IPFW(8) MidnightBSD System Manager’s Manual IPFW(8)

NAME

ipfw — IP firewall and traffic shaper control program

SYNOPSIS

ipfw [−cq] add rule
ipfw
[−acdefnNStT] [set N] {list show} [rule first-last ...]
ipfw
[−f −q] [set N] flush
ipfw
[−q] [set N] {delete zero resetlog} [number ...]
ipfw enable
{firewall altq one_pass debug verbose dyn_keepalive}
ipfw disable
{firewall altq one_pass debug verbose dyn_keepalive}

ipfw set [

disable number ... ] [enable number ...]

ipfw set move [rule] number to number
ipfw set swap
number number
ipfw set show

ipfw table number add addr[

/masklen ] [value]

ipfw table number delete addr[/masklen]
ipfw table
number flush
ipfw table
number list

ipfw {pipe queue} number config config-options
ipfw
[−s [field]] {pipe queue} {delete list show} [number ...]

ipfw nat number config config-options

ipfw [−cfnNqS] [

−p preproc [
preproc-flags
] ] pathname

DESCRIPTION

The ipfw utility is the user interface for controlling the ipfw(4) firewall and the dummynet(4) traffic shaper in FreeBSD.

An ipfw configuration, or ruleset, is made of a list of rules numbered from 1 to 65535. Packets are passed to ipfw from a number of different places in the protocol stack (depending on the source and destination of the packet, it is possible that ipfw is invoked multiple times on the same packet). The packet passed to the firewall is compared against each of the rules in the firewall ruleset. When a match is found, the action corresponding to the matching rule is performed.

Depending on the action and certain system settings, packets can be reinjected into the firewall at some rule after the matching one for further processing.

An ipfw ruleset always includes a default rule (numbered 65535) which cannot be modified or deleted, and matches all packets. The action associated with the default rule can be either deny or allow depending on how the kernel is configured.

If the ruleset includes one or more rules with the keep-state or limit option, then ipfw assumes a stateful behaviour, i.e., upon a match it will create dynamic rules matching the exact parameters (addresses and ports) of the matching packet.

These dynamic rules, which have a limited lifetime, are checked at the first occurrence of a check-state, keep-state or limit rule, and are typically used to open the firewall on-demand to legitimate traffic only. See the STATEFUL FIREWALL and EXAMPLES Sections below for more information on the stateful behaviour of ipfw.

All rules (including dynamic ones) have a few associated counters: a packet count, a byte count, a log count and a timestamp indicating the time of the last match. Counters can be displayed or reset with ipfw commands.

Rules can be added with the add command; deleted individually or in groups with the delete command, and globally (except those in set 31) with the flush command; displayed, optionally with the content of the counters, using the show and list commands. Finally, counters can be reset with the zero and resetlog commands.

Also, each rule belongs to one of 32 different sets , and there are ipfw commands to atomically manipulate sets, such as enable, disable, swap sets, move all rules in a set to another one, delete all rules in a set. These can be useful to install temporary configurations, or to test them. See Section SETS OF RULES for more information on sets.

The following options are available:

−a

While listing, show counter values. The show command just implies this option.

−b

Only show the action and the comment, not the body of a rule. Implies −c.

−c

When entering or showing rules, print them in compact form, i.e., without the optional "ip from any to any" string when this does not carry any additional information.

−d

While listing, show dynamic rules in addition to static ones.

−e

While listing, if the −d option was specified, also show expired dynamic rules.

−f

Do not ask for confirmation for commands that can cause problems if misused, i.e. flush. If there is no tty associated with the process, this is implied.

−n

Only check syntax of the command strings, without actually passing them to the kernel.

−N

Try to resolve addresses and service names in output.

−q

While adding, zeroing, resetlogging or flushing, be quiet about actions (implies −f). This is useful for adjusting rules by executing multiple ipfw commands in a script (e.g., ‘sh /etc/rc.firewall’), or by processing a file of many ipfw rules across a remote login session. It also stops a table add or delete from failing if the entry already exists or is not present. If a flush is performed in normal (verbose) mode (with the default kernel configuration), it prints a message. Because all rules are flushed, the message might not be delivered to the login session, causing the remote login session to be closed and the remainder of the ruleset to not be processed. Access to the console would then be required to recover.

−S

While listing rules, show the set each rule belongs to. If this flag is not specified, disabled rules will not be listed.

−s [field]

While listing pipes, sort according to one of the four counters (total or current packets or bytes).

−t

While listing, show last match timestamp (converted with ctime()).

−T

While listing, show last match timestamp (as seconds from the epoch). This form can be more convenient for postprocessing by scripts.

To ease configuration, rules can be put into a file which is processed using ipfw as shown in the last synopsis line. An absolute pathname must be used. The file will be read line by line and applied as arguments to the ipfw utility.

Optionally, a preprocessor can be specified using −p preproc where pathname is to be piped through. Useful preprocessors include cpp(1) and m4(1). If preproc does not start with a slash (‘/’) as its first character, the usual PATH name search is performed. Care should be taken with this in environments where not all file systems are mounted (yet) by the time ipfw is being run (e.g. when they are mounted over NFS). Once −p has been specified, any additional arguments as passed on to the preprocessor for interpretation. This allows for flexible configuration files (like conditionalizing them on the local hostname) and the use of macros to centralize frequently required arguments like IP addresses.

The ipfw pipe and queue commands are used to configure the traffic shaper, as shown in the TRAFFIC SHAPER (DUMMYNET) CONFIGURATION Section below.

If the world and the kernel get out of sync the ipfw ABI may break, preventing you from being able to add any rules. This can adversely effect the booting process. You can use ipfw disable firewall to temporarily disable the firewall to regain access to the network, allowing you to fix the problem.

PACKET FLOW

A packet is checked against the active ruleset in multiple places in the protocol stack, under control of several sysctl variables. These places and variables are shown below, and it is important to have this picture in mind in order to design a correct ruleset.

^ to upper layers V
| |
+----------->-----------+
^ V
[ip(6)_input] [ip(6)_output] net.inet(6).ip(6).fw.enable=1
| |
^ V
[ether_demux] [ether_output_frame] net.link.ether.ipfw=1
| |
+-->--[bdg_forward]-->--+ net.link.bridge.ipfw=1
^ V
| to devices |

As can be noted from the above picture, the number of times the same packet goes through the firewall can vary between 0 and 4 depending on packet source and destination, and system configuration.

Note that as packets flow through the stack, headers can be stripped or added to it, and so they may or may not be available for inspection. E.g., incoming packets will include the MAC header when ipfw is invoked from ether_demux(), but the same packets will have the MAC header stripped off when ipfw is invoked from ip_input() or ip6_input().

Also note that each packet is always checked against the complete ruleset, irrespective of the place where the check occurs, or the source of the packet. If a rule contains some match patterns or actions which are not valid for the place of invocation (e.g. trying to match a MAC header within ip_input or ip6_input ), the match pattern will not match, but a not operator in front of such patterns will cause the pattern to always match on those packets. It is thus the responsibility of the programmer, if necessary, to write a suitable ruleset to differentiate among the possible places. skipto rules can be useful here, as an example:

# packets from ether_demux or bdg_forward
ipfw add 10 skipto 1000 all from any to any layer2 in
# packets from ip_input
ipfw add 10 skipto 2000 all from any to any not layer2 in
# packets from ip_output
ipfw add 10 skipto 3000 all from any to any not layer2 out
# packets from ether_output_frame
ipfw add 10 skipto 4000 all from any to any layer2 out

(yes, at the moment there is no way to differentiate between ether_demux and bdg_forward).

SYNTAX

In general, each keyword or argument must be provided as a separate command line argument, with no leading or trailing spaces. Keywords are case-sensitive, whereas arguments may or may not be case-sensitive depending on their nature (e.g. uid’s are, hostnames are not).

