iptables.8

.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)
.\"        add EXTRA EXTENSIONS matches
.\"
.\" 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 \- IP packet filter administration
.SH SYNOPSIS
.BR "iptables -[ADC] " "chain rule-specification [options]"
.br
.BR "iptables -I " "chain [rulenum] rule-specification [options]"
.br
.BR "iptables -R " "chain rulenum rule-specification [options]"
.br
.BR "iptables -D " "chain rulenum [options]"
.br
.BR "iptables -[LFZ] " "[chain] [options]"
.br
.BR "iptables -N " "chain"
.br
.BR "iptables -X " "[chain]"
.br
.BR "iptables -P " "chain target [options]"
.br
.BR "iptables -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 current 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:
.TP
.B "filter"
This is the default table.  It contains the built-in chains INPUT (for
packets coming into the box itself), FORWARD (for packets being routed
through the box), and OUTPUT (for locally-generated packets).
.TP
.B "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).
.TP
.B "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).
.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
.br
 iptables -t nat -n -L
.br
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.
.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 is going to be 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 administrater 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
address, 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.
.SS tcp
These extensions are loaded if `--protocol tcp' is specified. It
provides the following options:
.TP
.BR "--source-port " "[!] \fIport\fP[:\fIport\fP]"
Source port or port range specification. This can either be a service
name or a port number. An inclusive range can also be specified,
using the format
.IR port : port .
If the first port is omitted, "0" is assumed; if the last is omitted,
"65535" is assumed.
If the second port greater then the first they will be swapped.
The flag
.B --sport
is a convenient alias for this option.
.TP
.BR "--destination-port " "[!] \fIport\fP[:\fIport\fP]"
Destination port or port range specification.  The flag
.B --dport
is a convenient alias for this option.
.TP
.BR "--tcp-flags " "[!] \fImask\fP \fIcomp\fP"
Match when the TCP flags are as specified.  The first argument is the
flags which we should examine, written as a comma-separated list, and
the second argument is a comma-separated list of flags which must be
set.  Flags are: 
.BR "SYN ACK FIN RST URG PSH ALL NONE" .
Hence the command
.br
 iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
.br
will only match packets with the SYN flag set, and the ACK, FIN and
RST flags unset.
.TP
.B "[!] --syn"
Only match TCP packets with the SYN bit set and the ACK and FIN bits
cleared.  Such packets are used to request TCP connection initiation;
for example, blocking such packets coming in an interface will prevent
incoming TCP connections, but outgoing TCP connections will be
unaffected.
It is equivalent to \fB--tcp-flags SYN,RST,ACK SYN\fP.
If the "!" flag precedes the "--syn", the sense of the
option is inverted.
.TP
.BR "--tcp-option " "[!] \fInumber\fP"
Match if TCP option set.
.TP
.BR "--mss " "\fIvalue\fP[:\fIvalue\fP]"
Match TCP SYN or SYN/ACK packets with the specified MSS value (or range),
which control the maximum packet size for that connection.
.SS udp
These extensions are loaded if `--protocol udp' is specified.  It
provides the following options:
.TP
.BR "--source-port " "[!] \fIport\fP[:\fIport\fP]"
Source port or port range specification.
See the description of the
.B --source-port
option of the TCP extension for details.
.TP
.BR "--destination-port " "[!] \fIport\fP[:\fIport\fP]"
Destination port or port range specification.
See the description of the
.B --destination-port
option of the TCP extension for details.
.SS icmp
This extension is loaded if `--protocol icmp' is specified.  It
provides the following option:
.TP
.BR "--icmp-type " "[!] \fItypename\fP"
This allows specification of the ICMP type, which can be a numeric
ICMP type, or one of the ICMP type names shown by the command
.br
 iptables -p icmp -h
.br
.SS mac
.TP
.BR "--mac-source " "[!] \fIaddress\fP"
Match source MAC address.  It must be of the form XX:XX:XX:XX:XX:XX.
Note that this only makes sense for packets coming from an Ethernet device
and entering the
.BR PREROUTING ,
.B FORWARD
or
.B INPUT
chains.
.SS limit
This module matches at a limited rate using a token bucket filter.
A rule using this extension will match until this limit is reached
(unless the `!' flag is used).  It can be used in combination with the
.B LOG
target to give limited logging, for example.
.TP
.BI "--limit " "rate"
Maximum average matching rate: specified as a number, with an optional
`/second', `/minute', `/hour', or `/day' suffix; the default is
3/hour.
