Oskar Andreasson - Iptables Tutorial 1.2.2

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

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

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

I have created a small sample script of how the recent match can be used, which you can find in the Recent-match.txt section.

Briefly, this is a poor replacement for the state engine available in netfilter. This version was created with a http server in mind, but will work with any TCP connection. First we have created two chains named http-recent and http-recent-final. The http-recent chain is used in the starting stages of the connection, and for the actual data transmission, while the http-recent-final chain is used for the last and final FIN/ACK, FIN handshake.

Warning!This is a very bad replacement for the built in state engine and can not handle all of the possibilities that the state engine can handle. However, it is a good example of what can be done with the recent match without being too specific. Do not use this example in a real world environment. It is slow, handles special cases badly, and should generally never be used more than as an example.

For example, it does not handle closed ports on connection, asyncronuous FIN handshake (where one of the connected parties closes down, while the other continues to send data), etc.

Let's follow a packet through the example ruleset. First a packet enters the INPUT chain, and we send it to the http-recent chain.

The first packet should be a SYN packet, and should not have the ACK,FIN or RST bits set. Hence it is matched using the --tcp-flags SYN,ACK,FIN,RST SYN line. At this point we add the connection to the httplist using -m recent --name httplist --set line. Finally we accept the packet.

After the first packet we should receive a SYN/ACK packet to acknowledge that the SYN packet was received. This can be matched using the --tcp-flags SYN,ACK,FIN,RST SYN,ACK line. FIN and RST should be illegal at this point as well. At this point we update the entry in the httplist using -m recent --name httplist --update and finally we ACCEPT the packet.

By now we should get a final ACK packet, from the original creater of the connection, to acknowledge the SYN/ACK sent by the server. SYN, FIN and RST are illegal at this point of the connection, so the line should look like --tcp-flags SYN,ACK,FIN,RST ACK. We update the list in exactly the same way as in the previous step, and ACCEPT it.

At this point the data transmission can start. The connection should never contain any SYN packet now, but it will contain ACK packets to acknowledge the data packets that are sent. Each time we see any packet like this, we update the list and ACCEPT the packets.