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IPTABLES(8)			iptables 1.4.6			   IPTABLES(8)

NAME
       iptables — administration tool for IPv4 packet filtering and NAT

SYNOPSIS
       iptables [-t table] {-A|-D} chain rule-specification

       iptables [-t table] -I chain [rulenum] rule-specification

       iptables [-t table] -R chain rulenum rule-specification

       iptables [-t table] -D chain rulenum

       iptables [-t table] -S [chain [rulenum]]

       iptables [-t table] {-F|-L|-Z} [chain [rulenum]] [options...]

       iptables [-t table] -N chain

       iptables [-t table] -X [chain]

       iptables [-t table] -P chain target

       iptables [-t table] -E old-chain-name new-chain-name

       rule-specification = [matches...] [target]

       match = -m matchname [per-match-options]

       target = -j targetname [per-target-options]

DESCRIPTION
       Iptables	 is  used  to set up, maintain, and inspect the tables of IPv4
       packet filter rules in the Linux kernel.	 Several different tables  may
       be  defined.   Each  table contains a number of built-in chains and may
       also contain user-defined chains.

       Each chain is a list of rules which can match a set of  packets.	  Each
       rule specifies what to do with a packet that matches.  This is called a
       `target', which may be a jump to a user-defined chain in the  same  ta‐
       ble.

TARGETS
       A  firewall  rule specifies criteria for a packet and a target.	If the
       packet does not match, the next rule in the chain is the	 examined;  if
       it does match, then the next rule is specified by the value of the tar‐
       get, which can be the name of a user-defined chain or one of  the  spe‐
       cial values ACCEPT, DROP, QUEUE or RETURN.

       ACCEPT  means to let the packet through.	 DROP means to drop the packet
       on the floor.  QUEUE means to pass the packet to userspace.   (How  the
       packet can be received by a userspace process differs by the particular
       queue handler.  2.4.x and  2.6.x	 kernels  up  to  2.6.13  include  the
       ip_queue	 queue handler.	 Kernels 2.6.14 and later additionally include
       the nfnetlink_queue queue handler.  Packets with a target of QUEUE will
       be  sent	 to queue number '0' in this case. Please also see the NFQUEUE
       target as described  later  in  this  man  page.)   RETURN  means  stop
       traversing  this	 chain	and  resume  at	 the next rule in the previous
       (calling) chain.	 If the end of a built-in chain is reached or  a  rule
       in a built-in chain with target RETURN is matched, the target specified
       by the chain policy determines the fate of the packet.

TABLES
       There are currently three independent tables (which tables are  present
       at  any time depends on the kernel configuration options and which mod‐
       ules are present).

       -t, --table table
	      This option specifies the packet matching table which  the  com‐
	      mand  should operate on.	If the kernel is configured with auto‐
	      matic module loading, an attempt will be made to load the appro‐
	      priate module for that table if it is not already there.

	      The tables are as follows:

	      filter:
		  This	is  the	 default table (if no -t option is passed). It
		  contains the built-in chains INPUT (for packets destined  to
		  local	 sockets),  FORWARD  (for packets being routed through
		  the box), and OUTPUT (for locally-generated packets).

	      nat:
		  This table is consulted when a packet	 that  creates	a  new
		  connection  is encountered.  It consists of three built-ins:
		  PREROUTING (for altering packets as soon as they  come  in),
		  OUTPUT  (for altering locally-generated packets before rout‐
		  ing), and POSTROUTING (for  altering	packets	 as  they  are
		  about to go out).

	      mangle:
		  This table is used for specialized packet alteration.	 Until
		  kernel 2.4.17 it had two built-in  chains:  PREROUTING  (for
		  altering  incoming  packets  before routing) and OUTPUT (for
		  altering locally-generated packets before  routing).	 Since
		  kernel  2.4.18,  three  other	 built-in chains are also sup‐
		  ported: INPUT (for packets coming into the box itself), FOR‐
		  WARD	(for  altering	packets being routed through the box),
		  and POSTROUTING (for altering packets as they are  about  to
		  go out).

	      raw:
		  This	table  is  used mainly for configuring exemptions from
		  connection tracking in combination with the NOTRACK  target.
		  It registers at the netfilter hooks with higher priority and
		  is thus called before ip_conntrack, or any other IP  tables.
		  It  provides	the following built-in chains: PREROUTING (for
		  packets arriving via	any  network  interface)  OUTPUT  (for
		  packets generated by local processes)

OPTIONS
       The options that are recognized by iptables can be divided into several
       different groups.

   COMMANDS
       These options specify the desired action to perform. Only one  of  them
       can be specified on the command line unless otherwise stated below. For
       long versions of the command and option names, you  need	 to  use  only
       enough  letters	to  ensure that iptables can differentiate it from all
       other options.

       -A, --append chain rule-specification
	      Append one or more rules to the end of the selected chain.  When
	      the  source  and/or  destination	names resolve to more than one
	      address, a rule will be added for each possible address combina‐
	      tion.

       -D, --delete chain rule-specification
       -D, --delete chain rulenum
	      Delete one or more rules from the selected chain.	 There are two
	      versions of this command: the rule can be specified as a	number
	      in  the  chain  (starting	 at 1 for the first rule) or a rule to
	      match.

       -I, --insert chain [rulenum] rule-specification
	      Insert one or more rules in the selected chain as the given rule
	      number.	So,  if	 the  rule  number is 1, the rule or rules are
	      inserted at the head of the chain.  This is also the default  if
	      no rule number is specified.

       -R, --replace chain rulenum rule-specification
	      Replace a rule in the selected chain.  If the source and/or des‐
	      tination names resolve to multiple addresses, the	 command  will
	      fail.  Rules are numbered starting at 1.

       -L, --list [chain]
	      List  all rules in the selected chain.  If no chain is selected,
	      all chains are listed. Like every	 other	iptables  command,  it
	      applies  to  the specified table (filter is the default), so NAT
	      rules get listed by
	       iptables -t nat -n -L
	      Please note that it is often used with the -n option,  in	 order
	      to  avoid	 long reverse DNS lookups.  It is legal to specify the
	      -Z (zero) option as well, in which case  the  chain(s)  will  be
	      atomically  listed  and zeroed.  The exact output is affected by
	      the other arguments given. The exact rules are suppressed	 until
	      you use
	       iptables -L -v

       -S, --list-rules [chain]
	      Print all rules in the selected chain.  If no chain is selected,
	      all chains are printed like iptables-save. Like every other ipt‐
	      ables  command, it applies to the specified table (filter is the
	      default).

       -F, --flush [chain]
	      Flush the selected chain (all the chains in the table if none is
	      given).	This  is  equivalent  to deleting all the rules one by
	      one.

       -Z, --zero [chain [rulenum]]
	      Zero the packet and byte counters in all	chains,	 or  only  the
	      given  chain,  or only the given rule in a chain. It is legal to
	      specify the -L, --list (list) option as well, to see  the	 coun‐
	      ters immediately before they are cleared. (See above.)

       -N, --new-chain chain
	      Create  a	 new user-defined chain by the given name.  There must
	      be no target of that name already.

       -X, --delete-chain [chain]
	      Delete the optional user-defined chain specified.	 There must be
	      no  references  to  the chain.  If there are, you must delete or
	      replace the referring rules before the  chain  can  be  deleted.
	      The  chain  must	be  empty,  i.e. not contain any rules.	 If no
	      argument is given, it will attempt to delete  every  non-builtin
	      chain in the table.

       -P, --policy chain target
	      Set  the policy for the chain to the given target.  See the sec‐
	      tion TARGETS for the legal targets.   Only  built-in  (non-user-
	      defined)	chains	can  have  policies,  and neither built-in nor
	      user-defined chains can be policy targets.

       -E, --rename-chain old-chain new-chain
	      Rename the user specified chain to the user supplied name.  This
	      is cosmetic, and has no effect on the structure of the table.

