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IPF(5)                        File Formats Manual                       IPF(5)

       ipf, ipf.conf, ipf6.conf - IP packet filter rule syntax

       A rule file for ipf may have any name or even be stdin.  As ipfstat
       produces parsable rules as output when displaying the internal kernel
       filter lists, it is quite plausible to use its output to feed back into
       ipf.  Thus, to remove all filters on input packets, the following could
       be done:

       # ipfstat -i | ipf -rf -

       The format used by ipf for construction of filtering rules can be
       described using the following grammar in BNF:
       filter-rule = [ insert ] action in-out [ options ] [ tos ] [ ttl ]
                  [ proto ] ip [ group ].

       insert    = "@" decnumber .
       action    = block | "pass" | log | "count" | skip | auth | call .
       in-out    = "in" | "out" .
       options   = [ log ] [ tag ] [ "quick" ] [ "on" interface-name [ dup ]
                  [ froute ] [ replyto ] ] .
       tos  = "tos" decnumber | "tos" hexnumber .
       ttl  = "ttl" decnumber .
       proto     = "proto" protocol .
       ip   = srcdst [ flags ] [ with withopt ] [ icmp ] [ keep ] .
       group     = [ "head" decnumber ] [ "group" decnumber ] .

       block     = "block" [ return-icmp[return-code] | "return-rst" ] .
       log  = "log" [ "body" ] [ "first" ] [ "or-block" ] [ "level" loglevel ] .
       tag     = "tag" tagid .
       skip = "skip" decnumber .
       auth = "auth" | "preauth" .
       call = "call" [ "now" ] function-name .
       dup  = "dup-to" interface-name [ ":" ipaddr ] .
       froute    = "fastroute" | "to" interface-name [ ":" ipaddr ] .
       replyto = "reply-to" interface-name [ ":" ipaddr ] .
       protocol = "tcp/udp" | "udp" | "tcp" | "icmp" | decnumber .
       srcdst    = "all" | fromto .
       fromto    = "from" [ "!" ] object "to" [ "!" ] object .

       return-icmp = "return-icmp" | "return-icmp-as-dest" .
       return-code = "(" icmp-code ")" .
       object    = addr [ port-comp | port-range ] .
       addr = "any" | nummask | host-name [ "mask" ipaddr | "mask" hexnumber ] .
       addr = "any" | "<thishost>" | nummask |
              host-name [ "mask" ipaddr | "mask" hexnumber ] .
       port-comp = "port" compare port-num .
       port-range = "port" port-num range port-num .
       flags     = "flags" flag { flag } [ "/" flag { flag } ] .
       with = "with" | "and" .
       icmp = "icmp-type" icmp-type [ "code" decnumber ] .
       return-code = "(" icmp-code ")" .
       keep = "keep" "state" [ "(" state-options ")" ] | "keep" "frags" .
       loglevel = facility"."priority | priority .

       nummask   = host-name [ "/" decnumber ] .
       host-name = ipaddr | hostname | "any" .
       ipaddr    = host-num "." host-num "." host-num "." host-num .
       host-num = digit [ digit [ digit ] ] .
       port-num = service-name | decnumber .
       state-options = state-opts [ "," state-options ] .

