[iptables.git] / 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)
.\"
.\" 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 destined to local sockets),
.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).
.TP
.BR "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:
.B PREROUTING
(for packets arriving via any network interface)
.B OUTPUT
(for packets generated by local processes)
.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@
.SS account
Account traffic for all hosts in defined network/netmask.

Features:

- long (one counter per protocol TCP/UDP/IMCP/Other) and short statistics

- one iptables rule for all hosts in network/netmask

- loading/saving counters (by reading/writting to procfs entries)

.TP
.BI "--aaddr " "network/netmask"
defines network/netmask for which make statistics.
.TP
.BI "--aname " "name"
defines name of list where statistics will be kept. If no is
specified DEFAULT will be used.
.TP
.B "--ashort"
table will colect only short statistics (only total counters
without splitting it into protocols.
.P
Example usage:

account traffic for/to 192.168.0.0/24 network into table mynetwork:

# iptables -A FORWARD -m account --aname mynetwork --aaddr 192.168.0.0/24

account traffic for/to WWW serwer for 192.168.0.0/24 network into table mywwwserver:

# iptables -A INPUT -p tcp --dport 80
  -m account --aname mywwwserver --aaddr 192.168.0.0/24 --ashort

# iptables -A OUTPUT -p tcp --sport 80
  -m account --aname mywwwserver --aaddr 192.168.0.0/24 --ashort

read counters:

# cat /proc/net/ipt_account/mynetwork
# cat /proc/net/ipt_account/mywwwserver

set counters:

# echo "ip = 192.168.0.1 packets_src = 0" > /proc/net/ipt_account/mywwserver

Webpage:
  http://www.barbara.eu.org/~quaker/ipt_account/
.SS addrtype
This module matches packets based on their 
.B address type.
Address types are used within the kernel networking stack and categorize
addresses into various groups.  The exact definition of that group depends on the specific layer three protocol.
.TP
The following address types are possible:
.TP
.BI "UNSPEC"
an unspecified address (i.e. 0.0.0.0)
.BI "UNICAST"
an unicast address
.BI "LOCAL"
a local address
.BI "BROADCAST"
a broadcast address
.BI "ANYCAST"
an anycast packet
.BI "MULTICAST"
a multicast address
.BI "BLACKHOLE"
a blackhole address
.BI "UNREACHABLE"
an unreachable address
.BI "PROHIBIT"
a prohibited address
.BI "THROW"
FIXME
.BI "NAT"
FIXME
.BI "XRESOLVE"
FIXME
.TP
.BI "--src-type " "type"
Matches if the source address is of given type
.TP
.BI "--dst-type " "type"
Matches if the destination address is of given type
.SS ah
This module matches the SPIs in AH header of IPSec packets.
.TP
.BR "--ahspi " "[!] \fIspi\fP[:\fIspi\fP]"
.SS childlevel
This is an experimental module.  It matches on whether the 
packet is part of a master connection or one of its children (or grandchildren,
etc).  For instance, most packets are level 0.  FTP data transfer is level 1.
.TP
.BR "--childlevel " "[!] \fIlevel\fP"
.SS comment
Allows you to add comments (up to 256 characters) to any rule.
.TP
.BI "--comment " "comment"
.TP
Example:
iptables -A INPUT -s 192.168.0.0/16 -m comment --comment "A privatized IP block"
.SS condition
This matches if a specific /proc filename is '0' or '1'.
.TP
.BI "--condition " "[!] filename"
Match on boolean value stored in /proc/net/ipt_condition/filename file
.SS connbytes
Match by how many bytes or packets a connection (or one of the two
flows constituting the connection) have tranferred so far, or by
average bytes per packet.

The counters are 64bit and are thus not expected to overflow ;)

The primary use is to detect long-lived downloads and mark them to be
scheduled using a lower priority band in traffic control.

