ipf(SFF)

Description

Grammar

   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" ] .
   call	= "call" [ "now" ] function-name .
   skip	= "skip" decnumber .
   dup	= "dup-to" interface-name[":"ipaddr] .
   froute	= "fastroute" | "to" interface-name .
   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")" .
   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  .
   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" .
   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" .

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

   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 nth entry in the current list. This is especially useful when modifying and testing active filter rulesets. See ipf(ADMN) for more information.

Actions

auth

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 unrecognized 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 is used 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.

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.

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) 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.

pass

flags 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(TC).

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>

auth

preauth

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.

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 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.

Matching parameters

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, for example:

   ... 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 conjuction 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.

Keep history

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 psuedo-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.

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.

See ipl(ADMP) for the format of records written to this device. The ipmon(TC) 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

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

   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

   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

   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

   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

See also

Standards conformance