CONTENTS

SYNOPSIS

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
.Brq Cm pipe | queue number config config-options ipfw [-s [field]]
.Brq Cm pipe | queue
.Brq Cm delete | list | show [number ...]

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.

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

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

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 ACTIONS

RULE BODY

"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 ipfw1. In ipfw2 any match pattern (including MAC headers, IP protocols, addresses and ports) can be specified in the options section.

Rule fields have the following meaning:

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

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

RULE OPTIONS (MATCH PATTERNS)

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

LOOKUP TABLES

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)).

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


.Brq Cm rule Ar rule-number | old-set

to new-set

Also, you can atomically swap two rulesets with the command

ipfw

set swap first-set second-set

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"

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:

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:

The following parameters can be configured for a queue:

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

• 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

SYSCTL VARIABLES

EXAMPLES

"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 ipfw2 syntax to specify 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 ipfw1 syntax would require a separate rule for each IP in the above example.

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"

"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

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"

"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"

HISTORY

AUTHORS

BUGS