path: root/sbin/ipf/ipf.5

.\"	$OpenBSD: ipf.5,v 1.8 1997/04/09 11:44:27 kstailey Exp $
.TH IPF 5
.SH NAME
ipf \- IP packet filter rule syntax
.SH DESCRIPTION
.PP
A rule file for \fBipf\fP may have any name or even be stdin.  As
\fBipfstat\fP produces parsable rules as output when displaying the internal
kernel filter lists, it is quite plausible to use its output to feed back
into \fBipf\fP.  Thus, to remove all filters on input packets, the following
could be done:
.nf

\fC# ipfstat \-i | ipf \-rf \-\fP
.fi
.SH GRAMMAR
.PP
The format used by \fBipf\fP for construction of filtering rules can be
described using the following grammar in BNF:
\fC
.nf
filter-rule = [ insert ] action in-out [ options ] [ match ] [ keep ]

insert	= "@" decnumber .
action	= block | "pass" | log | "count" | call .
in-out	= "in" | "out" .
options	= [ log ] [ "quick" ] [ "on" interface-name [ dup ] [ froute ] ] .
match	= [ tos ] [ ttl ] [ proto ] [ ip ] .
keep  	= "keep state" | "keep frags" .

block	= "block" [ "return-icmp"[return-code] | "return-rst" ] .
log   	= "log" [ "body" ] [ "first" ] [ "or-block" ] .
call  	= "call" [ "now" ] function-name .

dup   	= "dup-to" interface-name[":"ipaddr] .
froute	= "fastroute" | "to" interface-name .

tos   	= "tos" decnumber | "tos" hexnumber .
ttl   	= "ttl" decnumber .
proto	= "proto" protocol .
ip    	= srcdst [ flags ] [ with withopt ] [ icmp ] [ keep ] .

protocol = "tcp/udp" | "udp" | "tcp" | "icmp" | decnumber .
srcdst	= "all" | fromto .
fromto	= "from" object "to" object .

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")" .

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" ipopts  .
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 = decnumber | "net-unr" | "host-unr" | "proto-unr" | "port-unr" |
	    "needfrag" | "srcfail" | "net-unk" | "host-unk" | "isolate" |
	    "net-prohib" | "host-prohib" | "net-tos" | "host-tos" .
optlist	= "nop" | "rr" | "zsu" | "mtup" | "mtur" | "encode" | "ts" | "tr" |
	  "sec" | "lsrr" | "e-sec" | "cipso" | "satid" | "ssrr" | "addext" |
	  "visa" | "imitd" | "eip" | "finn" .