       -h     Help.   Give a (currently very brief) description of the command
	      syntax.

   PARAMETERS
       The following parameters make up a rule specification (as used  in  the
       add, delete, insert, replace and append commands).

       [!] -p, --protocol protocol
	      The  protocol of the rule or of the packet to check.  The speci‐
	      fied protocol can be one of tcp, udp, udplite,  icmp,  esp,  ah,
	      sctp  or	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 pro‐
	      tocol inverts the test.  The number zero is equivalent  to  all.
	      Protocol	all  will  match  with	all  protocols and is taken as
	      default when this option is omitted.

       [!] -s, --source address[/mask][,...]
	      Source specification. Address can be either a  network  name,  a
	      hostname,	 a  network  IP	 address  (with	 /mask), or a plain IP
	      address. Hostnames will be resolved once only, before  the  rule
	      is  submitted  to	 the  kernel.  Please note that specifying any
	      name to be resolved with a remote query such as DNS is a	really
	      bad idea.	 The mask can be either a network mask or a plain num‐
	      ber, specifying the number of 1's at the left side of  the  net‐
	      work  mask.   Thus, a mask of 24 is equivalent to 255.255.255.0.
	      A "!" argument before  the  address  specification  inverts  the
	      sense  of	 the  address.	The  flag  --src  is an alias for this
	      option.  Multiple addresses can  be  specified,  but  this  will
	      expand  to  multiple  rules (when adding with -A), or will cause
	      multiple rules to be deleted (with -D).

       [!] -d, --destination address[/mask][,...]
	      Destination  specification.   See	 the  description  of  the  -s
	      (source)	flag  for  a  detailed description of the syntax.  The
	      flag --dst is an alias for this option.

       -j, --jump target
	      This specifies the target of the rule; i.e., what to do  if  the
	      packet  matches  it.   The  target  can  be a user-defined chain
	      (other than the one this rule is in), one of the special builtin
	      targets  which  decide the fate of the packet immediately, or an
	      extension (see EXTENSIONS below).	 If this option is omitted  in
	      a rule (and -g is not used), then matching the rule will have no
	      effect on the packet's fate, but the counters on the  rule  will
	      be incremented.

       -g, --goto chain
	      This  specifies  that  the  processing should continue in a user
	      specified chain. Unlike the --jump option return will  not  con‐
	      tinue  processing	 in  this  chain but instead in the chain that
	      called us via --jump.

       [!] -i, --in-interface name
	      Name of an interface via which a packet was received  (only  for
	      packets  entering	 the  INPUT,  FORWARD  and PREROUTING chains).
	      When the "!" argument is used before  the	 interface  name,  the
	      sense  is	 inverted.   If the interface name ends in a "+", then
	      any interface which begins with this name will match.   If  this
	      option is omitted, any interface name will match.

       [!] -o, --out-interface name
	      Name of an interface via which a packet is going to be sent (for
	      packets entering the FORWARD, OUTPUT  and	 POSTROUTING  chains).
	      When  the	 "!"  argument	is used before the interface name, the
	      sense is inverted.  If the interface name ends in	 a  "+",  then
	      any  interface  which begins with this name will match.  If this
	      option is omitted, any interface name will match.

       [!] -f, --fragment
	      This means that the rule only refers to second and further frag‐
	      ments  of fragmented packets.  Since there is no way to tell the
	      source or destination ports of such a  packet  (or  ICMP	type),
	      such a packet will not match any rules which specify them.  When
	      the "!" argument precedes the "-f"  flag,	 the  rule  will  only
	      match head fragments, or unfragmented packets.

       -c, --set-counters packets bytes
	      This enables the administrator to initialize the packet and byte
	      counters of a rule (during INSERT, APPEND, REPLACE operations).

   OTHER OPTIONS
       The following additional options can be specified:

       -v, --verbose
	      Verbose output.  This option makes the  list  command  show  the
	      interface	 name,	the  rule options (if any), and the TOS masks.
	      The packet and byte counters are also listed,  with  the	suffix
	      'K',  'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 multipli‐
	      ers respectively (but see the -x	flag  to  change  this).   For
	      appending,  insertion,  deletion	and  replacement,  this causes
	      detailed information on the rule or rules to be printed.

       -n, --numeric
	      Numeric output.  IP addresses and port numbers will  be  printed
	      in  numeric format.  By default, the program will try to display
	      them as host names, network names, or services (whenever	appli‐
	      cable).

       -x, --exact
	      Expand  numbers.	Display the exact value of the packet and byte
	      counters, instead of only the rounded number in  K's  (multiples
	      of  1000)	 M's (multiples of 1000K) or G's (multiples of 1000M).
	      This option is only relevant for the -L command.

       --line-numbers
	      When listing rules, add line numbers to the  beginning  of  each
	      rule, corresponding to that rule's position in the chain.

       --modprobe=command
	      When adding or inserting rules into a chain, use command to load
	      any necessary modules (targets, match extensions, etc).

MATCH EXTENSIONS
       iptables can use extended packet matching modules.  These are loaded in
       two  ways:  implicitly, when -p or --protocol is specified, or with the
       -m or --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  mod‐
       ules  in	 one  line, and you can use the -h or --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 "!" to invert the sense of the match.

   addrtype
       This module matches packets based on their 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.

       The following address types are possible:

       UNSPEC an unspecified address (i.e. 0.0.0.0)

       UNICAST
	      an unicast address

       LOCAL  a local address

       BROADCAST
	      a broadcast address

       ANYCAST
	      an anycast packet

       MULTICAST
	      a multicast address

       BLACKHOLE
	      a blackhole address

       UNREACHABLE
	      an unreachable address

       PROHIBIT
	      a prohibited address

       THROW  FIXME

       NAT    FIXME

       XRESOLVE

       [!] --src-type type
	      Matches if the source address is of given type

       [!] --dst-type type
	      Matches if the destination address is of given type

       --limit-iface-in
	      The address type checking can be limited to  the	interface  the
	      packet  is  coming in. This option is only valid in the PREROUT‐
	      ING, INPUT and FORWARD chains. It cannot be specified  with  the
	      --limit-iface-out option.

       --limit-iface-out
	      The  address  type  checking can be limited to the interface the
	      packet is going out. This option is only valid in the  POSTROUT‐
	      ING,  OUTPUT and FORWARD chains. It cannot be specified with the
	      --limit-iface-in option.

   ah
       This module matches the SPIs in Authentication header of IPsec packets.

       [!] --ahspi spi[:spi]

   cluster
       Allows you to deploy gateway and back-end load-sharing clusters without
       the need of load-balancers.

       This  match requires that all the nodes see the same packets. Thus, the
       cluster match decides if this node has to handle	 a  packet  given  the
       following options:

       --cluster-total-nodes num
	      Set number of total nodes in cluster.

       [!] --cluster-local-node num
	      Set the local node number ID.

       [!] --cluster-local-nodemask mask
	      Set  the	local  node  number  ID	 mask. You can use this option
	      instead of --cluster-local-node.

       --cluster-hash-seed value
	      Set seed value of the Jenkins hash.