The transfered bytes per connection can also be viewed through
/proc/net/ip_conntrack and accessed via ctnetlink
.TP
[\fB!\fR]\fB --connbytes \fIfrom\fB:\fR[\fIto\fR]
match packets from a connection whose packets/bytes/average packet
size is more than FROM and less than TO bytes/packets. if TO is
omitted only FROM check is done. "!" is used to match packets not
falling in the range.
.TP
\fB--connbytes-dir\fR [\fBoriginal\fR|\fBreply\fR|\fBboth\fR]
which packets to consider
.TP
\fB--connbytes-mode\fR [\fBpackets\fR|\fBbytes\fR|\fBavgpkt\fR]
whether to check the amount of packets, number of bytes transferred or
the average size (in bytes) of all packets received so far. Note that
when "both" is used together with "avgpkt", and data is going (mainly)
only in one direction (for example HTTP), the average packet size will
be about half of the actual data packets.
.TP
Example:
iptables .. -m connbytes --connbytes 10000:100000 --connbytes-dir both --connbytes-mode bytes ...
.SS connlimit
Allows you to restrict the number of parallel TCP connections to a
server per client IP address (or address block).
.TP
[\fB!\fR] \fB--connlimit-above \fIn\fR
match if the number of existing tcp connections is (not) above n
.TP
.BI "--connlimit-mask " "bits"
group hosts using mask
.P
Examples:
.TP
# allow 2 telnet connections per client host
iptables -p tcp --syn --dport 23 -m connlimit --connlimit-above 2 -j REJECT
.TP
# you can also match the other way around:
iptables -p tcp --syn --dport 23 -m connlimit ! --connlimit-above 2 -j ACCEPT
.TP
# limit the nr of parallel http requests to 16 per class C sized \
network (24 bit netmask)
iptables -p tcp --syn --dport 80 -m connlimit --connlimit-above 16
--connlimit-mask 24 -j REJECT
.SS connmark
This module matches the netfilter mark field associated with a connection
(which can be set using the
.B CONNMARK
target below).
.TP
.BI "--mark " "value[/mask]"
Matches packets in connections with the given mark value (if a mask is
specified, this is logically ANDed with the mark before the
comparison).
.SS connrate
This module matches the current transfer rate in a connection.
.TP
.BI "--connrate " "[!] [\fIfrom\fP]:[\fIto\fP]"
Match against the current connection transfer rate being within 'from'
and 'to' bytes per second. When the "!" argument is used before the
range, the sense of the match is inverted.
.SS conntrack
This module, when combined with connection tracking, allows access to
more connection tracking information than the "state" match.
(this module is present only if iptables was compiled under a kernel
supporting this feature)
.TP
.BI "--ctstate " "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.
.B SNAT
A virtual state, matching if the original source address differs from
the reply destination.
.B DNAT
A virtual state, matching if the original destination differs from the
reply source.
.TP
.BI "--ctproto " "proto"
Protocol to match (by number or name)
.TP
.BI "--ctorigsrc " "[!] \fIaddress\fP[/\fImask\fP]"
Match against original source address
.TP
.BI "--ctorigdst " "[!] \fIaddress\fP[/\fImask\fP]"
Match against original destination address
.TP
.BI "--ctreplsrc " "[!] \fIaddress\fP[/\fImask\fP]"
Match against reply source address
.TP
.BI "--ctrepldst " "[!] \fIaddress\fB[/\fImask\fP]"
Match against reply destination address
.TP
.BI "--ctstatus " "[\fINONE|EXPECTED|SEEN_REPLY|ASSURED\fP][,...]"
Match against internal conntrack states
.TP
.BI "--ctexpire " "\fItime\fP[\fI:time\fP]"
Match remaining lifetime in seconds against given value
or range of values (inclusive)
.SS dscp
This module matches the 6 bit DSCP field within the TOS field in the
IP header.  DSCP has superseded TOS within the IETF.
.TP
.BI "--dscp " "value"
Match against a numeric (decimal or hex) value [0-32].
.TP
.BI "--dscp-class " "\fIDiffServ Class\fP"
Match the DiffServ class. This value may be any of the
BE, EF, AFxx or CSx classes.  It will then be converted
into it's according numeric value.
.SS dstlimit
This module allows you to limit the packet per second (pps) rate on a per
destination IP or per destination port base.  As opposed to the `limit' match,
every destination ip / destination port has it's own limit.
.TP
.BI "--dstlimit " "avg"
Maximum average match rate (packets per second unless followed by /sec /minute /hour /day postfixes).
.TP
.BI "--dstlimit-mode " "mode"
The limiting hashmode.  Is the specified limit per
.B dstip, dstip-dstport
tuple, 
.B srcip-dstip
tuple, or per
.B srcipdstip-dstport
tuple.
.TP
.BI "--dstlimit-name " "name"
Name for /proc/net/ipt_dstlimit/* file entry
.TP
.BI "[" "--dstlimit-burst " "burst" "]"
Number of packets to match in a burst.  Default: 5
.TP
.BI "[" "--dstlimit-htable-size " "size" "]"
Number of buckets in the hashtable
.TP
.BI "[" "--dstlimit-htable-max " "max" "]"
Maximum number of entries in the hashtable
.TP
.BI "[" "--dstlimit-htable-gcinterval " "interval" "]"
Interval between garbage collection runs of the hashtable (in miliseconds).
Default is 1000 (1 second).
.TP
.BI "[" "--dstlimit-htable-expire " "time"
After which time are idle entries expired from hashtable (in miliseconds)?
Default is 10000 (10 seconds).
.SS ecn
This allows you to match the ECN bits of the IPv4 and TCP header.  ECN is the Explicit Congestion Notification mechanism as specified in RFC3168
.TP
.BI "--ecn-tcp-cwr"
This matches if the TCP ECN CWR (Congestion Window Received) bit is set.
.TP
.BI "--ecn-tcp-ece"
This matches if the TCP ECN ECE (ECN Echo) bit is set.
.TP
.BI "--ecn-ip-ect " "num"
This matches a particular IPv4 ECT (ECN-Capable Transport). You have to specify
a number between `0' and `3'.
.SS esp
This module matches the SPIs in ESP header of IPSec packets.
.TP
.BR "--espspi " "[!] \fIspi\fP[:\fIspi\fP]"
.SS fuzzy
This module matches a rate limit based on a fuzzy logic controller [FLC]
.TP
.BI "--lower-limit  "number"
Specifies the lower limit (in packets per second).
.TP
.BI "--upper-limit " "number"
Specifies the upper limit (in packets per second).
.SS hashlimit
This patch adds a new match called 'hashlimit'.
The idea is to have something like 'limit', but either per
destination-ip or per (destip,destport) tuple.

