.0.0.0/24,127.0.0.1/8
           ipfw -q add deny src-ip 10.0.0.0/24, 127.0.0.1/8
           ipfw "-q add deny src-ip 10.0.0.0/24, 127.0.0.1/8"

RULE FORMAT
     The format of firewall rules is the following:

           [rule_number] [set set_number] [prob match_probability] action
           [log [logamount number]] [altq queue] [{tag | untag} number] body

     where the body of the rule specifies which information is used for
     filtering packets, among the following:

        Layer-2 header fields                 When available
        IPv4 and IPv6 Protocol                SCTP, TCP, UDP, ICMP, etc.
        Source and dest. addresses and ports
        Direction                             See Section PACKET FLOW
        Transmit and receive interface        By name or address
        Misc. IP header fields                Version, type of service, datagram
                                              length, identification,
                                              fragmentation flags, Time To Live
        IP options
        IPv6 Extension headers                Fragmentation, Hop-by-Hop options,
                                              Routing Headers, Source routing
                                              rthdr0, Mobile IPv6 rthdr2, IPSec
                                              options.
        IPv6 Flow-ID
        Misc. TCP header fields               TCP flags (SYN, FIN, ACK, RST,
                                              etc.), sequence number,
                                              acknowledgment number, window
        TCP options
        ICMP types                            for ICMP packets
        ICMP6 types                           for ICMP6 packets
        User/group ID                         When the packet can be associated
                                              with a local socket.
        Divert status                         Whether a packet came from a
                                              divert socket (e.g., natd(8)).
        Fib annotation state                  Whether a packet has been tagged
                                              for using a specific FIB (routing
                                              table) in future forwarding
                                              decisions.

     Note that some of the above information, e.g. source MAC or IP addresses
     and TCP/UDP ports, can be easily spoofed, so filtering on those fields
     alone might not guarantee the desired results.

     rule_number
             Each rule is associated with a rule_number in the range 1..65535,
             with the latter reserved for the default rule.  Rules are checked
             sequentially by rule number.  Multiple rules can have the same
             number, in which case they are checked (and listed) according to
             the order in which they have been added.  If a rule is entered
             without specifying a number, the kernel will assign one in such a
             way that the rule becomes the last one before the default rule.
             Automatic rule numbers are assigned by incrementing the last non-
             default rule number by the value of the sysctl variable
             net.inet.ip.fw.autoinc_step which defaults to 100.  If this is not
             possible (e.g. because we would go beyond the maximum allowed rule
             number), the number of the last non-default value is used instead.

     set set_number
             Each rule is associated with a set_number in the range 0..31.  Sets
             can be individually disabled and enabled, so this parameter is of
             fundamental importance for atomic ruleset manipulation.  It can be
             also used to simplify deletion of groups of rules.  If a rule is
             entered without specifying a set number, set 0 will be used.
             Set 31 is special in that it cannot be disabled, and rules in set
             31 are not deleted by the ipfw flush command (but you can delete
             them with the ipfw delete set 31 command).  Set 31 is also used for
             the default rule.

     prob match_probability
             A match is only declared with the specified probability (floating
             point number between 0 and 1).  This can be useful for a number of
             applications such as random packet drop or (in conjunction with
             dummynet) to simulate the effect of multiple paths leading to out-
             of-order packet delivery.

             Note: this condition is checked before any other condition,
             including ones such as keep-state or check-state which might have
             side effects.

     log [logamount number]
             Packets matching a rule with the log keyword will be made available
             for logging in two ways: if the sysctl variable
             net.inet.ip.fw.verbose is set to 0 (default), one can use bpf(4)
             attached to the ipfw0 pseudo interface.  This pseudo interface can
             be created manually after a system boot by using the following
             command:

                   # ifconfig ipfw0 create

             Or, automatically at boot time by adding the following line to the
             rc.conf(5) file:

                   firewall_logif="YES"

             There is zero overhead when no bpf(4) is attached to the pseudo
             interface.

             If net.inet.ip.fw.verbose is set to 1, packets will be logged to
             syslogd(8) with a LOG_SECURITY facility up to a maximum of
             logamount packets.  If no logamount is specified, the limit is
             taken from the sysctl variable net.inet.ip.fw.verbose_limit.  In
             both cases, a value of 0 means unlimited logging.

             Once the limit is reached, logging can be re-enabled by clearing
             the logging counter or the packet counter for that entry, see the
             resetlog command.

             Note: logging is done after all other packet matching conditions
             have been successfully verified, and before performing the final
             action (accept, deny, etc.) on the packet.

     tag number
             When a packet matches a rule with the tag keyword, the numeric tag
             for the given number in the range 1..65534 will be attached to the
             packet.  The tag acts as an internal marker (it is not sent out
             over the wire) that can be used to identify these packets later on.
             This can be used, for example, to provide trust between interfaces
             and to start doing policy-based filtering.  A packet can have
             multiple tags at the same time.  Tags are "sticky", meaning once a
             tag is applied to a packet by a matching rule it exists until
             explicit removal.  Tags are kept with the packet everywhere within
             the kernel, but are lost when packet leaves the kernel, for
             example, on transmitting packet out to the network or sending
             packet to a divert(4) socket.

             To check for previously applied tags, use the tagged rule option.
             To delete previously applied tag, use the untag keyword.

             Note: since tags are kept with the packet everywhere in
             kernelspace, they can be set and unset anywhere in the kernel
             network subsystem (using the mbuf_tags(9) facility), not only by
             means of the ipfw(4) tag and untag keywords.  For example, there
             can be a specialized netgraph(4) node doing traffic analyzing and
             tagging for later inspecting in firewall.

     untag number
             When a packet matches a rule with the untag keyword, the tag with
             the number number is searched among the tags attached to this
             packet and, if found, removed from it.  Other tags bound to packet,
             if present, are left untouched.

     altq queue
             When a packet matches a rule with the altq keyword, the ALTQ
             identifier for the given queue (see altq(4)) will be attached.
             Note that this ALTQ tag is only meaningful for packets going "out"
             of IPFW, and not being rejected or going to divert sockets.  Note
             that if there is insufficient memory at the time the packet is
             processed, it will not be tagged, so it is wise to make your ALTQ
             "default" queue policy account for this.  If multiple altq rules
             match a single packet, only the first one adds the ALTQ
             classification tag.  In doing so, traffic may be shaped by using
             count altq queue rules for classification early in the ruleset,
             then later applying the filtering decision.  For example,
             check-state and keep-state rules may come later and provide the
             actual filtering decisions in addition to the fallback ALTQ tag.

             You must run pfctl(8) to set up the queues before IPFW will be able
             to look them up by name, and if the ALTQ disciplines are
             rearranged, the rules in containing the queue identifiers in the
             kernel will likely have gone stale and need to be reloaded.  Stale
             queue identifiers will probably result in misclassification.

             All system ALTQ processing can be turned on or off via ipfw enable
             altq and ipfw disable altq.  The usage of net.inet.ip.fw.one_pass
             is irrelevant to ALTQ traffic shaping, as the actual rule action is
             followed always after adding an ALTQ tag.

   RULE ACTIONS
     A rule can be associated with one of the following actions, which will be
     executed when the packet matches the body of the rule.

     allow | accept | pass | permit
             Allow packets that match rule.  The search terminates.

     check-state [:flowname | :any]
             Checks the packet against the dynamic ruleset.  If a match is
             found, execute the action associated with the rule which generated
             this dynamic rule, otherwise move to the next rule.
             Check-state rules do not have a body.  If no check-state rule is
             found, the dynamic ruleset is checked at the first keep-state or
             limit rule.  The :flowname is symbolic name assigned to dynamic
             rule by keep-state opcode.  The special flowname :any can be used
             to ignore states flowname when matching.  The :default keyword is
             special name used for compatibility with old rulesets.

     count   Update counters for all packets that match rule.  The search
             continues with the next rule.

     deny | drop
             Discard packets that match this rule.  The search terminates.

     divert port
             Divert packets that match this rule to the divert(4) socket bound
             to port port.  The search terminates.

     fwd | forward ipaddr | tablearg[,port]
             Change the next-hop on matching packets to ipaddr, which can be an
             IP address or a host name.  The next hop can also be supplied by
             the last table looked up for the packet by using the tablearg
             keyword instead of an explicit address.  The search terminates if
             this rule matches.

             If ipaddr is a local address, then matching packets will be
             forwarded to port (or the port number in the packet if one is not
             specified in the rule) on the local machine.
             If ipaddr is not a local address, then the port number (if
             specified) is ignored, and the packet will be forwarded to the
             remote address, using the route as found in the local routing table
             for that IP.
             A fwd rule will not match layer-2 packets (those received on
             ether_input, ether_output, or bridged).
             The fwd action does not change the contents of the packet at all.
             In particular, the destination address remains unmodified, so
             packets forwarded to another system will usually be rejected by
             that system unless there is a matching rule on that system to
             capture them.  For packets forwarded locally, the local address of
             the socket will be set to the original destination address of the
             packet.  This makes the netstat(1) entry look rather weird but is
             intended for use with transparent proxy servers.

     nat nat_nr | tablearg
             Pass packet to a nat instance (for network address translation,
             address redirect, etc.): see the NETWORK ADDRESS TRANSLATION (NAT)
             Section for further information.

     nat64lsn name
             Pass packet to a stateful NAT64 instance (for IPv6/IPv4 network
             address and protocol translation): see the IPv6/IPv4 NETWORK
             ADDRESS AND PROTOCOL TRANSLATION Section for further information.

     nat64stl name
             Pass packet to a stateless NAT64 instance (for IPv6/IPv4 network
             address and protocol translation): see the IPv6/IPv4 NETWORK
             ADDRESS AND PROTOCOL TRANSLATION Section for further information.

     nat64clat name
             Pass packet to a CLAT NAT64 instance (for client-side IPv6/IPv4
             network address and protocol translation): see the IPv6/IPv4
             NETWORK ADDRESS AND PROTOCOL TRANSLATION Section for further
             information.

     nptv6 name
             Pass packet to a NPTv6 instance (for IPv6-to-IPv6 network prefix
             translation): see the IPv6-to-IPv6 NETWORK PREFIX TRANSLATION
             (NPTv6) Section for further information.

     pipe pipe_nr
             Pass packet to a dummynet “pipe” (for bandwidth limitation, delay,
             etc.).  See the TRAFFIC SHAPER (DUMMYNET) CONFIGURATION Section for
             further information.  The search terminates; however, on exit from
             the pipe and if the sysctl(8) variable net.inet.ip.fw.one_pass is
             not set, the packet is passed again to the firewall code starting
             from the next rule.

     queue queue_nr
             Pass packet to a dummynet “queue” (for bandwidth limitation using
             WF2Q+).

     reject  (Deprecated).  Synonym for unreach host.

     reset   Discard packets that match this rule, and if the packet is a TCP
             packet, try to send a TCP reset (RST) notice.  The search
             terminates.

     reset6  Discard packets that match this rule, and if the packet is a TCP
             packet, try to send a TCP reset (RST) notice.  The search
             terminates.

     skipto number | tablearg
             Skip all subsequent rules numbered less than number.  The search
             continues with the first rule numbered number or higher.  It is
             possible to use the tablearg keyword with a skipto for a computed
             skipto.  Skipto may work either in O(log(N)) or in O(1) depending
             on amount of memory and/or sysctl variables.  See the SYSCTL
             VARIABLES section for more details.

     call number | tablearg
             The current rule number is saved in the internal stack and ruleset
             processing continues with the first rule numbered number or higher.
             If later a rule with the return action is encountered, the
             processing returns to the first rule with number of this call rule
             plus one or higher (the same behaviour as with packets returning
             from divert(4) socket after a divert action).  This could be used
             to make somewhat like an assembly language “subroutine” calls to
             rules with common checks for different interfaces, etc.

             Rule with any number could be called, not just forward jumps as
             with skipto.  So, to prevent endless loops in case of mistakes,
             both call and return actions don't do any jumps and simply go to
             the next rule if memory cannot be allocated or stack
             overflowed/underflowed.

             Internally stack for rule numbers is implemented using mbuf_tags(9)
             facility and currently has size of 16 entries.  As mbuf tags are
             lost when packet leaves the kernel, divert should not be used in
             subroutines to avoid endless loops and other undesired effects.

     return  Takes rule number saved to internal stack by the last call action
             and returns ruleset processing to the first rule with number
             greater than number of corresponding call rule.  See description of
             the call action for more details.

             Note that return rules usually end a “subroutine” and thus are
             unconditional, but ipfw command-line utility currently requires
             every action except check-state to have body.  While it is
             sometimes useful to return only on some packets, usually you want
             to print just “return” for readability.  A workaround for this is
             to use new syntax and -c switch:

                   # Add a rule without actual body
                   ipfw add 2999 return via any

                   # List rules without "from any to any" part
                   ipfw -c list

             This cosmetic annoyance may be fixed in future releases.

     tee port
             Send a copy of packets matching this rule to the divert(4) socket
             bound to port port.  The search continues with the next rule.

     unreach code
             Discard packets that match this rule, and try to send an ICMP
             unreachable notice with code code, where code is a number from 0 to
             255, or one of these aliases: net, host, protocol, port, needfrag,
             srcfail, net-unknown, host-unknown, isolated, net-prohib,
             host-prohib, tosnet, toshost, filter-prohib, host-precedence or
             precedence-cutoff.  The search terminates.

     unreach6 code
             Discard packets that match this rule, and try to send an ICMPv6
             unreachable notice with code code, where code is a number from 0,
             1, 3 or 4, or one of these aliases: no-route, admin-prohib, address
             or port.  The search terminates.

     netgraph cookie
             Divert packet into netgraph with given cookie.  The search
             terminates.  If packet is later returned from netgraph it is either
             accepted or continues with the next rule, depending on
             net.inet.ip.fw.one_pass sysctl variable.

     ngtee cookie
             A copy of packet is diverted into netgraph, original packet
             continues with the next rule.  See ng_ipfw(4) for more information
             on netgraph and ngtee actions.

     setfib fibnum | tablearg
             The packet is tagged so as to use the FIB (routing table) fibnum in
             any subsequent forwarding decisions.  In the current
             implementation, this is limited to the values 0 through 15, see
             setfib(2).  Processing continues at the next rule.  It is possible
             to use the tablearg keyword with setfib.  If the tablearg value is
             not within the compiled range of fibs, the packet's fib is set to
             0.

     setdscp DSCP | number | tablearg
             Set specified DiffServ codepoint for an IPv4/IPv6 packet.
             Processing continues at the next rule.  Supported values are:

             cs0 (000000), cs1 (001000), cs2 (010000), cs3 (011000), cs4
             (100000), cs5 (101000), cs6 (110000), cs7 (111000), af11 (001010),
             af12 (001100), af13 (001110), af21 (010010), af22 (010100), af23
             (010110), af31 (011010), af32 (011100), af33 (011110), af41
             (100010), af42 (100100), af43 (100110), ef (101110), be (000000).
             Additionally, DSCP value can be specified by number (0..63).  It is
             also possible to use the tablearg keyword with setdscp.  If the
             tablearg value is not within the 0..63 range, lower 6 bits of
             supplied value are used.

     tcp-setmss mss
             Set the Maximum Segment Size (MSS) in the TCP segment to value mss.
             The kernel module ipfw_pmod should be loaded or kernel should have
             options IPFIREWALL_PMOD to be able use this action.  This command
             does not change a packet if original MSS value is lower than
             specified value.  Both TCP over IPv4 and over IPv6 are supported.
             Regardless of matched a packet or not by the tcp-setmss rule, the
             search continues with the next rule.

     reass   Queue and reassemble IPv4 fragments.  If the packet is not
             fragmented, counters are updated and processing continues with the
             next rule.  If the packet is the last logical fragment, the packet
             is reassembled and, if net.inet.ip.fw.one_pass is set to 0,
             processing continues with the next rule.  Otherwise, the packet is
             allowed to pass and the search terminates.  If the packet is a
             fragment in the middle of a logical group of fragments, it is
             consumed and processing stops immediately.