       Example:

	      iptables -A PREROUTING -t mangle	-i  eth1  -m  cluster  --clus‐
	      ter-total-nodes  2  --cluster-local-node	1  --cluster-hash-seed
	      0xdeadbeef -j MARK --set-mark 0xffff

	      iptables -A PREROUTING -t mangle	-i  eth2  -m  cluster  --clus‐
	      ter-total-nodes  2  --cluster-local-node	1  --cluster-hash-seed
	      0xdeadbeef -j MARK --set-mark 0xffff

	      iptables -A PREROUTING -t mangle -i eth1 -m mark ! --mark 0xffff
	      -j DROP

	      iptables -A PREROUTING -t mangle -i eth2 -m mark ! --mark 0xffff
	      -j DROP

       And the following commands to make all nodes see the same packets:

	      ip maddr add 01:00:5e:00:01:01 dev eth1

	      ip maddr add 01:00:5e:00:01:02 dev eth2

	      arptables -A OUTPUT -o eth1 --h-length 6 -j mangle --mangle-mac-
	      s 01:00:5e:00:01:01

	      arptables	 -A  INPUT  -i	eth1  --h-length  6  --destination-mac
	      01:00:5e:00:01:01 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27

	      arptables -A OUTPUT  -o  eth2  --h-length	 6  -j	mangle	--man‐
	      gle-mac-s 01:00:5e:00:01:02

	      arptables	 -A  INPUT  -i	eth2  --h-length  6  --destination-mac
	      01:00:5e:00:01:02 -j mangle --mangle-mac-d 00:zz:yy:xx:5a:27

       In the case of TCP connections, pickup facility has to be  disabled  to
       avoid marking TCP ACK packets coming in the reply direction as valid.

	      echo 0 > /proc/sys/net/netfilter/nf_conntrack_tcp_loose

   comment
       Allows you to add comments (up to 256 characters) to any rule.

       --comment comment

       Example:
	      iptables -A INPUT -s 192.168.0.0/16 -m comment --comment "A pri‐
	      vatized IP block"

   connbytes
       Match by how many bytes or packets a connection	(or  one  of  the  two
       flows  constituting the connection) has transferred so far, or by aver‐
       age bytes per packet.

       The counters are 64-bit 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  transferred	 bytes per connection can also be viewed through `con‐
       ntrack -L` and accessed via ctnetlink.

       NOTE that for connections which have  no	 accounting  information,  the
       match  will  always return false. The "net.netfilter.nf_conntrack_acct"
       sysctl flag  controls  whether  new  connections	 will  be  byte/packet
       counted.	 Existing  connection  flows  will not be gaining/losing a/the
       accounting structure when be sysctl flag is flipped.

       [!] --connbytes from[:to]
	      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.

       --connbytes-dir {original|reply|both}
	      which packets to consider

       --connbytes-mode {packets|bytes|avgpkt}
	      whether  to  check the amount of packets, number of bytes trans‐
	      ferred 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.

       Example:
	      iptables	  ..	-m    connbytes	   --connbytes	  10000:100000
	      --connbytes-dir both --connbytes-mode bytes ...

   connlimit
       Allows  you  to restrict the number of parallel connections to a server
       per client IP address (or client address block).

       [!] --connlimit-above n
	      Match if the number of existing connections is (not) above n.

       --connlimit-mask prefix_length
	      Group hosts using the prefix length. For IPv4, this  must	 be  a
	      number  between  (including)  0  and 32. For IPv6, between 0 and
	      128.

       Examples:

       # allow 2 telnet connections per client host
	      iptables	-A  INPUT  -p  tcp  --syn  --dport  23	-m   connlimit
	      --connlimit-above 2 -j REJECT

       # you can also match the other way around:
	      iptables	-A  INPUT  -p  tcp  --syn  --dport  23	-m connlimit !
	      --connlimit-above 2 -j ACCEPT

       # limit the number 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

       # limit the number of parallel HTTP requests to 16 for the  link	 local
       network
	      (ipv6)  ip6tables	 -p  tcp  --syn	 --dport  80  -s  fe80::/64 -m
	      connlimit --connlimit-above 16 --connlimit-mask 64 -j REJECT

   connmark
       This module matches the netfilter mark field associated with a  connec‐
       tion (which can be set using the CONNMARK target below).

       [!] --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).

   conntrack
       This  module,  when combined with connection tracking, allows access to
       the connection tracking state for this packet/connection.

       [!] --ctstate statelist
	      statelist is a comma separated list of the connection states  to
	      match.  Possible states are listed below.

       [!] --ctproto l4proto
	      Layer-4 protocol to match (by number or name)

       [!] --ctorigsrc address[/mask]

       [!] --ctorigdst address[/mask]

       [!] --ctreplsrc address[/mask]

       [!] --ctrepldst address[/mask]
	      Match against original/reply source/destination address

       [!] --ctorigsrcport port

       [!] --ctorigdstport port

       [!] --ctreplsrcport port

       [!] --ctrepldstport port
	      Match    against	  original/reply    source/destination	  port
	      (TCP/UDP/etc.) or GRE key.

       [!] --ctstatus statelist
	      statuslist is a comma separated list of the connection  statuses
	      to match.	 Possible statuses are listed below.

       [!] --ctexpire time[:time]
	      Match remaining lifetime in seconds against given value or range
	      of values (inclusive)

       --ctdir {ORIGINAL|REPLY}
	      Match packets that are flowing in the  specified	direction.  If
	      this  flag  is  not  specified  at  all, matches packets in both
	      directions.

       States for --ctstate:

       INVALID
	      meaning that the packet is associated with no known connection

       NEW    meaning that the packet has started a new connection, or	other‐
	      wise  associated with a connection which has not seen packets in
	      both directions, and

       ESTABLISHED
	      meaning that the packet is associated with  a  connection	 which
	      has seen packets in both directions,

       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.

       SNAT   A virtual state, matching if the original source address differs
	      from the reply destination.

       DNAT   A virtual state, matching if the	original  destination  differs
	      from the reply source.

       Statuses for --ctstatus:

       NONE   None of the below.

       EXPECTED
	      This  is	an expected connection (i.e. a conntrack helper set it
	      up)

       SEEN_REPLY
	      Conntrack has seen packets in both directions.

       ASSURED
	      Conntrack entry should never be early-expired.

       CONFIRMED
	      Connection is confirmed: originating packet has left box.

   dccp
       [!] --source-port,--sport port[:port]

       [!] --destination-port,--dport port[:port]

       [!] --dccp-types mask
	      Match when the DCCP packet type is one of 'mask'.	 'mask'	 is  a
	      comma-separated list of packet types.  Packet types are: REQUEST
	      RESPONSE DATA ACK DATAACK	 CLOSEREQ  CLOSE  RESET	 SYNC  SYNCACK
	      INVALID.

       [!] --dccp-option number
	      Match if DCP option set.

   dscp
       This module matches the 6 bit DSCP field within the TOS field in the IP
       header.	DSCP has superseded TOS within the IETF.

       [!] --dscp value
	      Match against a numeric (decimal or hex) value [0-63].

       [!] --dscp-class class
	      Match the DiffServ class. This value may be any of the  BE,  EF,
	      AFxx or CSx classes.  It will then be converted into its accord‐
	      ing numeric value.

   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

       [!] --ecn-tcp-cwr
	      This matches if the TCP ECN CWR (Congestion Window Received) bit
	      is set.

       [!] --ecn-tcp-ece
	      This matches if the TCP ECN ECE (ECN Echo) bit is set.

       [!] --ecn-ip-ect num
	      This  matches a particular IPv4 ECT (ECN-Capable Transport). You
	      have to specify a number between `0' and `3'.

   esp
       This module matches the SPIs in ESP header of IPsec packets.

       [!] --espspi spi[:spi]

   hashlimit
       hashlimit uses hash buckets to express a rate limiting match (like  the
       limit  match)  for a group of connections using a single iptables rule.
       Grouping can be done per-hostgroup (source and/or destination  address)
       and/or  per-port.  It  gives  you the ability to express "N packets per
       time quantum per group":

       matching on source host
	      "1000 packets per second for every host in 192.168.0.0/16"

       matching on source prot
	      "100 packets per second for every service of 192.168.1.1"

       matching on subnet
	      "10000 packets per minute for every /28 subnet in 10.0.0.0/8"

       A hash limit option (--hashlimit-upto, --hashlimit-above)  and  --hash‐
       limit-name are required.

       --hashlimit-upto amount[/second|/minute|/hour|/day]
	      Match  if	 the  rate  is below or equal to amount/quantum. It is
	      specified as a number, with an optional time quantum suffix; the
	      default is 3/hour.