.TP
.BI "--limit-burst " "number"
Maximum initial number of packets to match: this number gets
recharged by one every time the limit specified above is not reached,
up to this number; the default is 5.
.SS multiport
This module matches a set of source or destination ports.  Up to 15
ports can be specified.  It can only be used in conjunction with
.B "-p tcp"
or
.BR "-p udp" .
.TP
.BR "--source-ports " "\fIport\fP[,\fIport\fP[,\fIport\fP...]]"
Match if the source port is one of the given ports.  The flag
.B --sports
is a convenient alias for this option.
.TP
.BR "--destination-ports " "\fIport\fP[,\fIport\fP[,\fIport\fP...]]"
Match if the destination port is one of the given ports.  The flag
.B --dports
is a convenient alias for this option.
.TP
.BR "--ports " "\fIport\fP[,\fIport\fP[,\fIport\fP...]]"
Match if the both the source and destination ports are equal to each
other and to one of the given ports.
.SS mark
This module matches the netfilter mark field associated with a packet
(which can be set using the
.B MARK
target below).
.TP
.BR "--mark " "\fIvalue\fP[/\fImask\fP]"
Matches packets with the given unsigned mark value (if a mask is
specified, this is logically ANDed with the mask before the
comparison).
.SS owner
This module attempts to match various characteristics of the packet
creator, for locally-generated packets.  It is only valid in the
.B OUTPUT
chain, and even this some packets (such as ICMP ping responses) may
have no owner, and hence never match.
.TP
.BI "--uid-owner " "userid"
Matches if the packet was created by a process with the given
effective user id.
.TP
.BI "--gid-owner " "groupid"
Matches if the packet was created by a process with the given
effective group id.
.TP
.BI "--pid-owner " "processid"
Matches if the packet was created by a process with the given
process id.
.TP
.BI "--sid-owner " "sessionid"
Matches if the packet was created by a process in the given session
group.
.TP
.BI "--cmd-owner " "name"
Matches if the packet was created by a process with the given command name.
(this option is present only if iptables was compiled under a kernel
supporting this feature)
.SS state
This module, when combined with connection tracking, allows access to
the connection tracking state for this packet.
.TP
.BI "--state " "state"
Where state is a comma separated list of the connection states to
match.  Possible states are 
.B INVALID
meaning that the packet is associated with no known connection,
.B ESTABLISHED
meaning that the packet is associated with a connection which has seen
packets in both directions,
.B NEW
meaning that the packet has started a new connection, or otherwise
associated with a connection which has not seen packets in both
directions, and
.B RELATED
meaning that the packet is starting a new connection, but is
associated with an existing connection, such as an FTP data transfer,
or an ICMP error.
.SS tos
This module matches the 8 bits of Type of Service field in the IP
header (ie. including the precedence bits). 
.TP
.BI "--tos " "tos"
The argument is either a standard name, (use
.br
 iptables -m tos -h
.br
to see the list), or a numeric value to match.
.SS ah
This module matches the SPIs in AH header of IPSec packets.
.TP
.BR "--ahspi " "[!] \fIspi\fP[:\fIspi\fP]"
.SS esp
This module matches the SPIs in ESP header of IPSec packets.
.TP
.BR "--espspi " "[!] \fIspi\fP[:\fIspi\fP]"
.SS length
This module matches the length of a packet against a specific value
or range of values.
.TP
.BR "--length " "\fIlength\fP[:\fIlength\fP]"
.SS ttl
This module matches the time to live field in the IP header.
.TP
.BI "--ttl " "ttl"
Matches the given TTL value.
.SS owner
This module attempts to match various characteristics of the packet
creator, for locally-generated packets.  It is only valid in the
.B OUTPUT
chain, and even this some packets (such as ICMP ping responses) may
have no owner, and hence never match.  This is regarded as experimental.
.TP
.BI "--uid-owner " "userid"
Matches if the packet was created by a process with the given
effective user id.
.TP
.BI "--gid-owner " "groupid"
Matches if the packet was created by a process with the given
effective group id.
.TP
.BI "--pid-owner " "processid"
Matches if the packet was created by a process with the given
process id.
.TP
.BI "--sid-owner " "sessionid"
Matches if the packet was created by a process in the given session
group.
.SS unclean
This module takes no options, but attempts to match packets which seem
malformed or unusual.  This is regarded as experimental.