It gives you the ability to express
.IP
 '1000 packets per second for every host in 192.168.0.0/16'
.IP
 '100 packets per second for every service of 192.168.1.1'
.P
with a single iptables rule.
.TP
.BI "--hashlimit " "rate"
A rate just like the limit match
.TP
.BI "--hashlimit-burst " "num"
Burst value, just like limit match
.TP
.BI "--hashlimit-mode " "destip | destip-destport"
Limit per IP or per port
.TP
.BI "--hashlimit-name " "foo"
The name for the /proc/net/ipt_hashlimit/foo entry
.TP
.BI "--hashlimit-htable-size " "num"
The number of buckets of the hash table
.TP
.BI "--hashlimit-htable-max " "num"
Maximum entries in the hash
.TP
.BI "--hashlimit-htable-expire " "num"
After how many miliseconds do hash entries expire
.TP
.BI "--hashlimit-htable-gcinterval " "num"
How many miliseconds between garbage collection intervals
.SS helper
This module matches packets related to a specific conntrack-helper.
.TP
.BI "--helper " "string"
Matches packets related to the specified conntrack-helper.
.RS
.PP
string can be "ftp" for packets related to a ftp-session on default port.
For other ports append -portnr to the value, ie. "ftp-2121".
.PP
Same rules apply for other conntrack-helpers.
.RE
.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
.nf
 iptables -p icmp -h
.fi
.SS iprange
This matches on a given arbitrary range of IPv4 addresses
.TP
.BI "[!]" "--src-range " "ip-ip"
Match source IP in the specified range.
.TP
.BI "[!]" "--dst-range " "ip-ip"
Match destination IP in the specified range.
.SS ipv4options
Match on IPv4 header options like source routing, record route,
timestamp and router-alert.
.TP
.B "--ssrr"
To match packets with the flag strict source routing.
.TP
.B "--lsrr"
To match packets with the flag loose source routing.
.TP
.B "--no-srr"
To match packets with no flag for source routing.
.TP
.B "\fR[\fB!\fR]\fB --rr"
To match packets with the RR flag.
.TP
.B "\fR[\fB!\fR]\fB --ts"
To match packets with the TS flag.
.TP
.B "\fR[\fB!\fR]\fB --ra"
To match packets with the router-alert option.
.TP
.B "\fR[\fB!\fR]\fB --any-opt"
To match a packet with at least one IP option, or no IP option
at all if ! is chosen.
.TP
Examples:
.TP
$ iptables -A input -m ipv4options --rr -j DROP
will drop packets with the record-route flag.
.TP
$ iptables -A input -m ipv4options --ts -j DROP
will drop packets with the timestamp flag.
.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 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 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 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 mport
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 multiport
This module matches a set of source or destination ports.  Up to 15
ports can be specified.  A port range (port:port) counts as two
ports.  It can only be used in conjunction with
.B "-p tcp"
or
.BR "-p udp" .
.TP
.BR "--source-ports " "\fI[!] port\fP[,\fIport\fP[,\fIport:port\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 " "\fI[!] port\fP[,\fIport\fP[,\fIport:port\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 " "\fI[!] port\fP[,\fIport\fP[,\fIport:port\fP...]]"
Match if either the source or destination ports are equal to one of
the given ports.
.SS nth
This module matches every `n'th packet
.TP
.BI "--every " "value"
Match every `value' packet
.TP
.BI "[" "--counter " "num" "]"
Use internal counter number `num'.  Default is `0'.
.TP
.BI "[" "--start " "num" "]"
Initialize the counter at the number `num' insetad of `0'.  Most between `0'
and `value'-1.
.TP
.BI "[" "--packet " "num" "]"
Match on `num' packet.  Most be between `0' and `value'-1.
.SS osf
The idea of passive OS fingerprint matching exists for quite a long time,
but was created as extension fo OpenBSD pf only some weeks ago.
Original idea was lurked in some OpenBSD mailing list (thanks
grange@open...) and than adopted for Linux netfilter in form of this code.