       --hashlimit-above amount[/second|/minute|/hour|/day]
	      Match if the rate is above amount/quantum.

       --hashlimit-burst amount
	      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.

       --hashlimit-mode {srcip|srcport|dstip|dstport},...
	      A comma-separated list of objects to take into consideration. If
	      no --hashlimit-mode option is given, hashlimit acts like	limit,
	      but at the expensive of doing the hash housekeeping.

       --hashlimit-srcmask prefix
	      When  --hashlimit-mode  srcip  is	 used,	all  source  addresses
	      encountered will be grouped according to the given prefix length
	      and  the	so-created subnet will be subject to hashlimit. prefix
	      must be between (inclusive) 0 and 32. Note that --hashlimit-src‐
	      mask 0 is basically doing the same thing as not specifying srcip
	      for --hashlimit-mode, but is technically more expensive.

       --hashlimit-dstmask prefix
	      Like --hashlimit-srcmask, but for destination addresses.

       --hashlimit-name foo
	      The name for the /proc/net/ipt_hashlimit/foo entry.

       --hashlimit-htable-size buckets
	      The number of buckets of the hash table

       --hashlimit-htable-max entries
	      Maximum entries in the hash.

       --hashlimit-htable-expire msec
	      After how many milliseconds do hash entries expire.

       --hashlimit-htable-gcinterval msec
	      How many milliseconds between garbage collection intervals.

   helper
       This module matches packets related to a specific conntrack-helper.

       [!] --helper string
	      Matches packets related to the specified conntrack-helper.

	      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".

	      Same rules apply for other conntrack-helpers.

   icmp
       This extension can be used if `--protocol icmp' is specified.  It  pro‐
       vides the following option:

       [!] --icmp-type {type[/code]|typename}
	      This  allows  specification  of  the  ICMP  type, which can be a
	      numeric ICMP type, type/code pair, or one of the ICMP type names
	      shown by the command
	       iptables -p icmp -h

   iprange
       This matches on a given arbitrary range of IP addresses.

       [!] --src-range from[-to]
	      Match source IP in the specified range.

       [!] --dst-range from[-to]
	      Match destination IP in the specified range.

   length
       This  module  matches  the  length of the layer-3 payload (e.g. layer-4
       packet) of a packet against a specific value or range of values.

       [!] --length length[:length]

   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
       LOG target to give limited logging, for example.

       [!] --limit rate[/second|/minute|/hour|/day]
	      Maximum  average	matching  rate: specified as a number, with an
	      optional `/second', `/minute', `/hour', or  `/day'  suffix;  the
	      default is 3/hour.

       --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.

   mac
       [!] --mac-source address
	      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 PREROUTING, FOR‐
	      WARD or INPUT chains.

   mark
       This module matches the netfilter mark field associated with  a	packet
       (which can be set using the MARK target below).

       [!] --mark value[/mask]
	      Matches packets with the given unsigned mark value (if a mask is
	      specified, this is logically ANDed with the mask before the com‐
	      parison).

   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 -p tcp or -p udp.

       [!] --source-ports,--sports port[,port|,port:port]...
	      Match  if	 the  source port is one of the given ports.  The flag
	      --sports is a convenient alias for this option.  Multiple	 ports
	      or  port ranges are separated using a comma, and a port range is
	      specified using a colon.	53,1024:65535  would  therefore	 match
	      ports 53 and all from 1024 through 65535.

       [!] --destination-ports,--dports port[,port|,port:port]...
	      Match  if	 the  destination port is one of the given ports.  The
	      flag --dports is a convenient alias for this option.

       [!] --ports port[,port|,port:port]...
	      Match if either the source or destination ports are equal to one
	      of the given ports.

   owner
       This  module  attempts  to  match various characteristics of the packet
       creator, for locally generated packets. This match is only valid in the
       OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket
       associated with them. Packets from kernel threads do have a socket, but
       usually no owner.

       [!] --uid-owner username

       [!] --uid-owner userid[-userid]
	      Matches if the packet socket's file structure (if it has one) is
	      owned by the given user. You may also specify a  numerical  UID,
	      or an UID range.

       [!] --gid-owner groupname

       [!] --gid-owner groupid[-groupid]
	      Matches  if  the	packet socket's file structure is owned by the
	      given group.  You may also specify a numerical  GID,  or	a  GID
	      range.

       [!] --socket-exists
	      Matches if the packet is associated with a socket.

   physdev
       This  module  matches  on  the  bridge  port  input  and output devices
       enslaved to a bridge device. This module is a part of  the  infrastruc‐
       ture that enables a transparent bridging IP firewall and is only useful
       for kernel versions above version 2.5.44.

       [!] --physdev-in name
	      Name of a bridge port via which a packet is received  (only  for
	      packets  entering	 the INPUT, FORWARD and 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.

       [!] --physdev-out name
	      Name  of	a  bridge  port via which a packet is going to be sent
	      (for  packets  entering  the  FORWARD,  OUTPUT  and  POSTROUTING
	      chains).	 If  the interface name ends in a "+", then any inter‐
	      face which begins with this name will match. Note	 that  in  the
	      nat and mangle OUTPUT chains one cannot match on the bridge out‐
	      put port, however one can in the filter  OUTPUT  chain.  If  the
	      packet  won't  leave  by a bridge device or if it is yet unknown
	      what the output device will be, then the packet won't match this
	      option, unless '!' is used.

       [!] --physdev-is-in
	      Matches if the packet has entered through a bridge interface.

       [!] --physdev-is-out
	      Matches if the packet will leave through a bridge interface.

       [!] --physdev-is-bridged
	      Matches  if  the	packet	is  being bridged and therefore is not
	      being routed.  This is only useful in the FORWARD and  POSTROUT‐
	      ING chains.

   pkttype
       This module matches the link-layer packet type.

       [!] --pkt-type {unicast|broadcast|multicast}

   policy
       This modules matches the policy used by IPsec for handling a packet.

       --dir {in|out}
	      Used  to	select whether to match the policy used for decapsula‐
	      tion or the policy that will be used for encapsulation.	in  is
	      valid  in the PREROUTING, INPUT and FORWARD chains, out is valid
	      in the POSTROUTING, OUTPUT and FORWARD chains.

       --pol {none|ipsec}
	      Matches if the packet is subject to IPsec processing.

       --strict
	      Selects whether to match the exact policy or match if  any  rule
	      of the policy matches the given policy.

       [!] --reqid id
	      Matches the reqid of the policy rule. The reqid can be specified
	      with setkey(8) using unique:id as level.

       [!] --spi spi
	      Matches the SPI of the SA.

       [!] --proto {ah|esp|ipcomp}
	      Matches the encapsulation protocol.

       [!] --mode {tunnel|transport}
	      Matches the encapsulation mode.

       [!] --tunnel-src addr[/mask]
	      Matches the source end-point address of a tunnel mode SA.	  Only
	      valid with --mode tunnel.

       [!] --tunnel-dst addr[/mask]
	      Matches  the  destination end-point address of a tunnel mode SA.
	      Only valid with --mode tunnel.

       --next Start the next element in the policy specification. Can only  be
	      used with --strict.

   quota
       Implements  network  quotas  by	decrementing  a byte counter with each
       packet.

       --quota bytes
	      The quota in bytes.

   rateest
       The rate estimator can match on estimated rates	as  collected  by  the
       RATEEST	target.	 It supports matching on absolute bps/pps values, com‐
       paring two rate estimators and matching on the difference  between  two
       rate estimators.

       --rateest1 name
	      Name of the first rate estimator.

       --rateest2 name
	      Name of the second rate estimator (if difference is to be calcu‐
	      lated).

       --rateest-delta
	      Compare difference(s) to given rate(s)

       --rateest1-bps value

       --rateest2-bps value
	      Compare bytes per second.