In ipfw2 you can introduce spaces after commas ’,’ to make the line more readable. You can also put the entire command (including flags) into a single argument. E.g., the following forms are equivalent:

ipfw -q add deny src-ip 10.0.0.0/24,127.0.0.1/8
ipfw -q add deny src-ip 10.0.0.0/24, 127.0.0.1/8
ipfw "-q add deny src-ip 10.0.0.0/24, 127.0.0.1/8"

RULE FORMAT

The format of ipfw rules is the following:

[rule_number] [set set_number] [prob match_probability] action [log [logamount number]] [altq queue] [

{
tag 
untagnumber ] body

where the body of the rule specifies which information is used for filtering packets, among the following:

Layer-2 header fields

When available

IPv4 and IPv6 Protocol

TCP, UDP, ICMP, etc.

Source and dest. addresses and ports
Direction

See Section PACKET FLOW

Transmit and receive interface

By name or address

Misc. IP header fields

Version, type of service, datagram length, identification, fragment flag (non-zero IP offset), Time To Live

IP options
IPv6 Extension headers

Fragmentation, Hop-by-Hop options, Routing Headers, Source routing rthdr0, Mobile IPv6 rthdr2, IPSec options.

IPv6 Flow-ID
Misc. TCP header fields

TCP flags (SYN, FIN, ACK, RST, etc.), sequence number, acknowledgment number, window

TCP options
ICMP types

for ICMP packets

ICMP6 types

for ICMP6 packets

User/group ID

When the packet can be associated with a local socket.

Divert status

Whether a packet came from a divert socket (e.g., natd(8)).

Note that some of the above information, e.g. source MAC or IP addresses and TCP/UDP ports, could easily be spoofed, so filtering on those fields alone might not guarantee the desired results.

rule_number

Each rule is associated with a rule_number in the range 1..65535, with the latter reserved for the default rule. Rules are checked sequentially by rule number. Multiple rules can have the same number, in which case they are checked (and listed) according to the order in which they have been added. If a rule is entered without specifying a number, the kernel will assign one in such a way that the rule becomes the last one before the default rule. Automatic rule numbers are assigned by incrementing the last non-default rule number by the value of the sysctl variable net.inet.ip.fw.autoinc_step which defaults to 100. If this is not possible (e.g. because we would go beyond the maximum allowed rule number), the number of the last non-default value is used instead.

set set_number

Each rule is associated with a set_number in the range 0..31. Sets can be individually disabled and enabled, so this parameter is of fundamental importance for atomic ruleset manipulation. It can be also used to simplify deletion of groups of rules. If a rule is entered without specifying a set number, set 0 will be used.
Set 31 is special in that it cannot be disabled, and rules in set 31 are not deleted by the ipfw flush command (but you can delete them with the ipfw delete set 31 command). Set 31 is also used for the default rule.

prob match_probability

A match is only declared with the specified probability (floating point number between 0 and 1). This can be useful for a number of applications such as random packet drop or (in conjunction with dummynet(4)) to simulate the effect of multiple paths leading to out-of-order packet delivery.

Note: this condition is checked before any other condition, including ones such as keep-state or check-state which might have side effects.

log [logamount number]

When a packet matches a rule with the log keyword, a message will be logged to syslogd(8) with a LOG_SECURITY facility. The logging only occurs if the sysctl variable net.inet.ip.fw.verbose is set to 1 (which is the default when the kernel is compiled with IPFIREWALL_VERBOSE) and the number of packets logged so far for that particular rule does not exceed the logamount parameter. If no logamount is specified, the limit is taken from the sysctl variable net.inet.ip.fw.verbose_limit. In both cases, a value of 0 removes the logging limit.

Once the limit is reached, logging can be re-enabled by clearing the logging counter or the packet counter for that entry, see the resetlog command.

Note: logging is done after all other packet matching conditions have been successfully verified, and before performing the final action (accept, deny, etc.) on the packet.

tag number

When a packet matches a rule with the tag keyword, the numeric tag for the given number in the range 1..65534 will be attached to the packet. The tag acts as an internal marker (it is not sent out over the wire) that can be used to identify these packets later on. This can be used, for example, to provide trust between interfaces and to start doing policy-based filtering. A packet can have mutiple tags at the same time. Tags are "sticky", meaning once a tag is applied to a packet by a matching rule it exists until explicit removal. Tags are kept with the packet everywhere within the kernel, but are lost when packet leaves the kernel, for example, on transmitting packet out to the network or sending packet to a divert(4) socket.

To check for previously applied tags, use the tagged rule option. To delete previously applied tag, use the untag keyword.

Note: since tags are kept with the packet everywhere in kernelspace, they can be set and unset anywhere in kernel network subsystem (using mbuf_tags(9) facility), not only by means of ipfw(4) tag and untag keywords. For example, there can be a specialized netgraph(4) node doing traffic analyzing and tagging for later inspecting in firewall.

untag number

When a packet matches a rule with the untag keyword, the tag with the number number is searched among the tags attached to this packet and, if found, removed from it. Other tags bound to packet, if present, are left untouched.

altq queue

When a packet matches a rule with the altq keyword, the ALTQ identifier for the given queue (see altq(4)) will be attached. Note that this ALTQ tag is only meaningful for packets going "out" of IPFW, and not being rejected or going to divert sockets. Note that if there is insufficient memory at the time the packet is processed, it will not be tagged, so it is wise to make your ALTQ "default" queue policy account for this. If multiple altq rules match a single packet, only the first one adds the ALTQ classification tag. In doing so, traffic may be shaped by using count altq queue rules for classification early in the ruleset, then later applying the filtering decision. For example, check-state and keep-state rules may come later and provide the actual filtering decisions in addition to the fallback ALTQ tag.

You must run pfctl(8) to set up the queues before IPFW will be able to look them up by name, and if the ALTQ disciplines are rearranged, the rules in containing the queue identifiers in the kernel will likely have gone stale and need to be reloaded. Stale queue identifiers will probably result in misclassification.

All system ALTQ processing can be turned on or off via ipfw enable altq and ipfw disable altq. The usage of net.inet.ip.fw.one_pass is irrelevant to ALTQ traffic shaping, as the actual rule action is followed always after adding an ALTQ tag.

RULE ACTIONS
A rule can be associated with one of the following actions, which will be executed when the packet matches the body of the rule.

allow | accept | pass | permit

Allow packets that match rule. The search terminates.

check-state

Checks the packet against the dynamic ruleset. If a match is found, execute the action associated with the rule which generated this dynamic rule, otherwise move to the next rule.
Check-state
rules do not have a body. If no check-state rule is found, the dynamic ruleset is checked at the first keep-state or limit rule.

count

Update counters for all packets that match rule. The search continues with the next rule.

deny | drop

Discard packets that match this rule. The search terminates.

divert port

Divert packets that match this rule to the divert(4) socket bound to port port. The search terminates.

fwd | forward ipaddr | tablearg[,port]

Change the next-hop on matching packets to ipaddr, which can be an IP address or a host name. The next hop can also be supplied by the last table looked up for the packet by using the tablearg keyword instead of an explicit address. The search terminates if this rule matches.

If ipaddr is a local address, then matching packets will be forwarded to port (or the port number in the packet if one is not specified in the rule) on the local machine.
If ipaddr is not a local address, then the port number (if specified) is ignored, and the packet will be forwarded to the remote address, using the route as found in the local routing table for that IP.
A fwd rule will not match layer-2 packets (those received on ether_input, ether_output, or bridged).
The fwd action does not change the contents of the packet at all. In particular, the destination address remains unmodified, so packets forwarded to another system will usually be rejected by that system unless there is a matching rule on that system to capture them. For packets forwarded locally, the local address of the socket will be set to the original destination address of the packet. This makes the netstat(1) entry look rather weird but is intended for use with transparent proxy servers.