Original fingerprint table was created by Michal Zalewski <lcamtuf@coredump.cx>.

This module compares some data(WS, MSS, options and it's order, ttl,
df and others) from first SYN packet (actually from packets with SYN
bit set) with dynamically loaded OS fingerprints.
.TP
.B "--log 1/0" 
If present, OSF will log determined genres even if they don't match
desired one.    
0 - log all determined entries, 
1 - only first one.

In syslog you find something like this:
.IP
ipt_osf: Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> 11.22.33.44:139
.IP
ipt_osf: Unknown: 16384:106:1:48:020405B401010402 44.33.22.11:1239 -> 11.22.33.44:80
.TP
.B "--smart"
if present, OSF will use some smartness to determine remote OS.
OSF will use initial TTL only if source of connection is in our local network.
.TP
.B "--netlink"
If present, OSF will log all events also through netlink NETLINK_NFLOG groupt 1.
.TP
.BI "--genre " "[!] string"
Match a OS genre by passive fingerprinting
.P
Example:

#iptables -I INPUT -j ACCEPT -p tcp -m osf --genre Linux --log 1 --smart

NOTE: -p tcp is obviously required as it is a TCP match.

Fingerprints can be loaded and read through /proc/sys/net/ipv4/osf file.
One can flush all fingerprints with following command:
.IP
echo -en FLUSH > /proc/sys/net/ipv4/osf
.P
Only one fingerprint per open/write/close.

Fingerprints can be downloaded from http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os
.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)
.TP
.B NOTE: pid, sid and command matching are broken on SMP
.SS physdev
This module matches on the bridge port input and output devices enslaved
to a bridge device. This module is a part of the infrastructure that enables
a transparent bridging IP firewall and is only useful for kernel versions
above version 2.5.44.
.TP
.B --physdev-in name
Name of a bridge port via which a packet is received (only for
packets entering the
.BR INPUT ,
.B FORWARD
and
.B PREROUTING
chains). If the interface name ends in a "+", then any
interface which begins with this name will match. If the packet didn't arrive
through a bridge device, this packet won't match this option, unless '!' is used.
.TP
.B --physdev-out name
Name of a bridge port via which a packet is going to be sent (for packets
entering the
.BR FORWARD ,
.B OUTPUT
and
.B POSTROUTING
chains).  If the interface name ends in a "+", then any
interface which begins with this name will match. Note that in the
.BR nat " and " mangle
.B OUTPUT
chains one cannot match on the bridge output port, however one can in the
.B "filter OUTPUT"
chain. If the packet won't leave by a bridge device or it is yet unknown what
the output device will be, then the packet won't match this option, unless
'!' is used.
.TP
.B --physdev-is-in
Matches if the packet has entered through a bridge interface.
.TP
.B --physdev-is-out
Matches if the packet will leave through a bridge interface.
.TP
.B --physdev-is-bridged
Matches if the packet is being bridged and therefore is not being routed.
This is only useful in the FORWARD and POSTROUTING chains.
.SS pkttype
This module matches the link-layer packet type.
.TP
.BI "--pkt-type " "[\fIunicast\fP|\fIbroadcast\fP|\fImulticast\fP]"
.SS psd
Attempt to detect TCP and UDP port scans. This match was derived from
Solar Designer's scanlogd.
.TP
.BI "--psd-weight-threshold " "threshold"
Total weight of the latest TCP/UDP packets with different
destination ports coming from the same host to be treated as port
scan sequence.
.TP
.BI "--psd-delay-threshold " "delay"
Delay (in hundredths of second) for the packets with different
destination ports coming from the same host to be treated as
possible port scan subsequence.
.TP
.BI "--psd-lo-ports-weight " "weight"
Weight of the packet with privileged (<=1024) destination port.
.TP
.BI "--psd-hi-ports-weight " "weight"
Weight of the packet with non-priviliged destination port.
.SS quota
Implements network quotas by decrementing a byte counter with each
packet.
.TP
.BI "--quota " "bytes"
The quota in bytes.
.P
KNOWN BUGS: this does not work on SMP systems.
.SS random
This module randomly matches a certain percentage of all packets.
.TP
.BI "--average " "percent"
Matches the given percentage.  If omitted, a probability of 50% is set. 
.SS realm
This matches the routing realm.  Routing realms are used in complex routing
setups involving dynamic routing protocols like BGP.
.TP
.BI "--realm " "[!]" "value[/mask]"
Matches a given realm number (and optionally mask).
.SS recent
Allows you to dynamically create a list of IP addresses and then match
against that list in a few different ways.