       --rateest1-pps value

       --rateest2-pps value
	      Compare packets per second.

       [!] --rateest-lt
	      Match if rate is less than given rate/estimator.

       [!] --rateest-gt
	      Match if rate is greater than given rate/estimator.

       [!] --rateest-eq
	      Match if rate is equal to given rate/estimator.

       Example: This is what can be used to route  outgoing  data  connections
       from  an	 FTP server over two lines based on the available bandwidth at
       the time the data connection was started:

       # Estimate outgoing rates

       iptables -t mangle -A POSTROUTING -o  eth0  -j  RATEEST	--rateest-name
       eth0 --rateest-interval 250ms --rateest-ewma 0.5s

       iptables	 -t  mangle  -A	 POSTROUTING -o ppp0 -j RATEEST --rateest-name
       ppp0 --rateest-interval 250ms --rateest-ewma 0.5s

       # Mark based on available bandwidth

       iptables -t mangle -A balance -m	 conntrack  --ctstate  NEW  -m	helper
       --helper	 ftp -m rateest --rateest-delta --rateest1 eth0 --rateest-bps1
       2.5mbit --rateest-gt --rateest2 ppp0 --rateest-bps2 2mbit  -j  CONNMARK
       --set-mark 1

       iptables	 -t  mangle  -A	 balance  -m conntrack --ctstate NEW -m helper
       --helper ftp -m rateest --rateest-delta --rateest1 ppp0	--rateest-bps1
       2mbit  --rateest-gt  --rateest2 eth0 --rateest-bps2 2.5mbit -j CONNMARK
       --set-mark 2

       iptables -t mangle -A balance -j CONNMARK --restore-mark

   realm
       This matches the routing realm.	Routing realms	are  used  in  complex
       routing setups involving dynamic routing protocols like BGP.

       [!] --realm value[/mask]
	      Matches  a  given	 realm	number (and optionally mask). If not a
	      number, value can be a named realm from  /etc/iproute2/rt_realms
	      (mask can not be used in that case).

   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.

       --name name
	      Specify the list to use for the commands. If no  name  is	 given
	      then DEFAULT will be used.

       [!] --set
	      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).

       --rsource
	      Match/save the source address of each packet in the recent  list
	      table. This is the default.

       --rdest
	      Match/save  the destination address of each packet in the recent
	      list table.

       [!] --rcheck
	      Check if the source address of the packet is  currently  in  the
	      list.

       [!] --update
	      Like  --rcheck,  except it will update the "last seen" timestamp
	      if it matches.

       [!] --remove
	      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.

       [!] --seconds seconds
	      This  option must be used in conjunction with one of --rcheck or
	      --update. 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.

       [!] --hitcount hits
	      This option must be used in conjunction with one of --rcheck  or
	      --update.	 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 --seconds	 to  create  an	 even  narrower	 match
	      requiring a certain number of hits within a specific time frame.
	      The maximum value for the hitcount parameter  is	given  by  the
	      "ip_pkt_list_tot"	 parameter  of	the  xt_recent	kernel module.
	      Exceeding this value on the command line will cause the rule  to
	      be rejected.

       --rttl This option may only be used in conjunction with one of --rcheck
	      or --update. When used, this will narrow the match to only  hap‐
	      pen  when	 the address is in the list and the TTL of the current
	      packet matches that of the packet which hit the --set rule. This
	      may  be  useful  if  you	have problems with people faking their
	      source address in order to DoS you via this module by  disallow‐
	      ing others access to your site by sending bogus packets to you.

       Examples:

	      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

       Steve's	ipt_recent  website  (http://snowman.net/projects/ipt_recent/)
       also has some examples of usage.

       /proc/net/xt_recent/* are the current lists of addresses	 and  informa‐
       tion about each entry of each list.

       Each  file  in /proc/net/xt_recent/ can be read from to see the current
       list or written two using the following commands to modify the list:

       echo +addr >/proc/net/xt_recent/DEFAULT
	      to add addr to the DEFAULT list

       echo -addr >/proc/net/xt_recent/DEFAULT
	      to remove addr from the DEFAULT list

       echo / >/proc/net/xt_recent/DEFAULT
	      to flush the DEFAULT list (remove all entries).

       The module itself accepts parameters, defaults shown:

       ip_list_tot=100
	      Number of addresses remembered per table.

       ip_pkt_list_tot=20
	      Number of packets per address remembered.

       ip_list_hash_size=0
	      Hash table size. 0 means to calculate it based  on  ip_list_tot,
	      default: 512.

       ip_list_perms=0644
	      Permissions for /proc/net/xt_recent/* files.

       ip_list_uid=0
	      Numerical UID for ownership of /proc/net/xt_recent/* files.

       ip_list_gid=0
	      Numerical GID for ownership of /proc/net/xt_recent/* files.

   sctp
       [!] --source-port,--sport port[:port]

       [!] --destination-port,--dport port[:port]

       [!] --chunk-types {all|any|only} chunktype[:flags] [...]
	      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
	      DATA		    U B E u b e
	      ABORT		    T t
	      SHUTDOWN_COMPLETE	    T t

	      (lowercase means flag should be "off", uppercase means "on")

       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

   set
       This module matches IP sets which can be defined by ipset(8).

       [!] --match-set setname flag[,flag]...
	      where flags are the comma separated list of src and/or dst spec‐
	      ifications  and there can be no more than six of them. Hence the
	      command

	       iptables -A FORWARD -m set --match-set test src,dst

	      will match packets, for which (if the set type is ipportmap) the
	      source  address  and  destination	 port pair can be found in the
	      specified set. If the set type of the specified  set  is	single
	      dimension (for example ipmap), then the command will match pack‐
	      ets for which the source address can be found in	the  specified
	      set.

       The  option --match-set can be replaced by --set if that does not clash
       with an option of other extensions.

   socket
       This matches if an open socket can be found by doing a socket lookup on
       the packet.

   state
       This  module,  when combined with connection tracking, allows access to
       the connection tracking state for this packet.

       [!] --state state
	      Where state is a comma separated list of the  connection	states
	      to  match.   Possible states are 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, ESTABLISHED meaning that the packet is associ‐
	      ated  with  a  connection	 which has seen packets in both direc‐
	      tions, 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 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.

   statistic
       This module matches packets based on some statistic condition.  It sup‐
       ports two distinct modes settable with the --mode option.

       Supported options:

       --mode mode
	      Set  the matching mode of the matching rule, supported modes are
	      random and nth.

       --probability p
	      Set the probability from 0 to 1 for  a  packet  to  be  randomly
	      matched. It works only with the random mode.

       --every n
	      Match  one  packet  every nth packet. It works only with the nth
	      mode (see also the --packet option).

       --packet p
	      Set the initial counter value (0 <= p <= n-1, default 0) for the
	      nth mode.

   string
       This  modules  matches  a  given	 string by using some pattern matching
       strategy. It requires a linux kernel >= 2.6.14.

       --algo {bm|kmp}
	      Select the pattern matching strategy. (bm = Boyer-Moore,	kmp  =
	      Knuth-Pratt-Morris)

       --from offset
	      Set the offset from which it starts looking for any matching. If
	      not passed, default is 0.

       --to offset
	      Set the offset from which it starts looking for any matching. If
	      not passed, default is the packet size.

       [!] --string pattern
	      Matches the given pattern.

       [!] --hex-string pattern
	      Matches the given pattern in hex notation.

   tcp
       These  extensions can be used if `--protocol tcp' is specified. It pro‐
       vides the following options:

       [!] --source-port,--sport port[:port]
	      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 first:last.  If the  first  port  is
	      omitted,	"0"  is	 assumed;  if  the last is omitted, "65535" is
	      assumed.	If the first port is greater than the second one  they
	      will  be	swapped.   The	flag --sport is a convenient alias for
	      this option.