             Fragment handling can be tuned via net.inet.ip.maxfragpackets and
             net.inet.ip.maxfragsperpacket which limit, respectively, the
             maximum number of processable fragments (default: 800) and the
             maximum number of fragments per packet (default: 16).

             NOTA BENE: since fragments do not contain port numbers, they should
             be avoided with the reass rule.  Alternatively, direction-based
             (like in / out ) and source-based (like via ) match patterns can be
             used to select fragments.

             Usually a simple rule like:

                   # reassemble incoming fragments
                   ipfw add reass all from any to any in

             is all you need at the beginning of your ruleset.

     abort   Discard packets that match this rule, and if the packet is an SCTP
             packet, try to send an SCTP packet containing an ABORT chunk.  The
             search terminates.

     abort6  Discard packets that match this rule, and if the packet is an SCTP
             packet, try to send an SCTP packet containing an ABORT chunk.  The
             search terminates.

   RULE BODY
     The body of a rule contains zero or more patterns (such as specific source
     and destination addresses or ports, protocol options, incoming or outgoing
     interfaces, etc.)  that the packet must match in order to be recognised.
     In general, the patterns are connected by (implicit) and operators -- i.e.,
     all must match in order for the rule to match.  Individual patterns can be
     prefixed by the not operator to reverse the result of the match, as in

           ipfw add 100 allow ip from not 1.2.3.4 to any

     Additionally, sets of alternative match patterns (or-blocks) can be
     constructed by putting the patterns in lists enclosed between parentheses (
     ) or braces { }, and using the or operator as follows:

           ipfw add 100 allow ip from { x or not y or z } to any

     Only one level of parentheses is allowed.  Beware that most shells have
     special meanings for parentheses or braces, so it is advisable to put a
     backslash \ in front of them to prevent such interpretations.

     The body of a rule must in general include a source and destination address
     specifier.  The keyword any can be used in various places to specify that
     the content of a required field is irrelevant.

     The rule body has the following format:

           [proto from src to dst] [options]

     The first part (proto from src to dst) is for backward compatibility with
     earlier versions of FreeBSD.  In modern FreeBSD any match pattern
     (including MAC headers, IP protocols, addresses and ports) can be specified
     in the options section.

     Rule fields have the following meaning:

     proto: protocol | { protocol or ... }

     protocol: [not] protocol-name | protocol-number
             An IP protocol specified by number or name (for a complete list see
             /etc/protocols), or one of the following keywords:

             ip4 | ipv4
                     Matches IPv4 packets.

             ip6 | ipv6
                     Matches IPv6 packets.

             ip | all
                     Matches any packet.

             The ipv6 in proto option will be treated as inner protocol.  And,
             the ipv4 is not available in proto option.

             The { protocol or ... } format (an or-block) is provided for
             convenience only but its use is deprecated.

     src and dst: {addr | { addr or ... }} [[not] ports]
             An address (or a list, see below) optionally followed by ports
             specifiers.

             The second format (or-block with multiple addresses) is provided
             for convenience only and its use is discouraged.

     addr: [not] {any | me | me6 | table(name[,value]) | addr-list | addr-set}

             any     Matches any IP address.

             me      Matches any IP address configured on an interface in the
                     system.

             me6     Matches any IPv6 address configured on an interface in the
                     system.  The address list is evaluated at the time the
                     packet is analysed.

             table(name[,value])
                     Matches any IPv4 or IPv6 address for which an entry exists
                     in the lookup table number.  If an optional 32-bit unsigned
                     value is also specified, an entry will match only if it has
                     this value.  See the LOOKUP TABLES section below for more
                     information on lookup tables.

     addr-list: ip-addr[,addr-list]

     ip-addr:
             A host or subnet address specified in one of the following ways:

             numeric-ip | hostname
                     Matches a single IPv4 address, specified as dotted-quad or
                     a hostname.  Hostnames are resolved at the time the rule is
                     added to the firewall list.

             addr/masklen
                     Matches all addresses with base addr (specified as an IP
                     address, a network number, or a hostname) and mask width of
                     masklen bits.  As an example, 1.2.3.4/25 or 1.2.3.0/25 will
                     match all IP numbers from 1.2.3.0 to 1.2.3.127 .

             addr:mask
                     Matches all addresses with base addr (specified as an IP
                     address, a network number, or a hostname) and the mask of
                     mask, specified as a dotted quad.  As an example,
                     1.2.3.4:255.0.255.0 or 1.0.3.0:255.0.255.0 will match
                     1.*.3.*.  This form is advised only for non-contiguous
                     masks.  It is better to resort to the addr/masklen format
                     for contiguous masks, which is more compact and less error-
                     prone.

     addr-set: addr[/masklen]{list}

     list: {num | num-num}[,list]
             Matches all addresses with base address addr (specified as an IP
             address, a network number, or a hostname) and whose last byte is in
             the list between braces { } .  Note that there must be no spaces
             between braces and numbers (spaces after commas are allowed).
             Elements of the list can be specified as single entries or ranges.
             The masklen field is used to limit the size of the set of
             addresses, and can have any value between 24 and 32.  If not
             specified, it will be assumed as 24.
             This format is particularly useful to handle sparse address sets
             within a single rule.  Because the matching occurs using a bitmask,
             it takes constant time and dramatically reduces the complexity of
             rulesets.
             As an example, an address specified as 1.2.3.4/24{128,35-55,89} or
             1.2.3.0/24{128,35-55,89} will match the following IP addresses:
             1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .

     addr6-list: ip6-addr[,addr6-list]

     ip6-addr:
             A host or subnet specified one of the following ways:

             numeric-ip | hostname
                     Matches a single IPv6 address as allowed by inet_pton(3) or
                     a hostname.  Hostnames are resolved at the time the rule is
                     added to the firewall list.

             addr/masklen
                     Matches all IPv6 addresses with base addr (specified as
                     allowed by inet_pton(3) or a hostname) and mask width of
                     masklen bits.

             addr/mask
                     Matches all IPv6 addresses with base addr (specified as
                     allowed by inet_pton(3) or a hostname) and the mask of
                     mask, specified as allowed by inet_pton(3).  As an example,
                     fe::640:0:0/ffff::ffff:ffff:0:0 will match
                     fe:*:*:*:0:640:*:*.  This form is advised only for non-
                     contiguous masks.  It is better to resort to the
                     addr/masklen format for contiguous masks, which is more
                     compact and less error-prone.

             No support for sets of IPv6 addresses is provided because IPv6
             addresses are typically random past the initial prefix.

     ports: {port | port-port}[,ports]
             For protocols which support port numbers (such as SCTP, TCP and
             UDP), optional ports may be specified as one or more ports or port
             ranges, separated by commas but no spaces, and an optional not
             operator.  The ‘-’ notation specifies a range of ports (including
             boundaries).

             Service names (from /etc/services) may be used instead of numeric
             port values.  The length of the port list is limited to 30 ports or
             ranges, though one can specify larger ranges by using an or-block
             in the options section of the rule.

             A backslash (‘\’) can be used to escape the dash (‘-’) character in
             a service name (from a shell, the backslash must be typed twice to
             avoid the shell itself interpreting it as an escape character).

                   ipfw add count tcp from any ftp\\-data-ftp to any

             Fragmented packets which have a non-zero offset (i.e., not the
             first fragment) will never match a rule which has one or more port
             specifications.  See the frag option for details on matching
             fragmented packets.

   RULE OPTIONS (MATCH PATTERNS)
     Additional match patterns can be used within rules.  Zero or more of these
     so-called options can be present in a rule, optionally prefixed by the not
     operand, and possibly grouped into or-blocks.

     The following match patterns can be used (listed in alphabetical order):

     // this is a comment.
             Inserts the specified text as a comment in the rule.  Everything
             following // is considered as a comment and stored in the rule.
             You can have comment-only rules, which are listed as having a count
             action followed by the comment.

     bridged
             Alias for layer2.

     defer-immediate-action | defer-action
             A rule with this option will not perform normal action upon a
             match.  This option is intended to be used with record-state or
             keep-state as the dynamic rule, created but ignored on match, will
             work as intended.  Rules with both record-state and
             defer-immediate-action create a dynamic rule and continue with the
             next rule without actually performing the action part of this rule.
             When the rule is later activated via the state table, the action is
             performed as usual.

     diverted
             Matches only packets generated by a divert socket.

     diverted-loopback
             Matches only packets coming from a divert socket back into the IP
             stack input for delivery.

     diverted-output
             Matches only packets going from a divert socket back outward to the
             IP stack output for delivery.

     dst-ip ip-address
             Matches IPv4 packets whose destination IP is one of the address(es)
             specified as argument.

     {dst-ip6 | dst-ipv6} ip6-address
             Matches IPv6 packets whose destination IP is one of the address(es)
             specified as argument.

     dst-port ports
             Matches IP packets whose destination port is one of the port(s)
             specified as argument.

     established
             Matches TCP packets that have the RST or ACK bits set.

     ext6hdr header
             Matches IPv6 packets containing the extended header given by
             header.  Supported headers are:

             Fragment, (frag), Hop-to-hop options (hopopt), any type of Routing
             Header (route), Source routing Routing Header Type 0 (rthdr0),
             Mobile IPv6 Routing Header Type 2 (rthdr2), Destination options
             (dstopt), IPSec authentication headers (ah), and IPsec encapsulated
             security payload headers (esp).

     fib fibnum
             Matches a packet that has been tagged to use the given FIB (routing
             table) number.

     flow table(name[,value])
             Search for the flow entry in lookup table name.  If not found, the
             match fails.  Otherwise, the match succeeds and tablearg is set to
             the value extracted from the table.

             This option can be useful to quickly dispatch traffic based on
             certain packet fields.  See the LOOKUP TABLES section below for
             more information on lookup tables.

     flow-id labels
             Matches IPv6 packets containing any of the flow labels given in
             labels.  labels is a comma separated list of numeric flow labels.

     frag spec
             Matches IPv4 packets whose ip_off field contains the comma
             separated list of IPv4 fragmentation options specified in spec.
             The recognized options are: df (don't fragment), mf (more
             fragments), rf (reserved fragment bit) offset (non-zero fragment
             offset).  The absence of a particular options may be denoted with a
             ‘!’.

             Empty list of options defaults to matching on non-zero fragment
             offset.  Such rule would match all not the first fragment
             datagrams, both IPv4 and IPv6.  This is a backward compatibility
             with older rulesets.

     gid group
             Matches all TCP or UDP packets sent by or received for a group.  A
             group may be specified by name or number.

     jail jail
             Matches all TCP or UDP packets sent by or received for the jail
             whose ID or name is jail.

     icmptypes types
             Matches ICMP packets whose ICMP type is in the list types.  The
             list may be specified as any combination of individual types
             (numeric) separated by commas.  Ranges are not allowed.  The
             supported ICMP types are:

             echo reply (0), destination unreachable (3), source quench (4),
             redirect (5), echo request (8), router advertisement (9), router
             solicitation (10), time-to-live exceeded (11), IP header bad (12),
             timestamp request (13), timestamp reply (14), information request
             (15), information reply (16), address mask request (17) and address
             mask reply (18).

     icmp6types types
             Matches ICMP6 packets whose ICMP6 type is in the list of types.
             The list may be specified as any combination of individual types
             (numeric) separated by commas.  Ranges are not allowed.

     in | out
             Matches incoming or outgoing packets, respectively.  in and out are
             mutually exclusive (in fact, out is implemented as not in).

     ipid id-list
             Matches IPv4 packets whose ip_id field has value included in
             id-list, which is either a single value or a list of values or
             ranges specified in the same way as ports.

     iplen len-list
             Matches IP packets whose total length, including header and data,
             is in the set len-list, which is either a single value or a list of
             values or ranges specified in the same way as ports.

     ipoptions spec
             Matches packets whose IPv4 header contains the comma separated list
             of options specified in spec.  The supported IP options are:

             ssrr (strict source route), lsrr (loose source route), rr (record
             packet route) and ts (timestamp).  The absence of a particular
             option may be denoted with a ‘!’.

     ipprecedence precedence
             Matches IPv4 packets whose precedence field is equal to precedence.

     ipsec   Matches packets that have IPSEC history associated with them (i.e.,
             the packet comes encapsulated in IPSEC, the kernel has IPSEC
             support, and can correctly decapsulate it).

             Note that specifying ipsec is different from specifying proto ipsec
             as the latter will only look at the specific IP protocol field,
             irrespective of IPSEC kernel support and the validity of the IPSEC
             data.

