Iptables Tutorial 1.2.2 - Oskar Andreasson

What's next?

This chapter has discussed the iptables-save and iptables-restore programs to some extent and how they can be used. Both applications are distributed with the iptables package, and can be used to quickly save large rulesets and then inserting them into the kernel again.

The next chapter will take a look at the syntax of a iptables rule and how to write properly formatted rule-sets. It will also show some basic good coding styles to adhere to, as required.

Chapter 9. How a rule is built

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.

Basics of the iptables command

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.

Tables

The -t option specifies which table to use. Per default, the filter table is used. We may specify one of the following tables with the -t option. Do note that this is an extremely brief summary of some of the contents of the Traversing of tables and chains chapter.

Table 9-1. Tables

Table Explanation nat The nat table is used mainly for Network Address Translation. "NAT"ed packets get their IP addresses altered, according to our rules. Packets in a stream only traverse this table once. We assume that the first packet of a stream is allowed. The rest of the packets in the same stream are automatically "NAT"ed or Masqueraded etc, and will be subject to the same actions as the first packet. These will, in other words, not go through this table again, but will nevertheless be treated like the first packet in the stream. This is the main reason why you should not do any filtering in this table, which we will discuss at greater length further on. The PREROUTING chain is used to alter packets as soon as they get in to the firewall. The OUTPUT chain is used for altering locally generated packets (i.e., on the firewall) before they get to the routing decision. Finally we have the POSTROUTING chain which is used to alter packets just as they are about to leave the firewall. mangle This table is used mainly for mangling packets. Among other things, we can change the contents of different packets and that of their headers. Examples of this would be to change the TTL, TOS or MARK. Note that the MARK is not really a change to the packet, but a mark value for the packet is set in kernel space. Other rules or programs might use this mark further along in the firewall to filter or do advanced routing on; tc is one example. The table consists of five built in chains, the PREROUTING, POSTROUTING, OUTPUT, INPUT and FORWARD chains. PREROUTING is used for altering packets just as they enter the firewall and before they hit the routing decision. POSTROUTING is used to mangle packets just after all routing decisions have been made. OUTPUT is used for altering locally generated packets after they enter the routing decision. INPUT is used to alter packets after they have been routed to the local computer itself, but before the user space application actually sees the data. FORWARD is used to mangle packets after they have hit the first routing decision, but before they actually hit the last routing decision. Note that mangle can't be used for any kind of Network Address Translation or Masquerading, the nat table was made for these kinds of operations. filter The filter table should be used exclusively for filtering packets. For example, we could DROP, LOG, ACCEPT or REJECT packets without problems, as we can in the other tables. There are three chains built in to this table. The first one is named FORWARD and is used on all non-locally generated packets that are not destined for our local host (the firewall, in other words). INPUT is used on all packets that are destined for our local host (the firewall) and OUTPUT is finally used for all locally generated packets. raw The raw table and its chains are used before any other tables in netfilter. It was introduced to use the NOTRACK target. This table is rather new and is only available, if compiled, with late 2.6 kernels and later. The raw table contains two chains. The PREROUTING and OUTPUT chain, where they will handle packets before they hit any of the other netfilter subsystems. The PREROUTING chain can be used for all incoming packets to this machine, or that are forwarded, while the OUTPUT chain can be used to alter the locally generated packets before they hit any of the other netfilter subsystems.

The above details should have explained the basics about the three different tables that are available. They should be used for totally different purposes, and you should know what to use each chain for. If you do not understand their usage, you may well dig a pit for yourself in your firewall, into which you will fall as soon as someone finds it and pushes you into it. We have already discussed the requisite tables and chains in more detail within the Traversing of tables and chains chapter. If you do not understand this fully, I advise you to go back and read through it again.

Commands

In this section we will cover all the different commands and what can be done with them. The command tells iptables what to do with the rest of the rule that we send to the parser. Normally we would want either to add or delete something in some table or another. The following commands are available to iptables:

Table 9-2. Commands

Command -A, --append Example iptables -A INPUT ... Explanation This command appends the rule to the end of the chain. The rule will in other words always be put last in the rule-set and hence be checked last, unless you append more rules later on. Command -D, --delete Example iptables -D INPUT --dport 80 -j DROP, iptables -D INPUT 1 Explanation This command deletes a rule in a chain. This could be done in two ways; either by entering the whole rule to match (as in the first example), or by specifying the rule number that you want to match. If you use the first method, your entry must match the entry in the chain exactly. If you use the second method, you must match the number of the rule you want to delete. The rules are numbered from the top of each chain, starting with number 1. Command -R, --replace Example iptables -R INPUT 1 -s 192.168.0.1 -j DROP Explanation This command replaces the old entry at the specified line. It works in the same way as the --delete command, but instead of totally deleting the entry, it will replace it with a new entry. The main use for this might be while you're experimenting with iptables. Command -I, --insert Example iptables -I INPUT 1 --dport 80 -j ACCEPT Explanation Insert a rule somewhere in a chain. The rule is inserted as the actual number that we specify. In other words, the above example would be inserted as rule 1 in the INPUT chain, and hence from now on it would be the very first rule in the chain. Command -L, --list Example iptables -L INPUT Explanation This command lists all the entries in the specified chain. In the above case, we would list all the entries in the INPUT chain. It's also legal to not specify any chain at all. In the last case, the command would list all the chains in the specified table (To specify a table, see the Tables section). The exact output is affected by other options sent to the parser, for example the -n and -v options, etc. Command -F, --flush Example iptables -F INPUT Explanation This command flushes all rules from the specified chain and is equivalent to deleting each rule one by one, but is quite a bit faster. The command can be used without options, and will then delete all rules in all chains within the specified table. Command -Z, --zero Example iptables -Z INPUT Explanation This command tells the program to zero all counters in a specific chain, or in all chains. If you have used the -v option with the -L command, you have probably seen the packet counter at the beginning of each field. To zero this packet counter, use the -Z option. This option works the same as -L, except that -Z won't list the rules. If -L and -Z is used together (which is legal), the chains will first be listed, and then the packet counters are zeroed. Command -N, --new-chain Example iptables -N allowed Explanation This command tells the kernel to create a new chain of the specified name in the specified table. In the above example we create a chain called allowed. Note that there must not already be a chain or target of the same name. Command -X, --delete-chain Example iptables -X allowed Explanation This command deletes the specified chain from the table. For this command to work, there must be no rules that refer to the chain that is to be deleted. In other words, you would have to replace or delete all rules referring to the chain before actually deleting the chain. If this command is used without any options, all chains but those built in to the specified table will be deleted. Command -P, --policy Example iptables -P INPUT DROP Explanation This command tells the kernel to set a specified default target, or policy, on a chain. All packets that don't match any rule will then be forced to use the policy of the chain. Legal targets are DROP and ACCEPT (There might be more, mail me if so). Command -E, --rename-chain Example iptables -E allowed disallowed Explanation The -E command tells iptables to change the first name of a chain, to the second name. In the example above we would, in other words, change the name of the chain from allowed to disallowed. Note that this will not affect the actual way the table will work. It is, in other words, just a cosmetic change to the table.

You should always enter a complete command line, unless you just want to list the built-in help for iptables or get the version of the command. To get the version, use the -v option and to get the help message, use the -h option. As usual, in other words. Next comes a few options that can be used with various different commands. Note that we tell you with which commands the options can be used and what effect they will have. Also note that we do not include any options here that affect rules or matches. Instead, we'll take a look at matches and targets in a later section of this chapter.