.SH TARGET EXTENSIONS
iptables can use extended target modules: the following are included
in the standard distribution.
.SS LOG
Turn on kernel logging of matching packets.  When this option is set
for a rule, the Linux kernel will print some information on all
matching packets (like most IP header fields) via the kernel log
(where it can be read with
.I dmesg
or 
.IR syslogd (8)).
This is a "non-terminating target", i.e. rule traversal continues at
the next rule.  So if you want to LOG the packets you refuse, use two
separate rules with the same matching criterias, first using target LOG
then DROP (or REJECT).
.TP
.BI "--log-level " "level"
Level of logging (numeric or see \fIsyslog.conf\fP(5)).
.TP
.BI "--log-prefix " "prefix"
Prefix log messages with the specified prefix; up to 29 letters long,
and useful for distinguishing messages in the logs.
.TP
.B --log-tcp-sequence
Log TCP sequence numbers. This is a security risk if the log is
readable by users.
.TP
.B --log-tcp-options
Log options from the TCP packet header.
.TP
.B --log-ip-options
Log options from the IP packet header.
.SS MARK
This is used to set the netfilter mark value associated with the
packet.  It is only valid in the
.B mangle
table.  It can for example be used in conjunction with iproute2.
.TP
.BI "--set-mark " "mark"
.SS REJECT
This is used to send back an error packet in response to the matched
packet: otherwise it is equivalent to 
.B DROP
so it is a terminating TARGET, ending rule traversal.
This target is only valid in the
.BR INPUT ,
.B FORWARD
and
.B OUTPUT
chains, and user-defined chains which are only called from those
chains.  The following option controls the nature of the error packet
returned:
.TP
.BI "--reject-with " "type"
The type given can be 
.BR icmp-net-unreachable ,
.BR icmp-host-unreachable ,
.BR icmp-port-unreachable ,
.BR icmp-proto-unreachable ,
.BR "icmp-net-prohibited or"
.BR icmp-host-prohibited ,
which return the appropriate ICMP error message (\fBport-unreachable\fP is
the default).  The option
.B tcp-reset
can be used on rules which only match the TCP protocol: this causes a
TCP RST packet to be sent back.  This is mainly useful for blocking 
.I ident
(113/tcp) probes which frequently occur when sending mail to broken mail
hosts (which won't accept your mail otherwise).
.SS TOS
This is used to set the 8-bit Type of Service field in the IP header.
It is only valid in the
.B mangle
table.
.TP
.BI "--set-tos " "tos"
You can use a numeric TOS values, or use
.br
 iptables -j TOS -h
.br
to see the list of valid TOS names.
.SS MIRROR
This is an experimental demonstration target which inverts the source
and destination fields in the IP header and retransmits the packet.
It is only valid in the
.BR INPUT ,
.B FORWARD
and 
.B PREROUTING
chains, and user-defined chains which are only called from those
chains.  Note that the outgoing packets are
.B NOT
seen by any packet filtering chains, connection tracking or NAT, to
avoid loops and other problems.
.SS SNAT
This target is only valid in the 
.B nat
table, in the 
.B POSTROUTING
chain.  It specifies that the source address of the packet should be
modified (and all future packets in this connection will also be
mangled), and rules should cease being examined.  It takes one option:
.TP
.BR "--to-source  " "\fIipaddr\fP[-\fIipaddr\fP][:\fIport\fP-\fIport\fP]"
which can specify a single new source IP address, an inclusive range
of IP addresses, and optionally, a port range (which is only valid if
the rule also specifies
.B "-p tcp"
or
.BR "-p udp" ).
If no port range is specified, then source ports below 512 will be
mapped to other ports below 512: those between 512 and 1023 inclusive
will be mapped to ports below 1024, and other ports will be mapped to
1024 or above. Where possible, no port alteration will occur.
.SS DNAT
This target is only valid in the 
.B nat
table, in the 
.B PREROUTING
and
.B OUTPUT
chains, and user-defined chains which are only called from those
chains.  It specifies that the destination address of the packet
should be modified (and all future packets in this connection will
also be mangled), and rules should cease being examined.  It takes one
option:
.TP
.BR "--to-destination " "\fIipaddr\fP[-\fIipaddr\fP][:\fIport\fP-\fIport\fP]"
which can specify a single new destination IP address, an inclusive
range of IP addresses, and optionally, a port range (which is only
valid if the rule also specifies
.B "-p tcp"
or
.BR "-p udp" ).
