Options Used in iptables Commands

Rules that allow packets to be filtered by the kernel are put into place by running the iptables command with a number of options after it that identify the types of packets being filtered, the source or destination of those packets, and what to do with the packet if it matches the rule. The options used with a particular iptables rule must be grouped logically, based on the purpose and conditions of the overall rule, in order for the rule to be valid.


A powerful aspect of iptables is that multiple tables can be used to decide the fate of a particular packet, depending upon the type of packet being monitored and what is to be done with the packet. Thanks to the extensible nature of iptables, specialized tables can be created and stored in the /etc/modules/<kernel-version>/kernel/net/ipv4/netfilter/ directory to meet special goals. Think of iptables as being able to run multiple sets of ipchains rules in defined chains, with each set fulfilling a particular role.

The default table, named filter, contains the standard built-in INPUT, OUTPUT, and FORWARD chains. This is somewhat similar to the standard chains in use with ipchains. However, by default, iptables also includes two additional tables that perform specific packet filtering jobs. The nat table can be used to modify the source and destination addresses recorded in packets, and the mangle table allows you to alter packets in specialized ways.

Each table contains default chains that perform necessary tasks based on the purpose of the table, but you can easily set up new chains in each of the tables.


Many iptables commands have the following structure:

iptables [-t <table-name>] <command> <chain-name> <parameter-1> \
         <option-1> <parameter-n> <option-n>

In this example, the <table-name> option allows the user to select a table other than the default filter table to use with the command. The <command> option is the center of the command, dictating a specific action to perform, such as appending or deleting a rule from a particular chain, which is specified by the <chain-name> option. Following the <chain-name> are pairs of parameters and options that actually define the way the rule will work and what will happen when a packet matches the rule.

When looking at the structure of an iptables command, it is important to remember that, unlike most other commands, the length and complexity of an iptables command can change based on its purpose. A simple command to remove a rule from a chain can be very short, while a command designed to filter packets from a particular subnet using a variety of specific parameters and options can be rather lengthy. One way to think about iptables commands is to recognize that some parameters and options used may create the need to use other parameters and options to further specify the previous option's request. In order to construct a valid rule, this must continue until every parameter and option that requires another set of options is satisfied.

Type iptables -h to see a comprehensive list of iptables command structures.


Commands tell iptables to perform a specific action, and only one command is allowed per iptables command string. Except for the help command, all commands are written in upper-case characters.

The iptables commands are:


Once certain iptables commands are specified, including those used to add, append, delete, insert, or replace rules within a particular chain, parameters are required to begin the construction of the packet filtering rule.

Match Options

Different network protocols provide specialized matching options which may be set in specific ways to match a particular packet using that protocol. Of course, the protocol must first be specified in the iptables command, such as using -p tcp <protocol-name>, to make the options for that protocol available.

TCP Protocol

These match options are available for the TCP protocol (-p tcp):

  • --dport — Sets the destination port for the packet. You can use either a network service name (such as www or smtp), port number, or range of port numbers to configure this option. To browse the names and aliases of network services and the port numbers they use, view the /etc/services file. You can also use --destination-port to specify this match option.

    To specify a specific range of port numbers, separate the two numbers with a colon (:), such as -p tcp --dport 3000:3200. The largest valid range is 0:65535.

    You may also use an exclamation point character (!) as a flag after the --dport option to tell iptables to match all packets which do not use that network service or port.

  • --sport — Sets the source port of the packet, using the same options as --dport. You can also use --source-port to specify this match option.

  • --syn — Applies to all TCP packets designed to initiate communication, commonly called SYN packets. Any packets that carry a data payload are not touched. Placing an exclamation point character (!) as a flag after the --syn option causes all non-SYN packets to be matched.

  • --tcp-flags — Allows TCP packets with specific bits, or flags, set to be matched with a rule. The --tcp-flags match option accepts two parameters after it, which are flags for the various bits arranged in a comma-separated list. The first parameter is the mask, which sets the flags to be examined on the packet. The second parameter refers to the flags that must be set in the packet to make a match. The possible flags are ACK, FIN, PSH, RST, SYN, and URG. In addition, ALL and NONE can also be used to match every flag or none of them.

