ipf (SFF)
NAME
ipf, ipf.conf, ipf6.conf - IP packet filter rule syntax
DESCRIPTION
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 -
GRAMMAR
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" | "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).
FILTER RULES
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(ADMN) for more information.
ACTIONS
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 adminis-
tratively prohibited. This is done by enclosing the
ICMP code associated with it in parenthesis directly
following return-icmp or return-icmp-as-dest as fol-
lows:
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 ipfstat(ADMN).
call this action is used to invoke the named function in the
kernel, which must conform to a specific calling inter-
face. 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 informa-
tion to validate. The packet is held for a period of
time in an internal buffer whilst it waits for the pro-
gram 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 ker-
nel to drop it if from an unrecognised source.
preauth
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 defin-
ing 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 pro-
cessing) 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.
OPTIONS
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 pro-
cess 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 inter-
face 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 key-
word is a synonym for this option.
MATCHING PARAMETERS
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 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 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 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 accom-
panied 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 con-
secutive 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 ser-
vice 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 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. Alter-
natively, 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:
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 specifi-
cation of combinations, such as "SFR", that would not
normally be generated by law-abiding TCP implementa-
tions. 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".
icmp-type
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 abbre-
viation 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.
KEEP HISTORY
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 pack-
ets.
frags
keeps information on fragmented packets, to be applied
to later fragments.
allowing packets which match these to flow straight through,
rather than going through the access control list.
GROUPS
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.
LOGGING
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 con-
tents 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.
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 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(ADMP) for the format of records written to this device.
The ipmon(ADMN) program can be used to read and format this
log.
EXAMPLES
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 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 pro-
cessed by group 100, there is no need to respecify the
interface name. Likewise, we could further breakup process-
ing 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.
FILES
/dev/ipauth
/dev/ipl
/dev/ipstate
/etc/hosts
/etc/services
SEE ALSO
ipftest(ADMN), mkfilters(ADMN), ipf(SFF), ipnat(SFF),
ipf(ADMN), ipfstat(TC)
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