Note The pid, sid and command matching is broken in SMP kernels since they use different process lists for each processor. It might be fixed in the future however
Packet type match
The packet type match is used to match packets based on their type. I.e., are they destined to a specific person, to everyone or to a specific group of machines or users. These three groups are generally called unicast, broadcast and multicast, as discussed in the TCP/IP repetition chapter. The match is loaded by using -m pkttype.
Table 10-25. Packet type match options
| Match | --pkt-type |
| Kernel | 2.3, 2.4, 2.5 and 2.6 |
| Example | iptables -A OUTPUT -m pkttype --pkt-type unicast |
| Explanation | The --pkt-type match is used to tell the packet type match which packet type to match. It can either take unicast , broadcast or multicast as an argument, as in the example. It can also be inverted by using a ! like this: -m pkttype --pkt-type ! broadcast, which will match all other packet types. |
Realm match
The realm match is used to match packets based on the routing realm that they are part of. Routing realms are used in Linux for complex routing scenarios and setups such as when using BGP et cetera. The realm match is loaded by adding the -m realm keyword to the commandline.
A routing realm is used in Linux to classify routes into logical groups of routes. In most dedicated routers today, the Routing Information Base (RIB) and the forwarding engine are very close to eachother. Inside the kernel for example. Since Linux isn't really a dedicated routing system, it has been forced to separate its RIB and Forwarding Information Base (FIB). The RIB lives in userspace and the FIB lives inside kernelspace. Because of this separation, it becomes quite resourceheavy to do quick searches in the RIB. The routing realm is the Linux solution to this, and actually makes the system more flexible and richer.
The Linux realms can be used together with BGP and other routing protocols that delivers huge amounts of routes. The routing daemon can then sort the routes by their prefix, aspath, or source for example, and put them in different realms. The realm is numeric, but can also be named through the /etc/iproute2/rt_realms file.
Table 10-26. Realm match options
| Match | --realm |
| Kernel | 2.6 |
| Example | iptables -A OUTPUT -m realm --realm 4 |
| Explanation | This option matches the realm number and optionally a mask. If this is not a number, it will also try and resolve the realm from the /etc/iproute2/rt_realms file also. If a named realm is used, no mask may be used. The match may also be inverted by setting an exclamation sign, for example --realm ! cosmos. |
Recent match
The recent match is a rather large and complex matching system, which allows us to match packets based on recent events that we have previously matched. For example, if we would see an outgoing IRC connection, we could set the IP addresses into a list of hosts, and have another rule that allows identd requests back from the IRC server within 15 seconds of seeing the original packet.
Before we can take a closer look at the match options, let's try and explain a little bit how it works. First of all, we use several different rules to accomplish the use of the recent match. The recent match uses several different lists of recent events. The default list being used is the DEFAULT list. We create a new entry in a list with the set option, so once a rule is entirely matched (the set option is always a match), we also add an entry in the recent list specified. The list entry contains a timestamp, and the source IP address used in the packet that triggered the set option. Once this has happened, we can use a series of different recent options to match on this information, as well as update the entries timestamp, et cetera.
Finally, if we would for some reason want to remove a list entry, we would do this using the --remove match option from the recent match. All rules using the recent match, must load the recent module (-m recent) as usual. Before we go on with an example of the recent match, let's take a look at all the options.
Table 10-27. Recent match options
| Match | --name |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A OUTPUT -m recent --name examplelist |
| Explanation | The name option gives the name of the list to use. Per default the DEFAULT list is used, which is probably not what we want if we are using more than one list. |
| Match | --set |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A OUTPUT -m recent --set |
| Explanation | This creates a new list entry in the named recent list, which contains a timestamp and the source IP address of the host that triggered the rule. This match will always return success, unless it is preceded by a ! sign, in which case it will return failure. |
| Match | --rcheck |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A OUTPUT -m recent --name examplelist --rcheck |
| Explanation | The --rcheck option will check if the source IP address of the packet is in the named list. If it is, the match will return true, otherwise it returns false. The option may be inverted by using the ! sign. In the later case, it will return true if the source IP address is not in the list, and false if it is in the list. |
| Match | --update |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A OUTPUT -m recent --name examplelist --update |
| Explanation | This match is true if the source combination is available in the specified list and it also updates the last-seen time in the list. This match may also be reversed by setting the ! mark in front of the match. For example, ! --update. |
| Match | --remove |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --remove |
| Explanation | This match will try to find the source address of the packet in the list, and returns true if the packet is there. It will also remove the corresponding list entry from the list. The command is also possible to inverse with the ! sign. |
| Match | --seconds |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --check --seconds 60 |
| Explanation | This match is only valid together with the --check and --update matches. The --seconds match is used to specify how long since the "last seen" column was updated in the recent list. If the last seen column was older than this amount in seconds, the match returns false. Other than this the recent match works as normal, so the source address must still be in the list for a true return of the match. |
| Match | --hitcount |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --check --hitcount 20 |
| Explanation | The --hitcount match must be used together with the --check or --update matches and it will limit the match to only include packets that have seen at least the hitcount amount of packets. If this match is used together with the --seconds match, it will require the specified hitcount packets to be seen in the specific timeframe. This match may also be reversed by adding a ! sign in front of the match. Together with the --seconds match, this means that a maximum of this amount of packets may have been seen during the specified timeframe. If both of the matches are inversed, then a maximum of this amount of packets may have been seen during the last minumum of seconds. |
| Match | --rttl |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --check --rttl |
| Explanation | The --rttl match is used to verify that the TTL value of the current packet is the same as the original packet that was used to set the original entry in the recent list. This can be used to verify that people are not spoofing their source address to deny others access to your servers by making use of the recent match. |
| Match | --rsource |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --rsource |
| Explanation | The --rsource match is used to tell the recent match to save the source address and port in the recent list. This is the default behavior of the recent match. |
| Match | --rdest |
| Kernel | 2.4, 2.5 and 2.6 |
| Example | iptables -A INPUT -m recent --name example --rdest |
| Explanation | The --rdest match is the opposite of the --rsource match in that it tells the recent match to save the destination address and port to the recent list. |