Manual: iptables
IPTABLES(8) iptables 1.8.11 IPTABLES(8)
NAME
iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering and NAT
SYNOPSIS
iptables [-t table] {-A|-C|-D|-V} chain rule-specification
ip6tables [-t table] {-A|-C|-D|-V} 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 policy
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 config‐
ured 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 four
built-ins: PREROUTING (for altering packets as soon as they come in), INPUT (for altering packets destined
for local sockets), 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 pack‐
ets 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 NO‐
TRACK target. It registers at the netfilter hooks with higher priority and is thus called before ip_con‐
ntrack, or any other IP tables. It provides the following built-in chains: PREROUTING (for packets arriv‐
ing via any network interface) and 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 SEC‐
MARK 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 en‐
sure 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 ipta‐
bles 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
or iptables-save(8).
-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 chain specified. There must be no references to the chain. If there are, you must delete or re‐
place 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 delete all empty chains in the table. Empty builtin chains can only
be deleted with iptables-nft.
-P, --policy chain target
Set the policy for the built-in (non-user-defined) chain to the given target. The policy target must be ei‐
ther 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 struc‐
ture 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 com‐
mands).
-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 op‐
tion allows IPv4 and IPv6 rules in a single rule file for use with both iptables-restore and ip6tables-re‐
store.
-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 proto‐
cols and is taken as default when this option is omitted. Note that, in ip6tables, IPv6 extension headers ex‐
cept 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. Mul‐
tiple 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 syn‐
tax. 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 MATCH AND TARGET 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 coun‐
ters on the rule will be incremented.
-g, --goto chain
This specifies that the processing should continue in a user specified chain. Unlike with 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 PRE‐
ROUTING chains). When the "!" argument is used before the interface name, the sense is inverted. If the in‐
terface 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, in‐
sertion, 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: Specified twice, iptables-legacy
will dump table info and entries in libiptc, iptables-nft dumps rules in netlink (VM code) presentation.
Specified three times, iptables-nft will also dump any netlink messages sent to kernel.
-V, --version
Show program version and the kernel API used.
-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 rele‐
vant 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 exten‐
sions, etc).
LOCK FILE
iptables uses the /run/xtables.lock file to take an exclusive lock at launch.
The XTABLES_LOCKFILE environment variable can be used to override the default setting.
MATCH AND TARGET EXTENSIONS
iptables can use extended packet matching and target modules. A list of these is available in the iptables-exten‐
sions(8) manpage.
DIAGNOSTICS
Various error messages are printed to standard error. The exit code is 0 for correct functioning. Errors which ap‐
pear to be caused by invalid or abused command line parameters cause an exit code of 2. Errors which indicate an in‐
compatibility between kernel and user space cause an exit code of 3. Errors which indicate a resource problem, such
as a busy lock, failing memory allocation or error messages from kernel cause an exit code of 4. Finally, other er‐
rors cause an exit code of 1.
BUGS
Bugs? What's this? ;-) Well, you might want to have a look at https://bugzilla.netfilter.org/ iptables will exit im‐
mediately with an error code of 111 if it finds that it was called as a setuid-to-root program. iptables cannot be
used safely in this manner because it trusts the shared libraries (matches, targets) loaded at run time, the search
path can be set using environment variables.
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 OUT‐
PUT 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 avoid much of the confusion over the combination of IP mas‐
querading 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-ex‐
tensions-HOWTO details the extensions that are not in the standard distribution, and the netfilter-hacking-HOWTO de‐
tails the netfilter internals.
See https://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, Pablo Neira Ayuso, Eric Leblond, Florian Westphal and Arturo Borrero
Gonzalez. 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 .
VERSION
This manual page applies to iptables/ip6tables 1.8.11.
iptables 1.8.11 IPTABLES(8)