IPFW(8)

NAME
ipfw - controlling utility for IP firewall

SYNOPSIS
ipfw file
ipfw [ -f | -q] flush
ipfw [ -q] zero [number ...]
ipfw delete number ...
ipfw [-aftN] list
ipfw [ -ftN] show
ipfw [ -q] add [number] action [log] proto from src to dst [via name | ipno] [options]

DESCRIPTION
If used as shown in the first synopsis line, the file will be read line by line and applied as arguments to the ipfw command.

The ipfw code works by going through the rule-list for each packet, until a match is found. All rules have two associated counters, a packet count and a byte count. These counters are updated when a packet matches the rule.

The rules are ordered by a ``line-number'' from 1 to 65534 that is used to order and delete rules. Rules are tried in increasing order, and the first rule that matches a packet applies. Multiple rules may share the same number and apply in the order in which they were added.

If a rule is added without a number, it is numbered 100 higher than the previous rule. If the highest defined rule number is greater than 65434, new rules are appended to the last rule.

The delete operation deletes the first rule with number number, if any.

The list command prints out the current rule set.

The show command is equivalent to `ipfw -a list'.

The zero operation zeroes the counters associated with rule number number.

The flush operation removes all rules.

One rule is always present:

    65535 deny all from any to any

This rule is the default policy, i.e., don't allow anything at all. Your job in setting up rules is to modify this policy to match your needs.

The following options are available:

-a  While listing, show counter values. This option is the only way to see accounting records.

-f  Don't ask for confirmation for commands that can cause problems if misused (ie; flush). Note, if there is no tty associated with the process, this is implied.

-q  While adding 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. If a flush is performed in normal (verbose) mode, it prints a message. Because all rules are flushed, the message cannot be delivered to the login session, the login session is closed and the remainder of the ruleset is not processed. Access to the console is required to recover.

-t  While listing, show last match timestamp.

-N  Try to resolve addresses and service names in output.

action:

allow
    Allow packets that match rule. The search terminates. Aliases are pass, permit, and accept.

deny
    Discard packets that match this rule. The search terminates. Drop is an alias for deny.

reject
    (Deprecated.) Discard packets that match this rule, and try to send an ICMP host unreachable notice. The search terminates.

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 zero 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.

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

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

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

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.

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

If a packet matches more than one divert and/or tee rule, all but the last are ignored.

If the kernel was compiled with IPFIREWALL_VERBOSE, then when a packet matches a rule with the ``log'' keyword a message will be printed on the console. If the kernel was compiled with the IPFIREWALL_VERBOSE_LIMIT option, then logging will cease after the number of packets specified by the option are received for that particular chain entry. Logging may then be re-enabled by clearing the packet counter for that entry.

Console logging and the log limit are adjustable dynamically through the sysctl(8) interface.

proto:

ip  All packets match. The alias all has the same effect.

tcp Only TCP packets match.

udp Only UDP packets match.

icmp
    Only ICMP packets match.

<number|name>
    Only packets for the specified protocol matches (see /etc/protocols for a complete list).

src and dst:

<address/mask>
    [ports]

The <address/mask> may be specified as:

ipno
    An ipnumber of the form 1.2.3.4. Only this exact ip number match the rule.

ipno/bits
    An ipnumber with a mask width of the form 1.2.3.4/24. In this case all ip numbers from 1.2.3.0 to 1.2.3.255 will match.

ipno:mask
    An ipnumber with a mask width of the form 1.2.3.4:255.255.240.0. In this case all ip numbers from 1.2.0.0 to 1.2.15.255 will match.

The sense of the match can be inverted by preceding an address with the ``not'' modifier, causing all other addresses to be matched instead. This does not affect the selection of port numbers.

With the TCP and UDP protocols, optional ports may be specified as:

{port|port-port}[,port[,...]]

Service names (from /etc/services) may be used instead of numeric port values. A range may only be specified as the first value, and the length of the port list is limited to IP_FW_MAX_PORTS (as defined in /usr/src/sys/netinet/ip_fw.h) ports.

Rules can apply to packets when they are incoming, or outgoing, or both. The in keyword indicates the rule should only match incoming packets. The out keyword indicates the rule should only match outgoing packets.

To match packets going through a certain interface, specify the interface using via:

via ifX
    Packet must be going through interface ifX.

via if*
    Packet must be going through interface ifX, where X is any unit number.

via any
    Packet must be going through some interface.

via ipno
    Packet must be going through the interface having IP address ipno.

The via keyword causes the interface to always be checked. If recv or xmit is used instead of via, then the only 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 100 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 invalid) whenver xmit is used. Specifying via together with xmit or recv is invalid.

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.

Additional options:

frag
    Matches if the packet is a fragment and this is not the first fragment of the datagram.

in  Matches if this packet was on the way in.

out Matches if this packet was on the way out.

ipoptions spec
    Matches if the IP 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 ``!''.

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

setup
    Matches packets that have the SYN bit set but no ACK bit. TCP packets only.

tcpflags spec
    Matches 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 ``!''.

icmptypes types
    Matches if the ICMP type is in the list types. The list may be specified as any combination of ranges or individual types separated by commas.

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.

•   Don't forget the loopback interface.

FINE POINTS
There is one kind of packet that the firewall will always discard, that is an IP fragment with a fragment offset of one. This is a valid packet, but it only has one use, to try to circumvent firewalls.

If you are logged in over a network, loading the LKM version of ipfw is probably not as straightforward as you would think. I recommend this command line:

    modload /lkm/ipfw_mod.o && \
    ipfw add 32000 allow all from any to any

Along the same lines, doing an

    ipfw flush

in similar surroundings is also a bad idea.

PACKET DIVERSION
A divert socket bound to the specified port will receive all packets diverted to that port; see divert(4). If no socket is bound to the destination port, or if the kernel wasn't compiled with divert socket support, diverted packets are dropped.

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

    ipfw add deny tcp from hacker.evil.org to wolf.tambov.su 23

This one disallows any connection from the entire hackers network to my host:

    ipfw addf deny all from 123.45.67.0/24 to my.host.org

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

    ipfw -at l

or in short form without timestamps:

    ipfw -a l

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

    ipfw divert 5000 all from 192.168.2.0/24 to any in

SEE ALSO
ip(4), ipfirewall(4), divert(4), protocols(5), services(5), reboot(8), syslogd(8), sysctl(8)

BUGS

WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!WARNING!!

This program can put your computer in rather unusable state. When using it for the first time, work on the console of the computer, and do NOT do anything you don't understand.

When manipulating/adding chain entries, service and protocol names are not accepted.

Incoming packet fragments diverted by divert are reassembled before delivery to the socket, whereas fragments diverted via tee are not.

Port aliases containing dashes cannot be first in a list.

AUTHORS
Ugen J. S. Antsilevich, Poul-Henning Kamp, Alex Nash, Archie Cobbs. API based upon code written by Daniel Boulet for BSDI.

HISTORY
ipfw first appeared in FreeBSD 2.0.