IPF(5) Standards, Environments, and Macros IPF(5)


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


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

# ipfstat -i | ipf -rf -


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

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

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

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

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

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

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

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

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


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

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

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


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

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

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

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

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

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

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.

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

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


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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

There is a special case for the hostname any which is taken to be (see below for mask syntax) and matches all IP addresses. Only
the presence of "any" has an implied mask, in all other situations, a
hostname MUST be accompanied by a mask. It is possible to give "any" a
hostmask, but in the context of this language, it is 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
destination, then it is only applied to TCP and UDP packets. If there is
no proto match parameter, packets from both protocols are compared. This
is equivalent to "proto tcp/udp". When composing port comparisons,
either the service name or an integer port number may be used. Port
comparisons may be done in a number of forms, with a number of comparison
operators, or port ranges may be specified. When the port appears as part
of the from object, it matches the source port number, when it appears as
part of the to object, it matches the destination port number. See the
examples for more information.

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

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

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

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

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

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


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

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

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

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

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


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

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

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

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


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

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

head <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 thereafter
matches state information.

indicates that, if for some reason the filter is unable to log the
packet (such as the log reader being too slow) then the rule
should be interpreted as if the action was block for this 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(5) 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

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.




ipnat(5), ipfilter(7), ipf(8), ipfstat(8)

March 18, 2015 IPF(5)