             Further note that this flag is silently ignored in kernels without
             IPSEC support.  It does not affect rule processing when given and
             the rules are handled as if with no ipsec flag.

     iptos spec
             Matches IPv4 packets whose tos field contains the comma separated
             list of service types specified in spec.  The supported IP types of
             service are:

             lowdelay (IPTOS_LOWDELAY), throughput (IPTOS_THROUGHPUT),
             reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST),
             congestion (IPTOS_ECN_CE).  The absence of a particular type may be
             denoted with a ‘!’.

     dscp spec[,spec]
             Matches IPv4/IPv6 packets whose DS field value is contained in spec
             mask.  Multiple values can be specified via the comma separated
             list.  Value can be one of keywords used in setdscp action or exact
             number.

     ipttl ttl-list
             Matches IPv4 packets whose time to live is included in ttl-list,
             which is either a single value or a list of values or ranges
             specified in the same way as ports.

     ipversion ver
             Matches IP packets whose IP version field is ver.

     keep-state [:flowname]
             Upon a match, the firewall will create a dynamic rule, whose
             default behaviour is to match bidirectional traffic between source
             and destination IP/port using the same protocol.  The rule has a
             limited lifetime (controlled by a set of sysctl(8) variables), and
             the lifetime is refreshed every time a matching packet is found.
             The :flowname is used to assign additional to addresses, ports and
             protocol parameter to dynamic rule.  It can be used for more
             accurate matching by check-state rule.  The :default keyword is
             special name used for compatibility with old rulesets.

     layer2  Matches only layer2 packets, i.e., those passed to ipfw from
             ether_demux() and ether_output_frame().

     limit {src-addr | src-port | dst-addr | dst-port} N [:flowname]
             The firewall will only allow N connections with the same set of
             parameters as specified in the rule.  One or more of source and
             destination addresses and ports can be specified.

     lookup {dst-ip | dst-port | src-ip | src-port | uid | jail} name
             Search an entry in lookup table name that matches the field
             specified as argument.  If not found, the match fails.  Otherwise,
             the match succeeds and tablearg is set to the value extracted from
             the table.

             This option can be useful to quickly dispatch traffic based on
             certain packet fields.  See the LOOKUP TABLES section below for
             more information on lookup tables.

     { MAC | mac } dst-mac src-mac
             Match packets with a given dst-mac and src-mac addresses, specified
             as the any keyword (matching any MAC address), or six groups of hex
             digits separated by colons, and optionally followed by a mask
             indicating the significant bits.  The mask may be specified using
             either of the following methods:

             1.      A slash (/) followed by the number of significant bits.
                     For example, an address with 33 significant bits could be
                     specified as:

                           MAC 10:20:30:40:50:60/33 any

             2.      An ampersand (&) followed by a bitmask specified as six
                     groups of hex digits separated by colons.  For example, an
                     address in which the last 16 bits are significant could be
                     specified as:

                           MAC 10:20:30:40:50:60&00:00:00:00:ff:ff any

                     Note that the ampersand character has a special meaning in
                     many shells and should generally be escaped.
             Note that the order of MAC addresses (destination first, source
             second) is the same as on the wire, but the opposite of the one
             used for IP addresses.

     mac-type mac-type
             Matches packets whose Ethernet Type field corresponds to one of
             those specified as argument.  mac-type is specified in the same way
             as port numbers (i.e., one or more comma-separated single values or
             ranges).  You can use symbolic names for known values such as vlan,
             ipv4, ipv6.  Values can be entered as decimal or hexadecimal (if
             prefixed by 0x), and they are always printed as hexadecimal (unless
             the -N option is used, in which case symbolic resolution will be
             attempted).

     proto protocol
             Matches packets with the corresponding IP protocol.

     record-state
             Upon a match, the firewall will create a dynamic rule as if
             keep-state was specified.  However, this option doesn't imply an
             implicit check-state in contrast to keep-state.

     recv | xmit | via {ifX | if* | table(name[,value]) | ipno | any}
             Matches packets received, transmitted or going through,
             respectively, the interface specified by exact name (ifX), by
             device name (if*), by IP address, or through some interface.  Table
             name may be used to match interface by its kernel ifindex.  See the
             LOOKUP TABLES section below for more information on lookup tables.

             The via keyword causes the interface to always be checked.  If recv
             or xmit is used instead of via, then only the receive or transmit
             interface (respectively) is checked.  By specifying both, it is
             possible to match packets based on both receive and transmit
             interface, e.g.:

                   ipfw add deny ip from any to any out recv ed0 xmit ed1

             The recv interface can be tested on either incoming or outgoing
             packets, while the xmit interface can only be tested on outgoing
             packets.  So out is required (and in is invalid) whenever xmit is
             used.

             A packet might not have a receive or transmit interface: packets
             originating from the local host have no receive interface, while
             packets destined for the local host have no transmit interface.

     set-limit {src-addr | src-port | dst-addr | dst-port} N
             Works like limit but does not have an implicit check-state attached
             to it.

     setup   Matches TCP packets that have the SYN bit set but no ACK bit.  This
             is the short form of “tcpflags syn,!ack”.

     sockarg
             Matches packets that are associated to a local socket and for which
             the SO_USER_COOKIE socket option has been set to a non-zero value.
             As a side effect, the value of the option is made available as
             tablearg value, which in turn can be used as skipto or pipe number.

     src-ip ip-address
             Matches IPv4 packets whose source IP is one of the address(es)
             specified as an argument.

     src-ip6 ip6-address
             Matches IPv6 packets whose source IP is one of the address(es)
             specified as an argument.

     src-port ports
             Matches IP packets whose source port is one of the port(s)
             specified as argument.

     tagged tag-list
             Matches packets whose tags are included in tag-list, which is
             either a single value or a list of values or ranges specified in
             the same way as ports.  Tags can be applied to the packet using tag
             rule action parameter (see it's description for details on tags).

     tcpack ack
             TCP packets only.  Match if the TCP header acknowledgment number
             field is set to ack.

     tcpdatalen tcpdatalen-list
             Matches TCP packets whose length of TCP data is tcpdatalen-list,
             which is either a single value or a list of values or ranges
             specified in the same way as ports.

     tcpflags spec
             TCP packets only.  Match if the TCP header contains the comma
             separated list of flags specified in spec.  The supported TCP flags
             are:

             fin, syn, rst, psh, ack and urg.  The absence of a particular flag
             may be denoted with a ‘!’.  A rule which contains a tcpflags
             specification can never match a fragmented packet which has a non-
             zero offset.  See the frag option for details on matching
             fragmented packets.

     tcpmss tcpmss-list
             Matches TCP packets whose MSS (maximum segment size) value is set
             to tcpmss-list, which is either a single value or a list of values
             or ranges specified in the same way as ports.

     tcpseq seq
             TCP packets only.  Match if the TCP header sequence number field is
             set to seq.

     tcpwin tcpwin-list
             Matches TCP packets whose  header window field is set to
             tcpwin-list, which is either a single value or a list of values or
             ranges specified in the same way as ports.

     tcpoptions spec
             TCP packets only.  Match if the TCP header contains the comma
             separated list of options specified in spec.  The supported TCP
             options are:

             mss (maximum segment size), window (tcp window advertisement), sack
             (selective ack), ts (rfc1323 timestamp) and cc (rfc1644 t/tcp
             connection count).  The absence of a particular option may be
             denoted with a ‘!’.

     uid user
             Match all TCP or UDP packets sent by or received for a user.  A
             user may be matched by name or identification number.

     verrevpath
             For incoming packets, a routing table lookup is done on the
             packet's source address.  If the interface on which the packet
             entered the system matches the outgoing interface for the route,
             the packet matches.  If the interfaces do not match up, the packet
             does not match.  All outgoing packets or packets with no incoming
             interface match.

             The name and functionality of the option is intentionally similar
             to the Cisco IOS command:

                   ip verify unicast reverse-path

             This option can be used to make anti-spoofing rules to reject all
             packets with source addresses not from this interface.  See also
             the option antispoof.

     versrcreach
             For incoming packets, a routing table lookup is done on the
             packet's source address.  If a route to the source address exists,
             but not the default route or a blackhole/reject route, the packet
             matches.  Otherwise, the packet does not match.  All outgoing
             packets match.

             The name and functionality of the option is intentionally similar
             to the Cisco IOS command:

                   ip verify unicast source reachable-via any

             This option can be used to make anti-spoofing rules to reject all
             packets whose source address is unreachable.

     antispoof
             For incoming packets, the packet's source address is checked if it
             belongs to a directly connected network.  If the network is
             directly connected, then the interface the packet came on in is
             compared to the interface the network is connected to.  When
             incoming interface and directly connected interface are not the
             same, the packet does not match.  Otherwise, the packet does match.
             All outgoing packets match.

             This option can be used to make anti-spoofing rules to reject all
             packets that pretend to be from a directly connected network but do
             not come in through that interface.  This option is similar to but
             more restricted than verrevpath because it engages only on packets
             with source addresses of directly connected networks instead of all
             source addresses.

LOOKUP TABLES
     Lookup tables are useful to handle large sparse sets of addresses or other
     search keys (e.g., ports, jail IDs, interface names).  In the rest of this
     section we will use the term ``key''.  Table name needs to match the
     following spec: table-name.  Tables with the same name can be created in
     different sets.  However, rule links to the tables in set 0 by default.
     This behavior can be controlled by net.inet.ip.fw.tables_sets variable.
     See the SETS OF RULES section for more information.  There may be up to
     65535 different lookup tables.

     The following table types are supported:

     table-type: addr | iface | number | flow

     table-key: addr[/masklen] | iface-name | number | flow-spec

     flow-spec: flow-field[,flow-spec]

     flow-field: src-ip | proto | src-port | dst-ip | dst-port

     addr    Matches IPv4 or IPv6 address.  Each entry is represented by an
             addr[/masklen] and will match all addresses with base addr
             (specified as an IPv4/IPv6 address, or a hostname) and mask width
             of masklen bits.  If masklen is not specified, it defaults to 32
             for IPv4 and 128 for IPv6.  When looking up an IP address in a
             table, the most specific entry will match.

     iface   Matches interface names.  Each entry is represented by string
             treated as interface name.  Wildcards are not supported.

     number  Matches protocol ports, uids/gids or jail IDs.  Each entry is
             represented by 32-bit unsigned integer.  Ranges are not supported.

     flow    Matches packet fields specified by flow type suboptions with table
             entries.

     Tables require explicit creation via create before use.

     The following creation options are supported:

     create-options: create-option | create-options

     create-option: type table-type | valtype value-mask | algo algo-desc |
             limit number | locked | missing | or-flush

     type    Table key type.

     valtype
             Table value mask.

     algo    Table algorithm to use (see below).

     limit   Maximum number of items that may be inserted into table.

     locked  Restrict any table modifications.

     missing
             Do not fail if table already exists and has exactly same options as
             new one.

     or-flush
             Flush existing table with same name instead of returning error.
             Implies missing so existing table must be compatible with new one.

     Some of these options may be modified later via modify keyword.  The
     following options can be changed:

     modify-options: modify-option | modify-options

     modify-option: limit number

     limit   Alter maximum number of items that may be inserted into table.

     Additionally, table can be locked or unlocked using lock or unlock
     commands.

     Tables of the same type can be swapped with each other using swap name
     command.  Swap may fail if tables limits are set and data exchange would
     result in limits hit.  Operation is performed atomically.

     One or more entries can be added to a table at once using add command.
     Addition of all items are performed atomically.  By default, error in
     addition of one entry does not influence addition of other entries.
     However, non-zero error code is returned in that case.  Special atomic
     keyword may be specified before add to indicate all-or-none add request.

     One or more entries can be removed from a table at once using delete
     command.  By default, error in removal of one entry does not influence
     removing of other entries.  However, non-zero error code is returned in
     that case.

     It may be possible to check what entry will be found on particular
     table-key using lookup table-key command.  This functionality is optional
     and may be unsupported in some algorithms.

     The following operations can be performed on one or all tables:

     list    List all entries.

     flush   Removes all entries.

     info    Shows generic table information.

     detail  Shows generic table information and algo-specific data.

     The following lookup algorithms are supported:

     algo-desc: algo-name | algo-name algo-data

     algo-name: addr: radix | addr: hash | iface: array | number: array | flow:
             hash

     addr: radix
             Separate Radix trees for IPv4 and IPv6, the same way as the routing
             table (see route(4)).  Default choice for addr type.

     addr:hash
             Separate auto-growing hashes for IPv4 and IPv6.  Accepts entries
             with the same mask length specified initially via addr:hash
             masks=/v4,/v6 algorithm creation options.  Assume /32 and /128
             masks by default.  Search removes host bits (according to mask)
             from supplied address and checks resulting key in appropriate hash.
             Mostly optimized for /64 and byte-ranged IPv6 masks.

     iface:array
             Array storing sorted indexes for entries which are presented in the
             system.  Optimized for very fast lookup.

     number:array
             Array storing sorted u32 numbers.

     flow:hash
             Auto-growing hash storing flow entries.  Search calculates hash on
             required packet fields and searches for matching entries in
             selected bucket.

     The tablearg feature provides the ability to use a value, looked up in the
     table, as the argument for a rule action, action parameter or rule option.
     This can significantly reduce number of rules in some configurations.  If
     two tables are used in a rule, the result of the second (destination) is
     used.

     Each record may hold one or more values according to value-mask.  This mask
     is set on table creation via valtype option.  The following value types are
     supported:

     value-mask: value-type[,value-mask]

     value-type: skipto | pipe | fib | nat | dscp | tag | divert |
             netgraph | limit | ipv4

     skipto  rule number to jump to.

     pipe    Pipe number to use.

     fib     fib number to match/set.

     nat     nat number to jump to.

     dscp    dscp value to match/set.

     tag     tag number to match/set.

     divert  port number to divert traffic to.

     netgraph
             hook number to move packet to.

     limit   maximum number of connections.

     ipv4    IPv4 nexthop to fwd packets to.

     ipv6    IPv6 nexthop to fwd packets to.

     The tablearg argument can be used with the following actions: nat, pipe,
     queue, divert, tee, netgraph, ngtee, fwd, skipto, setfib, action
     parameters: tag, untag, rule options: limit, tagged.

     When used with the skipto action, the user should be aware that the code
     will walk the ruleset up to a rule equal to, or past, the given number.