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" .
.fi
.PP
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 \fBflags\fP for non-TCP packets).
.SH FILTER RULES
.PP
The "briefest" valid rules are (currently) no-ops and are of the form:
.nf
       block in
       pass in
       log in
       count in
.fi
.PP
These are supposed to be the same as, but currently differ from:
.\" XXX How, why do they differ??
.nf
       block in all
       pass in from any to any
       log in all
       count in all
.fi
.PP
Filter rules are checked in order, with the last matching rule
determining the fate of the packet (but see the \fBquick\fP option,
below).
.PP
Filters are installed by default at the end of the kernel's filter
lists, prepending the rule with \fB@n\fP 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(1) for more
information.
.SH ACTIONS
.PP
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 recognized:
.TP
.B 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 (\fBreturn-icmp\fP) or a TCP "reset"
(\fBreturn-rst\fP).  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.
.TP
.B pass
will flag the packet to be let through the filter.  
.TP
.B 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.
.TP
.B 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).
.TP
.B call
this action is used to invoke the named function in the kernel, which
must conform to a specific calling interface. Customized actions and
semantics can thus be implemented to supplement those available. This
feature is for use by knowledgeable hackers, and is not currently
documented.
.PP
The next word must be either \fBin\fP or \fBout\fP.  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.
.SH OPTIONS
.PP
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:
.TP
.B log
indicates that, should this be the last matching rule, the packet
header will be written to the \fBipl\fP log (as described in the
LOGGING section below).
.TP
.B 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
\fBquick\fP, 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.
.IP
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.
.TP
.B 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.
.IP
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.
.TP
.B 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.
.TP
.B 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 \fBfastroute\fP keyword is a
synonym for this option.
.SH MATCHING PARAMETERS
.PP 
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:
.TP
.B 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.
.\" XXX TOS mask??  not in grammar!
.TP
.B 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.
.TP
.B proto
allows a specific protocol to be matched against.  All protocol names
found in \fB/etc/protocols\fP are recognized 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.
.IP
The special protocol keyword \fBtcp/udp\fP 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.
.\" XXX grammar should reflect this (/etc/protocols)
.PP
The \fBfrom\fP and \fBto\fP keywords are used to match against IP
addresses (and optionally port numbers). Rules must specify BOTH
source and destination parameters.
.PP 
IP addresses may be specified in one of two ways: as a numerical
address\fB/\fPmask, or as a hostname \fBmask\fP 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 recognized.  Note that having your filter rules depend on DNS
results can introduce an avenue of attack, and is discouraged.
.PP
There is a special case for the hostname \fBany\fP 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.
.PP
The numerical format "x\fB/\fPy" 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 \fBmask\fP 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).
.PP
If a \fBport\fP match is included, for either or both of source and
destination, then it is only applied to
.\" XXX - "may only be" ? how does this apply to other protocols? will it not match, or will it be ignored?
TCP and UDP packets. If there is no \fBproto\fP match parameter,
packets from both protocols are compared. This is equivalent to "proto
tcp/udp".  When composing \fBport\fP 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. See the examples for more information.
.PP
The \fBall\fP keyword is essentially a synonym for "from any to any"
with no other match parameters.
.PP
Following the source and destination matching parameters, the
following additional parameters may be used:
.TP
.B 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 \fBwith ipopts\fP. To match packets that are too short to contain
a complete header, use \fBwith short\fP. To match fragmented packets,
use \fBwith frag\fP.  For more specific filtering on IP options,
individual options can be listed.
.IP
Before any parameter used after the \fBwith\fP keyword, the word
\fBnot\fP or \fBno\fP may be inserted to cause the filter rule to only
match if the option(s) is not present.
.IP
Multiple consecutive \fBwith\fP clauses are allowed.  Alternatively,
the keyword \fBand\fP may be used in place of \fBwith\fP, 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.
.\" XXX describe the options more specifically in a separate section
.TP
.B 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:
.LP
.nf
        F - FIN
        S - SYN
        R - RST
        P - PUSH
        A - ACK
        U - URG
.fi
.IP
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.:
.LP
.nf
	... 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".
.fi
.TP
.B icmp-type
is only effective when used with \fBproto icmp\fP and must NOT be used
in conjunction with \fBflags\fP.  There are a number of types, which can be
referred to by an abbreviation recognized 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.
.SH KEEP HISTORY
.PP
The last parameter which can be set for a filter rule is whether on not to
record historical information for that packet, and what sort to keep. The following information can be kept:
.TP
.B state
keeps information about the flow of a communication session. State can
be kept for TCP, UDP, and ICMP packets.
.TP
.B frags
keeps information on fragmented packets, to be applied to later
fragments.
.PP
allowing packets which match these to flow straight through, rather
than going through the access control list.
.SH LOGGING
.PP
When a packet is logged, with either the \fBlog\fP action or option,
the headers of the packet are written to the \fBipl\fP packet logging
pseudo-device. Immediately following the \fBlog\fP keyword, the
following qualifiers may be used (in order):
.TP
.B body
indicates that the first 128 bytes of the packet contents will be
logged after the headers. 
.TP
.B first
??
.TP
.B 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 \fBblock\fP for this packet.
.PP
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.
.SH EXAMPLES
.PP
The \fBquick\fP option is good for rules such as:
\fC
.nf
block in quick from any to any with ipopts
.fi
.PP
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.
.PP
The "fall-through" rule parsing allows for effects such as this:
.LP
.nf
        block in from any to any port < 6000
        pass in from any to any port >= 6000
        block in from any to port > 6003
.fi
.PP
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:
.LP
.nf
        block in from any to any port 6000 <> 6003
        pass in from any to any port 5999 >< 6004
.fi
.PP
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:
.LP
.nf
        pass in quick from any to any port < 1024
.fi
.PP
would be needed before the first block.  
.SH FILES
\fI/etc/services\fP -- port names
.br
\fI/etc/hosts\fP -- host names
.br
\fI/usr/share/ipf\fP -- sample configuration files
.SH SEE ALSO
ipf(1), ipftest(1), ipf(4), hosts(5), services(5)