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IPF(5)									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 ] [ "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
       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 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.

	      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
       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).

       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  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.

       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,
	      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  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 (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,
       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
	      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
       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 <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-
       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.

	      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
       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 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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