To enable fwd a custom kernel needs to be compiled with the option options IPFIREWALL_FORWARD.

nat nat_nr

Pass packet to a nat instance (for network address translation, address redirect, etc.): see the NETWORK ADDRESS TRANSLATION (NAT) Section for further information.

pipe pipe_nr

Pass packet to a dummynet(4) ‘‘pipe’’ (for bandwidth limitation, delay, etc.). See the TRAFFIC SHAPER (DUMMYNET) CONFIGURATION Section for further information. The search terminates; however, on exit from the pipe and if the sysctl(8) variable net.inet.ip.fw.one_pass is not set, the packet is passed again to the firewall code starting from the next rule.

queue queue_nr

Pass packet to a dummynet(4) ‘‘queue’’ (for bandwidth limitation using WF2Q+).

reject

(Deprecated). Synonym for unreach host.

reset

Discard packets that match this rule, and if the packet is a TCP packet, try to send a TCP reset (RST) notice. The search terminates.

reset6

Discard packets that match this rule, and if the packet is a TCP packet, try to send a TCP reset (RST) notice. The search terminates.

skipto number

Skip all subsequent rules numbered less than number. The search continues with the first rule numbered number or higher.

tee port

Send a copy of packets matching this rule to the divert(4) socket bound to port port. The search continues with the next rule.

unreach code

Discard packets that match this rule, and try to send an ICMP unreachable notice with code code, where code is a number from 0 to 255, or one of these aliases: net, host, protocol, port, needfrag, srcfail, net-unknown, host-unknown, isolated, net-prohib, host-prohib, tosnet, toshost, filter-prohib, host-precedence or precedence-cutoff. The search terminates.

unreach6 code

Discard packets that match this rule, and try to send an ICMPv6 unreachable notice with code code, where code is a number from 0, 1, 3 or 4, or one of these aliases: no-route, admin-prohib, address or port. The search terminates.

netgraph cookie

Divert packet into netgraph with given cookie. The search terminates. If packet is later returned from netgraph it is either accepted or continues with the next rule, depending on net.inet.ip.fw.one_pass sysctl variable.

ngtee cookie

A copy of packet is diverted into netgraph, original packet is either accepted or continues with the next rule, depending on net.inet.ip.fw.one_pass sysctl variable. See ng_ipfw(4) for more information on netgraph and ngtee actions.

RULE BODY
The body of a rule contains zero or more patterns (such as specific source and destination addresses or ports, protocol options, incoming or outgoing interfaces, etc.) that the packet must match in order to be recognised. In general, the patterns are connected by (implicit) and operators -- i.e., all must match in order for the rule to match. Individual patterns can be prefixed by the not operator to reverse the result of the match, as in

ipfw add 100 allow ip from not 1.2.3.4 to any

Additionally, sets of alternative match patterns (or-blocks) can be constructed by putting the patterns in lists enclosed between parentheses ( ) or braces { }, and using the or operator as follows:

ipfw add 100 allow ip from { x or not y or z } to any

Only one level of parentheses is allowed. Beware that most shells have special meanings for parentheses or braces, so it is advisable to put a backslash \ in front of them to prevent such interpretations.

The body of a rule must in general include a source and destination address specifier. The keyword any can be used in various places to specify that the content of a required field is irrelevant.

The rule body has the following format:

[proto from src to dst] [options]

The first part (proto from src to dst) is for backward compatibility with earlier versions of FreeBSD. In modern FreeBSD any match pattern (including MAC headers, IP protocols, addresses and ports) can be specified in the options section.

Rule fields have the following meaning:

proto: protocol | { protocol or ... }

protocol: [
not
] protocol-name | protocol-number

An IP protocol specified by number or name (for a complete list see /etc/protocols), or one of the following keywords:

ip4 | ipv4

Matches IPv4 packets.

ip6 | ipv6

Matches IPv6 packets.

ip | all

Matches any packet.

The ipv6 in proto option will be treated as inner protocol. And, the ipv4 is not available in proto option.

The { protocol or ... } format (an or-block) is provided for convenience only but its use is deprecated.

src and dst: {
addr
| { addr or ... }} [[
not
] ports]

An address (or a list, see below) optionally followed by ports specifiers.

The second format (or-block with multiple addresses) is provided for convenience only and its use is discouraged.

addr: [
not
] {
any
| me | me6 | table(number[,value]) | addr-list | addr-set}

any

matches any IP address.

me

matches any IP address configured on an interface in the system.

me6

matches any IPv6 address configured on an interface in the system. The address list is evaluated at the time the packet is analysed.

table(number[,value])

Matches any IPv4 address for which an entry exists in the lookup table number. If an optional 32-bit unsigned value is also specified, an entry will match only if it has this value. See the LOOKUP TABLES section below for more information on lookup tables.

addr-list: ip-addr[,addr-list]

ip-addr:

A host or subnet address specified in one of the following ways:

numeric-ip | hostname

Matches a single IPv4 address, specified as dotted-quad or a hostname. Hostnames are resolved at the time the rule is added to the firewall list.

addr/masklen

Matches all addresses with base addr (specified as an IP address, a network number, or a hostname) and mask width of masklen bits. As an example, 1.2.3.4/25 or 1.2.3.0/25 will match all IP numbers from 1.2.3.0 to 1.2.3.127 .

addr:mask

Matches all addresses with base addr (specified as an IP address, a network number, or a hostname) and the mask of mask, specified as a dotted quad. As an example, 1.2.3.4:255.0.255.0 or 1.0.3.0:255.0.255.0 will match 1.*.3.*. This form is advised only for non-contiguous masks. It is better to resort to the addr/masklen format for contiguous masks, which is more compact and less error-prone.

addr-set: addr[
/masklen]{list}

list: {
num
| num-num}[,list]

Matches all addresses with base address addr (specified as an IP address, a network number, or a hostname) and whose last byte is in the list between braces { } . Note that there must be no spaces between braces and numbers (spaces after commas are allowed). Elements of the list can be specified as single entries or ranges. The masklen field is used to limit the size of the set of addresses, and can have any value between 24 and 32. If not specified, it will be assumed as 24.
This format is particularly useful to handle sparse address sets within a single rule. Because the matching occurs using a bitmask, it takes constant time and dramatically reduces the complexity of rulesets.
As an example, an address specified as 1.2.3.4/24{128,35-55,89} or 1.2.3.0/24{128,35-55,89} will match the following IP addresses:
1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .

addr6-list: ip6-addr[,addr6-list]

ip6-addr:

A host or subnet specified one of the following ways:

numeric-ip | hostname

Matches a single IPv6 address as allowed by inet_pton(3) or a hostname. Hostnames are resolved at the time the rule is added to the firewall list.

addr/masklen

Matches all IPv6 addresses with base addr (specified as allowed by inet_pton or a hostname) and mask width of masklen bits.

No support for sets of IPv6 addresses is provided because IPv6 addresses are typically random past the initial prefix.

ports: {
port
| port-port}[,ports]

For protocols which support port numbers (such as TCP and UDP), optional ports may be specified as one or more ports or port ranges, separated by commas but no spaces, and an optional not operator. The ‘-’ notation specifies a range of ports (including boundaries).

Service names (from /etc/services) may be used instead of numeric port values. The length of the port list is limited to 30 ports or ranges, though one can specify larger ranges by using an or-block in the options section of the rule.

A backslash (‘\’) can be used to escape the dash (‘-’) character in a service name (from a shell, the backslash must be typed twice to avoid the shell itself interpreting it as an escape character).

ipfw add count tcp from any ftp\\-data-ftp to any

Fragmented packets which have a non-zero offset (i.e., not the first fragment) will never match a rule which has one or more port specifications. See the frag option for details on matching fragmented packets.

