Mac match

The MAC (Ethernet Media Access Control) match can be used to match packets based on their MAC source address. As of writing this documentation, this match is a little bit limited, however, in the future this may be more evolved and may be more useful. This match can be used to match packets on the source MAC address only as previously said.

Note Do note that to use this module we explicitly load it with the -m mac option. The reason that I am saying this is that a lot of people wonder if it should not be -m mac-source, which it should not.

Table 10-21. Mac match options

Match	--mac-source
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m mac --mac-source 00:00:00:00:00:01
Explanation	XX:XX:XX:XX:XX:XX

Mark match

The mark match extension is used to match packets based on the marks they have set. A mark is a special field, only maintained within the kernel, that is associated with the packets as they travel through the computer. Marks may be used by different kernel routines for such tasks as traffic shaping and filtering. As of today, there is only one way of setting a mark in Linux, namely the MARK target in iptables. This was previously done with the FWMARK target in ipchains, and this is why people still refer to FWMARK in advanced routing areas. The mark field is currently set to an unsigned integer, or 4294967296 possible values on a 32 bit system. In other words, you are probably not going to run into this limit for quite some time.

Table 10-22. Mark match options

Match	--mark
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -t mangle -A INPUT -m mark --mark 1
Explanation	This match is used to match packets that have previously been marked. Marks can be set with the MARK target which we will discuss in the next section. All packets traveling through Netfilter get a special mark field associated with them. Note that this mark field is not in any way propagated, within or outside the packet. It stays inside the computer that made it. If the mark field matches the mark, it is a match. The mark field is an unsigned integer, hence there can be a maximum of 4294967296 different marks. You may also use a mask with the mark. The mark specification would then look like, for example, --mark 1/1. If a mask is specified, it is logically AND ed with the mark specified before the actual comparison.

Multiport match

The multiport match extension can be used to specify multiple destination ports and port ranges. Without the possibility this match gives, you would have to use multiple rules of the same type, just to match different ports.

Note You can not use both standard port matching and multiport matching at the same time, for example you can't write: --sport 1024:63353 -m multiport --dport 21,23,80. This will simply not work. What in fact happens, if you do, is that iptables honors the first element in the rule, and ignores the multiport instruction.

Table 10-23. Multiport match options

Match	--source-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --source-port 22,53,80,110
Explanation	This match matches multiple source ports. A maximum of 15 separate ports may be specified. The ports must be comma delimited, as in the above example. The match may only be used in conjunction with the -p tcp or -p udp matches. It is mainly an enhanced version of the normal --source-port match.
Match	--destination-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --destination-port 22,53,80,110
Explanation	This match is used to match multiple destination ports. It works exactly the same way as the above mentioned source port match, except that it matches destination ports. It too has a limit of 15 ports and may only be used in conjunction with -p tcp and -p udp.
Match	--port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --port 22,53,80,110
Explanation	This match extension can be used to match packets based both on their destination port and their source port. It works the same way as the --source-port and --destination-port matches above. It can take a maximum of 15 ports and can only be used in conjunction with -p tcp and -p udp. Note that the --port match will only match packets coming in from and going to the same port, for example, port 80 to port 80, port 110 to port 110 and so on.

Owner match

The owner match extension is used to match packets based on the identity of the process that created them. The owner can be specified as the process ID either of the user who issued the command in question, that of the group, the process, the session, or that of the command itself. This extension was originally written as an example of what iptables could be used for. The owner match only works within the OUTPUT chain, for obvious reasons: It is pretty much impossible to find out any information about the identity of the instance that sent a packet from the other end, or where there is an intermediate hop to the real destination. Even within the OUTPUT chain it is not very reliable, since certain packets may not have an owner. Notorious packets of that sort are (among other things) the different ICMP responses. ICMP responses will never match.

Table 10-24. Owner match options

Match	--cmd-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --cmd-owner httpd
Explanation	This is the command owner match, and is used to match based on the command name of the process that is sending the packet. In the example, httpd is matched. This match may also be inverted by using an exclamation sign, for example -m owner ! --cmd-owner ssh.
Match	--uid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --uid-owner 500
Explanation	This packet match will match if the packet was created by the given User ID (UID). This could be used to match outgoing packets based on who created them. One possible use would be to block any other user than root from opening new connections outside your firewall. Another possible use could be to block everyone but the http user from sending packets from the HTTP port.
Match	--gid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --gid-owner 0
Explanation	This match is used to match all packets based on their Group ID (GID). This means that we match all packets based on what group the user creating the packets is in. This could be used to block all but the users in the network group from getting out onto the Internet or, as described above, only to allow members of the http group to create packets going out from the HTTP port.
Match	--pid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --pid-owner 78
Explanation	Pid-owner.txt
Match	--sid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --sid-owner 100
Explanation	Sid-owner.txt