       [!] --destination-port,--dport port[:port]
	      Destination port or port range specification.  The flag  --dport
	      is a convenient alias for this option.

       [!] --tcp-flags mask comp
	      Match  when  the TCP flags are as specified.  The first argument
	      mask is the flags which we should examine, written as  a	comma-
	      separated	 list,	and  the second argument comp is a comma-sepa‐
	      rated list of flags which must be set.  Flags are: SYN  ACK  FIN
	      RST URG PSH ALL NONE.  Hence the command
	       iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST SYN
	      will  only match packets with the SYN flag set, and the ACK, FIN
	      and RST flags unset.

       [!] --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 connec‐
	      tion 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
	      --tcp-flags  SYN,RST,ACK,FIN  SYN.  If the "!" flag precedes the
	      "--syn", the sense of the option is inverted.

       [!] --tcp-option number
	      Match if TCP option set.

   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.

       [!] --mss value[:value]
	      Match a given TCP MSS value or range.

   time
       This  matches  if the packet arrival time/date is within a given range.
       All options are optional, but are ANDed when specified.

       --datestart YYYY[-MM[-DD[Thh[:mm[:ss]]]]]

       --datestop YYYY[-MM[-DD[Thh[:mm[:ss]]]]]

	      Only match during the given time, which must be in ISO 8601  "T"
	      notation.	  The  possible	 time  range is 1970-01-01T00:00:00 to
	      2038-01-19T04:17:07.

	      If --datestart or --datestop are not specified, it will  default
	      to 1970-01-01 and 2038-01-19, respectively.

       --timestart hh:mm[:ss]

       --timestop hh:mm[:ss]

	      Only  match during the given daytime. The possible time range is
	      00:00:00 to 23:59:59. Leading zeroes are allowed (e.g.  "06:03")
	      and correctly interpreted as base-10.

       [!] --monthdays day[,day...]

	      Only match on the given days of the month. Possible values are 1
	      to 31. Note that specifying 31  will  of	course	not  match  on
	      months  which  do	 not have a 31st day; the same goes for 28- or
	      29-day February.

       [!] --weekdays day[,day...]

	      Only match on the given weekdays. Possible values are Mon,  Tue,
	      Wed,  Thu,  Fri,	Sat, Sun, or values from 1 to 7, respectively.
	      You may also use two-character variants (Mo, Tu, etc.).

       --utc

	      Interpret the times given for --datestart, --datestop,  --times‐
	      tart and --timestop to be UTC.

       --localtz

	      Interpret	 the times given for --datestart, --datestop, --times‐
	      tart and --timestop to be local kernel time. (Default)

       EXAMPLES. To match on weekends, use:

	      -m time --weekdays Sa,Su

       Or, to match (once) on a national holiday block:

	      -m time --datestart 2007-12-24 --datestop 2007-12-27

       Since the stop time is actually inclusive, you would need the following
       stop time to not match the first second of the new day:

	      -m      time     --datestart     2007-01-01T17:00	    --datestop
	      2007-01-01T23:59:59

       During lunch hour:

	      -m time --timestart 12:30 --timestop 13:30

       The fourth Friday in the month:

	      -m time --weekdays Fr --monthdays 22,23,24,25,26,27,28

       (Note that this exploits a certain mathematical	property.  It  is  not
       possible	 to  say "fourth Thursday OR fourth Friday" in one rule. It is
       possible with multiple rules, though.)

   tos
       This module matches the 8-bit Type of Service field in the IPv4	header
       (i.e.   including  the  "Precedence" bits) or the (also 8-bit) Priority
       field in the IPv6 header.

       [!] --tos value[/mask]
	      Matches packets with the given TOS mark  value.  If  a  mask  is
	      specified,  it  is  logically ANDed with the TOS mark before the
	      comparison.

       [!] --tos symbol
	      You can specify a symbolic name when using  the  tos  match  for
	      IPv4.  The list of recognized TOS names can be obtained by call‐
	      ing iptables with -m tos -h.  Note that this implies a  mask  of
	      0x3F, i.e. all but the ECN bits.

   ttl
       This module matches the time to live field in the IP header.

       --ttl-eq ttl
	      Matches the given TTL value.

       --ttl-gt ttl
	      Matches if TTL is greater than the given TTL value.

       --ttl-lt ttl
	      Matches if TTL is less than the given TTL value.

   u32
       U32  tests  whether quantities of up to 4 bytes extracted from a packet
       have specified values. The specification of what to extract is  general
       enough to find data at given offsets from tcp headers or payloads.

       [!] --u32 tests
	      The  argument amounts to a program in a small language described
	      below.

	      tests := location "=" value | tests "&&" location "=" value

	      value := range | value "," range

	      range := number | number ":" number

       a single number, n, is interpreted the same as n:n. n:m is  interpreted
       as the range of numbers >=n and <=m.

	   location := number | location operator number

	   operator := "&" | "<<" | ">>" | "@"

       The  operators &, <<, >> and && mean the same as in C.  The = is really
       a set membership operator and the value syntax describes a set.	The  @
       operator is what allows moving to the next header and is described fur‐
       ther below.

       There are currently some artificial implementation limits on  the  size
       of the tests:

	   *  no more than 10 of "=" (and 9 "&&"s) in the u32 argument

	   *  no more than 10 ranges (and 9 commas) per value

	   *  no more than 10 numbers (and 9 operators) per location

       To describe the meaning of location, imagine the following machine that
       interprets it. There are three registers:

	      A is of type char *, initially the address of the IP header

	      B and C are unsigned 32 bit integers, initially zero

       The instructions are:

	      number B = number;

	      C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)

	      &number C = C & number

	      << number C = C << number

	      >> number C = C >> number

	      @number A = A + C; then do the instruction number

       Any access of memory outside [skb->data,skb->end] causes the  match  to
       fail.  Otherwise the result of the computation is the final value of C.

       Whitespace is allowed but not required in the tests. However, the char‐
       acters that do occur there are likely to require shell quoting,	so  it
       is a good idea to enclose the arguments in quotes.

       Example:

	      match IP packets with total length >= 256

	      The IP header contains a total length field in bytes 2-3.

	      --u32 "0 & 0xFFFF = 0x100:0xFFFF"

	      read bytes 0-3

	      AND  that	 with 0xFFFF (giving bytes 2-3), and test whether that
	      is in the range [0x100:0xFFFF]

       Example: (more realistic, hence more complicated)

	      match ICMP packets with icmp type 0

	      First test that it is an ICMP packet, true iff byte 9 (protocol)
	      = 1

	      --u32 "6 & 0xFF = 1 && ...

	      read  bytes  6-9,	 use & to throw away bytes 6-8 and compare the
	      result to 1. Next test that it is not a  fragment.  (If  so,  it
	      might be part of such a packet but we cannot always tell.) N.B.:
	      This test is generally needed if	you  want  to  match  anything
	      beyond  the IP header. The last 6 bits of byte 6 and all of byte
	      7 are 0 iff this is a complete packet (not a fragment). Alterna‐
	      tively, you can allow first fragments by only testing the last 5
	      bits of byte 6.

	       ... 4 & 0x3FFF = 0 && ...

	      Last test: the first byte past the IP header (the	 type)	is  0.
	      This  is	where we have to use the @syntax. The length of the IP
	      header (IHL) in 32 bit words is stored in the right half of byte
	      0 of the IP header itself.

