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 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:
filter:
This is the default table (if no -t option is passed). It
contains the built-in chains INPUT (for packets destined to
local sockets), FORWARD (for packets being routed through
the box), and OUTPUT (for locally-generated packets).
nat:
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
supported: 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 Discretionary 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.
-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. Like 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
-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 deleting 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 built-in (non-user-defined) chain to the
given target. The policy target must be either ACCEPT or DROP.
-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 IPv4 and IPv6 rules in a
single rule file for use with both iptables-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 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, 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. "all" will match with all
protocols and is taken as default when this option is omitted.
Note that, in ip6tables, IPv6 extension headers except esp are
not allowed. esp and ipv6-nonext can be used with Kernel
version 2.6.11 or later. The number zero is equivalent to all,
which means that you cannot test the protocol field for the
value 0 directly. To match on a HBH header, even if it were the
last, you cannot use -p 0, but always need -m hbh.
[!] -s, --source address[/mask][,...]
Source specification. Address can be either a network name, a
hostname, a network IP address (with /mask), or a plain IP
address. Hostnames will be resolved once only, before the rule
is submitted to the kernel. Please note that specifying any
name to be resolved with a remote query such as DNS is a really
bad idea. The mask can be either an ipv4 network mask (for
iptables) or a plain number, specifying the number of 1's at the
left side of the network mask. Thus, an iptables 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. 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 available 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:
-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. -v may
be specified multiple times to possibly emit more detailed debug
statements.
-w, --wait [seconds]
Wait for the xtables lock. To prevent multiple instances of the
program from running concurrently, an attempt will be made to
obtain an exclusive lock at launch. By default, the program
will exit if the lock cannot be obtained. This option will make
the program wait (indefinitely or for optional seconds) until
the exclusive lock can be obtained.
-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).
MATCH AND TARGET EXTENSIONS
iptables can use extended packet matching and target modules. A list of these is available in the iptables-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
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.
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 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/.
AUTHORS
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: Jozsef Kadlecsik, Patrick McHardy, Pablo Neira Ayuso, Eric Leblond and Florian Westphal. Emeritus Core Team members are: Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, James Morris, Harald Welte and Rusty Russell. Man page originally written by Herve Eychenne <rv@wallfire.org>.
VERSION
This manual page applies to iptables/ip6tables 1.6.0.
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