       state-opts = "age" decnumber [ "/" decnumber ] | "strict" |
                    "no-icmp-err" | "limit" decnumber | "newisn" | "sync" .
       withopt = [ "not" | "no" ] opttype [ withopt ] .
       opttype = "ipopts" | "short" | "frag" | "opt" optname .
       optname   = ipopts [ "," optname ] .
       ipopts  = optlist | "sec-class" [ secname ] .
       secname   = seclvl [ "," secname ] .
       seclvl  = "unclass" | "confid" | "reserv-1" | "reserv-2" | "reserv-3" |
              "reserv-4" | "secret" | "topsecret" .
       icmp-type = "unreach" | "echo" | "echorep" | "squench" | "redir" |
                "timex" | "paramprob" | "timest" | "timestrep" | "inforeq" |
                "inforep" | "maskreq" | "maskrep"  | decnumber .
       icmp-code = decumber | "net-unr" | "host-unr" | "proto-unr" | "port-unr" |
                "needfrag" | "srcfail" | "net-unk" | "host-unk" | "isolate" |
                "net-prohib" | "host-prohib" | "net-tos" | "host-tos" |
                "filter-prohib" | "host-preced" | "cutoff-preced" .
       optlist   = "nop" | "rr" | "zsu" | "mtup" | "mtur" | "encode" | "ts" |
              "tr" | "sec" | "lsrr" | "e-sec" | "cipso" | "satid" | "ssrr" |
              "addext" | "visa" | "imitd" | "eip" | "finn" .
       facility = "kern" | "user" | "mail" | "daemon" | "auth" | "syslog" |
               "lpr" | "news" | "uucp" | "cron" | "ftp" | "authpriv" |
               "audit" | "logalert" | "local0" | "local1" | "local2" |
               "local3" | "local4" | "local5" | "local6" | "local7" .
       priority = "emerg" | "alert" | "crit" | "err" | "warn" | "notice" |
               "info" | "debug" .

       hexnumber = "0" "x" hexstring .
       hexstring = hexdigit [ hexstring ] .
       decnumber = digit [ decnumber ] .

       compare = "=" | "!=" | "<" | ">" | "<=" | ">=" | "eq" | "ne" | "lt" |
              "gt" | "le" | "ge" .
       range     = "<>" | "><" .
       hexdigit = digit | "a" | "b" | "c" | "d" | "e" | "f" .
       digit     = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" .
       flag = "F" | "S" | "R" | "P" | "A" | "U" .

       This syntax is somewhat simplified for readability, some combinations
       that match this grammar are disallowed by the software because they do
       not make sense (such as tcp flags for non-TCP packets).

       The "briefest" valid rules are (currently) no-ops and are of the form:
              block in all
              pass in all
              log out all
              count in all

       Filter rules are checked in order, with the last matching rule
       determining the fate of the packet (but see the quick option, below).

       Filters are installed by default at the end of the kernel's filter
       lists, prepending the rule with @n will cause it to be inserted as the
       n'th entry in the current list. This is especially useful when
       modifying and testing active filter rulesets. See ipf(8) for more

       The action indicates what to do with the packet if it matches the rest
       of the filter rule. Each rule MUST have an action. The following
       actions are recognised:

       block  indicates that the packet should be flagged to be dropped. In
              response to blocking a packet, the filter may be instructed to
              send a reply packet, either an ICMP packet (return-icmp), an
              ICMP packet masquerading as being from the original packet's
              destination (return-icmp-as-dest), or a TCP "reset" (return-
              rst).  An ICMP packet may be generated in response to any IP
              packet, and its type may optionally be specified, but a TCP
              reset may only be used with a rule which is being applied to TCP
              packets.  When using return-icmp or return-icmp-as-dest, it is
              possible to specify the actual unreachable `type'.  That is,
              whether it is a network unreachable, port unreachable or even
              administratively prohibited. This is done by enclosing the ICMP
              code associated with it in parenthesis directly following
              return-icmp or return-icmp-as-dest as follows:
                      block return-icmp(11) ...

       Would return a Type-Of-Service (TOS) ICMP unreachable error.

       pass   will flag the packet to be let through the filter.

       log    causes the packet to be logged (as described in the LOGGING
              section below) and has no effect on whether the packet will be
              allowed through the filter.

       count  causes the packet to be included in the accounting statistics
              kept by the filter, and has no effect on whether the packet will
              be allowed through the filter. These statistics are viewable
              with ipfstat(8).

       call   this action is used to invoke the named function in the kernel,
              which must conform to a specific calling interface. Customised
              actions and semantics can thus be implemented to supplement
              those available. This feature is for use by knowledgeable
              hackers, and is not currently documented.

       skip <&lt;n>&gt;
              causes the filter to skip over the next n filter rules.  If a
              rule is inserted or deleted inside the region being skipped
              over, then the value of n is adjusted appropriately.