	       ... 0 >> 22 & 0x3C @ 0 >> 24 = 0"

	      The  first 0 means read bytes 0-3, >>22 means shift that 22 bits
	      to the right. Shifting 24 bits would give	 the  first  byte,  so
	      only  22	bits is four times that plus a few more bits. &3C then
	      eliminates the two extra bits on the right and  the  first  four
	      bits  of	the  first  byte.  For instance, if IHL=5, then the IP
	      header is 20 (4 x 5) bytes long. In this case, bytes 0-1 are (in
	      binary)	xxxx0101   yyzzzzzz,  >>22  gives  the	10  bit	 value
	      xxxx0101yy and &3C gives 010100. @ means to use this number as a
	      new  offset  into	 the packet, and read four bytes starting from
	      there. This is the first 4 bytes of the ICMP payload,  of	 which
	      byte 0 is the ICMP type. Therefore, we simply shift the value 24
	      to the right to throw out all but the first byte and compare the
	      result with 0.

       Example:

	      TCP payload bytes 8-12 is any of 1, 2, 5 or 8

	      First we test that the packet is a tcp packet (similar to ICMP).

	      --u32 "6 & 0xFF = 6 && ...

	      Next, test that it is not a fragment (same as above).

	       ... 0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8"

	      0>>22&3C as above computes the number of bytes in the IP header.
	      @ makes this the new offset into the packet, which is the	 start
	      of the TCP header. The length of the TCP header (again in 32 bit
	      words) is the left half of  byte	12  of	the  TCP  header.  The
	      12>>26&3C	 computes  this	 length	 in  bytes  (similar to the IP
	      header before). "@" makes this the  new  offset,	which  is  the
	      start  of	 the  TCP  payload. Finally, 8 reads bytes 8-12 of the
	      payload and = checks whether the result is any of 1, 2, 5 or 8.

   udp
       These extensions can be used if `--protocol udp' is specified. It  pro‐
       vides the following options:

       [!] --source-port,--sport port[:port]
	      Source port or port range specification.	See the description of
	      the --source-port option of the TCP extension for details.

       [!] --destination-port,--dport port[:port]
	      Destination port or port range specification.  See the  descrip‐
	      tion  of	the --destination-port option of the TCP extension for
	      details.

TARGET EXTENSIONS
       iptables can use extended target modules: the following are included in
       the standard distribution.

   AUDIT
       This  target  allows  the  logging  of  matching packets with the audit
       facility. When this option is set for a rule,  the  Linux  kernel  will
       generate	 an  audit  entry for each matching packet which holds details
       about the packet depending on the protocol of the packet.

       Data that is always logged includes the action applied  to  the	packet
       (action=	 audit	information),  the  inbound interface (inif=), and the
       outbound interface (outif=).

       If the matched packet from the ARP layer, the source (smac=) and desti‐
       nation  (dmac=) MAC address as well as the protocol type (macproto=) is
       logged.

       When using the target with IPv4 and IPv6 packets, the  source  (saddr=)
       and  destination	 (daddr=)  IP  address,	 as  well as the protocol type
       (proto=) is logged. In addition, the audit trail identifies whether the
       packet is truncated (truncate=1) or whether it is fragmented (frag=1).

       In case the IP packet encapsulates TCP or UDP data, the source (sport=)
       and destination (dport=) port information is audited.

       If the IP packet encapsulates ICMP data, the ICMP type (icmptype=)  and
       the ICMP code (icmpcode=) is logged.

       --type type
	      Sets  the	 action	 type  to be audited. Valid options are either
	      accept, drop, or reject.

   CLASSIFY
       This module allows you to set the skb->priority value (and  thus	 clas‐
       sify the packet into a specific CBQ class).

       --set-class major:minor
	      Set  the	major  and  minor  class  value. The values are always
	      interpreted as hexadecimal even if no 0x prefix is given.

   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.

       --new  Create  a	 new  ClusterIP.   You	always have to set this on the
	      first rule for a given ClusterIP.

       --hashmode mode
	      Specify the hashing mode.	 Has to	 be  one  of  sourceip,	 sour‐
	      ceip-sourceport, sourceip-sourceport-destport.

       --clustermac mac
	      Specify the ClusterIP MAC address. Has to be a link-layer multi‐
	      cast address

       --total-nodes num
	      Number of total nodes within this cluster.

       --local-node num
	      Local node number within this cluster.

       --hash-init rnd
	      Specify the random seed used for hash initialization.

   CONNMARK
       This module sets the netfilter mark value associated with a connection.
       The mark is 32 bits wide.

       --set-xmark value[/mask]
	      Zero out the bits given by mask and XOR value into the ctmark.

       --save-mark [--nfmask nfmask] [--ctmask ctmask]
	      Copy  the	 packet	 mark (nfmark) to the connection mark (ctmark)
	      using the given masks. The new nfmark  value  is	determined  as
	      follows:

	      ctmark = (ctmark & ~ctmask) ^ (nfmark & nfmask)

	      i.e.  ctmask  defines what bits to clear and nfmask what bits of
	      the nfmark to XOR into the ctmark. ctmask and nfmask default  to
	      0xFFFFFFFF.

       --restore-mark [--nfmask nfmask] [--ctmask ctmask]
	      Copy  the	 connection  mark (ctmark) to the packet mark (nfmark)
	      using the given masks. The new ctmark  value  is	determined  as
	      follows:

	      nfmark = (nfmark & ~nfmask) ^ (ctmark & ctmask);

	      i.e.  nfmask  defines what bits to clear and ctmask what bits of
	      the ctmark to XOR into the nfmark. ctmask and nfmask default  to
	      0xFFFFFFFF.

	      --restore-mark is only valid in the mangle table.

       The following mnemonics are available for --set-xmark:

       --and-mark bits
	      Binary  AND  the	ctmark	with  bits.  (Mnemonic for --set-xmark
	      0/invbits, where invbits is the binary negation of bits.)

       --or-mark bits
	      Binary OR	 the  ctmark  with  bits.  (Mnemonic  for  --set-xmark
	      bits/bits.)

       --xor-mark bits
	      Binary  XOR  the	ctmark	with  bits.  (Mnemonic for --set-xmark
	      bits/0.)

       --set-mark value[/mask]
	      Set the connection mark. If a mask is specified then only	 those
	      bits set in the mask are modified.

       --save-mark [--mask mask]
	      Copy  the	 nfmark	 to  the  ctmark. If a mask is specified, only
	      those bits are copied.

       --restore-mark [--mask mask]
	      Copy the ctmark to the nfmark. If	 a  mask  is  specified,  only
	      those bits are copied. This is only valid in the mangle table.

   CONNSECMARK
       This  module  copies  security markings from packets to connections (if
       unlabeled), and from connections back to packets (also  only  if	 unla‐
       beled).	 Typically  used in conjunction with SECMARK, it is only valid
       in the mangle table.

       --save If the packet has a security marking, copy it to the  connection
	      if the connection is not marked.

       --restore
	      If  the packet does not have a security marking, and the connec‐
	      tion does, copy the security marking from the connection to  the
	      packet.

   DNAT
       This  target is only valid in the nat table, in the PREROUTING and OUT‐
       PUT 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:

       --to-destination [ipaddr][-ipaddr][:port[-port]]
	      which can specify a single new destination IP address, an inclu‐
	      sive  range of IP addresses, and optionally, a port range (which
	      is only valid if the rule also specifies -p tcp or -p udp).   If
	      no port range is specified, then the destination port will never
	      be modified. If no IP address is specified then only the	desti‐
	      nation port will be modified.

	      In  Kernels  up  to  2.6.10 you can add several --to-destination
	      options. For those kernels, if you specify more than one	desti‐
	      nation   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
	      addresses.  Later Kernels (>= 2.6.11-rc1) don't have the ability
	      to NAT to multiple ranges anymore.

       --random
	      If  option --random is used then port mapping will be randomized
	      (kernel >= 2.6.22).

       --persistent
	      Gives a client the  same	source-/destination-address  for  each
	      connection.   This  supersedes the SAME target. Support for per‐
	      sistent mappings is available from 2.6.29-rc2.

   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.

       --set-dscp value
	      Set the DSCP field to a numerical value (can be decimal or hex)

       --set-dscp-class class
	      Set the DSCP field to a DiffServ class.

