iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering
and NAT
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 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 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 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
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 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 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 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.
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).
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: 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 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 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.
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/ iptables will exit immediately 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.
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: 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 <
[email protected]>.
This manual page applies to iptables/ip6tables 1.8.9.