     See the EXAMPLES Section for example usage of tables and the tablearg
     keyword.

SETS OF RULES
     Each rule or table belongs to one of 32 different sets , numbered 0 to 31.
     Set 31 is reserved for the default rule.

     By default, rules or tables are put in set 0, unless you use the set N
     attribute when adding a new rule or table.  Sets can be individually and
     atomically enabled or disabled, so this mechanism permits an easy way to
     store multiple configurations of the firewall and quickly (and atomically)
     switch between them.

     By default, tables from set 0 are referenced when adding rule with table
     opcodes regardless of rule set.  This behavior can be changed by setting
     net.inet.ip.fw.tables_sets variable to 1.  Rule's set will then be used for
     table references.

     The command to enable/disable sets is

           ipfw set [disable number ...] [enable number ...]

     where multiple enable or disable sections can be specified.  Command
     execution is atomic on all the sets specified in the command.  By default,
     all sets are enabled.

     When you disable a set, its rules behave as if they do not exist in the
     firewall configuration, with only one exception:

           dynamic rules created from a rule before it had been disabled will
           still be active until they expire.  In order to delete dynamic rules
           you have to explicitly delete the parent rule which generated them.

     The set number of rules can be changed with the command

           ipfw set move {rule rule-number | old-set} to new-set

     Also, you can atomically swap two rulesets with the command

           ipfw set swap first-set second-set

     See the EXAMPLES Section on some possible uses of sets of rules.

STATEFUL FIREWALL
     Stateful operation is a way for the firewall to dynamically create rules
     for specific flows when packets that match a given pattern are detected.
     Support for stateful operation comes through the check-state, keep-state,
     record-state, limit and set-limit options of rules.

     Dynamic rules are created when a packet matches a keep-state, record-state,
     limit or set-limit rule, causing the creation of a dynamic rule which will
     match all and only packets with a given protocol between a src-ip/src-port
     dst-ip/dst-port pair of addresses (src and dst are used here only to denote
     the initial match addresses, but they are completely equivalent
     afterwards).  Rules created by keep-state option also have a :flowname
     taken from it.  This name is used in matching together with addresses,
     ports and protocol.  Dynamic rules will be checked at the first
     check-state, keep-state or limit occurrence, and the action performed upon
     a match will be the same as in the parent rule.

     Note that no additional attributes other than protocol and IP addresses and
     ports and :flowname are checked on dynamic rules.

     The typical use of dynamic rules is to keep a closed firewall
     configuration, but let the first TCP SYN packet from the inside network
     install a dynamic rule for the flow so that packets belonging to that
     session will be allowed through the firewall:

           ipfw add check-state :OUTBOUND
           ipfw add allow tcp from my-subnet to any setup keep-state :OUTBOUND
           ipfw add deny tcp from any to any

     A similar approach can be used for UDP, where an UDP packet coming from the
     inside will install a dynamic rule to let the response through the
     firewall:

           ipfw add check-state :OUTBOUND
           ipfw add allow udp from my-subnet to any keep-state :OUTBOUND
           ipfw add deny udp from any to any

     Dynamic rules expire after some time, which depends on the status of the
     flow and the setting of some sysctl variables.  See Section SYSCTL
     VARIABLES for more details.  For TCP sessions, dynamic rules can be
     instructed to periodically send keepalive packets to refresh the state of
     the rule when it is about to expire.

     See Section EXAMPLES for more examples on how to use dynamic rules.

TRAFFIC SHAPER (DUMMYNET) CONFIGURATION
     ipfw is also the user interface for the dummynet traffic shaper, packet
     scheduler and network emulator, a subsystem that can artificially queue,
     delay or drop packets emulating the behaviour of certain network links or
     queueing systems.

     dummynet operates by first using the firewall to select packets using any
     match pattern that can be used in ipfw rules.  Matching packets are then
     passed to either of two different objects, which implement the traffic
     regulation:

         pipe    A pipe emulates a link with given bandwidth and propagation
                 delay, driven by a FIFO scheduler and a single queue with
                 programmable queue size and packet loss rate.  Packets are
                 appended to the queue as they come out from ipfw, and then
                 transferred in FIFO order to the link at the desired rate.

         queue   A queue is an abstraction used to implement packet scheduling
                 using one of several packet scheduling algorithms.  Packets
                 sent to a queue are first grouped into flows according to a
                 mask on the 5-tuple.  Flows are then passed to the scheduler
                 associated to the queue, and each flow uses scheduling
                 parameters (weight and others) as configured in the queue
                 itself.  A scheduler in turn is connected to an emulated link,
                 and arbitrates the link's bandwidth among backlogged flows
                 according to weights and to the features of the scheduling
                 algorithm in use.

     In practice, pipes can be used to set hard limits to the bandwidth that a
     flow can use, whereas queues can be used to determine how different flows
     share the available bandwidth.

     A graphical representation of the binding of queues, flows, schedulers and
     links is below.

                            (flow_mask|sched_mask)  sched_mask
                    +---------+   weight Wx  +-------------+
                    |         |->-[flow]-->--|             |-+
               -->--| QUEUE x |   ...        |             | |
                    |         |->-[flow]-->--| SCHEDuler N | |
                    +---------+              |             | |
                        ...                  |             +--[LINK N]-->--
                    +---------+   weight Wy  |             | +--[LINK N]-->--
                    |         |->-[flow]-->--|             | |
               -->--| QUEUE y |   ...        |             | |
                    |         |->-[flow]-->--|             | |
                    +---------+              +-------------+ |
                                               +-------------+
     It is important to understand the role of the SCHED_MASK and FLOW_MASK,
     which are configured through the commands
           ipfw sched N config mask SCHED_MASK ...
     and
           ipfw queue X config mask FLOW_MASK ....

     The SCHED_MASK is used to assign flows to one or more scheduler instances,
     one for each value of the packet's 5-tuple after applying SCHED_MASK.  As
     an example, using ``src-ip 0xffffff00'' creates one instance for each /24
     destination subnet.

     The FLOW_MASK, together with the SCHED_MASK, is used to split packets into
     flows.  As an example, using ``src-ip 0x000000ff'' together with the
     previous SCHED_MASK makes a flow for each individual source address.  In
     turn, flows for each /24 subnet will be sent to the same scheduler
     instance.

     The above diagram holds even for the pipe case, with the only restriction
     that a pipe only supports a SCHED_MASK, and forces the use of a FIFO
     scheduler (these are for backward compatibility reasons; in fact,
     internally, a dummynet's pipe is implemented exactly as above).

     There are two modes of dummynet operation: “normal” and “fast”.  The
     “normal” mode tries to emulate a real link: the dummynet scheduler ensures
     that the packet will not leave the pipe faster than it would on the real
     link with a given bandwidth.  The “fast” mode allows certain packets to
     bypass the dummynet scheduler (if packet flow does not exceed pipe's
     bandwidth).  This is the reason why the “fast” mode requires less CPU
     cycles per packet (on average) and packet latency can be significantly
     lower in comparison to a real link with the same bandwidth.  The default
     mode is “normal”.  The “fast” mode can be enabled by setting the
     net.inet.ip.dummynet.io_fast sysctl(8) variable to a non-zero value.

   PIPE, QUEUE AND SCHEDULER CONFIGURATION
     The pipe, queue and scheduler configuration commands are the following:

           pipe number config pipe-configuration

           queue number config queue-configuration

           sched number config sched-configuration

     The following parameters can be configured for a pipe:

     bw bandwidth | device
             Bandwidth, measured in [K|M|G]{bit/s|Byte/s}.

             A value of 0 (default) means unlimited bandwidth.  The unit must
             immediately follow the number, as in

                   ipfw pipe 1 config bw 300Kbit/s

             If a device name is specified instead of a numeric value, as in

                   ipfw pipe 1 config bw tun0

             then the transmit clock is supplied by the specified device.  At
             the moment only the tun(4) device supports this functionality, for
             use in conjunction with ppp(8).

     delay ms-delay
             Propagation delay, measured in milliseconds.  The value is rounded
             to the next multiple of the clock tick (typically 10ms, but it is a
             good practice to run kernels with “options HZ=1000” to reduce the
             granularity to 1ms or less).  The default value is 0, meaning no
             delay.

     burst size
             If the data to be sent exceeds the pipe's bandwidth limit (and the
             pipe was previously idle), up to size bytes of data are allowed to
             bypass the dummynet scheduler, and will be sent as fast as the
             physical link allows.  Any additional data will be transmitted at
             the rate specified by the pipe bandwidth.  The burst size depends
             on how long the pipe has been idle; the effective burst size is
             calculated as follows: MAX( size , bw * pipe_idle_time).

     profile filename
             A file specifying the additional overhead incurred in the
             transmission of a packet on the link.

             Some link types introduce extra delays in the transmission of a
             packet, e.g., because of MAC level framing, contention on the use
             of the channel, MAC level retransmissions and so on.  From our
             point of view, the channel is effectively unavailable for this
             extra time, which is constant or variable depending on the link
             type.  Additionally, packets may be dropped after this time (e.g.,
             on a wireless link after too many retransmissions).  We can model
             the additional delay with an empirical curve that represents its
             distribution.

                         cumulative probability
                         1.0 ^
                             |
                         L   +-- loss-level          x
                             |                 ******
                             |                *
                             |           *****
                             |          *
                             |        **
                             |       *
                             +-------*------------------->
                                         delay
             The empirical curve may have both vertical and horizontal lines.
             Vertical lines represent constant delay for a range of
             probabilities.  Horizontal lines correspond to a discontinuity in
             the delay distribution: the pipe will use the largest delay for a
             given probability.

             The file format is the following, with whitespace acting as a
             separator and '#' indicating the beginning a comment:

             name identifier
                     optional name (listed by "ipfw pipe show") to identify the
                     delay distribution;

             bw value
                     the bandwidth used for the pipe.  If not specified here, it
                     must be present explicitly as a configuration parameter for
                     the pipe;

             loss-level L
                     the probability above which packets are lost.  (0.0 <= L <=
                     1.0, default 1.0 i.e., no loss);

             samples N
                     the number of samples used in the internal representation
                     of the curve (2..1024; default 100);

             delay prob | prob delay
                     One of these two lines is mandatory and defines the format
                     of the following lines with data points.

             XXX YYY
                     2 or more lines representing points in the curve, with
                     either delay or probability first, according to the chosen
                     format.  The unit for delay is milliseconds.  Data points
                     do not need to be sorted.  Also, the number of actual lines
                     can be different from the value of the "samples" parameter:
                     ipfw utility will sort and interpolate the curve as needed.

             Example of a profile file:

                   name    bla_bla_bla
                   samples 100
                   loss-level    0.86
                   prob    delay
                   0       200     # minimum overhead is 200ms
                   0.5     200
                   0.5     300
                   0.8     1000
                   0.9     1300
                   1       1300
                   #configuration file end

     The following parameters can be configured for a queue:

     pipe pipe_nr
             Connects a queue to the specified pipe.  Multiple queues (with the
             same or different weights) can be connected to the same pipe, which
             specifies the aggregate rate for the set of queues.

     weight weight
             Specifies the weight to be used for flows matching this queue.  The
             weight must be in the range 1..100, and defaults to 1.

     The following case-insensitive parameters can be configured for a
     scheduler:

     type {fifo | wf2q+ | rr | qfq | fq_codel | fq_pie}
             specifies the scheduling algorithm to use.
             fifo    is just a FIFO scheduler (which means that all packets are
                     stored in the same queue as they arrive to the scheduler).
                     FIFO has O(1) per-packet time complexity, with very low
                     constants (estimate 60-80ns on a 2GHz desktop machine) but
                     gives no service guarantees.
             wf2q+   implements the WF2Q+ algorithm, which is a Weighted Fair
                     Queueing algorithm which permits flows to share bandwidth
                     according to their weights.  Note that weights are not
                     priorities; even a flow with a minuscule weight will never
                     starve.  WF2Q+ has O(log N) per-packet processing cost,
                     where N is the number of flows, and is the default
                     algorithm used by previous versions dummynet's queues.
             rr      implements the Deficit Round Robin algorithm, which has
                     O(1) processing costs (roughly, 100-150ns per packet) and
                     permits bandwidth allocation according to weights, but with
                     poor service guarantees.
             qfq     implements the QFQ algorithm, which is a very fast variant
                     of WF2Q+, with similar service guarantees and O(1)
                     processing costs (roughly, 200-250ns per packet).
             fq_codel
                     implements the FQ-CoDel (FlowQueue-CoDel) scheduler/AQM
                     algorithm, which uses a modified Deficit Round Robin
                     scheduler to manage two lists of sub-queues (old sub-queues
                     and new sub-queues) for providing brief periods of priority
                     to lightweight or short burst flows.  By default, the total
                     number of sub-queues is 1024.  FQ-CoDel's internal,
                     dynamically created sub-queues are controlled by separate
                     instances of CoDel AQM.
             fq_pie  implements the FQ-PIE (FlowQueue-PIE) scheduler/AQM
                     algorithm, which similar to fq_codel but uses per sub-queue
                     PIE AQM instance to control the queue delay.

             fq_codel inherits AQM parameters and options from codel (see
             below), and fq_pie inherits AQM parameters and options from pie
             (see below).  Additionally, both of fq_codel and fq_pie have shared
             scheduler parameters which are:

             quantum
                     m specifies the quantum (credit) of the scheduler.  m is
                     the number of bytes a queue can serve before being moved to
                     the tail of old queues list.  The default is 1514 bytes,
                     and the maximum acceptable value is 9000 bytes.

             limit   m specifies the hard size limit (in unit of packets) of all
                     queues managed by an instance of the scheduler.  The
                     default value of m is 10240 packets, and the maximum
                     acceptable value is 20480 packets.

             flows   m specifies the total number of flow queues (sub-queues)
                     that fq_* creates and manages.  By default, 1024 sub-queues
                     are created when an instance of the fq_{codel/pie}
                     scheduler is created.  The maximum acceptable value is
                     65536.

             Note that any token after fq_codel or fq_pie is considered a
             parameter for fq_{codel/pie}.  So, ensure all scheduler
             configuration options not related to fq_{codel/pie} are written
             before fq_codel/fq_pie tokens.

     In addition to the type, all parameters allowed for a pipe can also be
     specified for a scheduler.

     Finally, the following parameters can be configured for both pipes and
     queues:

     buckets hash-table-size
           Specifies the size of the hash table used for storing the various
           queues.  Default value is 64 controlled by the sysctl(8) variable
           net.inet.ip.dummynet.hash_size, allowed range is 16 to 65536.

     mask mask-specifier
           Packets sent to a given pipe or queue by an ipfw rule can be further
           classified into multiple flows, each of which is then sent to a
           different dynamic pipe or queue.  A flow identifier is constructed by
           masking the IP addresses, ports and protocol types as specified with
           the mask options in the configuration of the pipe or queue.  For each
           different flow identifier, a new pipe or queue is created with the
           same parameters as the original object, and matching packets are sent
           to it.

           Thus, when dynamic pipes are used, each flow will get the same
           bandwidth as defined by the pipe, whereas when dynamic queues are
           used, each flow will share the parent's pipe bandwidth evenly with
           other flows generated by the same queue (note that other queues with
           different weights might be connected to the same pipe).
           Available mask specifiers are a combination of one or more of the
           following:

           dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask, dst-port mask,
           src-port mask, flow-id mask, proto mask or all,

           where the latter means all bits in all fields are significant.

     noerror
           When a packet is dropped by a dummynet queue or pipe, the error is
           normally reported to the caller routine in the kernel, in the same
           way as it happens when a device queue fills up.  Setting this option
           reports the packet as successfully delivered, which can be needed for
           some experimental setups where you want to simulate loss or
           congestion at a remote router.

     plr packet-loss-rate
           Packet loss rate.  Argument packet-loss-rate is a floating-point
           number between 0 and 1, with 0 meaning no loss, 1 meaning 100% loss.
           The loss rate is internally represented on 31 bits.

     queue {slots | sizeKbytes}
           Queue size, in slots or KBytes.  Default value is 50 slots, which is
           the typical queue size for Ethernet devices.  Note that for slow
           speed links you should keep the queue size short or your traffic
           might be affected by a significant queueing delay.  E.g., 50 max-
           sized Ethernet packets (1500 bytes) mean 600Kbit or 20s of queue on a
           30Kbit/s pipe.  Even worse effects can result if you get packets from
           an interface with a much larger MTU, e.g. the loopback interface with
           its 16KB packets.  The sysctl(8) variables
           net.inet.ip.dummynet.pipe_byte_limit and
           net.inet.ip.dummynet.pipe_slot_limit control the maximum lengths that
           can be specified.

     red | gred w_q/min_th/max_th/max_p
           [ecn] Make use of the RED (Random Early Detection) queue management
           algorithm.  w_q and max_p are floating point numbers between 0 and 1
           (inclusive), while min_th and max_th are integer numbers specifying
           thresholds for queue management (thresholds are computed in bytes if
           the queue has been defined in bytes, in slots otherwise).  The two
           parameters can also be of the same value if needed.  The dummynet
           also supports the gentle RED variant (gred) and ECN (Explicit
           Congestion Notification) as optional.  Three sysctl(8) variables can
           be used to control the RED behaviour:

           net.inet.ip.dummynet.red_lookup_depth
                   specifies the accuracy in computing the average queue when
                   the link is idle (defaults to 256, must be greater than zero)

           net.inet.ip.dummynet.red_avg_pkt_size
                   specifies the expected average packet size (defaults to 512,
                   must be greater than zero)

           net.inet.ip.dummynet.red_max_pkt_size
                   specifies the expected maximum packet size, only used when
                   queue thresholds are in bytes (defaults to 1500, must be
                   greater than zero).

     codel [target time] [interval time] [ecn | noecn]
           Make use of the CoDel (Controlled-Delay) queue management algorithm.
           time is interpreted as milliseconds by default but seconds (s),
           milliseconds (ms) or microseconds (us) can be specified instead.
           CoDel drops or marks (ECN) packets depending on packet sojourn time
           in the queue.  target time (5ms by default) is the minimum acceptable
           persistent queue delay that CoDel allows.  CoDel does not drop
           packets directly after packets sojourn time becomes higher than
           target time but waits for interval time (100ms default) before
           dropping.  interval time should be set to maximum RTT for all
           expected connections.  ecn enables (disabled by default) packet
           marking (instead of dropping) for ECN-enabled TCP flows when queue
           delay becomes high.

           Note that any token after codel is considered a parameter for CoDel.
           So, ensure all pipe/queue configuration options are written before
           codel token.

           The sysctl(8) variables net.inet.ip.dummynet.codel.target and
           net.inet.ip.dummynet.codel.interval can be used to set CoDel default
           parameters.

     pie [target time] [tupdate time] [alpha n] [beta n] [max_burst time]
           [max_ecnth n] [ecn | noecn] [capdrop | nocapdrop] [drand | nodrand]
           [onoff] [dre | ts]
           Make use of the PIE (Proportional Integral controller Enhanced) queue
           management algorithm.  PIE drops or marks packets depending on a
           calculated drop probability during en-queue process, with the aim of
           achieving high throughput while keeping queue delay low.  At regular
           time intervals of tupdate time (15ms by default) a background process
           (re)calculates the probability based on queue delay deviations from
           target time (15ms by default) and queue delay trends.  PIE
           approximates current queue delay by using a departure rate estimation
           method, or (optionally) by using a packet timestamp method similar to
           CoDel.  time is interpreted as milliseconds by default but seconds
           (s), milliseconds (ms) or microseconds (us) can be specified instead.
           The other PIE parameters and options are as follows:

           alpha n
                   n is a floating point number between 0 and 7 which specifies
                   the weight of queue delay deviations that is used in drop
                   probability calculation.  0.125 is the default.

           beta n  n is a floating point number between 0 and 7 which specifies
                   is the weight of queue delay trend that is used in drop
                   probability calculation.  1.25 is the default.

           max_burst time
                   The maximum period of time that PIE does not drop/mark
                   packets.  150ms is the default and 10s is the maximum value.

           max_ecnth n
                   Even when ECN is enabled, PIE drops packets instead of
                   marking them when drop probability becomes higher than ECN
                   probability threshold max_ecnth n , the default is 0.1 (i.e
                   10%) and 1 is the maximum value.

           ecn | noecn
                   enable or disable ECN marking for ECN-enabled TCP flows.
                   Disabled by default.

           capdrop | nocapdrop
                   enable or disable cap drop adjustment.  Cap drop adjustment
                   is enabled by default.

           drand | nodrand
                   enable or disable drop probability de-randomisation.  De-
                   randomisation eliminates the problem of dropping packets too
                   close or too far.  De-randomisation is enabled by default.

           onoff   enable turning PIE on and off depending on queue load.  If
                   this option is enabled, PIE turns on when over 1/3 of queue
                   becomes full.  This option is disabled by default.

           dre | ts
                   Calculate queue delay using departure rate estimation dre or
                   timestamps ts.  dre is used by default.

           Note that any token after pie is considered a parameter for PIE.  So
           ensure all pipe/queue the configuration options are written before
           pie token.  sysctl(8) variables can be used to control the pie
           default parameters.  See the SYSCTL VARIABLES section for more
           details.

     When used with IPv6 data, dummynet currently has several limitations.
     Information necessary to route link-local packets to an interface is not
     available after processing by dummynet so those packets are dropped in the
     output path.  Care should be taken to ensure that link-local packets are
     not passed to dummynet.

CHECKLIST
     Here are some important points to consider when designing your rules:

     Remember that you filter both packets going in and out.  Most
         connections need packets going in both directions.

     Remember to test very carefully.  It is a good idea to be near the
         console when doing this.  If you cannot be near the console, use an
         auto-recovery script such as the one in
         /usr/share/examples/ipfw/change_rules.sh.

     Do not forget the loopback interface.

FINE POINTS
     There are circumstances where fragmented datagrams are unconditionally
         dropped.  TCP packets are dropped if they do not contain at least 20
         bytes of TCP header, UDP packets are dropped if they do not contain a
         full 8 byte UDP header, and ICMP packets are dropped if they do not
         contain 4 bytes of ICMP header, enough to specify the ICMP type, code,
         and checksum.  These packets are simply logged as “pullup failed” since
         there may not be enough good data in the packet to produce a meaningful
         log entry.

     Another type of packet is unconditionally dropped, a TCP packet with a
         fragment offset of one.  This is a valid packet, but it only has one
         use, to try to circumvent firewalls.  When logging is enabled, these
         packets are reported as being dropped by rule -1.

     If you are logged in over a network, loading the kld(4) version of ipfw
         is probably not as straightforward as you would think.  The following
         command line is recommended:

               kldload ipfw && \
               ipfw add 32000 allow ip from any to any

         Along the same lines, doing an

               ipfw flush

         in similar surroundings is also a bad idea.

     The ipfw filter list may not be modified if the system security level
         is set to 3 or higher (see init(8) for information on system security
         levels).

PACKET DIVERSION
     A divert(4) socket bound to the specified port will receive all packets
     diverted to that port.  If no socket is bound to the destination port, or
     if the divert module is not loaded, or if the kernel was not compiled with
     divert socket support, the packets are dropped.

NETWORK ADDRESS TRANSLATION (NAT)
     ipfw support in-kernel NAT using the kernel version of libalias(3).  The
     kernel module ipfw_nat should be loaded or kernel should have options
     IPFIREWALL_NAT to be able use NAT.

     The nat configuration command is the following:

           nat nat_number config nat-configuration

     The following parameters can be configured:

     ip ip_address
             Define an ip address to use for aliasing.

     if nic  Use ip address of NIC for aliasing, dynamically changing it if
             NIC's ip address changes.

     log     Enable logging on this nat instance.

     deny_in
             Deny any incoming connection from outside world.

     same_ports
             Try to leave the alias port numbers unchanged from the actual local
             port numbers.

     unreg_only
             Traffic on the local network not originating from a RFC 1918
             unregistered address spaces will be ignored.

     unreg_cgn
             Like unreg_only, but includes the RFC 6598 (Carrier Grade NAT)
             address range.

     reset   Reset table of the packet aliasing engine on address change.

     reverse
             Reverse the way libalias handles aliasing.

     proxy_only
             Obey transparent proxy rules only, packet aliasing is not
             performed.

     skip_global
             Skip instance in case of global state lookup (see below).

     Some specials value can be supplied instead of nat_number:

     global  Looks up translation state in all configured nat instances.  If an
             entry is found, packet is aliased according to that entry.  If no
             entry was found in any of the instances, packet is passed
             unchanged, and no new entry will be created.  See section MULTIPLE
             INSTANCES in natd(8) for more information.

     tablearg
             Uses argument supplied in lookup table.  See LOOKUP TABLES section
             below for more information on lookup tables.

     To let the packet continue after being (de)aliased, set the sysctl variable
     net.inet.ip.fw.one_pass to 0.  For more information about aliasing modes,
     refer to libalias(3).  See Section EXAMPLES for some examples about nat
     usage.

   REDIRECT AND LSNAT SUPPORT IN IPFW
     Redirect and LSNAT support follow closely the syntax used in natd(8).  See
     Section EXAMPLES for some examples on how to do redirect and lsnat.

   SCTP NAT SUPPORT
     SCTP nat can be configured in a similar manner to TCP through the ipfw
     command line tool.  The main difference is that sctp nat does not do port
     translation.  Since the local and global side ports will be the same, there
     is no need to specify both.  Ports are redirected as follows:

           nat nat_number config if nic redirect_port sctp
           ip_address [,addr_list] {[port | port-port] [,ports]}

     Most sctp nat configuration can be done in real-time through the sysctl(8)
     interface.  All may be changed dynamically, though the hash_table size will
     only change for new nat instances.  See SYSCTL VARIABLES for more info.

IPv6/IPv4 NETWORK ADDRESS AND PROTOCOL TRANSLATION
   Stateful translation
     ipfw supports in-kernel IPv6/IPv4 network address and protocol translation.
     Stateful NAT64 translation allows IPv6-only clients to contact IPv4 servers
     using unicast TCP, UDP or ICMP protocols.  One or more IPv4 addresses
     assigned to a stateful NAT64 translator are shared among several IPv6-only
     clients.  When stateful NAT64 is used in conjunction with DNS64, no changes
     are usually required in the IPv6 client or the IPv4 server.  The kernel
     module ipfw_nat64 should be loaded or kernel should have options
     IPFIREWALL_NAT64 to be able use stateful NAT64 translator.

     Stateful NAT64 uses a bunch of memory for several types of objects.  When
     IPv6 client initiates connection, NAT64 translator creates a host entry in
     the states table.  Each host entry uses preallocated IPv4 alias entry.
     Each alias entry has a number of ports group entries allocated on demand.
     Ports group entries contains connection state entries.  There are several
     options to control limits and lifetime for these objects.

     NAT64 translator follows RFC7915 when does ICMPv6/ICMP translation,
     unsupported message types will be silently dropped.  IPv6 needs several
     ICMPv6 message types to be explicitly allowed for correct operation.  Make
     sure that ND6 neighbor solicitation (ICMPv6 type 135) and neighbor
     advertisement (ICMPv6 type 136) messages will not be handled by translation
     rules.

     After translation NAT64 translator by default sends packets through
     corresponding netisr queue.  Thus translator host should be configured as
     IPv4 and IPv6 router.  Also this means, that a packet is handled by
     firewall twice.  First time an original packet is handled and consumed by
     translator, and then it is handled again as translated packet.  This
     behavior can be changed by sysctl variable
     net.inet.ip.fw.nat64_direct_output.  Also translated packet can be tagged
     using tag rule action, and then matched by tagged opcode to avoid loops and
     extra overhead.