RULE OPTIONS (MATCH PATTERNS)
Additional match patterns can be used within rules. Zero or more of these so-called options can be present in a rule, optionally prefixed by the not operand, and possibly grouped into or-blocks.

The following match patterns can be used (listed in alphabetical order):

// this is a comment.

Inserts the specified text as a comment in the rule. Everything following // is considered as a comment and stored in the rule. You can have comment-only rules, which are listed as having a count action followed by the comment.

bridged

Alias for layer2.

diverted

Matches only packets generated by a divert socket.

diverted-loopback

Matches only packets coming from a divert socket back into the IP stack input for delivery.

diverted-output

Matches only packets going from a divert socket back outward to the IP stack output for delivery.

dst-ip ip-address

Matches IPv4 packets whose destination IP is one of the address(es) specified as argument.

{
dst-ip6
| dst-ipv6} ip6-address

Matches IPv6 packets whose destination IP is one of the address(es) specified as argument.

dst-port ports

Matches IP packets whose destination port is one of the port(s) specified as argument.

established

Matches TCP packets that have the RST or ACK bits set.

ext6hdr header

Matches IPv6 packets containing the extended header given by header. Supported headers are:

Fragment, (frag), Hop-to-hop options (hopopt), any type of Routing Header (route), Source routing Routing Header Type 0 (rthdr0), Mobile IPv6 Routing Header Type 2 (rthdr2), Destination options (dstopt), IPSec authentication headers (ah), and IPSec encapsulated security payload headers (esp).

flow-id labels

Matches IPv6 packets containing any of the flow labels given in labels. labels is a comma seperate list of numeric flow labels.

frag

Matches packets that are fragments and not the first fragment of an IP datagram. Note that these packets will not have the next protocol header (e.g. TCP, UDP) so options that look into these headers cannot match.

gid group

Matches all TCP or UDP packets sent by or received for a group. A group may be specified by name or number.

jail prisonID

Matches all TCP or UDP packets sent by or received for the jail whos prison ID is prisonID.

icmptypes types

Matches ICMP packets whose ICMP type is in the list types. The list may be specified as any combination of individual types (numeric) separated by commas. Ranges are not allowed. The supported ICMP types are:

echo reply (0), destination unreachable (3), source quench (4), redirect (5), echo request (8), router advertisement (9), router solicitation (10), time-to-live exceeded (11), IP header bad (12), timestamp request (13), timestamp reply (14), information request (15), information reply (16), address mask request (17) and address mask reply (18).

icmp6types types

Matches ICMP6 packets whose ICMP6 type is in the list of types. The list may be specified as any combination of individual types (numeric) separated by commas. Ranges are not allowed.

in | out

Matches incoming or outgoing packets, respectively. in and out are mutually exclusive (in fact, out is implemented as not in).

ipid id-list

Matches IPv4 packets whose ip_id field has value included in id-list, which is either a single value or a list of values or ranges specified in the same way as ports.

iplen len-list

Matches IP packets whose total length, including header and data, is in the set len-list, which is either a single value or a list of values or ranges specified in the same way as ports.

ipoptions spec

Matches packets whose IPv4 header contains the comma separated list of options specified in spec. The supported IP options are:

ssrr (strict source route), lsrr (loose source route), rr (record packet route) and ts (timestamp). The absence of a particular option may be denoted with a ‘!’.

ipprecedence precedence

Matches IPv4 packets whose precedence field is equal to precedence.

ipsec

Matches packets that have IPSEC history associated with them (i.e., the packet comes encapsulated in IPSEC, the kernel has IPSEC support and IPSEC_FILTERTUNNEL option, and can correctly decapsulate it).

Note that specifying ipsec is different from specifying proto ipsec as the latter will only look at the specific IP protocol field, irrespective of IPSEC kernel support and the validity of the IPSEC data.

Further note that this flag is silently ignored in kernels without IPSEC support. It does not affect rule processing when given and the rules are handled as if with no ipsec flag.

iptos spec

Matches IPv4 packets whose tos field contains the comma separated list of service types specified in spec. The supported IP types of service are:

lowdelay (IPTOS_LOWDELAY), throughput (IPTOS_THROUGHPUT), reliability (IPTOS_RELIABILITY), mincost (IPTOS_MINCOST), congestion (IPTOS_CE). The absence of a particular type may be denoted with a ‘!’.

ipttl ttl-list

Matches IPv4 packets whose time to live is included in ttl-list, which is either a single value or a list of values or ranges specified in the same way as ports.

ipversion ver

Matches IP packets whose IP version field is ver.

keep-state

Upon a match, the firewall will create a dynamic rule, whose default behaviour is to match bidirectional traffic between source and destination IP/port using the same protocol. The rule has a limited lifetime (controlled by a set of sysctl(8) variables), and the lifetime is refreshed every time a matching packet is found.

layer2

Matches only layer2 packets, i.e., those passed to ipfw from ether_demux() and ether_output_frame().

limit {
src-addr
| src-port | dst-addr | dst-port} N

The firewall will only allow N connections with the same set of parameters as specified in the rule. One or more of source and destination addresses and ports can be specified. Currently, only IPv4 flows are supported.

{ MAC | mac } dst-mac src-mac

Match packets with a given dst-mac and src-mac addresses, specified as the any keyword (matching any MAC address), or six groups of hex digits separated by colons, and optionally followed by a mask indicating the significant bits. The mask may be specified using either of the following methods:

1.

A slash (/) followed by the number of significant bits. For example, an address with 33 significant bits could be specified as:

MAC 10:20:30:40:50:60/33 any

2.

An ampersand (&) followed by a bitmask specified as six groups of hex digits separated by colons. For example, an address in which the last 16 bits are significant could be specified as:

MAC 10:20:30:40:50:60&00:00:00:00:ff:ff any

Note that the ampersand character has a special meaning in many shells and should generally be escaped.

Note that the order of MAC addresses (destination first, source second) is the same as on the wire, but the opposite of the one used for IP addresses.

mac-type mac-type

Matches packets whose Ethernet Type field corresponds to one of those specified as argument. mac-type is specified in the same way as port numbers (i.e., one or more comma-separated single values or ranges). You can use symbolic names for known values such as vlan, ipv4, ipv6. Values can be entered as decimal or hexadecimal (if prefixed by 0x), and they are always printed as hexadecimal (unless the -N option is used, in which case symbolic resolution will be attempted).

proto protocol

Matches packets with the corresponding IP protocol.

recv | xmit | via {ifX | if* | ipno | any}

Matches packets received, transmitted or going through, respectively, the interface specified by exact name (ifX), by device name (if*), by IP address, or through some interface.

The via keyword causes the interface to always be checked. If recv or xmit is used instead of via, then only the receive or transmit interface (respectively) is checked. By specifying both, it is possible to match packets based on both receive and transmit interface, e.g.:

ipfw add deny ip from any to any out recv ed0 xmit ed1

The recv interface can be tested on either incoming or outgoing packets, while the xmit interface can only be tested on outgoing packets. So out is required (and in is invalid) whenever xmit is used.