       auth   this allows authentication to be performed by a user-space
              program running and waiting for packet information to validate.
              The packet is held for a period of time in an internal buffer
              whilst it waits for the program to return to the kernel the real
              flags for whether it should be allowed through or not.  Such a
              program might look at the source address and request some sort
              of authentication from the user (such as a password) before
              allowing the packet through or telling the kernel to drop it if
              from an unrecognised source.

              tells the filter that for packets of this class, it should look
              in the pre-authenticated list for further clarification.  If no
              further matching rule is found, the packet will be dropped (the
              FR_PREAUTH is not the same as FR_PASS).  If a further matching
              rule is found, the result from that is used in its instead.
              This might be used in a situation where a person logs in to the
              firewall and it sets up some temporary rules defining the access
              for that person.

       The next word must be either in or out.  Each packet moving through the
       kernel is either inbound (just been received on an interface, and
       moving towards the kernel's protocol processing) or outbound
       (transmitted or forwarded by the stack, and on its way to an
       interface). There is a requirement that each filter rule explicitly
       state which side of the I/O it is to be used on.

       The list of options is brief, and all are indeed optional. Where
       options are used, they must be present in the order shown here. These
       are the currently supported options:

       log    indicates that, should this be the last matching rule, the
              packet header will be written to the ipl log (as described in
              the LOGGING section below).

       tag tagid
              indicates that, if this rule causes the packet to be logged or
              entered in the state table, the tagid will be logged as part of
              the log entry.  This can be used to quickly match "similar"
              rules in scripts that post process the log files for e.g.
              generation of security reports or accounting purposes. The tagid
              is a 32 bit unsigned integer.

       quick  allows "short-cut" rules in order to speed up the filter or
              override later rules.  If a packet matches a filter rule which
              is marked as quick, this rule will be the last rule checked,
              allowing a "short-circuit" path to avoid processing later rules
              for this packet. The current status of the packet (after any
              effects of the current rule) will determine whether it is passed
              or blocked.

              If this option is missing, the rule is taken to be a "fall-
              through" rule, meaning that the result of the match (block/pass)
              is saved and that processing will continue to see if there are
              any more matches.

       on     allows an interface name to be incorporated into the matching
              procedure. Interface names are as printed by "netstat -i". If
              this option is used, the rule will only match if the packet is
              going through that interface in the specified direction
              (in/out). If this option is absent, the rule is taken to be
              applied to a packet regardless of the interface it is present on
              (i.e. on all interfaces).  Filter rulesets are common to all
              interfaces, rather than having a filter list for each interface.

              This option is especially useful for simple IP-spoofing
              protection: packets should only be allowed to pass inbound on
              the interface from which the specified source address would be
              expected, others may be logged and/or dropped.

       dup-to causes the packet to be copied, and the duplicate packet to be
              sent outbound on the specified interface, optionally with the
              destination IP address changed to that specified. This is useful
              for off-host logging, using a network sniffer.

       to     causes the packet to be moved to the outbound queue on the
              specified interface. This can be used to circumvent kernel
              routing decisions, and even to bypass the rest of the kernel
              processing of the packet (if applied to an inbound rule). It is
              thus possible to construct a firewall that behaves
              transparently, like a filtering hub or switch, rather than a
              router. The fastroute keyword is a synonym for this option.

       The keywords described in this section are used to describe attributes
       of the packet to be used when determining whether rules match or don't
       match. The following general-purpose attributes are provided for
       matching, and must be used in this order:

       tos    packets with different Type-Of-Service values can be filtered.
              Individual service levels or combinations can be filtered upon.
              The value for the TOS mask can either be represented as a hex
              number or a decimal integer value.

       ttl    packets may also be selected by their Time-To-Live value.  The
              value given in the filter rule must exactly match that in the
              packet for a match to occur.  This value can only be given as a
              decimal integer value.

       proto  allows a specific protocol to be matched against.  All protocol
              names found in /etc/protocols are recognised and may be used.
              However, the protocol may also be given as a DECIMAL number,
              allowing for rules to match your own protocols, or new ones
              which would out-date any attempted listing.

