Ubuntu Manpage: iptables/ip6tables — administration tool for IPv4/IPv6 packet filtering and NAT

Provided by: iptables_1.4.21-1ubuntu1_amd64

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 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.

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
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).

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.

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:  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/ip6tables @PACKAGE_AND_VERSION@.