IP firewall and traffic shaper control program.

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

Parameters

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

-S

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

-t

hile 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.

Remarks

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 behavior, 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 for more information on the stateful behavior 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 SETS OF RULES for more information on sets.

Additional information

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 charac- ter, 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 pre-processor 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 TRAFFIC SHAPER (DUMMYNET) CONFIGURATION.

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 rule- set, 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"

Rules

Rule format Rule actions Rule body Rule options Lookup tables Sets of rules

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 | untag} number] 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.

Syntax

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 occurs only 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 log amount 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.

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 ... }

1.2.3.128, 1.2.3.35 to 1.2.3.55, 1.2.3.89 .

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

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):

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 configura- tion, 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 dif- ferent objects, which implement the traffic regulation:

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 queue:

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

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,

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

The nat configuration command is the following:

nat nat_number config nat-configuration

The following parameters can be configured:

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 behavior 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:

Examples

There are far too many possible uses of ipfw so this section will give only 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 esidential 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

Requirements

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.

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; 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.

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.