IPTABLES(8) iptables 1.4.21 IPTABLES(8)
NAME
iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering and NAT
SYNOPSIS
iptables [-t table] {-A|-C|-D} chain rule-specification
ip6tables [-t table] {-A|-C|-D} chain rule-specification
iptables [-t table] -I chain [rulenum] rule-specification
iptables [-t table] -R chain rulenum rule-specification
iptables [-t table] -D chain rulenum
iptables [-t table] -S [chain [rulenum]]
iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]
iptables [-t table] -N chain
iptables [-t table] -X [chain]
iptables [-t table] -P chain target
iptables [-t table] -E old-chain-name new-chain-name
rule-specification = [matches...] [target]
match = -m matchname [per-match-options]
target = -j targetname [per-target-options]
DESCRIPTION
Iptables and ip6tables are used to set up, maintain, and inspect the tables of IPv4 and IPv6 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.
TARGETS
A firewall rule specifies criteria for a packet and a target. If the packet does not match, the next rule in
the chain is 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, one of the targets described in iptables-extensions(8), or one of the
special values ACCEPT, DROP or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet on the floor. 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.
TABLES
There are currently five independent tables (which tables are present at any time depends on the kernel con‐
figuration options and which modules are present).
-t, --table table
This table is consulted when a packet that creates a new connection is encountered. It consists of
three built-ins: PREROUTING (for altering packets as soon as they come in), OUTPUT (for altering
locally-generated packets before routing), and POSTROUTING (for altering packets as they are about
to go out). IPv6 NAT support is available since kernel 3.7.
mangle:
This table is used for specialized packet alteration. Until kernel 2.4.17 it had two built-in
chains: PREROUTING (for altering incoming packets before routing) and OUTPUT (for altering locally-
generated packets before routing). Since kernel 2.4.18, three other built-in chains are also sup‐
ported: INPUT (for packets coming into the box itself), FORWARD (for altering packets being routed
through the box), and POSTROUTING (for altering packets as they are about to go out).
raw:
This table is used mainly for configuring exemptions from connection tracking in combination with
the NOTRACK target. It registers at the netfilter hooks with higher priority and is thus called
before ip_conntrack, or any other IP tables. It provides the following built-in chains: PREROUTING
(for packets arriving via any network interface) OUTPUT (for packets generated by local processes)
security:
This table is used for Mandatory Access Control (MAC) networking rules, such as those enabled by
the SECMARK and CONNSECMARK targets. Mandatory Access Control is implemented by Linux Security
Modules such as SELinux. The security table is called after the filter table, allowing any Discre‐
tionary Access Control (DAC) rules in the filter table to take effect before MAC rules. This table
provides the following built-in chains: INPUT (for packets coming into the box itself), OUTPUT (for
altering locally-generated packets before routing), and FORWARD (for altering packets being routed
through the box).
OPTIONS
The options that are recognized by iptables and ip6tables can be divided into several different groups.
COMMANDS
These options specify the desired action to perform. Only one of them can be specified on the command line
unless otherwise stated below. For 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.
-C, --check chain rule-specification
Check whether a rule matching the specification does exist in the selected chain. This command uses the
same logic as -D to find a matching entry, but does not alter the existing iptables configuration and
uses its exit code to indicate success or failure.
-D, --delete chain rule-specification
-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.
iptables -L -v
-S, --list-rules [chain]
Print all rules in the selected chain. If no chain is selected, all chains are printed like iptables-
save. Like every other iptables command, it applies to the specified table (filter is the default).
-F, --flush [chain]
Flush the selected chain (all the chains in the table if none is given). This is equivalent to delet‐
ing all the rules one by one.
-Z, --zero [chain [rulenum]]
Zero the packet and byte counters in all chains, or only the given chain, or only the given rule in a
chain. 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.
PARAMETERS
The following parameters make up a rule specification (as used in the add, delete, insert, replace and append
commands).
-4, --ipv4
This option has no effect in iptables and iptables-restore. If a rule using the -4 option is inserted
with (and only with) ip6tables-restore, it will be silently ignored. Any other uses will throw an
error. This option allows to put both IPv4 and IPv6 rules in a single rule file for use with both ipta‐
bles-restore and ip6tables-restore.
-6, --ipv6
If a rule using the -6 option is inserted with (and only with) iptables-restore, it will be silently
ignored. Any other uses will throw an error. This option allows to put both IPv4 and IPv6 rules in a
single rule file for use with both iptables-restore and ip6tables-restore. This option has no effect
in ip6tables and ip6tables-restore.
[!] -p, --protocol protocol
The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp,
udplite, icmp, icmpv6,esp, ah, sctp, mh or the special keyword "all", or it can be a numeric value,
really bad idea. The mask can be either an ipv4 network mask (for iptables) or a plain number, speci‐
fying the number of 1's at the left side of the network mask. Thus, an iptables mask of 24 is equiva‐
lent 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. Multiple addresses can be specified, but this
will expand to multiple rules (when adding with -A), or will cause multiple rules to be deleted (with
-D).
[!] -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.
-m, --match match
Specifies a match to use, that is, an extension module that tests for a specific property. The set of
matches make up the condition under which a target is invoked. Matches are evaluated first to last as
specified on the command line and work in short-circuit fashion, i.e. if one extension yields false,
evaluation will stop.
-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 IPv4 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. This option is IPv4 specific, it is not avail‐
able in ip6tables.
-c, --set-counters packets bytes
This enables the administrator to initialize the packet and byte counters of a rule (during INSERT,
APPEND, REPLACE operations).
OTHER OPTIONS
The following additional options can be specified:
-n, --numeric
Numeric output. IP addresses and port numbers will be printed in numeric format. By default, the pro‐
gram 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 posi‐
tion in the chain.
--modprobe=command
When adding or inserting rules into a chain, use command to load any necessary modules (targets, match
extensions, etc).
MATCH AND TARGET EXTENSIONS
iptables can use extended packet matching and target modules. A list of these is available in the ipta‐
bles-extensions(8) manpage.
DIAGNOSTICS
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
Bugs? What's this? ;-) Well, you might want to have a look at http://bugzilla.netfilter.org/
COMPATIBILITY WITH IPCHAINS
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 respec‐
tively. 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.
SEE ALSO
iptables-apply(8), iptables-save(8), iptables-restore(8), iptables-extensions(8),
The packet-filtering-HOWTO details iptables usage for packet filtering, the NAT-HOWTO details NAT, the netfil‐
ter-extensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hack‐
ing-HOWTO details the netfilter internals.
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, Yasuyuki Kozakai, Jozsef Kadlecsik, Patrick
McHardy, James Morris, Pablo Neira Ayuso, Harald Welte and Rusty Russell.
Man page originally written by Herve Eychenne <[email protected]>.
VERSION
This manual page applies to iptables/ip6tables @PACKAGE_AND_VERSION@.
iptables 1.4.21 IPTABLES(8)
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