    For example, an iptables rule which contains -p tcp --tcp-flags ACK,FIN,SYN SYN will only match TCP packets that have the SYN flag set and the ACK and FIN flags unset.

    Like many other options, using the exclamation point character (!) after --tcp-flags reverses the effect of the match option, so that the second parameter's flags must not be set in order to match.

  • --tcp-option — Attempts to match with TCP-specific options that can be set within a particular packet. This match option can also be reversed with the exclamation point character (!).

UDP Protocol

These match options are available for the UDP protocol (-p udp):

  • --dport — Specifies the destination port of the UDP packet, using the service name, port number, or range of port numbers. The --destination-port match option may be used instead of --dport. See the --dport match option in the Section called TCP Protocol for various ways to use this option.

  • --sport — Specifies the source port of the UDP packet, using the service name, port number, or range of port numbers. The --source-port match option may be used instead of --sport. See the --dport match option in the Section called TCP Protocol for various ways to use this option.

ICMP Protocol

Packets using the Internet Control Message Protocol (ICMP) can be matched using the following option when -p icmp is specified:

  • --icmp-type — Sets the name or number of the ICMP type to match with the rule. A list of valid ICMP names can be seen by typing the iptables -p icmp -h command.

Modules with Additional Match Options

Additional match options are also available through modules loaded when the iptables command calls them. To use a match option module, you must load the module by name by including -m <module-name> in the iptables command.

A large number of modules are available by default. It is even possible to create your own modules to provide additional match option functionality. Many modules exist, but only the most popular ones are discussed here.

The limit module allows you to place a limit on how many packets will be matched to a particular rule. This is especially beneficial when logging rule matches so that a flood of matching packets will not fill up your logs with repetitive messages or use too many system resources.

  • --limit — Sets the number of matches for a particular range of time, specified with a number and time modifier arranged in a <number>/<time> format. For example, using --limit 5/hour only lets a rule match five times in a single hour.

    If a number and time modifier are not used, the default value of 3/hour is assumed.

  • --limit-burst — Sets a limit on the number of packets able to match a rule at one time. This option should be used in conjunction with the --limit option, and it accepts a number to set the burst threshold.

    If no number is specified, only five packets are initially able to match the rule.

The state module, which uses the --state match option, can match a packet with these particular connection states:

  • ESTABLISHED — The matching packet is associated with other packets in an established connection.

  • INVALID — The matching packet cannot be tied to a known connection.

  • NEW — The matching packet is either creating a new connection or is part of a two-way connection not previously seen.

  • RELATED — The matching packet is starting a new connection related in some way to an existing connection.

These connection states can be used in combination with one another by separating them with commas, such as -m state --state INVALID,NEW.

To specifically match a hardware MAC address of an Ethernet device, use the mac module, which accepts --mac-source plus a MAC address as an option. To exclude a MAC address from a rule, place an exclamation point (!) after the --mac-source match option.

To view other match options available through modules, see the iptables man page.

Target Options

Once a packet has matched a particular rule, the rule can direct the packet to a number of different targets that decide its fate and, possibly, take additional actions, such as logging the action. Additionally, each chain has a default target, which is used if none of the rules on that chain match a packet or if none of the rules which match the packet specify a target.

There are only a few standard targets available to decide what happens with the packet:

In addition to these standard targets, various other targets may be used with extensions called target modules. For more information about match option modules, see the Section called Modules with Additional Match Options.

There are many extended target modules, most of which only apply to specific tables or situations. A couple of the most popular target modules included by default in Red Hat Linux are:

Other target extensions, including several that are useful with masquerading using the nat table, can be found in the iptables man page.

Listing Options

The default list command, iptables -L, provides a very basic overview of the default filter table's current rile chains. Additional options exist that provide more information and arrange that information in specific ways: