This chapter and the upcoming three chapters will discuss at length how to build your own rules. A rule could be described as the directions the firewall will adhere to when blocking or permitting different connections and packets in a specific chain. Each line you write that's inserted in a chain should be considered a rule. We will also discuss the basic matches that are available, and how to use them, as well as the different targets and how we can construct new targets of our own (i.e.,new sub chains).
This chapter will deal with the raw basics of how a rule is created and how you write it and enter it so that it will be accepted by the userspace program iptables, the different tables, as well as the commands that you can issue to iptables. After that we will in the next chapter look at all the matches that are available to iptables, and then get more into detail of each type of target and jump.
As we have already explained, each rule is a line that the kernel looks at to find out what to do with a packet. If all the criteria - or matches - are met, we perform the target - or jump - instruction. Normally we would write our rules in a syntax that looks something like this:
iptables [-t table] command [match] [target/jump]
There is nothing that says that the target instruction has to be the last function in the line. However, you would usually adhere to this syntax to get the best readability. Anyway, most of the rules you'll see are written in this way. Hence, if you read someone else's script, you'll most likely recognize the syntax and easily understand the rule.
If you want to use a table other than the standard table, you could insert the table specification at the point at which [table] is specified. However, it is not necessary to state explicitly what table to use, since by default iptables uses the filter table on which to implement all commands. Neither do you have to specify the table at just this point in the rule. It could be set pretty much anywhere along the line. However, it is more or less standard to put the table specification at the beginning.
One thing to think about though: The command should always come first, or alternatively directly after the table specification. We use 'command' to tell the program what to do, for example to insert a rule or to add a rule to the end of the chain, or to delete a rule. We shall take a further look at this below.
The match is the part of the rule that we send to the kernel that details the specific character of the packet, what makes it different from all other packets. Here we could specify what IP address the packet comes from, from which network interface, the intended IP address, port, protocol or whatever. There is a heap of different matches that we can use that we will look closer at further on in this chapter.
Finally we have the target of the packet. If all the matches are met for a packet, we tell the kernel what to do with it. We could, for example, tell the kernel to send the packet to another chain that we've created ourselves, and which is part of this particular table. We could tell the kernel to drop the packet dead and do no further processing, or we could tell the kernel to send a specified reply to the sender. As with the rest of the content in this section, we'll look closer at it further on in the chapter.