IPTABLES(8) manual page
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iptables/ip6tables -- administration tool for
IPv4/IPv6 packet filtering and NAT
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]
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.
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.
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).
The options that are recognized by iptables and
ip6tables can be divided into several different groups.
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 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).
- -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 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 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, 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).
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
- 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 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).
iptables
can use extended packet matching and target modules. A list of these is
available in the iptables-extensions(8)
manpage.
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-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/.
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,
Yasuyuki Kozakai, Jozsef Kadlecsik, Patrick McHardy, James Morris, Pablo
Neira Ayuso, Harald Welte and Rusty Russell.
Man page originally written
by Herve Eychenne <rv@wallfire.org>.
This manual page applies to
iptables/ip6tables @PACKAGE_AND_VERSION@.
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