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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 de-
       scribed 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 .
       host-num	= digit	[ digit	[ digit	] ] .
       port-num	= service-name | decnumber .

       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	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-

       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  ac-
       tions 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 re-
	      set 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 re-
	      turn-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 al-
	      lowed 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 al-
	      lowing 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 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.

       The  list  of  options is brief,	and all	are indeed optional. Where op-
       tions 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).

       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, al-
	      lowing a "short-circuit" path to avoid  processing  later	 rules
	      for this packet. The current status of the packet	(after any ef-
	      fects 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  ap-
	      plied  to	 a packet regardless of	the interface it is present on
	      (i.e. on all interfaces).	 Filter	rulesets are common to all in-
	      terfaces,	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  in-
	      terface  from  which  the	 specified source address would	be ex-
	      pected, 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.

       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, al-
	      lowing  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 destina-
       tion parameters.

       IP addresses may	be specified in	one of two ways: as  a	numerical  ad-
       dress/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	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-

       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, ei-
       ther  the service name or an integer port number	may be used. Port com-
       parisons	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 ap-
       pears 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  op-
	      tions, 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	possi-
	      ble 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-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".

	      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.

       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  de-
       fault,  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-de-
       fault 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.

       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-de-
       vice.  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.

	      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 <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 ip-
       mon(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  op-
       tions 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 be-
       ing  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  re-
       sult  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 pro-
       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.


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



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