For example, you can create a `badguy' list out of people attempting
to connect to port 139 on your firewall and then DROP all future
packets from them without considering them.
.TP
.BI "--name " "name"
Specify the list to use for the commands. If no name is given then 'DEFAULT'
will be used.
.TP
[\fB!\fR] \fB--set\fR
This will add the source address of the packet to the list. If the
source address is already in the list, this will update the existing
entry. This will always return success (or failure if `!' is passed
in).
.TP
[\fB!\fR] \fB--rcheck\fR
Check if the source address of the packet is currently in
the list.
.TP
[\fB!\fR] \fB--update\fR
Like \fB--rcheck\fR, except it will update the "last seen" timestamp if it
matches.
.TP
[\fB!\fR] \fB--remove\fR
Check if the source address of the packet is currently in the list and
if so that address will be removed from the list and the rule will
return true. If the address is not found, false is returned.
.TP
[\fB!\fR] \fB--seconds \fIseconds\fR
This option must be used in conjunction with one of \fB--rcheck\fR or
\fB--update\fR. When used, this will narrow the match to only happen
when the address is in the list and was seen within the last given
number of seconds.
.TP
[\fB!\fR] \fB--hitcount \fIhits\fR
This option must be used in conjunction with one of \fB--rcheck\fR or
\fB--update\fR. When used, this will narrow the match to only happen
when the address is in the list and packets had been received greater
than or equal to the given value. This option may be used along with
\fB--seconds\fR to create an even narrower match requiring a certain
number of hits within a specific time frame.
.TP
\fB--rttl\fR
This option must be used in conjunction with one of \fB--rcheck\fR or
\fB--update\fR. When used, this will narrow the match to only happen
when the address is in the list and the TTL of the current packet
matches that of the packet which hit the \fB--set\fR rule. This may be
useful if you have problems with people faking their source address in
order to DoS you via this module by disallowing others access to your
site by sending bogus packets to you.
.P
Examples:
.IP
# iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP

# iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j DROP
.P
Official website (http://snowman.net/projects/ipt_recent/) also has
some examples of usage.

/proc/net/ipt_recent/* are the current lists of addresses and information 
about each entry of each list.

Each file in /proc/net/ipt_recent/ can be read from to see the current list
or written two using the following commands to modify the list:
.TP
echo xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Add to the DEFAULT list
.TP
echo -xx.xx.xx.xx > /proc/net/ipt_recent/DEFAULT
to Remove from the DEFAULT list
.TP
echo clear > /proc/net/ipt_recent/DEFAULT
to empty the DEFAULT list.
.P
The module itself accepts parameters, defaults shown:
.TP
.BI "ip_list_tot=" "100"
Number of addresses remembered per table
.TP
.BI "ip_pkt_list_tot=" "20"
Number of packets per address remembered
.TP
.BI "ip_list_hash_size=" "0"
Hash table size. 0 means to calculate it based on ip_list_tot, default: 512
.TP
.BI "ip_list_perms=" "0644"
Permissions for /proc/net/ipt_recent/* files
.TP
.BI "debug=" "0"
Set to 1 to get lots of debugging info
.SS sctp
.TP
\fB--source-port\fR,\fB--sport \fR[\fB!\fR] \fIport\fR[\fB:\fIport\fR]
.TP
\fB--destination-port\fR,\fB--dport \fR[\fB!\fR] \fIport\fR[\fB:\fIport\fR]
.TP
\fB--chunk-types\fR [\fB!\fR] \fBall\fR|\fBany\fR|\fBonly \fIchunktype\fR[\fB:\fIflags\fR] [...]
The flag letter in upper case indicates that the flag is to match if set,
in the lower case indicates to match if unset.

