Man page written by Herve Eychenne <rv@wallfire.org> (May 1999)
It is based on ipchains page.
TODO: add a word for protocol helpers (FTP, IRC, SNMP-ALG)

ipchains page by Paul ``Rusty'' Russell March 1997
Based on the original ipfwadm man page by Jos Vos <jos@xos.nl>

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iptables - administration tool for IPv4 packet filtering and NAT "iptables [-t table] -[AD] " "chain rule-specification [options]"

"iptables [-t table] -I " "chain [rulenum] rule-specification [options]"

"iptables [-t table] -R " "chain rulenum rule-specification [options]"

"iptables [-t table] -D " "chain rulenum [options]"

"iptables [-t table] -[LFZ] " "[chain] [options]"

"iptables [-t table] -N " "chain"

"iptables [-t table] -X " "[chain]"

"iptables [-t table] -P " "chain target [options]"

"iptables [-t table] -E " "old-chain-name new-chain-name"

Iptables is used to set up, maintain, and inspect the tables of IP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains. Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This is called a `target', which may be a jump to a user-defined chain in the same table. A firewall rule specifies criteria for a packet, and a target. If the packet does not match, the next rule in the chain is the examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain or one of the special values ACCEPT , DROP , QUEUE , or RETURN .

ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to userspace. (How the packet can be received by a userspace process differs by the particular queue handler. 2.4.x and 2.6.x kernels up to 2.6.13 include the ip_queue queue handler. Kernels 2.6.14 and later additionally include the nfnetlink_queue queue handler. Packets with a target of QUEUE will be sent to queue number '0' in this case. Please also see the NFQUEUE target as described later in this man page.) RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target specified by the chain policy determines the fate of the packet.

There are currently three independent tables (which tables are present at any time depends on the kernel configuration options and which modules are present).

"-t, --table " "table" This option specifies the packet matching table which the command should operate on. If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there. The tables are as follows:

The options that are recognized by iptables can be divided into several different groups. These options specify the specific action to perform. Only one of them can be specified on the command line unless otherwise specified below. For all the long versions of the command and option names, you need to use only enough letters to ensure that iptables can differentiate it from all other options.

"-A, --append " "chain rule-specification" Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination.

"-D, --delete " "chain rule-specification" .ns

"-D, --delete " "chain rulenum" Delete one or more rules from the selected chain. There are two versions of this command: the rule can be specified as a number in the chain (starting at 1 for the first rule) or a rule to match.

"-I, --insert " "chain [rulenum] rule-specification" Insert one or more rules in the selected chain as the given rule number. So, if the rule number is 1, the rule or rules are inserted at the head of the chain. This is also the default if no rule number is specified.

"-R, --replace " "chain rulenum rule-specification" Replace a rule in the selected chain. If the source and/or destination names resolve to multiple addresses, the command will fail. Rules are numbered starting at 1.

"-L, --list " "[chain]" List all rules in the selected chain. If no chain is selected, all chains are listed. As every other iptables command, it applies to the specified table (filter is the default), so NAT rules get listed by

 iptables -t nat -n -L

Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is affected by the other arguments given. The exact rules are suppressed until you use

 iptables -L -v

"-F, --flush " "[chain]" Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one by one.

"-Z, --zero " "[chain]" Zero the packet and byte counters in all chains. It is legal to specify the "-L, --list" (list) option as well, to see the counters immediately before they are cleared. (See above.)

"-N, --new-chain " "chain" Create a new user-defined chain by the given name. There must be no target of that name already.

"-X, --delete-chain " "[chain]" Delete the optional user-defined chain specified. There must be no references to the chain. If there are, you must delete or replace the referring rules before the chain can be deleted. The chain must be empty, i.e. not contain any rules. If no argument is given, it will attempt to delete every non-builtin chain in the table.

"-P, --policy " "chain target" Set the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.

"-E, --rename-chain " "old-chain new-chain" Rename the user specified chain to the user supplied name. This is cosmetic, and has no effect on the structure of the table.

-h Help. Give a (currently very brief) description of the command syntax.

The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).

"-p, --protocol " "[!] protocol" The protocol of the rule or of the packet to check. The specified protocol can be one of tcp , udp , icmp , or all , or it can be a numeric value, representing one of these protocols or a different one. A protocol name from /etc/protocols is also allowed. A "!" argument before the protocol inverts the test. The number zero is equivalent to all . Protocol all will match with all protocols and is taken as default when this option is omitted.