     The stateful NAT64 configuration command is the following:

           nat64lsn name create create-options

     The following parameters can be configured:

     prefix4 ipv4_prefix/plen
             The IPv4 prefix with mask defines the pool of IPv4 addresses used
             as source address after translation.  Stateful NAT64 module
             translates IPv6 source address of client to one IPv4 address from
             this pool.  Note that incoming IPv4 packets that don't have
             corresponding state entry in the states table will be dropped by
             translator.  Make sure that translation rules handle packets,
             destined to configured prefix.

     prefix6 ipv6_prefix/length
             The IPv6 prefix defines IPv4-embedded IPv6 addresses used by
             translator to represent IPv4 addresses.  This IPv6 prefix should be
             configured in DNS64.  The translator implementation follows
             RFC6052, that restricts the length of prefixes to one of following:
             32, 40, 48, 56, 64, or 96.  The Well-Known IPv6 Prefix 64:ff9b::
             must be 96 bits long.  The special ::/length prefix can be used to
             handle several IPv6 prefixes with one NAT64 instance.  The NAT64
             instance will determine a destination IPv4 address from prefix
             length.

     states_chunks number
             The number of states chunks in single ports group.  Each ports
             group by default can keep 64 state entries in single chunk.  The
             above value affects the maximum number of states that can be
             associated with single IPv4 alias address and port.  The value must
             be power of 2, and up to 128.

     host_del_age seconds
             The number of seconds until the host entry for a IPv6 client will
             be deleted and all its resources will be released due to
             inactivity.  Default value is 3600.

     pg_del_age seconds
             The number of seconds until a ports group with unused state entries
             will be released.  Default value is 900.

     tcp_syn_age seconds
             The number of seconds while a state entry for TCP connection with
             only SYN sent will be kept.  If TCP connection establishing will
             not be finished, state entry will be deleted.  Default value is 10.

     tcp_est_age seconds
             The number of seconds while a state entry for established TCP
             connection will be kept.  Default value is 7200.

     tcp_close_age seconds
             The number of seconds while a state entry for closed TCP connection
             will be kept.  Keeping state entries for closed connections is
             needed, because IPv4 servers typically keep closed connections in a
             TIME_WAIT state for a several minutes.  Since translator's IPv4
             addresses are shared among all IPv6 clients, new connections from
             the same addresses and ports may be rejected by server, because
             these connections are still in a TIME_WAIT state.  Keeping them in
             translator's state table protects from such rejects.  Default value
             is 180.

     udp_age seconds
             The number of seconds while translator keeps state entry in a
             waiting for reply to the sent UDP datagram.  Default value is 120.

     icmp_age seconds
             The number of seconds while translator keeps state entry in a
             waiting for reply to the sent ICMP message.  Default value is 60.

     log     Turn on logging of all handled packets via BPF through ipfwlog0
             interface.  ipfwlog0 is a pseudo interface and can be created after
             a boot manually with ifconfig command.  Note that it has different
             purpose than ipfw0 interface.  Translators sends to BPF an
             additional information with each packet.  With tcpdump you are able
             to see each handled packet before and after translation.

     -log    Turn off logging of all handled packets via BPF.

     allow_private
             Turn on processing private IPv4 addresses.  By default IPv6 packets
             with destinations mapped to private address ranges defined by
             RFC1918 are not processed.

     -allow_private
             Turn off private address handling in nat64 instance.

     To inspect a states table of stateful NAT64 the following command can be
     used:

           nat64lsn name show states

     Stateless NAT64 translator doesn't use a states table for translation and
     converts IPv4 addresses to IPv6 and vice versa solely based on the mappings
     taken from configured lookup tables.  Since a states table doesn't used by
     stateless translator, it can be configured to pass IPv4 clients to
     IPv6-only servers.

     The stateless NAT64 configuration command is the following:

           nat64stl name create create-options

     The following parameters can be configured:

     prefix6 ipv6_prefix/length
             The IPv6 prefix defines IPv4-embedded IPv6 addresses used by
             translator to represent IPv4 addresses.  This IPv6 prefix should be
             configured in DNS64.

     table4 table46
             The lookup table table46 contains mapping how IPv4 addresses should
             be translated to IPv6 addresses.

     table6 table64
             The lookup table table64 contains mapping how IPv6 addresses should
             be translated to IPv4 addresses.

     log     Turn on logging of all handled packets via BPF through ipfwlog0
             interface.

     -log    Turn off logging of all handled packets via BPF.

     allow_private
             Turn on processing private IPv4 addresses.  By default IPv6 packets
             with destinations mapped to private address ranges defined by
             RFC1918 are not processed.

     -allow_private
             Turn off private address handling in nat64 instance.

     Note that the behavior of stateless translator with respect to not matched
     packets differs from stateful translator.  If corresponding addresses was
     not found in the lookup tables, the packet will not be dropped and the
     search continues.

   XLAT464 CLAT translation
     XLAT464 CLAT NAT64 translator implements client-side stateless translation
     as defined in RFC6877 and is very similar to statless NAT64 translator
     explained above.  Instead of lookup tables it uses one-to-one mapping
     between IPv4 and IPv6 addresses using configured prefixes.  This mode can
     be used as a replacement of DNS64 service for applications that are not
     using it (e.g. VoIP) allowing them to access IPv4-only Internet over
     IPv6-only networks with help of remote NAT64 translator.

     The CLAT NAT64 configuration command is the following:

           nat64clat name create create-options

     The following parameters can be configured:

     clat_prefix ipv6_prefix/length
             The IPv6 prefix defines IPv4-embedded IPv6 addresses used by
             translator to represent source IPv4 addresses.

     plat_prefix ipv6_prefix/length
             The IPv6 prefix defines IPv4-embedded IPv6 addresses used by
             translator to represent destination IPv4 addresses.  This IPv6
             prefix should be configured on a remote NAT64 translator.

     log     Turn on logging of all handled packets via BPF through ipfwlog0
             interface.

     -log    Turn off logging of all handled packets via BPF.

     allow_private
             Turn on processing private IPv4 addresses.  By default nat64clat
             instance will not process IPv4 packets with destination address
             from private ranges as defined in RFC1918.

     -allow_private
             Turn off private address handling in nat64clat instance.

     Note that the behavior of CLAT translator with respect to not matched
     packets differs from stateful translator.  If corresponding addresses were
     not matched against prefixes configured, the packet will not be dropped and
     the search continues.

IPv6-to-IPv6 NETWORK PREFIX TRANSLATION (NPTv6)
     ipfw supports in-kernel IPv6-to-IPv6 network prefix translation as
     described in RFC6296.  The kernel module ipfw_nptv6 should be loaded or
     kernel should has options IPFIREWALL_NPTV6 to be able use NPTv6 translator.

     The NPTv6 configuration command is the following:

           nptv6 name create create-options

     The following parameters can be configured:

     int_prefix ipv6_prefix
             IPv6 prefix used in internal network.  NPTv6 module translates
             source address when it matches this prefix.

     ext_prefix ipv6_prefix
             IPv6 prefix used in external network.  NPTv6 module translates
             destination address when it matches this prefix.

     ext_if nic
             The NPTv6 module will use first global IPv6 address from interface
             nic as external prefix.  It can be useful when IPv6 prefix of
             external network is dynamically obtained.  ext_prefix and ext_if
             options are mutually exclusive.

     prefixlen length
             The length of specified IPv6 prefixes.  It must be in range from 8
             to 64.

     Note that the prefix translation rules are silently ignored when IPv6
     packet forwarding is disabled.  To enable the packet forwarding, set the
     sysctl variable net.inet6.ip6.forwarding to 1.

     To let the packet continue after being translated, set the sysctl variable
     net.inet.ip.fw.one_pass to 0.

LOADER TUNABLES
     Tunables can be set in loader(8) prompt, loader.conf(5) or kenv(1) before
     ipfw module gets loaded.

     net.inet.ip.fw.default_to_accept: 0
             Defines ipfw last rule behavior.  This value overrides options
             IPFW_DEFAULT_TO_(ACCEPT|DENY) from kernel configuration file.

     net.inet.ip.fw.tables_max: 128
             Defines number of tables available in ipfw.  Number cannot exceed
             65534.

SYSCTL VARIABLES
     A set of sysctl(8) variables controls the behaviour of the firewall and
     associated modules (dummynet, bridge, sctp nat).  These are shown below
     together with their default value (but always check with the sysctl(8)
     command what value is actually in use) and meaning:

     net.inet.ip.alias.sctp.accept_global_ootb_addip: 0
             Defines how the nat responds to receipt of global OOTB ASCONF-
             AddIP:

             0       No response (unless a partially matching association exists
                     - ports and vtags match but global address does not)

             1       nat will accept and process all OOTB global AddIP messages.

             Option 1 should never be selected as this forms a security risk.
             An attacker can establish multiple fake associations by sending
             AddIP messages.

     net.inet.ip.alias.sctp.chunk_proc_limit: 5
             Defines the maximum number of chunks in an SCTP packet that will be
             parsed for a packet that matches an existing association.  This
             value is enforced to be greater or equal than
             net.inet.ip.alias.sctp.initialising_chunk_proc_limit.  A high value
             is a DoS risk yet setting too low a value may result in important
             control chunks in the packet not being located and parsed.

     net.inet.ip.alias.sctp.error_on_ootb: 1
             Defines when the nat responds to any Out-of-the-Blue (OOTB) packets
             with ErrorM packets.  An OOTB packet is a packet that arrives with
             no existing association registered in the nat and is not an INIT or
             ASCONF-AddIP packet:

             0       ErrorM is never sent in response to OOTB packets.

             1       ErrorM is only sent to OOTB packets received on the local
                     side.

             2       ErrorM is sent to the local side and on the global side
                     ONLY if there is a partial match (ports and vtags match but
                     the source global IP does not).  This value is only useful
                     if the nat is tracking global IP addresses.

             3       ErrorM is sent in response to all OOTB packets on both the
                     local and global side (DoS risk).

             At the moment the default is 0, since the ErrorM packet is not yet
             supported by most SCTP stacks.  When it is supported, and if not
             tracking global addresses, we recommend setting this value to 1 to
             allow multi-homed local hosts to function with the nat.  To track
             global addresses, we recommend setting this value to 2 to allow
             global hosts to be informed when they need to (re)send an ASCONF-
             AddIP.  Value 3 should never be chosen (except for debugging) as
             the nat will respond to all OOTB global packets (a DoS risk).

     net.inet.ip.alias.sctp.hashtable_size: 2003
             Size of hash tables used for nat lookups (100 < prime_number >
             1000001).  This value sets the hash table size for any future
             created nat instance and therefore must be set prior to creating a
             nat instance.  The table sizes may be changed to suit specific
             needs.  If there will be few concurrent associations, and memory is
             scarce, you may make these smaller.  If there will be many
             thousands (or millions) of concurrent associations, you should make
             these larger.  A prime number is best for the table size.  The
             sysctl update function will adjust your input value to the next
             highest prime number.

     net.inet.ip.alias.sctp.holddown_time: 0
             Hold association in table for this many seconds after receiving a
             SHUTDOWN-COMPLETE.  This allows endpoints to correct shutdown
             gracefully if a shutdown_complete is lost and retransmissions are
             required.

     net.inet.ip.alias.sctp.init_timer: 15
             Timeout value while waiting for (INIT-ACK|AddIP-ACK).  This value
             cannot be 0.

     net.inet.ip.alias.sctp.initialising_chunk_proc_limit: 2
             Defines the maximum number of chunks in an SCTP packet that will be
             parsed when no existing association exists that matches that
             packet.  Ideally this packet will only be an INIT or ASCONF-AddIP
             packet.  A higher value may become a DoS risk as malformed packets
             can consume processing resources.

     net.inet.ip.alias.sctp.param_proc_limit: 25
             Defines the maximum number of parameters within a chunk that will
             be parsed in a packet.  As for other similar sysctl variables,
             larger values pose a DoS risk.

     net.inet.ip.alias.sctp.log_level: 0
             Level of detail in the system log messages (0 - minimal, 1 - event,
             2 - info, 3 - detail, 4 - debug, 5 - max debug).  May be a good
             option in high loss environments.

     net.inet.ip.alias.sctp.shutdown_time: 15
             Timeout value while waiting for SHUTDOWN-COMPLETE.  This value
             cannot be 0.

     net.inet.ip.alias.sctp.track_global_addresses: 0
             Enables/disables global IP address tracking within the nat and
             places an upper limit on the number of addresses tracked for each
             association:

             0       Global tracking is disabled

             >1      Enables tracking, the maximum number of addresses tracked
                     for each association is limited to this value

             This variable is fully dynamic, the new value will be adopted for
             all newly arriving associations, existing associations are treated
             as they were previously.  Global tracking will decrease the number
             of collisions within the nat at a cost of increased processing
             load, memory usage, complexity, and possible nat state problems in
             complex networks with multiple nats.  We recommend not tracking
             global IP addresses, this will still result in a fully functional
             nat.

     net.inet.ip.alias.sctp.up_timer: 300
             Timeout value to keep an association up with no traffic.  This
             value cannot be 0.

     net.inet.ip.dummynet.codel.interval: 100000
             Default codel AQM interval in microseconds.  The value must be in
             the range 1..5000000.

     net.inet.ip.dummynet.codel.target: 5000
             Default codel AQM target delay time in microseconds (the minimum
             acceptable persistent queue delay).  The value must be in the range
             1..5000000.

     net.inet.ip.dummynet.expire: 1
             Lazily delete dynamic pipes/queue once they have no pending
             traffic.  You can disable this by setting the variable to 0, in
             which case the pipes/queues will only be deleted when the threshold
             is reached.

     net.inet.ip.dummynet.fqcodel.flows: 1024
             Defines the default total number of flow queues (sub-queues) that
             fq_codel creates and manages.  The value must be in the range
             1..65536.

     net.inet.ip.dummynet.fqcodel.interval: 100000
             Default fq_codel scheduler/AQM interval in microseconds.  The value
             must be in the range 1..5000000.

     net.inet.ip.dummynet.fqcodel.limit: 10240
             The default hard size limit (in unit of packet) of all queues
             managed by an instance of the fq_codel scheduler.  The value must
             be in the range 1..20480.

     net.inet.ip.dummynet.fqcodel.quantum: 1514
             The default quantum (credit) of the fq_codel in unit of byte.  The
             value must be in the range 1..9000.

     net.inet.ip.dummynet.fqcodel.target: 5000
             Default fq_codel scheduler/AQM target delay time in microseconds
             (the minimum acceptable persistent queue delay).  The value must be
             in the range 1..5000000.

     net.inet.ip.dummynet.fqpie.alpha: 125
             The default alpha parameter (scaled by 1000) for fq_pie
             scheduler/AQM.  The value must be in the range 1..7000.

     net.inet.ip.dummynet.fqpie.beta: 1250
             The default beta parameter (scaled by 1000) for fq_pie
             scheduler/AQM.  The value must be in the range 1..7000.