If no port range is specified, then the destination port will never be
modified.
.SS MASQUERADE
This target is only valid in the 
.B nat
table, in the 
.B POSTROUTING
chain.  It should only be used with dynamically assigned IP (dialup)
connections: if you have a static IP address, you should use the SNAT
target.  Masquerading is equivalent to specifying a mapping to the IP
address of the interface the packet is going out, but also has the
effect that connections are 
.I forgotten
when the interface goes down.  This is the correct behavior when the
next dialup is unlikely to have the same interface address (and hence
any established connections are lost anyway).  It takes one option:
.TP
.BR "--to-ports " "\fIport\fP[-\fIport\fP]"
This specifies a range of source ports to use, overriding the default 
.B SNAT
source port-selection heuristics (see above).  This is only valid
if the rule also specifies
.B "-p tcp"
or
.BR "-p udp" .
.SS REDIRECT
This target is only valid in the 
.B nat
table, in the 
.B PREROUTING
and
.B OUTPUT
chains, and user-defined chains which are only called from those
chains.  It alters the destination IP address to send the packet to
the machine itself (locally-generated packets are mapped to the
127.0.0.1 address).  It takes one option:
.TP
.BR "--to-ports " "\fIport\fP[-\fIport\fP]"
This specifies a destination port or range of ports to use: without
this, the destination port is never altered.  This is only valid
if the rule also specifies
.B "-p tcp"
or
.BR "-p udp" .
.SS ULOG
This target provides userspace logging of matching packets.  When this
target is set for a rule, the Linux kernel will multicast this packet
through a
.IR netlink 
socket. One or more userspace processes may then subscribe to various 
multicast groups and receive the packets.
.TP
.BI "--ulog-nlgroup " "nlgroup"
This specifies the netlink group (1-32) to which the packet is sent.
Default value is 1.
.TP
.BI "--ulog-prefix " "prefix"
Prefix log messages with the specified prefix; up to 32 characters
long, and useful fro distinguishing messages in the logs.
.TP
.BI "--ulog-cprange " "size"
Number of bytes to be copied to userspace. A value of 0 always copies
the entire packet, regardless of its size. Default is 0.
.TP
.BI "--ulog-qthreshold " "size"
Number of packet to queue inside kernel. Setting this value to, e.g. 10
accumulates ten packets inside the kernel and transmits them as one
netlink multipart message to userspace.  Default is 1 (for backwards
compatibility).
.SS TCPMSS
This target allows to alter the MSS value of TCP SYN packets, to control
the maximum size for that connection (usually limiting it to your
outgoing interface's MTU minus 40). Of course, it can only be used
in conjunction with
.BR "-p tcp" .
.br
This target is used to overcome criminally braindead ISPs or servers
which block ICMP Fragmentation Needed packets.  The symptoms of this
problem are that everything works fine from your Linux
firewall/router, but machines behind it can never exchange large
packets:
.br
 1) Web browsers connect, then hang with no data received.
.br
 2) Small mail works fine, but large emails hang.
.br
 3) ssh works fine, but scp hangs after initial handshaking.
.br
Workaround: activate this option and add a rule to your firewall
configuration like:
.br
 iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
.br
             -j TCPMSS --clamp-mss-to-pmtu
.TP
.BI "--set-mss " "value"
Explicitly set MSS option to specified value.
.TP
.B "--clamp-mss-to-pmtu"
Automatically clamp MSS value to (path_MTU - 40).
.TP
These options are mutually exclusive.
.SH EXTRA EXTENSIONS
The following extensions are not included by default in the standard
distribution.
.SS TTL
This target is used to modify the time to live field in the IP header.
It is only valid in the 
.B mangle
table.
.TP
.BI "--ttl-set " "ttl"
Set the TTL to the given value.
.TP
.BI "--ttl-dec " "ttl"
Decrement the TTL by the given value.
.TP
.BI "--ttl-inc " "ttl"
Increment the TTL by the given value.
.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
Check is not implemented (yet).
.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; 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:
.br
 -j MASQ
.br
 -M -S
.br
 -M -L
.br
There are several other changes in iptables.
.SH SEE ALSO
The packet-filtering-HOWTO, which details more iptables usage for
packet filtering, the NAT-HOWTO, which details NAT,
and the netfilter-hacking-HOWTO which details the 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 match+target and libipulog.
.PP
The Netfilter Core Team is: Marc Boucher, Jozsef Kadlecsik, 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 ..