Iptables Tutorial 1.2.2

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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.


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