     net.inet.ip.dummynet.fqpie.flows: 1024
             Defines the default total number of flow queues (sub-queues) that
             fq_pie creates and manages.  The value must be in the range
             1..65536.

     net.inet.ip.dummynet.fqpie.limit: 10240
             The default hard size limit (in unit of packet) of all queues
             managed by an instance of the fq_pie scheduler.  The value must be
             in the range 1..20480.

     net.inet.ip.dummynet.fqpie.max_burst: 150000
             The default maximum period of microseconds that fq_pie
             scheduler/AQM does not drop/mark packets.  The value must be in the
             range 1..10000000.

     net.inet.ip.dummynet.fqpie.max_ecnth: 99
             The default maximum ECN probability threshold (scaled by 1000) for
             fq_pie scheduler/AQM.  The value must be in the range 1..7000.

     net.inet.ip.dummynet.fqpie.quantum: 1514
             The default quantum (credit) of the fq_pie in unit of byte.  The
             value must be in the range 1..9000.

     net.inet.ip.dummynet.fqpie.target: 15000
             The default target delay of the fq_pie in unit of microsecond.  The
             value must be in the range 1..5000000.

     net.inet.ip.dummynet.fqpie.tupdate: 15000
             The default tupdate of the fq_pie in unit of microsecond.  The
             value must be in the range 1..5000000.

     net.inet.ip.dummynet.hash_size: 64
             Default size of the hash table used for dynamic pipes/queues.  This
             value is used when no buckets option is specified when configuring
             a pipe/queue.

     net.inet.ip.dummynet.io_fast: 0
             If set to a non-zero value, the “fast” mode of dummynet operation
             (see above) is enabled.

     net.inet.ip.dummynet.io_pkt
             Number of packets passed to dummynet.

     net.inet.ip.dummynet.io_pkt_drop
             Number of packets dropped by dummynet.

     net.inet.ip.dummynet.io_pkt_fast
             Number of packets bypassed by the dummynet scheduler.

     net.inet.ip.dummynet.max_chain_len: 16
             Target value for the maximum number of pipes/queues in a hash
             bucket.  The product max_chain_len*hash_size is used to determine
             the threshold over which empty pipes/queues will be expired even
             when net.inet.ip.dummynet.expire=0.

     net.inet.ip.dummynet.red_lookup_depth: 256

     net.inet.ip.dummynet.red_avg_pkt_size: 512

     net.inet.ip.dummynet.red_max_pkt_size: 1500
             Parameters used in the computations of the drop probability for the
             RED algorithm.

     net.inet.ip.dummynet.pie.alpha: 125
             The default alpha parameter (scaled by 1000) for pie AQM.  The
             value must be in the range 1..7000.

     net.inet.ip.dummynet.pie.beta: 1250
             The default beta parameter (scaled by 1000) for pie AQM.  The value
             must be in the range 1..7000.

     net.inet.ip.dummynet.pie.max_burst: 150000
             The default maximum period of microseconds that pie AQM does not
             drop/mark packets.  The value must be in the range 1..10000000.

     net.inet.ip.dummynet.pie.max_ecnth: 99
             The default maximum ECN probability threshold (scaled by 1000) for
             pie AQM.  The value must be in the range 1..7000.

     net.inet.ip.dummynet.pie.target: 15000
             The default target delay of pie AQM in unit of microsecond.  The
             value must be in the range 1..5000000.

     net.inet.ip.dummynet.pie.tupdate: 15000
             The default tupdate of pie AQM in unit of microsecond.  The value
             must be in the range 1..5000000.

     net.inet.ip.dummynet.pipe_byte_limit: 1048576

     net.inet.ip.dummynet.pipe_slot_limit: 100
             The maximum queue size that can be specified in bytes or packets.
             These limits prevent accidental exhaustion of resources such as
             mbufs.  If you raise these limits, you should make sure the system
             is configured so that sufficient resources are available.

     net.inet.ip.fw.autoinc_step: 100
             Delta between rule numbers when auto-generating them.  The value
             must be in the range 1..1000.

     net.inet.ip.fw.curr_dyn_buckets: net.inet.ip.fw.dyn_buckets
             The current number of buckets in the hash table for dynamic rules
             (readonly).

     net.inet.ip.fw.debug: 1
             Controls debugging messages produced by ipfw.

     net.inet.ip.fw.default_rule: 65535
             The default rule number (read-only).  By the design of ipfw, the
             default rule is the last one, so its number can also serve as the
             highest number allowed for a rule.

     net.inet.ip.fw.dyn_buckets: 256
             The number of buckets in the hash table for dynamic rules.  Must be
             a power of 2, up to 65536.  It only takes effect when all dynamic
             rules have expired, so you are advised to use a flush command to
             make sure that the hash table is resized.

     net.inet.ip.fw.dyn_count: 3
             Current number of dynamic rules (read-only).

     net.inet.ip.fw.dyn_keepalive: 1
             Enables generation of keepalive packets for keep-state rules on TCP
             sessions.  A keepalive is generated to both sides of the connection
             every 5 seconds for the last 20 seconds of the lifetime of the
             rule.

     net.inet.ip.fw.dyn_max: 8192
             Maximum number of dynamic rules.  When you hit this limit, no more
             dynamic rules can be installed until old ones expire.

     net.inet.ip.fw.dyn_ack_lifetime: 300

     net.inet.ip.fw.dyn_syn_lifetime: 20

     net.inet.ip.fw.dyn_fin_lifetime: 1

     net.inet.ip.fw.dyn_rst_lifetime: 1

     net.inet.ip.fw.dyn_udp_lifetime: 5

     net.inet.ip.fw.dyn_short_lifetime: 30
             These variables control the lifetime, in seconds, of dynamic rules.
             Upon the initial SYN exchange the lifetime is kept short, then
             increased after both SYN have been seen, then decreased again
             during the final FIN exchange or when a RST is received.  Both
             dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower than 5
             seconds, the period of repetition of keepalives.  The firewall
             enforces that.

     net.inet.ip.fw.dyn_keep_states: 0
             Keep dynamic states on rule/set deletion.  States are relinked to
             default rule (65535).  This can be handly for ruleset reload.
             Turned off by default.

     net.inet.ip.fw.enable: 1
             Enables the firewall.  Setting this variable to 0 lets you run your
             machine without firewall even if compiled in.

     net.inet6.ip6.fw.enable: 1
             provides the same functionality as above for the IPv6 case.

     net.inet.ip.fw.one_pass: 1
             When set, the packet exiting from the dummynet pipe or from
             ng_ipfw(4) node is not passed though the firewall again.
             Otherwise, after an action, the packet is reinjected into the
             firewall at the next rule.

     net.inet.ip.fw.tables_max: 128
             Maximum number of tables.

     net.inet.ip.fw.verbose: 1
             Enables verbose messages.

     net.inet.ip.fw.verbose_limit: 0
             Limits the number of messages produced by a verbose firewall.

     net.inet6.ip6.fw.deny_unknown_exthdrs: 1
             If enabled packets with unknown IPv6 Extension Headers will be
             denied.

     net.link.ether.ipfw: 0
             Controls whether layer-2 packets are passed to ipfw.  Default is
             no.

     net.link.bridge.ipfw: 0
             Controls whether bridged packets are passed to ipfw.  Default is
             no.

     net.inet.ip.fw.nat64_debug: 0
             Controls debugging messages produced by ipfw_nat64 module.

     net.inet.ip.fw.nat64_direct_output: 0
             Controls the output method used by ipfw_nat64 module:

             0       A packet is handled by ipfw twice.  First time an original
                     packet is handled by ipfw and consumed by ipfw_nat64
                     translator.  Then translated packet is queued via netisr to
                     input processing again.

             1       A packet is handled by ipfw only once, and after
                     translation it will be pushed directly to outgoing
                     interface.

INTERNAL DIAGNOSTICS
     There are some commands that may be useful to understand current state of
     certain subsystems inside kernel module.  These commands provide debugging
     output which may change without notice.

     Currently the following commands are available as internal sub-options:

     iflist  Lists all interface which are currently tracked by ipfw with their
             in-kernel status.

     talist  List all table lookup algorithms currently available.

EXAMPLES
     There are far too many possible uses of ipfw so this Section will only give
     a small set of examples.

   BASIC PACKET FILTERING
     This command adds an entry which denies all tcp packets from
     cracker.evil.org to the telnet port of wolf.tambov.su from being forwarded
     by the host:

           ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet

     This one disallows any connection from the entire cracker's network to my
     host:

           ipfw add deny ip from 123.45.67.0/24 to my.host.org

     A first and efficient way to limit access (not using dynamic rules) is the
     use of the following rules:

           ipfw add allow tcp from any to any established
           ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup
           ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup
           ...
           ipfw add deny tcp from any to any

     The first rule will be a quick match for normal TCP packets, but it will
     not match the initial SYN packet, which will be matched by the setup rules
     only for selected source/destination pairs.  All other SYN packets will be
     rejected by the final deny rule.

     If you administer one or more subnets, you can take advantage of the
     address sets and or-blocks and write extremely compact rulesets which
     selectively enable services to blocks of clients, as below:

           goodguys="{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }"
           badguys="10.1.2.0/24{8,38,60}"

           ipfw add allow ip from ${goodguys} to any
           ipfw add deny ip from ${badguys} to any
           ... normal policies ...

     The verrevpath option could be used to do automated anti-spoofing by adding
     the following to the top of a ruleset:

           ipfw add deny ip from any to any not verrevpath in

     This rule drops all incoming packets that appear to be coming to the system
     on the wrong interface.  For example, a packet with a source address
     belonging to a host on a protected internal network would be dropped if it
     tried to enter the system from an external interface.

     The antispoof option could be used to do similar but more restricted anti-
     spoofing by adding the following to the top of a ruleset:

           ipfw add deny ip from any to any not antispoof in

     This rule drops all incoming packets that appear to be coming from another
     directly connected system but on the wrong interface.  For example, a
     packet with a source address of 192.168.0.0/24, configured on fxp0, but
     coming in on fxp1 would be dropped.

     The setdscp option could be used to (re)mark user traffic, by adding the
     following to the appropriate place in ruleset:

           ipfw add setdscp be ip from any to any dscp af11,af21

   SELECTIVE MIRRORING
     If your network has network traffic analyzer connected to your host
     directly via dedicated interface or remotely via RSPAN vlan, you can
     selectively mirror some Ethernet layer2 frames to the analyzer.

     First, make sure your firewall is already configured and runs.  Then,
     enable layer2 processing if not already enabled:

           sysctl net.link.ether.ipfw=1

     Next, load needed additional kernel modules:

           kldload ng_ether ng_ipfw

     Optionally, make system load these modules automatically at startup:

           sysrc kld_list+="ng_ether ng_ipfw"

     Next, configure ng_ipfw(4) kernel module to transmit mirrored copies of
     layer2 frames out via vlan900 interface:

           ngctl connect ipfw: vlan900: 1 lower

     Think of "1" here as of "mirroring instance index" and vlan900 is its
     destination.  You can have arbitrary number of instances.  Refer to
     ng_ipfw(4) for details.

     At last, actually start mirroring of selected frames using "instance 1".
     For frames incoming from em0 interface:

           ipfw add ngtee 1 ip from any to 192.168.0.1 layer2 in recv em0

     For frames outgoing to em0 interface:

           ipfw add ngtee 1 ip from any to 192.168.0.1 layer2 out xmit em0

     For both incoming and outgoing frames while flowing through em0:

           ipfw add ngtee 1 ip from any to 192.168.0.1 layer2 via em0

     Make sure you do not perform mirroring for already duplicated frames or
     kernel may hang as there is no safety net.

   DYNAMIC RULES
     In order to protect a site from flood attacks involving fake TCP packets,
     it is safer to use dynamic rules:

           ipfw add check-state
           ipfw add deny tcp from any to any established
           ipfw add allow tcp from my-net to any setup keep-state

     This will let the firewall install dynamic rules only for those connection
     which start with a regular SYN packet coming from the inside of our
     network.  Dynamic rules are checked when encountering the first occurrence
     of a check-state, keep-state or limit rule.  A check-state rule should
     usually be placed near the beginning of the ruleset to minimize the amount
     of work scanning the ruleset.  Your mileage may vary.

     For more complex scenarios with dynamic rules record-state and defer-action
     can be used to precisely control creation and checking of dynamic rules.
     Example of usage of these options are provided in NETWORK ADDRESS
     TRANSLATION (NAT) Section.

     To limit the number of connections a user can open you can use the
     following type of rules:

           ipfw add allow tcp from my-net/24 to any setup limit src-addr 10
           ipfw add allow tcp from any to me setup limit src-addr 4

     The former (assuming it runs on a gateway) will allow each host on a /24
     network to open at most 10 TCP connections.  The latter can be placed on a
     server to make sure that a single client does not use more than 4
     simultaneous connections.

     BEWARE: stateful rules can be subject to denial-of-service attacks by a
     SYN-flood which opens a huge number of dynamic rules.  The effects of such
     attacks can be partially limited by acting on a set of sysctl(8) variables
     which control the operation of the firewall.

     Here is a good usage of the list command to see accounting records and
     timestamp information:

           ipfw -at list

     or in short form without timestamps:

           ipfw -a list

     which is equivalent to:

           ipfw show

     Next rule diverts all incoming packets from 192.168.2.0/24 to divert port
     5000:

           ipfw divert 5000 ip from 192.168.2.0/24 to any in

   TRAFFIC SHAPING
     The following rules show some of the applications of ipfw and dummynet for
     simulations and the like.

     This rule drops random incoming packets with a probability of 5%:

           ipfw add prob 0.05 deny ip from any to any in

     A similar effect can be achieved making use of dummynet pipes:

           ipfw add pipe 10 ip from any to any
           ipfw pipe 10 config plr 0.05

     We can use pipes to artificially limit bandwidth, e.g. on a machine acting
     as a router, if we want to limit traffic from local clients on
     192.168.2.0/24 we do:

           ipfw add pipe 1 ip from 192.168.2.0/24 to any out
           ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

     note that we use the out modifier so that the rule is not used twice.
     Remember in fact that ipfw rules are checked both on incoming and outgoing
     packets.