A packet may not have a receive or transmit interface: packets originating from the local host have no receive interface, while packets destined for the local host have no transmit interface.

setup

Matches TCP packets that have the SYN bit set but no ACK bit. This is the short form of ‘‘tcpflags syn,!ack’’.

src-ip ip-address

Matches IPv4 packets whose source IP is one of the address(es) specified as an argument.

src-ip6 ip6-address

Matches IPv6 packets whose source IP is one of the address(es) specified as an argument.

src-port ports

Matches IP packets whose source port is one of the port(s) specified as argument.

tagged tag-list

Matches packets whose tags are included in tag-list, which is either a single value or a list of values or ranges specified in the same way as ports. Tags can be applied to the packet using tag rule action parameter (see it’s description for details on tags).

tcpack ack

TCP packets only. Match if the TCP header acknowledgment number field is set to ack.

tcpdatalen tcpdatalen-list

Matches TCP packets whose length of TCP data is tcpdatalen-list, which is either a single value or a list of values or ranges specified in the same way as ports.

tcpflags spec

TCP packets only. Match if the TCP header contains the comma separated list of flags specified in spec. The supported TCP flags are:

fin, syn, rst, psh, ack and urg. The absence of a particular flag may be denoted with a ‘!’. A rule which contains a tcpflags specification can never match a fragmented packet which has a non-zero offset. See the frag option for details on matching fragmented packets.

tcpseq seq

TCP packets only. Match if the TCP header sequence number field is set to seq.

tcpwin win

TCP packets only. Match if the TCP header window field is set to win.

tcpoptions spec

TCP packets only. Match if the TCP header contains the comma separated list of options specified in spec. The supported TCP options are:

mss (maximum segment size), window (tcp window advertisement), sack (selective ack), ts (rfc1323 timestamp) and cc (rfc1644 t/tcp connection count). The absence of a particular option may be denoted with a ‘!’.

uid user

Match all TCP or UDP packets sent by or received for a user. A user may be matched by name or identification number.

verrevpath

For incoming packets, a routing table lookup is done on the packet’s source address. If the interface on which the packet entered the system matches the outgoing interface for the route, the packet matches. If the interfaces do not match up, the packet does not match. All outgoing packets or packets with no incoming interface match.

The name and functionality of the option is intentionally similar to the Cisco IOS command:

ip verify unicast reverse-path

This option can be used to make anti-spoofing rules to reject all packets with source addresses not from this interface. See also the option antispoof.

versrcreach

For incoming packets, a routing table lookup is done on the packet’s source address. If a route to the source address exists, but not the default route or a blackhole/reject route, the packet matches. Otherwise, the packet does not match. All outgoing packets match.

The name and functionality of the option is intentionally similar to the Cisco IOS command:

ip verify unicast source reachable-via any

This option can be used to make anti-spoofing rules to reject all packets whose source address is unreachable.

antispoof

For incoming packets, the packet’s source address is checked if it belongs to a directly connected network. If the network is directly connected, then the interface the packet came on in is compared to the interface the network is connected to. When incoming interface and directly connected interface are not the same, the packet does not match. Otherwise, the packet does match. All outgoing packets match.

This option can be used to make anti-spoofing rules to reject all packets that pretend to be from a directly connected network but do not come in through that interface. This option is similar to but more restricted than verrevpath because it engages only on packets with source addresses of directly connected networks instead of all source addresses.

LOOKUP TABLES

Lookup tables are useful to handle large sparse address sets, typically from a hundred to several thousands of entries. There may be up to 128 different lookup tables, numbered 0 to 127.

Each entry is represented by an addr[/masklen] and will match all addresses with base addr (specified as an IP address or a hostname) and mask width of masklen bits. If masklen is not specified, it defaults to 32. When looking up an IP address in a table, the most specific entry will match. Associated with each entry is a 32-bit unsigned value, which can optionally be checked by a rule matching code. When adding an entry, if value is not specified, it defaults to 0.

An entry can be added to a table (add), removed from a table (delete), a table can be examined (list) or flushed (flush).

Internally, each table is stored in a Radix tree, the same way as the routing table (see route(4)).

Lookup tables currently support IPv4 addresses only.

The tablearg feature provides the ability to use a value, looked up in the table, as the argument for a rule action, action parameter or rule option. This can significantly reduce number of rules in some configurations. The tablearg argument can be used with the following actions: pipe, queue, divert, tee, netgraph, ngtee, fwd action parameters: tag, untag, rule options: limit, tagged.

When used with fwd it is possible to supply table entries with values that are in the form of IP addresses or hostnames. See the EXAMPLES Section for example usage of tables and the tablearg keyword.

SETS OF RULES

Each rule belongs to one of 32 different sets , numbered 0 to 31. Set 31 is reserved for the default rule.

By default, rules are put in set 0, unless you use the set N attribute when entering a new rule. Sets can be individually and atomically enabled or disabled, so this mechanism permits an easy way to store multiple configurations of the firewall and quickly (and atomically) switch between them. The command to enable/disable sets is

ipfw set [

disable number ... ] [enable number ...]

where multiple enable or disable sections can be specified. Command execution is atomic on all the sets specified in the command. By default, all sets are enabled.

When you disable a set, its rules behave as if they do not exist in the firewall configuration, with only one exception:

dynamic rules created from a rule before it had been disabled will still be active until they expire. In order to delete dynamic rules you have to explicitly delete the parent rule which generated them.

The set number of rules can be changed with the command

ipfw set move {rule rule-number | old-set} to new-set

Also, you can atomically swap two rulesets with the command

ipfw set swap first-set second-set

See the EXAMPLES Section on some possible uses of sets of rules.

STATEFUL FIREWALL

Stateful operation is a way for the firewall to dynamically create rules for specific flows when packets that match a given pattern are detected. Support for stateful operation comes through the check-state, keep-state and limit options of rules.

Dynamic rules are created when a packet matches a keep-state or limit rule, causing the creation of a dynamic rule which will match all and only packets with a given protocol between a src-ip/src-port dst-ip/dst-port pair of addresses (src and dst are used here only to denote the initial match addresses, but they are completely equivalent afterwards). Dynamic rules will be checked at the first check-state, keep-state or limit occurrence, and the action performed upon a match will be the same as in the parent rule.

Note that no additional attributes other than protocol and IP addresses and ports are checked on dynamic rules.

The typical use of dynamic rules is to keep a closed firewall configuration, but let the first TCP SYN packet from the inside network install a dynamic rule for the flow so that packets belonging to that session will be allowed through the firewall:

ipfw add check-state
ipfw add allow tcp from my-subnet to any setup keep-state
ipfw add deny tcp from any to any

A similar approach can be used for UDP, where an UDP packet coming from the inside will install a dynamic rule to let the response through the firewall:

ipfw add check-state
ipfw add allow udp from my-subnet to any keep-state
ipfw add deny udp from any to any

Dynamic rules expire after some time, which depends on the status of the flow and the setting of some sysctl variables. See Section SYSCTL VARIABLES for more details. For TCP sessions, dynamic rules can be instructed to periodically send keepalive packets to refresh the state of the rule when it is about to expire.

See Section EXAMPLES for more examples on how to use dynamic rules.

TRAFFIC SHAPER (DUMMYNET) CONFIGURATION

ipfw is also the user interface for the dummynet(4) traffic shaper.

dummynet operates by first using the firewall to classify packets and divide them into flows, using any match pattern that can be used in ipfw rules. Depending on local policies, a flow can contain packets for a single TCP connection, or from/to a given host, or entire subnet, or a protocol type, etc.

Packets belonging to the same flow are then passed to either of two different objects, which implement the traffic regulation:

pipe

A pipe emulates a link with given bandwidth, propagation delay, queue size and packet loss rate. Packets are queued in front of the pipe as they come out from the classifier, and then transferred to the pipe according to the pipe’s parameters.

queue

A queue is an abstraction used to implement the WF2Q+ (Worst-case Fair Weighted Fair Queueing) policy, which is an efficient variant of the WFQ policy.
The queue associates a weight and a reference pipe to each flow, and then all backlogged (i.e., with packets queued) flows linked to the same pipe share the pipe’s bandwidth proportionally to their weights. Note that weights are not priorities; a flow with a lower weight is still guaranteed to get its fraction of the bandwidth even if a flow with a higher weight is permanently backlogged.

In practice, pipes can be used to set hard limits to the bandwidth that a flow can use, whereas queues can be used to determine how different flow share the available bandwidth.

The pipe and queue configuration commands are the following:

pipe number config pipe-configuration

queue number config queue-configuration

The following parameters can be configured for a pipe:

bw bandwidth | device

Bandwidth, measured in [K|M]{bit/s|Byte/s}.

