iptables.8.in

.TH IPTABLES 8 "Mar 09, 2002" "" ""
.\"
.\" Man page written by Herve Eychenne <rv@wallfire.org> (May 1999)
.\" It is based on ipchains page.
.\" TODO: add a word for protocol helpers (FTP, IRC, SNMP-ALG)
.\"
.\" ipchains page by Paul ``Rusty'' Russell March 1997
.\" Based on the original ipfwadm man page by Jos Vos <jos@xos.nl>
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.SH NAME
iptables \- administration tool for IPv4 packet filtering and NAT
.SH SYNOPSIS
.BR "iptables [-t table] -[AD] " "chain rule-specification [options]"
.br
.BR "iptables [-t table] -I " "chain [rulenum] rule-specification [options]"
.br
.BR "iptables [-t table] -R " "chain rulenum rule-specification [options]"
.br
.BR "iptables [-t table] -D " "chain rulenum [options]"
.br
.BR "iptables [-t table] -[LFZ] " "[chain] [options]"
.br
.BR "iptables [-t table] -N " "chain"
.br
.BR "iptables [-t table] -X " "[chain]"
.br
.BR "iptables [-t table] -P " "chain target [options]"
.br
.BR "iptables [-t table] -E " "old-chain-name new-chain-name"
.SH DESCRIPTION
.B Iptables
is used to set up, maintain, and inspect the tables of IP 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.

.SH 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
target, which can be the name of a user-defined chain or one of the
special values 
.IR ACCEPT ,
.IR DROP ,
.IR QUEUE ,
or
.IR RETURN .
.PP
.I ACCEPT 
means to let the packet through.
.I DROP
means to drop the packet on the floor.
.I QUEUE
means to pass the packet to userspace (if supported by the kernel).
.I 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
.I RETURN
is matched, the target specified by the chain policy determines the
fate of the packet.
.SH TABLES
There are currently three independent tables (which tables are present
at any time depends on the kernel configuration options and which
modules are present).
.TP
.BI "-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:
.RS
.TP .4i
.BR "filter" :
This is the default table (if no -t option is passed).  It contains
the built-in chains
.B INPUT
(for packets coming into the box itself),
.B FORWARD
(for packets being routed through the box), and
.B OUTPUT
(for locally-generated packets).
.TP
.BR "nat" :
This table is consulted when a packet that creates a new
connection is encountered.  It consists of three built-ins:
.B PREROUTING
(for altering packets as soon as they come in),
.B OUTPUT
(for altering locally-generated packets before routing), and
.B POSTROUTING
(for altering packets as they are about to go out).
.TP
.BR "mangle" :
This table is used for specialized packet alteration.  Until kernel
2.4.17 it had two built-in chains:
.B PREROUTING
(for altering incoming packets before routing) and
.B OUTPUT
(for altering locally-generated packets before routing).
Since kernel 2.4.18, three other built-in chains are also supported:
.B INPUT
(for packets coming into the box itself),
.B FORWARD
(for altering packets being routed through the box), and
.B POSTROUTING
(for altering packets as they are about to go out).
.RE
.SH OPTIONS
The options that are recognized by
.B iptables
can be divided into several different groups.
.SS COMMANDS
These options specify the specific action to perform.  Only one of them
can be specified on the command line unless otherwise specified
below.  For all the long versions of the command and option names, you
need to use only enough letters to ensure that
.B iptables
can differentiate it from all other options.
.TP
.BI "-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.
.TP
.BI "-D, --delete " "chain rule-specification"
.ns
.TP
.BI "-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.
.TP
.BR "-I, --insert " "\fIchain\fP [\fIrulenum\fP] \fIrule-specification\fP"
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.
.TP
.BI "-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.
.TP
.BR "-L, --list " "[\fIchain\fP]"
List all rules in the selected chain.  If no chain is selected, all
chains are listed.  As every other iptables command, it applies to the
specified table (filter is the default), so NAT rules get listed by
.nf
 iptables -t nat -n -L
.fi
Please note that it is often used with the
.B -n
option, in order to avoid long reverse DNS lookups.
It is legal to specify the
.B -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
.nf
 iptables -L -v
.fi
.TP
.BR "-F, --flush " "[\fIchain\fP]"
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.
.TP
.BR "-Z, --zero " "[\fIchain\fP]"
Zero the packet and byte counters in all chains.  It is legal to
specify the
.B "-L, --list"
(list) option as well, to see the counters immediately before they are
cleared. (See above.)
.TP
.BI "-N, --new-chain " "chain"
Create a new user-defined chain by the given name.  There must be no
target of that name already.
.TP
.BR "-X, --delete-chain " "[\fIchain\fP]"
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.  If no argument is given, it
will attempt to delete every non-builtin chain in the table.
.TP
.BI "-P, --policy " "chain target"
Set the policy for the chain to the given target.  See the section
.B 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.
.TP
.BI "-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.
.TP
.B -h
Help.
Give a (currently very brief) description of the command syntax.
.SS PARAMETERS
The following parameters make up a rule specification (as used in the
add, delete, insert, replace and append commands).
.TP
.BR "-p, --protocol " "[!] \fIprotocol\fP"
The protocol of the rule or of the packet to check.
The specified protocol can be one of
.IR tcp ,
.IR udp ,
.IR icmp ,
or
.IR 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
.IR all .
Protocol
.I all
will match with all protocols and is taken as default when this
option is omitted.
.TP
.BR "-s, --source " "[!] \fIaddress\fP[/\fImask\fP]"
Source specification.
.I Address
can be either a network name, a hostname (please note that specifying
any name to be resolved with a remote query such as DNS is a really bad idea),
a network IP address (with /mask), or a plain IP address.
The
.I mask
can be either a network mask or a plain number,
specifying the number of 1's at the left side of the network mask.
Thus, a mask of
.I 24
is equivalent to
.IR 255.255.255.0 .
A "!" argument before the address specification inverts the sense of
the address. The flag
.B --src
is an alias for this option.
.TP
.BR "-d, --destination " "[!] \fIaddress\fP[/\fImask\fP]"
Destination specification. 
See the description of the
.B -s
(source) flag for a detailed description of the syntax.  The flag
.B --dst
is an alias for this option.
.TP
.BI "-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
.B EXTENSIONS
below).  If this
option is omitted in a rule, then matching the rule will have no
effect on the packet's fate, but the counters on the rule will be
incremented.
.TP
.BR "-i, --in-interface " "[!] \fIname\fP"
Name of an interface via which a packet was received (only for
packets entering the 
.BR INPUT ,
.B FORWARD
and
.B 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.
.TP
.BR "-o, --out-interface " "[!] \fIname\fP"
Name of an interface via which a packet is going to be sent (for packets
entering the
.BR FORWARD ,
.B OUTPUT
and
.B 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.
.TP
.B "[!] " "-f, --fragment"
This means that the rule only refers to second and further 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.
.TP
.BI "-c, --set-counters " "PKTS BYTES"
This enables the administrator to initialize the packet and byte
counters of a rule (during
.B INSERT,
.B APPEND,
.B REPLACE
operations).
.SS "OTHER OPTIONS"
The following additional options can be specified:
.TP
.B "-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
.B -x
flag to change this).
For appending, insertion, deletion and replacement, this causes
detailed information on the rule or rules to be printed.
.TP
.B "-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).
.TP
.B "-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 
.B -L
command.
.TP
.B "--line-numbers"
When listing rules, add line numbers to the beginning of each rule,
corresponding to that rule's position in the chain.
.TP
.B "--modprobe=command"
When adding or inserting rules into a chain, use
.B command
to load any necessary modules (targets, match extensions, etc).
.SH MATCH EXTENSIONS
iptables can use extended packet matching modules.  These are loaded
in two ways: implicitly, when
.B -p
or
.B --protocol
is specified, or with the
.B -m
or
.B --match
options, followed by the matching module name; after these, various
extra command line options become available, depending on the specific
module.  You can specify multiple extended match modules in one line,
and you can use the
.B -h
or
.B --help
options after the module has been specified to receive help specific
to that module.