"-s, --source " "[!] address[/mask]" Source specification. Address can be either a network name, a hostname (please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea), a network IP address (with /mask), or a plain IP address. The mask can be either a network mask or a plain number, specifying the number of 1's at the left side of the network mask. Thus, a mask of 24 is equivalent to 255.255.255.0 . A "!" argument before the address specification inverts the sense of the address. The flag --src is an alias for this option.

"-d, --destination " "[!] address[/mask]" Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The flag --dst is an alias for this option.

"-j, --jump " "target" This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain (other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EXTENSIONS below). If this option is omitted in a rule (and -g is not used), then matching the rule will have no effect on the packet's fate, but the counters on the rule will be incremented.

"-g, --goto " "chain" This specifies that the processing should continue in a user specified chain. Unlike the --jump option return will not continue processing in this chain but instead in the chain that called us via --jump.

"-i, --in-interface " "[!] name" Name of an interface via which a packet was received (only for packets entering the INPUT , FORWARD and PREROUTING chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then any interface which begins with this name will match. If this option is omitted, any interface name will match.

"-o, --out-interface " "[!] name" Name of an interface via which a packet is going to be sent (for packets entering the FORWARD , OUTPUT and POSTROUTING chains). When the "!" argument is used before the interface name, the sense is inverted. If the interface name ends in a "+", then any interface which begins with this name will match. If this option is omitted, any interface name will match.

"[!] " "-f, --fragment" This means that the rule only refers to second and further fragments of fragmented packets. Since there is no way to tell the source or destination ports of such a packet (or ICMP type), such a packet will not match any rules which specify them. When the "!" argument precedes the "-f" flag, the rule will only match head fragments, or unfragmented packets.

"-c, --set-counters " "PKTS BYTES" This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).

The following additional options can be specified:

"-v, --verbose" Verbose output. This option makes the list command show the interface name, the rule options (if any), and the TOS masks. The packet and byte counters are also listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipliers respectively (but see the -x flag to change this). For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.

"-n, --numeric" Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the program will try to display them as host names, network names, or services (whenever applicable).

"-x, --exact" Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (multiples of 1000) M's (multiples of 1000K) or G's (multiples of 1000M). This option is only relevant for the -L command.

"--line-numbers" When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.

"--modprobe=command" When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).

iptables can use extended packet matching modules. These are loaded in two ways: implicitly, when -p or --protocol is specified, or with the -m or --match options, followed by the matching module name; after these, various extra command line options become available, depending on the specific module. You can specify multiple extended match modules in one line, and you can use the -h or --help options after the module has been specified to receive help specific to that module. The following are included in the base package, and most of these can be preceded by a ! to invert the sense of the match. @MATCH@
iptables can use extended target modules: the following are included in the standard distribution. @TARGET@
Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which appear to be caused by invalid or abused command line parameters cause an exit code of 2, and other errors cause an exit code of 1. Bugs? What's this? ;-) Well, you might want to have a look at http://bugzilla.netfilter.org/ This iptables is very similar to ipchains by Rusty Russell. The main difference is that the chains INPUT and OUTPUT are only traversed for packets coming into the local host and originating from the local host respectively. Hence every packet only passes through one of the three chains (except loopback traffic, which involves both INPUT and OUTPUT chains); previously a forwarded packet would pass through all three.

The other main difference is that -i refers to the input interface; -o refers to the output interface, and both are available for packets entering the FORWARD chain.

The various forms of NAT have been separated out; iptables is a pure packet filter when using the default `filter' table, with optional extension modules. This should simplify much of the previous confusion over the combination of IP masquerading and packet filtering seen previously. So the following options are handled differently:

 -j MASQ
 -M -S
 -M -L

There are several other changes in iptables. iptables-save (8), iptables-restore (8), ip6tables (8), ip6tables-save (8), ip6tables-restore (8), libipq (3).

The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO details the netfilter internals.

See "http://www.netfilter.org/" .

Rusty Russell originally wrote iptables, in early consultation with Michael Neuling.

Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework in iptables, then wrote the mangle table, the owner match, the mark stuff, and ran around doing cool stuff everywhere.

James Morris wrote the TOS target, and tos match.

Jozsef Kadlecsik wrote the REJECT target.

Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as the TTL, DSCP, ECN matches and targets.

The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik, Patrick McHardy, James Morris, Harald Welte and Rusty Russell.

Man page originally written by Herve Eychenne <rv@wallfire.org>. .. and did I mention that we are incredibly cool people?
.. sexy, too ..
.. witty, charming, powerful ..
.. and most of all, modest ..