A value of 0 (default) means unlimited bandwidth. The unit must immediately follow the number, as in

ipfw pipe 1 config bw 300Kbit/s

If a device name is specified instead of a numeric value, as in

ipfw pipe 1 config bw tun0

then the transmit clock is supplied by the specified device. At the moment only the tun(4) device supports this functionality, for use in conjunction with ppp(8).

delay ms-delay

Propagation delay, measured in milliseconds. The value is rounded to the next multiple of the clock tick (typically 10ms, but it is a good practice to run kernels with ‘‘options HZ=1000’’ to reduce the granularity to 1ms or less). Default value is 0, meaning no delay.

The following parameters can be configured for a queue:

pipe pipe_nr

Connects a queue to the specified pipe. Multiple queues (with the same or different weights) can be connected to the same pipe, which specifies the aggregate rate for the set of queues.

weight weight

Specifies the weight to be used for flows matching this queue. The weight must be in the range 1..100, and defaults to 1.

Finally, the following parameters can be configured for both pipes and queues:

buckets hash-table-size

Specifies the size of the hash table used for storing the various queues. Default value is 64 controlled by the sysctl(8) variable net.inet.ip.dummynet.hash_size, allowed range is 16 to 65536.

mask mask-specifier

Packets sent to a given pipe or queue by an ipfw rule can be further classified into multiple flows, each of which is then sent to a different dynamic pipe or queue. A flow identifier is constructed by masking the IP addresses, ports and protocol types as specified with the mask options in the configuration of the pipe or queue. For each different flow identifier, a new pipe or queue is created with the same parameters as the original object, and matching packets are sent to it.

Thus, when dynamic pipes are used, each flow will get the same bandwidth as defined by the pipe, whereas when dynamic queues are used, each flow will share the parent’s pipe bandwidth evenly with other flows generated by the same queue (note that other queues with different weights might be connected to the same pipe).
Available mask specifiers are a combination of one or more of the following:

dst-ip mask, dst-ip6 mask, src-ip mask, src-ip6 mask, dst-port mask, src-port mask, flow-id mask, proto mask or all,

where the latter means all bits in all fields are significant.

noerror

When a packet is dropped by a dummynet queue or pipe, the error is normally reported to the caller routine in the kernel, in the same way as it happens when a device queue fills up. Setting this option reports the packet as successfully delivered, which can be needed for some experimental setups where you want to simulate loss or congestion at a remote router.

plr packet-loss-rate

Packet loss rate. Argument packet-loss-rate is a floating-point number between 0 and 1, with 0 meaning no loss, 1 meaning 100% loss. The loss rate is internally represented on 31 bits.

queue {slots | sizeKbytes}

Queue size, in slots or KBytes. Default value is 50 slots, which is the typical queue size for Ethernet devices. Note that for slow speed links you should keep the queue size short or your traffic might be affected by a significant queueing delay. E.g., 50 max-sized ethernet packets (1500 bytes) mean 600Kbit or 20s of queue on a 30Kbit/s pipe. Even worse effects can result if you get packets from an interface with a much larger MTU, e.g. the loopback interface with its 16KB packets.

red | gred w_q/min_th/max_th/max_p

Make use of the RED (Random Early Detection) queue management algorithm. w_q and max_p are floating point numbers between 0 and 1 (0 not included), while min_th and max_th are integer numbers specifying thresholds for queue management (thresholds are computed in bytes if the queue has been defined in bytes, in slots otherwise). The dummynet(4) also supports the gentle RED variant (gred). Three sysctl(8) variables can be used to control the RED behaviour:

net.inet.ip.dummynet.red_lookup_depth

specifies the accuracy in computing the average queue when the link is idle (defaults to 256, must be greater than zero)

net.inet.ip.dummynet.red_avg_pkt_size

specifies the expected average packet size (defaults to 512, must be greater than zero)

net.inet.ip.dummynet.red_max_pkt_size

specifies the expected maximum packet size, only used when queue thresholds are in bytes (defaults to 1500, must be greater than zero).

When used with IPv6 data, dummynet currently has several limitations. Information necessary to route link-local packets to an interface is not avalable after processing by dummynet so those packets are dropped in the output path. Care should be taken to insure that link-local packets are not passed to dummynet.

CHECKLIST

Here are some important points to consider when designing your rules:

Remember that you filter both packets going in and out. Most connections need packets going in both directions.

Remember to test very carefully. It is a good idea to be near the console when doing this. If you cannot be near the console, use an auto-recovery script such as the one in /usr/share/examples/ipfw/change_rules.sh.

Do not forget the loopback interface.

FINE POINTS

There are circumstances where fragmented datagrams are unconditionally dropped. TCP packets are dropped if they do not contain at least 20 bytes of TCP header, UDP packets are dropped if they do not contain a full 8 byte UDP header, and ICMP packets are dropped if they do not contain 4 bytes of ICMP header, enough to specify the ICMP type, code, and checksum. These packets are simply logged as ‘‘pullup failed’’ since there may not be enough good data in the packet to produce a meaningful log entry.

Another type of packet is unconditionally dropped, a TCP packet with a fragment offset of one. This is a valid packet, but it only has one use, to try to circumvent firewalls. When logging is enabled, these packets are reported as being dropped by rule -1.

If you are logged in over a network, loading the kld(4) version of ipfw is probably not as straightforward as you would think. I recommend the following command line:

kldload ipfw && \
ipfw add 32000 allow ip from any to any

Along the same lines, doing an

ipfw flush

in similar surroundings is also a bad idea.

The ipfw filter list may not be modified if the system security level is set to 3 or higher (see init(8) for information on system security levels).

PACKET DIVERSION

A divert(4) socket bound to the specified port will receive all packets diverted to that port. If no socket is bound to the destination port, or if the divert module is not loaded, or if the kernel was not compiled with divert socket support, the packets are dropped.

NETWORK ADDRESS TRANSLATION (NAT)

The nat configuration command is the following:

nat nat_number config nat-configuration

The following parameters can be configured:

ip ip_address

Define an ip address to use for aliasing.

if nic

Use ip addres of NIC for aliasing, dynamically changing it if NIC’s ip address change.

log

Enable logging on this nat instance.

deny_in

Deny any incoming connection from outside world.

same_ports

Try to leave the alias port numbers unchanged from the actual local port numbers.

unreg_only

Traffic on the local network not originating from an unregistered address spaces will be ignored.

reset

Reset table of the packet aliasing engine on address change.

reverse

Reverse the way libalias handles aliasing.

proxy_only

Obey transparent proxy rules only, packet aliasing is not performed.

To let the packet continue after being (de)aliased, set the sysctl variable net.inet.ip.fw.one_pass to 0. For more information about aliasing modes, refer to libalias(3) See Section EXAMPLES for some examples about nat usage.

REDIRECT AND LSNAT SUPPORT IN IPFW

Redirect and LSNAT support follow closely the syntax used in natd(8) See Section EXAMPLES for some examples on how to do redirect and lsnat.

SYSCTL VARIABLES

A set of sysctl(8) variables controls the behaviour of the firewall and associated modules (dummynet, bridge). These are shown below together with their default value (but always check with the sysctl(8) command what value is actually in use) and meaning:

net.inet.ip.dummynet.expire: 1

Lazily delete dynamic pipes/queue once they have no pending traffic. You can disable this by setting the variable to 0, in which case the pipes/queues will only be deleted when the threshold is reached.

net.inet.ip.dummynet.hash_size: 64

Default size of the hash table used for dynamic pipes/queues. This value is used when no buckets option is specified when configuring a pipe/queue.

net.inet.ip.dummynet.max_chain_len: 16

Target value for the maximum number of pipes/queues in a hash bucket. The product max_chain_len*hash_size is used to determine the threshold over which empty pipes/queues will be expired even when net.inet.ip.dummynet.expire=0.

net.inet.ip.dummynet.red_lookup_depth: 256

net.inet.ip.dummynet.red_avg_pkt_size: 512

net.inet.ip.dummynet.red_max_pkt_size: 1500

Parameters used in the computations of the drop probability for the RED algorithm.

net.inet.ip.fw.autoinc_step: 100

Delta between rule numbers when auto-generating them. The value must be in the range 1..1000.

net.inet.ip.fw.curr_dyn_buckets: net.inet.ip.fw.dyn_buckets

The current number of buckets in the hash table for dynamic rules (readonly).

net.inet.ip.fw.debug: 1

Controls debugging messages produced by ipfw.

net.inet.ip.fw.dyn_buckets: 256

The number of buckets in the hash table for dynamic rules. Must be a power of 2, up to 65536. It only takes effect when all dynamic rules have expired, so you are advised to use a flush command to make sure that the hash table is resized.

net.inet.ip.fw.dyn_count: 3

Current number of dynamic rules (read-only).

net.inet.ip.fw.dyn_keepalive: 1

Enables generation of keepalive packets for keep-state rules on TCP sessions. A keepalive is generated to both sides of the connection every 5 seconds for the last 20 seconds of the lifetime of the rule.

net.inet.ip.fw.dyn_max: 8192

Maximum number of dynamic rules. When you hit this limit, no more dynamic rules can be installed until old ones expire.

net.inet.ip.fw.dyn_ack_lifetime: 300

net.inet.ip.fw.dyn_syn_lifetime: 20

net.inet.ip.fw.dyn_fin_lifetime: 1

net.inet.ip.fw.dyn_rst_lifetime: 1

net.inet.ip.fw.dyn_udp_lifetime: 5

net.inet.ip.fw.dyn_short_lifetime: 30

These variables control the lifetime, in seconds, of dynamic rules. Upon the initial SYN exchange the lifetime is kept short, then increased after both SYN have been seen, then decreased again during the final FIN exchange or when a RST is received. Both dyn_fin_lifetime and dyn_rst_lifetime must be strictly lower than 5 seconds, the period of repetition of keepalives. The firewall enforces that.

net.inet.ip.fw.enable: 1

Enables the firewall. Setting this variable to 0 lets you run your machine without firewall even if compiled in.

net.inet6.ip6.fw.enable: 1

provides the same functionality as above for the IPv6 case.

net.inet.ip.fw.one_pass: 1

When set, the packet exiting from the dummynet(4) pipe or from ng_ipfw(4) node is not passed though the firewall again. Otherwise, after an action, the packet is reinjected into the firewall at the next rule.

net.inet.ip.fw.verbose: 1

Enables verbose messages.

net.inet.ip.fw.verbose_limit: 0

Limits the number of messages produced by a verbose firewall.

net.inet6.ip6.fw.deny_unknown_exthdrs: 1

If enabled packets with unknown IPv6 Extension Headers will be denied.

net.link.ether.ipfw: 0

Controls whether layer-2 packets are passed to ipfw. Default is no.

net.link.bridge.ipfw: 0

Controls whether bridged packets are passed to ipfw. Default is no.

EXAMPLES

There are far too many possible uses of ipfw so this Section will only give a small set of examples.

BASIC PACKET FILTERING
This command adds an entry which denies all tcp packets from cracker.evil.org to the telnet port of wolf.tambov.su from being forwarded by the host:

ipfw add deny tcp from cracker.evil.org to wolf.tambov.su telnet

This one disallows any connection from the entire cracker’s network to my host:

ipfw add deny ip from 123.45.67.0/24 to my.host.org

A first and efficient way to limit access (not using dynamic rules) is the use of the following rules:

ipfw add allow tcp from any to any established
ipfw add allow tcp from net1 portlist1 to net2 portlist2 setup
ipfw add allow tcp from net3 portlist3 to net3 portlist3 setup
...
ipfw add deny tcp from any to any

The first rule will be a quick match for normal TCP packets, but it will not match the initial SYN packet, which will be matched by the setup rules only for selected source/destination pairs. All other SYN packets will be rejected by the final deny rule.

If you administer one or more subnets, you can take advantage of the address sets and or-blocks and write extremely compact rulesets which selectively enable services to blocks of clients, as below:

goodguys="{ 10.1.2.0/24{20,35,66,18} or 10.2.3.0/28{6,3,11} }"
badguys="10.1.2.0/24{8,38,60}"
ipfw add allow ip from ${goodguys} to any
ipfw add deny ip from ${badguys} to any
... normal policies ...

The verrevpath option could be used to do automated anti-spoofing by adding the following to the top of a ruleset:

ipfw add deny ip from any to any not verrevpath in

This rule drops all incoming packets that appear to be coming to the system on the wrong interface. For example, a packet with a source address belonging to a host on a protected internal network would be dropped if it tried to enter the system from an external interface.

The antispoof option could be used to do similar but more restricted anti-spoofing by adding the following to the top of a ruleset:

ipfw add deny ip from any to any not antispoof in

This rule drops all incoming packets that appear to be coming from another directly connected system but on the wrong interface. For example, a packet with a source address of 192.168.0.0/24 , configured on fxp0 , but coming in on fxp1 would be dropped.

DYNAMIC RULES
In order to protect a site from flood attacks involving fake TCP packets, it is safer to use dynamic rules:

ipfw add check-state
ipfw add deny tcp from any to any established
ipfw add allow tcp from my-net to any setup keep-state

This will let the firewall install dynamic rules only for those connection which start with a regular SYN packet coming from the inside of our network. Dynamic rules are checked when encountering the first check-state or keep-state rule. A check-state rule should usually be placed near the beginning of the ruleset to minimize the amount of work scanning the ruleset. Your mileage may vary.

To limit the number of connections a user can open you can use the following type of rules:

ipfw add allow tcp from my-net/24 to any setup limit src-addr 10
ipfw add allow tcp from any to me setup limit src-addr 4

The former (assuming it runs on a gateway) will allow each host on a /24 network to open at most 10 TCP connections. The latter can be placed on a server to make sure that a single client does not use more than 4 simultaneous connections.

BEWARE: stateful rules can be subject to denial-of-service attacks by a SYN-flood which opens a huge number of dynamic rules. The effects of such attacks can be partially limited by acting on a set of sysctl(8) variables which control the operation of the firewall.

Here is a good usage of the list command to see accounting records and timestamp information:

ipfw -at list

or in short form without timestamps:

ipfw -a list

which is equivalent to:

ipfw show

Next rule diverts all incoming packets from 192.168.2.0/24 to divert port 5000:

ipfw divert 5000 ip from 192.168.2.0/24 to any in

TRAFFIC SHAPING
The following rules show some of the applications of ipfw and dummynet(4) for simulations and the like.

This rule drops random incoming packets with a probability of 5%:

ipfw add prob 0.05 deny ip from any to any in

A similar effect can be achieved making use of dummynet pipes:

ipfw add pipe 10 ip from any to any
ipfw pipe 10 config plr 0.05

We can use pipes to artificially limit bandwidth, e.g. on a machine acting as a router, if we want to limit traffic from local clients on 192.168.2.0/24 we do:

ipfw add pipe 1 ip from 192.168.2.0/24 to any out
ipfw pipe 1 config bw 300Kbit/s queue 50KBytes

note that we use the out modifier so that the rule is not used twice. Remember in fact that ipfw rules are checked both on incoming and outgoing packets.

Should we want to simulate a bidirectional link with bandwidth limitations, the correct way is the following:

ipfw add pipe 1 ip from any to any out
ipfw add pipe 2 ip from any to any in
ipfw pipe 1 config bw 64Kbit/s queue 10Kbytes
ipfw pipe 2 config bw 64Kbit/s queue 10Kbytes

The above can be very useful, e.g. if you want to see how your fancy Web page will look for a residential user who is connected only through a slow link. You should not use only one pipe for both directions, unless you want to simulate a half-duplex medium (e.g. AppleTalk, Ethernet, IRDA). It is not necessary that both pipes have the same configuration, so we can also simulate asymmetric links.

Should we want to verify network performance with the RED queue management algorithm:

ipfw add pipe 1 ip from any to any
ipfw pipe 1 config bw 500Kbit/s queue 100 red 0.002/30/80/0.1

Another typical application of the traffic shaper is to introduce some delay in the communication. This can significantly affect applications which do a lot of Remote Procedure Calls, and where the round-trip-time of the connection often becomes a limiting factor much more than bandwidth:

ipfw add pipe 1 ip from any to any out
ipfw add pipe 2 ip from any to any in
ipfw pipe 1 config delay 250ms bw 1Mbit/s
ipfw pipe 2 config delay 250ms bw 1Mbit/s

Per-flow queueing can be useful for a variety of purposes. A very simple one is counting traffic:

ipfw add pipe 1 tcp from any to any
ipfw add pipe 1 udp from any to any
ipfw add pipe 1 ip from any to any
ipfw pipe 1 config mask all

The above set of rules will create queues (and collect statistics) for all traffic. Because the pipes have no limitations, the only effect is collecting statistics. Note that we need 3 rules, not just the last one, because when ipfw tries to match IP packets it will not consider ports, so we would not see connections on separate ports as different ones.

A more sophisticated example is limiting the outbound traffic on a net with per-host limits, rather than per-network limits:

ipfw add pipe 1 ip from 192.168.2.0/24 to any out
ipfw add pipe 2 ip from any to 192.168.2.0/24 in
ipfw pipe 1 config mask src-ip 0x000000ff bw 200Kbit/s queue 20Kbytes
ipfw pipe 2 config mask dst-ip 0x000000ff bw 200Kbit/s queue 20Kbytes

LOOKUP TABLES
In the following example, we need to create several traffic bandwidth classes and we need different hosts/networks to fall into different classes. We create one pipe for each class and configure them accordingly. Then we create a single table and fill it with IP subnets and addresses. For each subnet/host we set the argument equal to the number of the pipe that it should use. Then we classify traffic using a single rule:

ipfw pipe 1 config bw 1000Kbyte/s
ipfw pipe 4 config bw 4000Kbyte/s
...
ipfw table 1 add 192.168.2.0/24 1
ipfw table 1 add 192.168.0.0/27 4
ipfw table 1 add 192.168.0.2 1
...
ipfw add pipe tablearg ip from table(1) to any

Using the fwd action, the table entries may include hostnames and IP addresses.

ipfw table 1 add 192.168.2.0/24 10.23.2.1
ipfw table 1 add 192.168.0.0/27 router1.dmz
...
ipfw add 100 fwd tablearg ip from any to table(1)

SETS OF RULES
To add a set of rules atomically, e.g. set 18:

ipfw set disable 18
ipfw add NN set 18 ... # repeat as needed
ipfw set enable 18

To delete a set of rules atomically the command is simply:

ipfw delete set 18

To test a ruleset and disable it and regain control if something goes wrong:

ipfw set disable 18
ipfw add NN set 18 ... # repeat as needed
ipfw set enable 18; echo done; sleep 30 && ipfw set disable 18

Here if everything goes well, you press control-C before the "sleep" terminates, and your ruleset will be left active. Otherwise, e.g. if you cannot access your box, the ruleset will be disabled after the sleep terminates thus restoring the previous situation.

To show rules of the specific set:

ipfw set 18 show

To show rules of the disabled set:

ipfw -S set 18 show

To clear a specific rule counters of the specific set:

ipfw set 18 zero NN

To delete a specific rule of the specific set:

ipfw set 18 delete NN

NAT, REDIRECT AND LSNAT
First redirect all the traffic to nat instance 123:

ipfw add nat 123 all from any to any

Then to configure nat instance 123 to alias all the outgoing traffic with ip 192.168.0.123, blocking all incoming connections, trying to keep same ports on both sides, clearing aliasing table on address change and keeping a log of traffic/link statistics:

ipfw nat 123 config ip 192.168.0.123 log deny_in reset same_ports

Or to change address of instance 123, aliasing table will be cleared (see reset option):

ipfw nat 123 config ip 10.0.0.1

To see configuration of nat instance 123:

ipfw nat 123 show config

To show logs of all the instances in range 111-999:

ipfw nat 111-999 show

To see configurations of all instances:

ipfw nat show config

Or a redirect rule with mixed modes could looks like:

ipfw nat 123 config redirect_addr 10.0.0.1 10.0.0.66
redirect_port tcp 192.168.0.1:80 500
redirect_proto udp 192.168.1.43 192.168.1.1
redirect_addr 192.168.0.10,192.168.0.11
10.0.0.100 # LSNAT
redirect_port tcp 192.168.0.1:80,192.168.0.10:22
500 # LSNAT

or it could be splitted in:

ipfw nat 1 config redirect_addr 10.0.0.1 10.0.0.66
ipfw nat 2 config redirect_port tcp 192.168.0.1:80 500
ipfw nat 3 config redirect_proto udp 192.168.1.43 192.168.1.1
ipfw nat 4 config redirect_addr 192.168.0.10,192.168.0.11,192.168.0.12
10.0.0.100
ipfw nat 5 config redirect_port tcp
192.168.0.1:80,192.168.0.10:22,192.168.0.20:25 500

SEE ALSO

cpp(1), m4(1), altq(4), divert(4), dummynet(4), if_bridge(4), ip(4), ipfirewall(4), ng_ipfw(4), protocols(5), services(5), init(8), kldload(8), reboot(8), sysctl(8), syslogd(8)

HISTORY

The ipfw utility first appeared in FreeBSD 2.0. dummynet(4) was introduced in FreeBSD 2.2.8. Stateful extensions were introduced in FreeBSD 4.0. ipfw2 was introduced in Summer 2002.

AUTHORS

Ugen J. S. Antsilevich,
Poul-Henning Kamp,
Alex Nash,
Archie Cobbs,
Luigi Rizzo.

API based upon code written by Daniel Boulet for BSDI.

In-kernel NAT support written by Paolo Pisati 〈piso@FreeBSD.org〉 as part of a Summer of Code 2005 project.

Work on dummynet(4) traffic shaper supported by Akamba Corp.

BUGS

The syntax has grown over the years and sometimes it might be confusing. Unfortunately, backward compatibility prevents cleaning up mistakes made in the definition of the syntax.

!!! WARNING !!!

Misconfiguring the firewall can put your computer in an unusable state, possibly shutting down network services and requiring console access to regain control of it.

Incoming packet fragments diverted by divert are reassembled before delivery to the socket. The action used on those packet is the one from the rule which matches the first fragment of the packet.

Packets diverted to userland, and then reinserted by a userland process may lose various packet attributes. The packet source interface name will be preserved if it is shorter than 8 bytes and the userland process saves and reuses the sockaddr_in (as does natd(8)); otherwise, it may be lost. If a packet is reinserted in this manner, later rules may be incorrectly applied, making the order of divert rules in the rule sequence very important.

Dummynet drops all packets with IPv6 link-local addresses.

Rules using uid or gid may not behave as expected. In particular, incoming SYN packets may have no uid or gid associated with them since they do not yet belong to a TCP connection, and the uid/gid associated with a packet may not be as expected if the associated process calls setuid(2) or similar system calls.

Rule syntax is subject to the command line environment and some patterns may need to be escaped with the backslash character or quoted appropriately.

Due to the architecture of libalias(3), ipfw nat is not compatible with the tcp segmentation offloading (TSO). Thus, to reliably nat your network traffic, please disable TSO on your NICs using ifconfig(8).

MidnightBSD 0.3 August 5, 2007 MidnightBSD 0.3