Chunk types: DATA INIT INIT_ACK SACK HEARTBEAT HEARTBEAT_ACK ABORT SHUTDOWN SHUTDOWN_ACK ERROR COOKIE_ECHO COOKIE_ACK ECN_ECNE ECN_CWR SHUTDOWN_COMPLETE ASCONF ASCONF_ACK

chunk type            available flags      
.br
DATA                  U B E u b e         
.br
ABORT                 T t                 
.br
SHUTDOWN_COMPLETE     T t                 

(lowercase means flag should be "off", uppercase means "on")
.P
Examples:

iptables -A INPUT -p sctp --dport 80 -j DROP

iptables -A INPUT -p sctp --chunk-types any DATA,INIT -j DROP

iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT
.SS set
This modules macthes IP sets which can be defined by ipset(8).
.TP
.BR "--set " "setname flag[,flag...]"
where flags are
.BR "src"
and/or
.BR "dst" 
and there can be no more than six of them. Hence the command
.nf
 iptables -A FORWARD -m set --set test src,dst
.fi
will match packets, for which (depending on the type of the set) the source
address or port number of the packet can be found in the specified set. If 
there is a binding belonging to the mached set element or there is a default 
binding for the given set, then the rule will match the packet only if 
additionally (depending on the type of the set) the destination address or 
port number of the packet can be found in the set according to the binding.
.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 could not be identified for some reason which
includes running out of memory and ICMP errors which don't correspond to any
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 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
.nf
 iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
.fi
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,RST 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,FIN 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 tcpmss
This matches the TCP MSS (maximum segment size) field of the TCP header.  You can only use this on TCP SYN or SYN/ACK packets, since the MSS is only negotiated during the TCP handshake at connection startup time.
.TP
.BI "[!] "--mss " "value[:value]"
Match a given TCP MSS value or range.
.SS time
This matches if the packet arrival time/date is within a given range. All options are facultative.
.TP
.BI " --timestart " "value"
Match only if it is after `value' (Inclusive, format: HH:MM ; default 00:00).
.TP
.BI "--timestop  " "value"
Match only if it is before `value' (Inclusive, format: HH:MM ; default 23:59).
.TP
.BI "--days " "listofdays"
Match only if today is one of the given days. (format: Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)
.TP
.BI "--datestart " "date"
Match only if it is after `date' (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 1970)
.TP
.BI "--datestop " "date"
Match only if it is before `date' (Inclusive, format: YYYY[:MM[:DD[:hh[:mm[:ss]]]]] ; h,m,s start from 0 ; default to 2037)
.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 ttl
This module matches the time to live field in the IP header.
.TP
.BI "--ttl-eq " "ttl"
Matches the given TTL value.
.TP
.BI "--ttl-gt " "ttl"
Matches if TTL is greater than the given TTL value.
.TP
.BI "--ttl-lt " "ttl"
Matches if TTL is less than the given TTL value.
.SS u32
U32 allows you to extract quantities of up to 4 bytes from a packet,
AND them with specified masks, shift them by specified amounts and
test whether the results are in any of a set of specified ranges.
The specification of what to extract is general enough to skip over
headers with lengths stored in the packet, as in IP or TCP header
lengths.