The following are included in the base package, and most of these can
be preceded by a
.B !
to invert the sense of the match.
.\" @MATCH@
.SH TARGET EXTENSIONS
iptables can use extended target modules: the following are included
in the standard distribution.
.\" @TARGET@
.SH 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.
.SH BUGS
Bugs?  What's this? ;-)
Well... the counters are not reliable on sparc64.
.SH COMPATIBILITY WITH IPCHAINS
This
.B iptables
is very similar to ipchains by Rusty Russell.  The main difference is
that the chains
.B INPUT
and
.B 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.
.PP
The other main difference is that
.B -i
refers to the input interface;
.B -o
refers to the output interface, and both are available for packets
entering the
.B FORWARD
chain.
.PP The various forms of NAT have been separated out; 
.B 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:
.nf
 -j MASQ
 -M -S
 -M -L
.fi
There are several other changes in iptables.
.SH SEE ALSO
.BR iptables-save (8),
.BR iptables-restore (8),
.BR ip6tables (8),
.BR ip6tables-save (8),
.BR ip6tables-restore (8).
.P
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.
.br
See
.BR "http://www.netfilter.org/" .
.SH AUTHORS
Rusty Russell wrote iptables, in early consultation with Michael
Neuling.
.PP
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.
.PP
James Morris wrote the TOS target, and tos match.
.PP
Jozsef Kadlecsik wrote the REJECT target.
.PP
Harald Welte wrote the ULOG target, TTL, DSCP, ECN matches and targets.
.PP
The Netfilter Core Team is: Marc Boucher, Martin Josefsson, Jozsef Kadlecsik, 
Patrick McHardy, James Morris, Harald Welte and Rusty Russell.
.PP
Man page written by Herve Eychenne <rv@wallfire.org>.
.\" .. and did I mention that we are incredibly cool people?
.\" .. sexy, too ..
.\" .. witty, charming, powerful ..
.\" .. and most of all, modest ..