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IPF(5)											   IPF(5)

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

DESCRIPTION
       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 -

GRAMMAR
       The format used by ipf for construction of filtering rules can be described using the fol-
       lowing 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).

FILTER RULES
       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
       information.

ACTIONS
       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 unreach-
	      able, port unreachable or even administratively prohibited. This is done by enclos-
	      ing  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 <n>
	      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.

       preauth
	      tells the filter that for packets of this class, it should look in the  pre-authen-
	      ticated  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 tem-
	      porary 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 pro-
       cessing)  or  outbound (transmitted or forwarded by the stack, and on its way to an inter-
       face). There is a requirement that each filter rule explicitly state which side of the I/O
       it is to be used on.

OPTIONS
       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  con-
	      tinue 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.

MATCHING PARAMETERS
       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-iden-
	      tical rules.

       The  from and to keywords are used to match against IP addresses (and optionally port num-
       bers). 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 0.0.0.0/0 (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 non-sensical.

       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 bit-
       masks (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  com-
       parisons,  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 parame-
       ters.

       Following the source and destination matching parameters, the following additional parame-
       ters 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  specifi-
	      cation 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".

       icmp-type
	      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 abbrevia-
	      tion 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.

KEEP HISTORY
       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 fragments.

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

GROUPS
       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 process-
       ing 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 <n>
	      indicates that a new group (number n) should be created.

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

LOGGING
       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  key-
       word, 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.

       or-block
	      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 packet.

       level <loglevel>
	      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.

EXAMPLES
       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.

FILES
       /dev/ipauth
       /dev/ipl
       /dev/ipstate
       /etc/hosts
       /etc/services
       /usr/share/examples/ipf	Directory with examples.

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

											   IPF(5)
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