Details and examples are in the kernel module source.
.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 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.
.\" @TARGET@
.SS BALANCE
This allows you to DNAT connections in a round-robin way over a given range of destination addresses.
.TP
.BI "--to-destination " "ipaddr-ipaddr"
Address range to round-robin over.
.SS CLASSIFY
This module allows you to set the skb->priority value (and thus classify the packet into a specific CBQ class).
.TP
.BI "--set-class " "MAJOR:MINOR"
Set the major and minor class value.
.SS CLUSTERIP
This module allows you to configure a simple cluster of nodes that share
a certain IP and MAC address without an explicit load balancer in front of
them.  Connections are statically distributed between the nodes in this
cluster.
.TP
.BI "--new "
Create a new ClusterIP.  You always have to set this on the first rule
for a given ClusterIP.
.TP
.BI "--hashmode " "mode"
Specify the hashing mode.  Has to be one of
.B sourceip, sourceip-sourceport, sourceip-sourceport-destport
.TP
.BI "--clustermac " "mac"
Specify the ClusterIP MAC address.  Has to be a link-layer multicast address
.TP
.BI "--total-nodes " "num"
Number of total nodes within this cluster.
.TP
.BI "--local-node " "num"
Local node number within this cluster.
.TP
.BI "--hash-init " "rnd"
Specify the random seed used for hash initialization.
.SS CONNMARK
This module sets the netfilter mark value associated with a connection
.TP
.B --set-mark mark[/mask]
Set connection mark. If a mask is specified then only those bits set in the
mask is modified.
.TP
.B --save-mark [--mask mask]
Copy the netfilter packet mark value to the connection mark. If a mask
is specified then only those bits are copied.
.TP
.B --restore-mark [--mask mask]
Copy the connection mark value to the packet. If a mask is specified
then only those bits are copied. This is only valid in the
.B mangle
table.
.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
type of 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.
.RS
.PP
You can add several --to-destination options.  If you specify more
than one destination address, either via an address range or multiple
--to-destination options, a simple round-robin (one after another in
cycle) load balancing takes place between these adresses.
.SS DSCP
This target allows to alter the value of the DSCP bits within the TOS
header of the IPv4 packet.  As this manipulates a packet, it can only
be used in the mangle table.
.TP
.BI "--set-dscp " "value"
Set the DSCP field to a numerical value (can be decimal or hex)
.TP
.BI "--set-dscp-class " "class"
Set the DSCP field to a DiffServ class.
.SS ECN
This target allows to selectively work around known ECN blackholes.
It can only be used in the mangle table.
.TP
.BI "--ecn-tcp-remove"
Remove all ECN bits from the TCP header.  Of course, it can only be used
in conjunction with
.BR "-p tcp" .
.SS IPMARK
Allows you to mark a received packet basing on its IP address. This
can replace many mangle/mark entries with only one, if you use
firewall based classifier.

This target is to be used inside the mangle table, in the PREROUTING,
POSTROUTING or FORWARD hooks.
.TP
.BI "--addr " "src/dst"
Use source or destination IP address.
.TP
.BI "--and-mask " "mask"
Perform bitwise `and' on the IP address and this mask.
.TP
.BI "--or-mask " "mask"
Perform bitwise `or' on the IP address and this mask.
.P
The order of IP address bytes is reversed to meet "human order of bytes":
192.168.0.1 is 0xc0a80001. At first the `and' operation is performed, then
`or'.

Examples:

We create a queue for each user, the queue number is adequate
to the IP address of the user, e.g.: all packets going to/from 192.168.5.2
are directed to 1:0502 queue, 192.168.5.12 -> 1:050c etc.

We have one classifier rule:
.IP
tc filter add dev eth3 parent 1:0 protocol ip fw
.P
Earlier we had many rules just like below:
.IP
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.2 -j MARK
--set-mark 0x10502
.IP
iptables -t mangle -A POSTROUTING -o eth3 -d 192.168.5.3 -j MARK
--set-mark 0x10503
.P
Using IPMARK target we can replace all the mangle/mark rules with only one:
.IP
iptables -t mangle -A POSTROUTING -o eth3 -j IPMARK --addr=dst
--and-mask=0xffff --or-mask=0x10000
.P
On the routers with hundreds of users there should be significant load
decrease (e.g. twice).
.SS IPV4OPTSSTRIP
Strip all the IP options from a packet.

The target doesn't take any option, and therefore is extremly easy to use :

# iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP
.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 criteria, 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.
.TP
.B --log-uid
Log the userid of the process which generated the packet.
.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 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 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 NETMAP
This target allows you to statically map a whole network of addresses onto
another network of addresses.  It can only be used from rules in the
.B nat
table.
.TP
.BI "--to "  "address[/mask]"
Network address to map to.  The resulting address will be constructed in the
following way: All 'one' bits in the mask are filled in from the new `address'.
All bits that are zero in the mask are filled in from the original address.
.SS NOTRACK
This target disables connection tracking for all packets matching that rule.
.TP
It can only be used in the
.B raw
table.
.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 redirects the packet to the machine itself by changing the
destination IP to the primary address of the incoming interface
(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 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
.nf
.B " icmp-net-unreachable"
.B " icmp-host-unreachable"
.B " icmp-port-unreachable"
.B " icmp-proto-unreachable"
.B " icmp-net-prohibited"
.B " icmp-host-prohibited or"
.B " icmp-admin-prohibited (*)"
.fi
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).
.TP
(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
.SS ROUTE
This is used to explicitly override the core network stack's routing decision.
.B mangle
table.
.TP
.BI "--oif " "ifname"
Route the packet through `ifname' network interface
.TP
.BI "--iif " "ifname"
Change the packet's incoming interface to `ifname'
.TP
.BI "--gw " "IP_address"
Route the packet via this gateway
.TP
.BI "--continue "
Behave like a non-terminating target and continue traversing the rules.  Not valid in combination with `--iif' or `--tee'
.TP
.BI "--tee "
Make a copy of the packet, and route that copy to the given destination. For the original, uncopied packet, behave like a non-terminating target and continue traversing the rules.  Not valid in combination with `--iif' or `--continue'
.SS SAME
Similar to SNAT/DNAT depending on chain: it takes a range of addresses
(`--to 1.2.3.4-1.2.3.7') and gives a client the same
source-/destination-address for each connection.
.TP
.BI "--to " "<ipaddr>-<ipaddr>"
Addresses to map source to. May be specified more than once for
multiple ranges.
.TP
.B "--nodst"
Don't use the destination-ip in the calculations when selecting the
new source-ip
.SS SET
This modules adds and/or deletes entries from IP sets which can be defined 
by ipset(8).
.TP
.BR "--add-set " "setname flag[,flag...]"
add the address(es)/port(s) of the packet to the sets
.TP
.BR "--del-set " "setname flag[,flag...]"
delete the address(es)/port(s) of the packet from the sets,
where flags are
.BR "src"
and/or
.BR "dst"
and there can be no more than six of them.
.TP
The bindings to follow must previously be defined in order to use 
multilevel adding/deleting by the SET target.
.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 type
of 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.
.RS
.PP
You can add several --to-source options.  If you specify more
than one source address, either via an address range or multiple
--to-source options, a simple round-robin (one after another in
cycle) takes place between these adresses.
.SS TARPIT
Captures and holds incoming TCP connections using no local
per-connection resources. Connections are accepted, but immediately
switched to the persist state (0 byte window), in which the remote
side stops sending data and asks to continue every 60-240 seconds.
Attempts to close the connection are ignored, forcing the remote side
to time out the connection in 12-24 minutes.

This offers similar functionality to LaBrea
<http://www.hackbusters.net/LaBrea/> but doesn't require dedicated
hardware or IPs. Any TCP port that you would normally DROP or REJECT
can instead become a tarpit.

To tarpit connections to TCP port 80 destined for the current machine:
.IP
iptables -A INPUT -p tcp -m tcp --dport 80 -j TARPIT
.P
To significantly slow down Code Red/Nimda-style scans of unused address
space, forward unused ip addresses to a Linux box not acting as a router
(e.g. "ip route 10.0.0.0 255.0.0.0 ip.of.linux.box" on a Cisco), enable IP
forwarding on the Linux box, and add:
.IP
iptables -A FORWARD -p tcp -j TARPIT
.IP
iptables -A FORWARD -j DROP
.TP
NOTE:
If you use the conntrack module while you are using TARPIT, you should
also use the NOTRACK target, or the kernel will unnecessarily allocate
resources for each TARPITted connection. To TARPIT incoming
connections to the standard IRC port while using conntrack, you could:
.IP
iptables -t raw -A PREROUTING -p tcp --dport 6667 -j NOTRACK
.IP
iptables -A INPUT -p tcp --dport 6667 -j TARPIT
.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:
.PD 0
.RS 0.1i
.TP 0.3i
1)
Web browsers connect, then hang with no data received.
.TP
2)
Small mail works fine, but large emails hang.
.TP
3)
ssh works fine, but scp hangs after initial handshaking.
.RE
.PD
Workaround: activate this option and add a rule to your firewall
configuration like:
.nf
 iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \\
             -j TCPMSS --clamp-mss-to-pmtu
.fi
.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.
.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
.nf
 iptables -j TOS -h
.fi
to see the list of valid TOS names.
.SS TRACE
This target has no options.  It just turns on 
.B packet tracing
for all packets that match this rule.
.SS TTL
This is used to modify the IPv4 TTL header field.  The TTL field determines
how many hops (routers) a packet can traverse until it's time to live is
exceeded.
.TP
Setting or incrementing the TTL field can potentially be very dangerous,
so it should be avoided at any cost.  
.TP
.B Don't ever set or increment the value on packets that leave your local network!
.B mangle
table.
.TP
.BI "--ttl-set " "value"
Set the TTL value to `value'.
.TP
.BI "--ttl-dec " "value"
Decrement the TTL value `value' times.
.TP
.BI "--ttl-inc " "value"
Increment the TTL value `value' times.
.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.
Like LOG, this is a "non-terminating target", i.e. rule traversal
continues at the next rule.
.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 for 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).
.br
.SS XOR
Encrypt TCP and UDP traffic using a simple XOR encryption
.TP
.BI "--key " "string"
Set key to "string"
.TP
.BI "--block-size"
Set block size
.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, you might want to have a look at http://bugzilla.netfilter.org/
.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 ..