IPF(5)IPF(5)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 ] [ "quick" ] [ "on" interface-name [ dup ] [ froute ] ] .
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" ] .
auth = "auth" | "preauth" .
log = "log" [ "body" ] [ "first" ] [ "or-block" ] [ "level" loglevel ] .
call = "call" [ "now" ] function-name .
skip = "skip" decnumber .
dup = "dup-to" interface-name[":"ipaddr] .
froute = "fastroute" | "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" .
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")" .
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 deter‐
mining 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 modify‐
ing and testing active filter rulesets. See ipf(8) for more informa‐
tion.
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
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 sec‐
tion 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 hack‐
ers, 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 pro‐
gram 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 mov‐
ing 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.
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).
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 protec‐
tion: 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 spec‐
ified 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 possi‐
ble 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.
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 match‐
ing, 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 destina‐
tion 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 recog‐
nised. 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 situa‐
tions, 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 destina‐
tion, 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 compari‐
son 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 num‐
ber. 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 possi‐
ble 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-abid‐
ing TCP implementations. However, to guard against weird aber‐
rations, 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 sig‐
nificant). 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/AUPRFSC" 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 lan‐
guage, 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 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.
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 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 there‐
after 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 infor‐
mation 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.
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 pro‐
cessing 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 qual‐
ifies all service/port names with the protocol specified.
FILES
/dev/ipauth
/dev/ipl
/dev/ipstate
/etc/hosts
/etc/services
SEE ALSOipftest(1), iptest(1), mkfilters(1), ipf(4), ipnat(5), ipf(8), ipfs‐
tat(8)IPF(5)
