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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 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 ] [ "quick" ] [ "on" interface-name [ dup ] [ froute ] ] .
       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" ] .
       auth    = "auth" | "preauth" .
       log  = "log" [ "body" ] [ "first" ] [ "or-block" ] [ "level" loglevel ] .
       call = "call" [ "now" ] function-name .
       skip = "skip" decnumber .
       dup  = "dup-to" interface-name[":"ipaddr] .
       froute	 = "fastroute" | "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" .
       object	 = addr [ port-comp | port-range ] .
       addr = "any" | 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" | "keep" "frags" .
       loglevel = facility"."priority | priority .

       nummask	 = host-name [ "/" decnumber ] .
       host-name = ipaddr | hostname | "any" .
       ipaddr	 = host-num "." host-num "." host-num "." host-num .
       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	deter-
       mining 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 modify-
       ing and testing active filter rulesets. See ipf(8)  for	more  informa-
       tion.

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
	      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 sec-
	      tion  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  hack-
	      ers, 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 pro-
	      gram 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-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 mov-
       ing 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.

OPTIONS

       The  list  of  options  is  brief,  and	all are indeed optional. Where
	      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  protec-
	      tion:  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 spec-
	      ified  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 possi-
	      ble 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 match-
       ing, 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,
       optionally port numbers). Rules must specify BOTH source  and  destina-
       tion 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 recog-
       nised.  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 situa-
       tions, 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 bitmasks (anthropomorphization;
       it's not just for breakfast anymore).

       If a port match is included, for either or both of source and  destina-
       tion,  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 compari-
       son 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  num-
       ber.  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
	       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-abid-
	      ing TCP implementations.	However, to guard against weird  aber-
	      rations,	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 sig-
	      nificant).  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/AUPRFSC" 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 abbreviation recognised by this lan-
	      guage, 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

       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 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	there-
	      after 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 infor-
	      mation 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

       cessing	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 qual-
       ifies all service/port names with the protocol specified.

FILES

       /dev/ipauth
       /dev/ipl
       /dev/ipstate
       /etc/hosts
       /etc/services

SEE ALSO

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

									IPF(5)

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