IPTABLES(8)iptables 1.4.20 IPTABLES(8)NAMEiptables — administration tool for IPv4 packet filtering and NAT
SYNOPSISiptables [-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 is used to set up, maintain, and inspect the tables of IPv4
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 ta‐
ble.
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 the examined; if
it does match, then the next rule is specified by the value of the tar‐
get, which can be the name of a user-defined chain or one of the spe‐
cial values ACCEPT, DROP, QUEUE or RETURN.
ACCEPT means to let the packet through. DROP means to drop the packet
on the floor. QUEUE means to pass the packet to userspace. (How the
packet can be received by a userspace process differs by the particular
queue handler. 2.4.x and 2.6.x kernels up to 2.6.13 include the
ip_queue queue handler. Kernels 2.6.14 and later additionally include
the nfnetlink_queue queue handler. Packets with a target of QUEUE will
be sent to queue number '0' in this case. Please also see the NFQUEUE
target as described later in this man page.) 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 mod‐
ules are present).
-t, --table table
This option specifies the packet matching table which the com‐
mand should operate on. If the kernel is configured with auto‐
matic module loading, an attempt will be made to load the appro‐
priate 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 rout‐
ing), and POSTROUTING (for altering packets as they are
about to go out).
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), FOR‐
WARD (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) net‐
working rules, such as those enabled by the SECMARK and
CONNSECMARK targets. Mandatory Access Control is imple‐
mented by Linux Security Modules such as SELinux. The secu‐
rity 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 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 combina‐
tion.
-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 des‐
tination 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 ipt‐
ables 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 coun‐
ters 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 sec‐
tion 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.
-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.
[!] -p, --protocol protocol
The protocol of the rule or of the packet to check. The speci‐
fied protocol can be one of tcp, udp, udplite, icmp, esp, ah,
sctp or the special keyword "all", or it can be a numeric value,
representing one of these protocols or a different one. A pro‐
tocol name from /etc/protocols is also allowed. A "!" argument
before the protocol inverts the test. The number zero is equiv‐
alent to all. "all" will match with all protocols and is taken
as default when this option is omitted.
[!] -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 a network mask or a plain num‐
ber, specifying the number of 1's at the left side of the net‐
work mask. Thus, a 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, eval‐
uation 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 con‐
tinue 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 frag‐
ments 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.
-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 multipli‐
ers 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 appli‐
cable).
-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 EXTENSIONSiptables 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 pre‐
viously. So the following options are handled differently:
-j MASQ
-M -S
-M -L
There are several other changes in iptables.
SEE ALSOiptables-apply(8), iptables-save(8), iptables-restore(8), ipta‐
bles-extensions(8), ip6tables(8), ip6tables-save(8),
ip6tables-restore(8), libipq(3).
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 netfil‐
ter-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 every‐
where.
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>.
VERSION
This manual page applies to iptables 1.4.20.
iptables 1.4.20IPTABLES(8)