              The special protocol keyword tcp/udp may be used to match either
              a TCP or a UDP packet, and has been added as a convenience to
              save duplication of otherwise-identical rules.

       The from and to keywords are used to match against IP addresses (and
       optionally port numbers). Rules must specify BOTH source and
       destination parameters.

       IP addresses may be specified in one of two ways: as a numerical
       address/mask, or as a hostname mask netmask.  The hostname may either
       be a valid hostname, from either the hosts file or DNS (depending on
       your configuration and library) or of the dotted numeric form.  There
       is no special designation for networks but network names are
       recognised.  Note that having your filter rules depend on DNS results
       can introduce an avenue of attack, and is discouraged.

       There is a special case for the hostname any which is taken to be (see below for mask syntax) and matches all IP addresses.
       Only the presence of "any" has an implied mask, in all other
       situations, a hostname MUST be accompanied by a mask.  It is possible
       to give "any" a hostmask, but in the context of this language, it is

       The numerical format "x/y" indicates that a mask of y consecutive 1
       bits set is generated, starting with the MSB, so a y value of 16 would
       give 0xffff0000. The symbolic "x mask y" indicates that the mask y is
       in dotted IP notation or a hexadecimal number of the form 0x12345678.
       Note that all the bits of the IP address indicated by the bitmask must
       match the address on the packet exactly; there isn't currently a way to
       invert the sense of the match, or to match ranges of IP addresses which
       do not express themselves easily as bitmasks (anthropomorphization;
       it's not just for breakfast anymore).

       If a port match is included, for either or both of source and
       destination, then it is only applied to TCP and UDP packets. If there
       is no proto match parameter, packets from both protocols are compared.
       This is equivalent to "proto tcp/udp".  When composing port
       comparisons, either the service name or an integer port number may be
       used. Port comparisons may be done in a number of forms, with a number
       of comparison operators, or port ranges may be specified. When the port
       appears as part of the from object, it matches the source port number,
       when it appears as part of the to object, it matches the destination
       port number.  See the examples for more information.

       The all keyword is essentially a synonym for "from any to any" with no
       other match parameters.

       Following the source and destination matching parameters, the following
       additional parameters may be used:

       with   is used to match irregular attributes that some packets may have
              associated with them.  To match the presence of IP options in
              general, use with ipopts. To match packets that are too short to
              contain a complete header, use with short. To match fragmented
              packets, use with frag.  For more specific filtering on IP
              options, individual options can be listed.

              Before any parameter used after the with keyword, the word not
              or no may be inserted to cause the filter rule to only match if
              the option(s) is not present.

              Multiple consecutive with clauses are allowed.  Alternatively,
              the keyword and may be used in place of with, this is provided
              purely to make the rules more readable ("with ... and ...").
              When multiple clauses are listed, all those must match to cause
              a match of the rule.

       flags  is only effective for TCP filtering.  Each of the letters
              possible represents one of the possible flags that can be set in
              the TCP header.  The association is as follows:

               F - FIN
               S - SYN
               R - RST
               P - PUSH
               A - ACK
               U - URG

              The various flag symbols may be used in combination, so that
              "SA" would represent a SYN-ACK combination present in a packet.
              There is nothing preventing the specification of combinations,
              such as "SFR", that would not normally be generated by law-
              abiding TCP implementations.  However, to guard against weird
              aberrations, it is necessary to state which flags you are
              filtering against.  To allow this, it is possible to set a mask
              indicating which TCP flags you wish to compare (i.e., those you
              deem significant).  This is done by appending "/<flags>" to the
              set of TCP flags you wish to match against, e.g.:

            ... flags S
                      # becomes "flags S/AUPRFS" and will match
                      # packets with ONLY the SYN flag set.

            ... flags SA
                      # becomes "flags SA/AUPRFS" and will match any
                      # packet with only the SYN and ACK flags set.

            ... flags S/SA
                      # will match any packet with just the SYN flag set
                      # out of the SYN-ACK pair; the common "establish"
                      # keyword action.  "S/SA" will NOT match a packet
                      # with BOTH SYN and ACK set, but WILL match "SFP".

              is only effective when used with proto icmp and must NOT be used
              in conjunction with flags.  There are a number of types, which
              can be referred to by an abbreviation recognised by this
              language, or the numbers with which they are associated can be
              used.  The most important from a security point of view is the
              ICMP redirect.

