.LP
.TH IPF 5
.SH NAME
ipf - IP packet filtering format.
.SH DESCRIPTION
.PP
A rule file for \fBipf\fP may have any name or even be stdin.  As
\fBipfstat\fP produces parseable 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
.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 ] [ tos ] [ ttl ]
              [ proto ] [ ip ] .

insert  = "@" decnumber
action  = block | "pass" | log | "count" .
in-out  = "in" | "out" .
options = [ log ] [ "quick" ] [ "on" interface-name] .
tos     = "tos" decnumber | "tos" hexnumber .
ttl     = "ttl" decnumber .
proto   = "proto" protocol .
ip      = srcdst [ flags ] [ with withopt ] [ icmp ] [ keep ] .

block   = "block" [ "return-icmp" [ return-code ] | "return-rst" ].
log	= "log" [ "body" ] [ "first" ] .
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 | 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 .
return-code = "(" icmp-code ")" .
keep	= "keep" "state" | "keep" "frags" .

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 .
ipopts  = "nop" | "rr" | "ts" | "security" | "sec-class" [ "=" seclvl ] |
	  "lsrr" | "satid" | "rsrr" .
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" .

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
The "briefest" valid rule is of the form:
.nf

       block in
       pass in
       log in
.fi
.PP
These can also be written like:
.nf
       block in all pass in from any to any
.fi
.PP
The action, one of either block, log or pass, indicates what to do with
the packet if it matches the rest of the filter rule.  Block indicates that
the packet should be dropped here and not let through, log write the packet
header to the \fBipl\fP packet logging psuedo-device (and has no further
effect on validity of packet to be allowed through the filter) and pass which
will allow the packet through.  Each rule MUST have one of these three
keywords.
.PP
In response to blocking a packet, the filter may be instructed to send a
reply packet, either an ICMP unreachable (\fBreturn-icmp\fP)or a TCP
"reset" (\fBreturn-rst\fP).  An ICMP packet may be generated in response
to any IP packet but a TCP reset may only be used with a rule which is
being applied to TCP packets.
.PP
When a packet header is logged with the \fBlog\fP keyword, the optional
\fBbody\fP keyword indicates that the first 128 bytes of the packet contents
will be logged to the \fBipl\fP packet logging psuedo-device after the
headers.
.PP
The next word must be either \fBin\fP or \fBout\fP.  As each packet moving
through the kernel is either an inbound packet or outbound, there is a
requirement that each filter rule be explicitly stated as to which side of
the IO it is to be used on.
.PP
The list of options is brief, and indeed all are optional.  The presence
of the \fBlog\fP option indicates, that should this be the last matching
rule, the packet header will be written to the \fBipl\fP log. The \fBquick\fP
option allows "short-cut" rules in order to speed up the filter.  If a
packet header matches a filter rule which is marked as \fBquick\fP, it will
result in a quick-match and stop processing at this point.  This 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, recording a match and also that the packet should be
blocked.  If this command is missing, the rule is taken to be a
"fall-through" rule, meaning that the result of the match is used
(block/pass) and that it will continue processing to see if there are any
more matches.  This 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.  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 affect a result
as a failed "block" 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.  Expect to see a "between" operator
as soon as I can work out how to fit in in.
.PP
The \fBon\fP command allows an interface name to be incorporated into the
matching procedure.  That it is a match and not actually associated with
the interface itself is a result of the way this was implemented.  Indeed,
there is nothing to stop you using this with every rule if you so wish.
If it is absent, the rule is taken to be applied to a packet regardless of
the interface it is present on.
.PP
The \fBall\fP command is essentially an alias for "from any to any" with
no other commands.
.PP
Using \fBtos\fP, packets with different service capabilities can be filtered
upon.  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.
.PP
Packets may also be selected by their \fBttl\fP 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.
.PP
The \fBproto\fP command allows a specific protocol to be matched against.
All protocol names found in \fB/etc/protocols\fP are recognised and maybe
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.
.PP
To match against BOTH source and destination addresses, the \fBfrom\fP and
\fBto\fP commands are used.  They both support a large variety of valid
syntaxes, including the "x/y" format.  There is a special case for the
hostname \fBany\fP which is taken to be 0.0.0.0/0 and matches all IP numbers.
If a \fBport\fP match is included, then it is only applied to TCP/UDP
packets.  If the \fBproto\fP command is left out, packets from both protocols
are compared.  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.
.PP
"x/y" indicates that a mask of y consecutive bits set is generated, starting
with the MSB, so a value of 16 would give 0xffff0000.
.PP
"x mask y" indicates that the mask y is in dotted IP notation or a hexadecimal
number of the form 0x12345678.
.PP
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
When composing
\fBport\fP comparisons, either the service name may be used or an integer
port number.
.PP
The \fBwith\fP command is used to nominate irregular attributes that some
packets ma have associated with them.  Alternatively, the keyword \fBand\fP
maybe used in place of \fBwith\fP.  This is provided to make the rules more
readable and serves no other purpose.  To filter IP options, in general,
use \fBipopts\fP.  For more specific filtering on IP options, individual
options can be listed.  When listed, all those listed must be found in a
packet to cause a match.
.PP
Before any option used after the \fBwith\fP keyword, the word \fBnot\fp
maybe inserted to cause the filter rule to only match if the option(s) is
not present.
.PP
The \fBflags\fP command 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
.PP
The various flag symbols maybe used in combination, so that "SA" would
represent a SYN-ACK combination present in a packet.  There is nothing
preventing combinations, such as "SFR".  However, to guard against weird
abberations, 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
wich to compare (ie those you deem significant).  This is done by appending
"/<flags>" to the set of TCP flags you wish to match against. eg:
.LP
.nf
	... flags S
			# becomes "flags S/AUPRFS" and will match a
			# packet 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
.PP
The next parameter set for the filter rule is the optional \fBicmp-type\fP.
It is only effective when used with \fB"proto icmp"\fP and must NOT be used
in conjuction with \fBflags\fP.  There are a number of types which can be
refered to by an abbreviation recognised by this language or the numbers
with which they are associated can be used.
.PP
The last parameter which can be set for a filter rule is whether on not to
record state information for that packet, and what sort to keep.  Either
information relating to the packet's `flow' or if fragment details can be
kept, allowing packets which match these to flow straight through, rather
than going through the access control list.
.SH FILES
/etc/services
/etc/hosts
.SH SEE ALSO
ipf(1), ipftest(1)