     Should we want to simulate a bidirectional link with bandwidth limitations,
     the correct way is the following:

           ipfw add pipe 1 ip from any to any out
           ipfw add pipe 2 ip from any to any in
           ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
           ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes

     The above can be very useful, e.g. if you want to see how your fancy Web
     page will look for a residential user who is connected only through a slow
     link.  You should not use only one pipe for both directions, unless you
     want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet, IRDA).  It
     is not necessary that both pipes have the same configuration, so we can
     also simulate asymmetric links.

     Should we want to verify network performance with the RED queue management
     algorithm:

           ipfw add pipe 1 ip from any to any
           ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

     Another typical application of the traffic shaper is to introduce some
     delay in the communication.  This can significantly affect applications
     which do a lot of Remote Procedure Calls, and where the round-trip-time of
     the connection often becomes a limiting factor much more than bandwidth:

           ipfw add pipe 1 ip from any to any out
           ipfw add pipe 2 ip from any to any in
           ipfw pipe 1 config delay 250ms bw 1Mbit/s
           ipfw pipe 2 config delay 250ms bw 1Mbit/s

     Per-flow queueing can be useful for a variety of purposes.  A very simple
     one is counting traffic:

           ipfw add pipe 1 tcp from any to any
           ipfw add pipe 1 udp from any to any
           ipfw add pipe 1 ip from any to any
           ipfw pipe 1 config mask all

     The above set of rules will create queues (and collect statistics) for all
     traffic.  Because the pipes have no limitations, the only effect is
     collecting statistics.  Note that we need 3 rules, not just the last one,
     because when ipfw tries to match IP packets it will not consider ports, so
     we would not see connections on separate ports as different ones.

     A more sophisticated example is limiting the outbound traffic on a net with
     per-host limits, rather than per-network limits:

           ipfw add pipe 1 ip from 192.168.2.0/24 to any out
           ipfw add pipe 2 ip from any to 192.168.2.0/24 in
           ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes
           ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes

   LOOKUP TABLES
     In the following example, we need to create several traffic bandwidth
     classes and we need different hosts/networks to fall into different
     classes.  We create one pipe for each class and configure them accordingly.
     Then we create a single table and fill it with IP subnets and addresses.
     For each subnet/host we set the argument equal to the number of the pipe
     that it should use.  Then we classify traffic using a single rule:

           ipfw pipe 1 config bw 1000Kbyte/s
           ipfw pipe 4 config bw 4000Kbyte/s
           ...
           ipfw table T1 create type addr
           ipfw table T1 add 192.168.2.0/24 1
           ipfw table T1 add 192.168.0.0/27 4
           ipfw table T1 add 192.168.0.2 1
           ...
           ipfw add pipe tablearg ip from 'table(T1)' to any

     Using the fwd action, the table entries may include hostnames and IP
     addresses.

           ipfw table T2 create type addr ftype ip
           ipfw table T2 add 192.168.2.0/24 10.23.2.1
           ipfw table T21 add 192.168.0.0/27 router1.dmz
           ...
           ipfw add 100 fwd tablearg ip from any to table(1)

     In the following example per-interface firewall is created:

           ipfw table IN create type iface valtype skipto,fib
           ipfw table IN add vlan20 12000,12
           ipfw table IN add vlan30 13000,13
           ipfw table OUT create type iface valtype skipto
           ipfw table OUT add vlan20 22000
           ipfw table OUT add vlan30 23000
           ..
           ipfw add 100 setfib tablearg ip from any to any recv 'table(IN)' in
           ipfw add 200 skipto tablearg ip from any to any recv 'table(IN)' in
           ipfw add 300 skipto tablearg ip from any to any xmit 'table(OUT)' out

     The following example illustrate usage of flow tables:

           ipfw table fl create type flow:src-ip,proto,dst-ip,dst-port
           ipfw table fl add 2a02:6b8:77::88,tcp,2a02:6b8:77::99,80 11
           ipfw table fl add 10.0.0.1,udp,10.0.0.2,53 12
           ..
           ipfw add 100 allow ip from any to any flow 'table(fl,11)' recv ix0

   SETS OF RULES
     To add a set of rules atomically, e.g. set 18:

           ipfw set disable 18
           ipfw add NN set 18 ...         # repeat as needed
           ipfw set enable 18

     To delete a set of rules atomically the command is simply:

           ipfw delete set 18

     To test a ruleset and disable it and regain control if something goes
     wrong:

           ipfw set disable 18
           ipfw add NN set 18 ...         # repeat as needed
           ipfw set enable 18; echo done; sleep 30 && ipfw set disable 18

     Here if everything goes well, you press control-C before the "sleep"
     terminates, and your ruleset will be left active.  Otherwise, e.g. if you
     cannot access your box, the ruleset will be disabled after the sleep
     terminates thus restoring the previous situation.

     To show rules of the specific set:

           ipfw set 18 show

     To show rules of the disabled set:

           ipfw -S set 18 show

     To clear a specific rule counters of the specific set:

           ipfw set 18 zero NN

     To delete a specific rule of the specific set:

           ipfw set 18 delete NN

   NAT, REDIRECT AND LSNAT
     First redirect all the traffic to nat instance 123:

           ipfw add nat 123 all from any to any

     Then to configure nat instance 123 to alias all the outgoing traffic with
     ip 192.168.0.123, blocking all incoming connections, trying to keep same
     ports on both sides, clearing aliasing table on address change and keeping
     a log of traffic/link statistics:

           ipfw nat 123 config ip 192.168.0.123 log deny_in reset same_ports

     Or to change address of instance 123, aliasing table will be cleared (see
     reset option):

           ipfw nat 123 config ip 10.0.0.1

     To see configuration of nat instance 123:

           ipfw nat 123 show config

     To show logs of all the instances in range 111-999:

           ipfw nat 111-999 show

     To see configurations of all instances:

           ipfw nat show config

     Or a redirect rule with mixed modes could looks like:

       ipfw nat 123 config redirect_addr 10.0.0.1 10.0.0.66
                                redirect_port tcp 192.168.0.1:80 500
                                redirect_proto udp 192.168.1.43 192.168.1.1
                                redirect_addr 192.168.0.10,192.168.0.11
                                           10.0.0.100  # LSNAT
                                redirect_port tcp 192.168.0.1:80,192.168.0.10:22
                                           500         # LSNAT

     or it could be split in:

       ipfw nat 1 config redirect_addr 10.0.0.1 10.0.0.66
       ipfw nat 2 config redirect_port tcp 192.168.0.1:80 500
       ipfw nat 3 config redirect_proto udp 192.168.1.43 192.168.1.1
       ipfw nat 4 config redirect_addr 192.168.0.10,192.168.0.11,192.168.0.12
                                                10.0.0.100
       ipfw nat 5 config redirect_port tcp
                               192.168.0.1:80,192.168.0.10:22,192.168.0.20:25 500

     Sometimes you may want to mix NAT and dynamic rules.  It could be achieved
     with record-state and defer-action options.  Problem is, you need to create
     dynamic rule before NAT and check it after NAT actions (or vice versa) to
     have consistent addresses and ports.  Rule with keep-state option will
     trigger activation of existing dynamic state, and action of such rule will
     be performed as soon as rule is matched.  In case of NAT and allow rule
     packet need to be passed to NAT, not allowed as soon is possible.

     There is example of set of rules to achieve this.  Bear in mind that this
     is example only and it is not very useful by itself.

     On way out, after all checks place this rules:

           ipfw add allow record-state skip-action
           ipfw add nat 1

     And on way in there should be something like this:

           ipfw add nat 1
           ipfw add check-state

     Please note, that first rule on way out doesn't allow packet and doesn't
     execute existing dynamic rules.  All it does, create new dynamic rule with
     allow action, if it is not created yet.  Later, this dynamic rule is used
     on way in by check-state rule.

   CONFIGURING CODEL, PIE, FQ-CODEL and FQ-PIE AQM
     codel and pie AQM can be configured for dummynet pipe or queue.

     To configure a pipe with codel AQM using default configuration for traffic
     from 192.168.0.0/24 and 1Mbits/s rate limit, we do:

           ipfw pipe 1 config bw 1mbits/s codel
           ipfw add 100 pipe 1 ip from 192.168.0.0/24 to any

     To configure a queue with codel AQM using different configurations
     parameters for traffic from 192.168.0.0/24 and 1Mbits/s rate limit, we do:

           ipfw pipe 1 config bw 1mbits/s
           ipfw queue 1 config pipe 1 codel target 8ms interval 160ms ecn
           ipfw add 100 queue 1 ip from 192.168.0.0/24 to any

     To configure a pipe with pie AQM using default configuration for traffic
     from 192.168.0.0/24 and 1Mbits/s rate limit, we do:

           ipfw pipe 1 config bw 1mbits/s pie
           ipfw add 100 pipe 1 ip from 192.168.0.0/24 to any

     To configure a queue with pie AQM using different configuration parameters
     for traffic from 192.168.0.0/24 and 1Mbits/s rate limit, we do:

           ipfw pipe 1 config bw 1mbits/s
           ipfw queue 1 config pipe 1 pie target 20ms tupdate 30ms ecn
           ipfw add 100 queue 1 ip from 192.168.0.0/24 to any

     fq_codel and fq_pie AQM can be configured for dummynet schedulers.

     To configure fq_codel scheduler using different configurations parameters
     for traffic from 192.168.0.0/24 and 1Mbits/s rate limit, we do:

           ipfw pipe 1 config bw 1mbits/s
           ipfw sched 1 config pipe 1 type fq_codel
           ipfw queue 1 config sched 1
           ipfw add 100 queue 1 ip from 192.168.0.0/24 to any

     To change fq_codel default configuration for a sched such as disable ECN
     and change the target to 10ms, we do:

           ipfw sched 1 config pipe 1 type fq_codel target 10ms noecn

     Similar to fq_codel, to configure fq_pie scheduler using different
     configurations parameters for traffic from 192.168.0.0/24 and 1Mbits/s rate
     limit, we do:

           ipfw pipe 1 config bw 1mbits/s
           ipfw sched 1 config pipe 1 type fq_pie
           ipfw queue 1 config sched 1
           ipfw add 100 queue 1 ip from 192.168.0.0/24 to any

     The configurations of fq_pie sched can be changed in a similar way as for
     fq_codel

SEE ALSO
     cpp(1), m4(1), altq(4), divert(4), dummynet(4), if_bridge(4), ip(4),
     ipfirewall(4), ng_ether(4), ng_ipfw(4), protocols(5), services(5), init(8),
     kldload(8), reboot(8), sysctl(8), syslogd(8), sysrc(8)

HISTORY
     The ipfw utility first appeared in FreeBSD 2.0.  dummynet was introduced in
     FreeBSD 2.2.8.  Stateful extensions were introduced in FreeBSD 4.0.  ipfw2
     was introduced in Summer 2002.

AUTHORS
     Ugen J. S. Antsilevich,
     Poul-Henning Kamp,
     Alex Nash,
     Archie Cobbs,
     Luigi Rizzo,
     Rasool Al-Saadi.

     API based upon code written by Daniel Boulet for BSDI.

     Dummynet has been introduced by Luigi Rizzo in 1997-1998.

     Some early work (1999-2000) on the dummynet traffic shaper supported by
     Akamba Corp.

     The ipfw core (ipfw2) has been completely redesigned and reimplemented by
     Luigi Rizzo in summer 2002.  Further actions and options have been added by
     various developers over the years.

     In-kernel NAT support written by Paolo Pisati <piso@FreeBSD.org> as part of
     a Summer of Code 2005 project.

     SCTP nat support has been developed by The Centre for Advanced Internet
     Architectures (CAIA) <http://www.caia.swin.edu.au>.  The primary developers
     and maintainers are David Hayes and Jason But.  For further information
     visit: ⟨http://www.caia.swin.edu.au/urp/SONATA⟩

     Delay profiles have been developed by Alessandro Cerri and Luigi Rizzo,
     supported by the European Commission within Projects Onelab and Onelab2.

     CoDel, PIE, FQ-CoDel and FQ-PIE AQM for Dummynet have been implemented by
     The Centre for Advanced Internet Architectures (CAIA) in 2016, supported by
     The Comcast Innovation Fund.  The primary developer is Rasool Al-Saadi.

BUGS
     The syntax has grown over the years and sometimes it might be confusing.
     Unfortunately, backward compatibility prevents cleaning up mistakes made in
     the definition of the syntax.

     !!! WARNING !!!

     Misconfiguring the firewall can put your computer in an unusable state,
     possibly shutting down network services and requiring console access to
     regain control of it.

     Incoming packet fragments diverted by divert are reassembled before
     delivery to the socket.  The action used on those packet is the one from
     the rule which matches the first fragment of the packet.

     Packets diverted to userland, and then reinserted by a userland process may
     lose various packet attributes.  The packet source interface name will be
     preserved if it is shorter than 8 bytes and the userland process saves and
     reuses the sockaddr_in (as does natd(8)); otherwise, it may be lost.  If a
     packet is reinserted in this manner, later rules may be incorrectly
     applied, making the order of divert rules in the rule sequence very
     important.

     Dummynet drops all packets with IPv6 link-local addresses.

     Rules using uid or gid may not behave as expected.  In particular, incoming
     SYN packets may have no uid or gid associated with them since they do not
     yet belong to a TCP connection, and the uid/gid associated with a packet
     may not be as expected if the associated process calls setuid(2) or similar
     system calls.

     Rule syntax is subject to the command line environment and some patterns
     may need to be escaped with the backslash character or quoted
     appropriately.

     Due to the architecture of libalias(3), ipfw nat is not compatible with the
     TCP segmentation offloading (TSO).  Thus, to reliably nat your network
     traffic, please disable TSO on your NICs using ifconfig(8).

     ICMP error messages are not implicitly matched by dynamic rules for the
     respective conversations.  To avoid failures of network error detection and
     path MTU discovery, ICMP error messages may need to be allowed explicitly
     through static rules.

     Rules using call and return actions may lead to confusing behaviour if
     ruleset has mistakes, and/or interaction with other subsystems (netgraph,
     dummynet, etc.) is used.  One possible case for this is packet leaving ipfw
     in subroutine on the input pass, while later on output encountering
     unpaired return first.  As the call stack is kept intact after input pass,
     packet will suddenly return to the rule number used on input pass, not on
     output one.  Order of processing should be checked carefully to avoid such
     mistakes.

BSD                              August 21, 2020                             BSD