filter_syntax

FILTER SYNTAX(5)              File Formats Manual             FILTER SYNTAX(5)



NAME
       rules.filter - Input format for filtergen packet filter compiler


INTRO
       This file describes the input syntax accepted by filtergen(8).


BASICS
       In general form, a filter rule is described by a direction, an
       interface, a target and (possibly empty) sets of matches and options.


       Simple rules will look like:

       direction interface match0 .. matchN target;

       for example:


       input eth0 source host1 dest host2 proto tcp dport http accept;

       Note that the elements of the rule can be placed in any order, with the
       exception that the interface must immediately follow the direction.
       Thus, this rule is equivalent to the above (though perhaps less
       readable):

       proto tcp source host1 dport http accept dest host2 input eth0;

       The semicolon separates rules.  It is optional before a closing brace
       or the end of a file.  Whitespace is not significant.  Anything after a
       hash ("#") on a line is ignored.


   DIRECTION
       A direction merely specifies whether to match packets being sent or
       received.  The only two directions available are "input" and "output".
       Forwarded packets will pass through both,


   INTERFACE
       This specifies which real or virtual network device to filter.  As far
       as filtergen is concerned, this is just a text string.  It must be the
       same as the device name on the target system.  Common names on Linux
       are "eth0", "eth1", ..., "ppp0", etc.  Other systems will have
       different naming rules.

       If you want to match all interfaces, you can specify "*" (without the
       quotes).


   TARGET
       A target notes what we do with a matching packet.  Universal options
       are accept and drop which, respectively, state that the packet should
       be allowed as normal, or thrown away.  Some backends support reject to
       throw away a packet, but send notification to the sender that it was
       denied, masq (on output rules only) to "masquerade" a packet - alter it
       so that it appears to come from the address of the sending interface -
       and proxy (and its deprecated alias redirect) to divert a connection
       via the local system.


   MATCHES
       The matches are the meat of the rule.  They apply a set of tests to a
       packet and decide if this rule will be used to process it.  Available
       matches are:

       source addr-range dest addr-range

       proto {tcp|udp|icmp|...} sport port-range dport port-range icmptype
       icmp-type

       Matches can be negated by prefixing them with a "!":

            input eth0 ! dest 10.0.0.3 reject;

       (note than not all backends can support this).


   OPTIONS
       Options affect the behaviour of the matcher or the target.  Currently
       implemented are log, which logs packets, local, which means only to
       check packets to or from this interface, forward which means the
       opposite of local, and oneway which causes the backend to omit rules
       which permit return packets.

       The log option can optionally specify a message to log matching packets
       with, where the backend supports it:

            input eth0 source {
                 10.0.0.0/8 192.168.0.0/16
            } log text "private addresses" drop;

       Note that the oneway option make have no effect when used with the -l
       command-line flag on backends which support it.


GROUPING
       Because it can get tedious to type:

            input eth0 source foo dest bar proto tcp dport http accept;
            input eth0 source foo dest bar proto tcp dport https accept;
            input eth0 source foo dest bar proto tcp dport nntp accept;
            input eth0 source foo dest bar proto tcp sport 1:1023 dport ssh accept;
            ...

       filter allows you to group rules with a set syntax:

            input eth0 source foo dest bar proto tcp {
                 dport http;
                 dport https;
                 dport nntp;
                 sport 1:1023 dport ssh;
            } accept;

       Matches which accept arguments can also be grouped:

            input eth0 source foo dest bar proto tcp {
                 dport {http https nntp};
                 sport 1:1023 dport ssh;
            } accept;


OUT-OF-LINE GROUPS
       It is commonly the case that both hosts and routers have long lists of
       similar looking rules to allow traffic between groups of hosts, as
       above.  What if we had another pair of hosts which needed a variety of
       services?  We could simply put the rule groups one after the other:

            input eth0 source foo dest bar proto tcp {
                 dport {http https nntp};
                 sport 1:1023 dport ssh;
            } accept;
            input eth0 source baz dest quux proto tcp {
                 dport {1264 1521 1984 8008 8080 26000};
            } accept;