       The second last parameter which can be set for a filter rule is whether
       or not to record historical information for that packet, and what sort
       to keep. The following information can be kept:

       state  keeps information about the flow of a communication session.
              State can be kept for TCP, UDP, and ICMP packets.

       frags  keeps information on fragmented packets, to be applied to later

       allowing packets which match these to flow straight through, rather
       than going through the access control list.

       The last pair of parameters control filter rule "grouping".  By
       default, all filter rules are placed in group 0 if no other group is
       specified.  To add a rule to a non-default group, the group must first
       be started by creating a group head.  If a packet matches a rule which
       is the head of a group, the filter processing then switches to the
       group, using that rule as the default for the group.  If quick is used
       with a head rule, rule processing isn't stopped until it has returned
       from processing the group.

       A rule may be both the head for a new group and a member of a non-
       default group (head and group may be used together in a rule).

       head <&lt;n>&gt;
              indicates that a new group (number n) should be created.

       group <&lt;n>&gt;
              indicates that the rule should be put in group (number n) rather
              than group 0.

       When a packet is logged, with either the log action or option, the
       headers of the packet are written to the ipl packet logging pseudo-
       device. Immediately following the log keyword, the following qualifiers
       may be used (in order):

       body   indicates that the first 128 bytes of the packet contents will
              be logged after the headers.

       first  If log is being used in conjunction with a "keep" option, it is
              recommended that this option is also applied so that only the
              triggering packet is logged and not every packet which
              thereafter matches state information.

              indicates that, if for some reason the filter is unable to log
              the packet (such as the log reader being too slow) then the rule
              should be interpreted as if the action was block for this

       level <&lt;loglevel>&gt;
              indicates what logging facility and priority, or just priority
              with the default facility being used, will be used to log
              information about this packet using ipmon's -s option.

       See ipl(4) for the format of records written to this device. The
       ipmon(8) program can be used to read and format this log.

       The quick option is good for rules such as:
       block in quick from any to any with ipopts

       which will match any packet with a non-standard header length (IP
       options present) and abort further processing of later rules, recording
       a match and also that the packet should be blocked.

       The "fall-through" rule parsing allows for effects such as this:

               block in from any to any port < 6000
               pass in from any to any port >= 6000
               block in from any to any port > 6003

       which sets up the range 6000-6003 as being permitted and all others
       being denied.  Note that the effect of the first rule is overridden by
       subsequent rules.  Another (easier) way to do the same is:

               block in from any to any port 6000 <> 6003
               pass in from any to any port 5999 >< 6004

       Note that both the "block" and "pass" are needed here to effect a
       result as a failed match on the "block" action does not imply a pass,
       only that the rule hasn't taken effect.  To then allow ports < 1024, a
       rule such as:

               pass in quick from any to any port < 1024

       would be needed before the first block.  To create a new group for
       processing all inbound packets on le0/le1/lo0, with the default being
       to block all inbound packets, we would do something like:

              block in all
              block in quick on le0 all head 100
              block in quick on le1 all head 200
              block in quick on lo0 all head 300

       and to then allow ICMP packets in on le0, only, we would do:

              pass in proto icmp all group 100

       Note that because only inbound packets on le0 are used processed by
       group 100, there is no need to respecify the interface name.  Likewise,
       we could further breakup processing of TCP, etc, as follows:

              block in proto tcp all head 110 group 100
              pass in from any to any port = 23 group 110

       and so on.  The last line, if written without the groups would be:

              pass in on le0 proto tcp from any to any port = telnet

       Note, that if we wanted to say "port = telnet", "proto tcp" would need
       to be specified as the parser interprets each rule on its own and
       qualifies all service/port names with the protocol specified.

       /usr/share/examples/ipf  Directory with examples.

       ipftest(1), iptest(1), mkfilters(1), ipf(4), ipnat(5), ipf(8),