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" | "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
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
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 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(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 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.
- 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
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 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 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:
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-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".
- 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 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
fragments.
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.
- 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(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
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.
/dev/ipauth
/dev/ipl
/dev/ipstate
/etc/hosts
/etc/services
ipftest(1), iptest(1), mkfilters(1), ipf(4), ipnat(5), ipf(8), ipfstat(8)