       The above generates 11 rules, and every additional port adds another
       rule through which packets will pass (well, ones which don't match any
       of the above).  The first four output rules have the same source and
       destination hosts and protocol, and we know that if it doesn't match
       those on the first rule, it won't on the next three, either.  Out-of-
       line groups  use this fact to streamline things somewhat:

            input eth0 source foo dest bar [
                 proto tcp {
                      dport {http https nntp};
                      sport 1:1023 dport ssh;
                 } accept;
            ];
            input eth0 source baz dest quux [
                 proto tcp { dport {1264 1521 1984 8008 8080 26000}; } accept;
            ];

       Where the underlying system supports it, everything inside the square
       brackets is moved into a separate "chain" (in ipchains and iptables-
       speak) or "group" (in ipfilter-speak).  Thus, any packet not matching
       "source foo dest bar" or "source baz dest quux" above will be checked
       against only two rules, not eleven.

       Note that matches which must appear together, like "proto tcp" and
       "sport 12345" need to be either both in the group, or both out of it.


INCLUDING OTHER FILES
       You can, if necessary, include other files containing filtergen
       statements in your filter files, with the include directive.  This is
       legal anywhere in the file, and acts as if you literally read the
       included file into the position where the include is placed.  This
       allows you to, for example, specify a common set of addresses (say,
       your monitoring system) and reference them in multiple locations:

            input eth0 {
                 proto tcp dport ssh source {
                      include monitoring-hosts.acl
                 } accept;

                 ...

                 # NRPE
                 proto tcp dport 5666 source {
                      include monitoring-hosts.acl
                 } accept;
            }

            output eth0 {
                 proto tcp sport ssh dest {
                      include monitoring-hosts.acl
                 } accept;

                 ...

                 # NRPE
                 proto tcp sport 5666 source {
                      include monitoring-hosts.acl
                 } accept;
            }

       Whilst you could improve this particular example without an include by
       grouping your ssh and NRPE ports together, you'd still have two places
       to edit when you changed your set of monitoring hosts.  Using include,
       you can have a single place to change when you change your monitoring
       hosts.

       You can also include a glob of files, rather than a single file, by
       using one of the shell globbing metacharacters '*', '?', or '['.  This
       will cause filtergen to expand that glob and parse the files as if you
       had included them one-by-one, in your locale's sorted order.  This is
       very handy if you configure your systems with an automated system,
       because you can dump a series of files into a directory depending on
       what classes are defined, and filtergen will pick them all up.  The
       sorting is to ensure that your rules appear in the order you want them,
       rather than some higgledy-piggledy mess.  Note that any directories
       that match your glob will not be recursively included in this mode.

       Finally, you can just provide a directory name, and filtergen will
       include all the files and directories under there, recursively
       (excluding hidden files, starting with a '.').  This method of
       including files is strongly deprecated.  There is no defined sort
       order, and if your editor leaves backup or temporary files around, or
       your configuration management system puts it's backups in the same
       directory as the original file (cfengine, I'm looking at you), you can
       find yourself including files you really didn't intend -- hence why it
       is a bad idea.  Instead, use a glob (above) with an explicit extension
       (we like *.fg) to perform your directory inclusions with minimal chance
       of accidents.



EXAMPLE
       Here's a fairly complete example, for a single-interface machine:

            #
            # Example filter for (for example) a mail server
            #

            # Unfortunately, we don't have time to audit the
            # communications which go on locally
            {input lo; output lo} accept;

            # But we want to be a bit more careful when speaking
            # to the outside world
            input eth0 {
                 # Sadly, we share a DMZ with Windows machines.
                 # Don't log their netbios noise
                 proto {tcp udp} source ournet/24 dport 137:139 drop;

                 proto tcp {
                      dport { smtp pop-3 } accept;
                      dport ssh source ournet/24 accept;
                      # We don't answer this, but don't want to
                      # cause timeouts by blocking it
                      dport auth reject;
                      log drop;
                 };
                 # We don't run any UDP (or other non-TCP)
                 # services
                 log drop;
            };
            output eth0 {
                 proto tcp {
                      dport { smtp auth } accept;
                      log drop;
                 };
                 # Outbound DNS is OK
                 proto udp dport domain dest { ns0 ns1 } accept;
                 log drop;
            };


SEE ALSO
       filtergen(8), filter_backends(7)



                                January 7, 2004               FILTER SYNTAX(5)