   ECN
       This target allows to selectively work around known ECN blackholes.  It
       can only be used in the mangle table.

       --ecn-tcp-remove
	      Remove all ECN bits from the TCP header.	Of course, it can only
	      be used in conjunction with -p tcp.

   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 match‐
       ing packets (like most IP header fields) via the kernel log  (where  it
       can be read with dmesg or syslogd(8)).  This is a "non-terminating tar‐
       get", 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).

       --log-level level
	      Level of logging (numeric or see syslog.conf(5)).

       --log-prefix prefix
	      Prefix log messages with the specified prefix; up to 29  letters
	      long, and useful for distinguishing messages in the logs.

       --log-tcp-sequence
	      Log  TCP sequence numbers. This is a security risk if the log is
	      readable by users.

       --log-tcp-options
	      Log options from the TCP packet header.

       --log-ip-options
	      Log options from the IP packet header.

       --log-uid
	      Log the userid of the process which generated the packet.

   MARK
       This target is used to set the Netfilter mark value associated with the
       packet.	 The  target can only be used in the mangle table. It can, for
       example, be used in conjunction with routing  based  on	fwmark	(needs
       iproute2). The mark field is 32 bits wide.

       --set-xmark value[/mask]
	      Zeroes out the bits given by mask and XORs value into the packet
	      mark ("nfmark"). If mask is omitted, 0xFFFFFFFF is assumed.

       --set-mark value[/mask]
	      Zeroes out the bits given by mask and ORs value into the	packet
	      mark. If mask is omitted, 0xFFFFFFFF is assumed.

       The following mnemonics are available:

       --and-mark bits
	      Binary  AND  the	nfmark	with  bits.  (Mnemonic for --set-xmark
	      0/invbits, where invbits is the binary negation of bits.)

       --or-mark bits
	      Binary OR	 the  nfmark  with  bits.  (Mnemonic  for  --set-xmark
	      bits/bits.)

       --xor-mark bits
	      Binary  XOR  the	nfmark	with  bits.  (Mnemonic for --set-xmark
	      bits/0.)

   MASQUERADE
       This target is only valid in the nat table, in the  POSTROUTING	chain.
       It  should  only	 be used with dynamically assigned IP (dialup) connec‐
       tions: 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	 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 any‐
       way).  It takes one option:

       --to-ports port[-port]
	      This specifies a range of source ports to	 use,  overriding  the
	      default SNAT source port-selection heuristics (see above).  This
	      is only valid if the rule also specifies -p tcp or -p udp.

       --random
	      Randomize source port mapping If option --random	is  used  then
	      port mapping will be randomized (kernel >= 2.6.21).

   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 INPUT, FORWARD and PREROUTING chains, and user-
       defined chains which are only called from those chains.	Note that  the
       outgoing	 packets  are NOT seen by any packet filtering chains, connec‐
       tion tracking or NAT, to avoid loops and other problems.

   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 nat table.

       --to address[/mask]
	      Network address to map to.  The resulting address will  be  con‐
	      structed	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.

   NFLOG
       This  target  provides logging of matching packets. When this target is
       set for a rule, the Linux kernel will pass the  packet  to  the	loaded
       logging	backend to log the packet. This is usually used in combination
       with nfnetlink_log as logging backend, which will multicast the	packet
       through	a netlink socket to the specified multicast group. One or more
       userspace processes may subscribe to the group to receive the  packets.
       Like LOG, this is a non-terminating target, i.e. rule traversal contin‐
       ues at the next rule.

       --nflog-group nlgroup
	      The netlink group (1 - 2^32-1) to which packets are (only appli‐
	      cable for nfnetlink_log). The default value is 0.

       --nflog-prefix prefix
	      A	 prefix string to include in the log message, up to 64 charac‐
	      ters long, useful for distinguishing messages in the logs.

       --nflog-range size
	      The number of bytes to be copied to userspace  (only  applicable
	      for  nfnetlink_log).  nfnetlink_log  instances may specify their
	      own range, this option overrides it.

       --nflog-threshold size
	      Number of packets to queue inside the kernel before sending them
	      to  userspace (only applicable for nfnetlink_log). Higher values
	      result in less overhead per packet, but increase delay until the
	      packets reach userspace. The default value is 1.

   NFQUEUE
       This  target  is an extension of the QUEUE target. As opposed to QUEUE,
       it allows you to put a packet into any specific	queue,	identified  by
       its  16-bit  queue  number.   It	 can only be used with Kernel versions
       2.6.14 or later, since it requires the nfnetlink_queue kernel  support.
       The queue-balance option was added in Linux 2.6.31.

       --queue-num value
	      This  specifies the QUEUE number to use. Valid queue numbers are
	      0 to 65535. The default value is 0.

       --queue-balance value:value
	      This specifies a range of queues to use. Packets are  then  bal‐
	      anced  across  the  given	 queues.  This is useful for multicore
	      systems: start multiple instances of the	userspace  program  on
	      queues  x, x+1, .. x+n and use "--queue-balance x:x+n".  Packets
	      belonging to the same connection are put into the same nfqueue.

   NOTRACK
       This target disables connection tracking for all packets matching  that
       rule.

       It can only be used in the raw table.

   RATEEST
       The RATEEST target collects statistics, performs rate estimation calcu‐
       lation and saves the results for later  evaluation  using  the  rateest
       match.

       --rateest-name name
	      Count  matched  packets into the pool referred to by name, which
	      is freely choosable.

       --rateest-interval amount{s|ms|us}
	      Rate measurement interval, in seconds, milliseconds or microsec‐
	      onds.

       --rateest-ewmalog value
	      Rate measurement averaging time constant.

   REDIRECT
       This  target is only valid in the nat table, in the PREROUTING and OUT‐
       PUT 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).

       --to-ports port[-port]
	      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 -p tcp or -p udp.

       --random
	      If  option --random is used then port mapping will be randomized
	      (kernel >= 2.6.22).

   REJECT
       This is used to send back an error packet in response  to  the  matched
       packet:	otherwise it is equivalent to DROP so it is a terminating TAR‐
       GET, ending rule traversal.  This target is only valid  in  the	INPUT,
       FORWARD	and  OUTPUT  chains,  and  user-defined	 chains which are only
       called from those chains.  The following option controls the nature  of
       the error packet returned:

       --reject-with type
	      The  type	 given can be icmp-net-unreachable, icmp-host-unreach‐
	      able,	  icmp-port-unreachable,       icmp-proto-unreachable,
	      icmp-net-prohibited,  icmp-host-prohibited or icmp-admin-prohib‐
	      ited  (*)	 which	return	the  appropriate  ICMP	error  message
	      (port-unreachable	 is the default).  The option 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 ident (113/tcp)	probes	which  frequently  occur  when
	      sending  mail to broken mail hosts (which won't accept your mail
	      otherwise).

       (*) Using icmp-admin-prohibited with kernels that  do  not  support  it
       will result in a plain DROP instead of REJECT

   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-/destina‐
       tion-address for each connection.

       N.B.: The DNAT target's --persistent option replaced the SAME target.

       --to ipaddr[-ipaddr]
	      Addresses	 to map source to. May be specified more than once for
	      multiple ranges.

       --nodst
	      Don't use the destination-ip in the calculations when  selecting
	      the new source-ip

       --random
	      Port  mapping will be forcibly randomized to avoid attacks based
	      on port prediction (kernel >= 2.6.21).

   SECMARK
       This is used to set the security mark value associated with the	packet
       for  use	 by  security subsystems such as SELinux.  It is only valid in
       the mangle table. The mark is 32 bits wide.

       --selctx security_context

   SET
       This modules adds and/or deletes entries from  IP  sets	which  can  be
       defined by ipset(8).

       --add-set setname flag[,flag...]
	      add the address(es)/port(s) of the packet to the sets

       --del-set setname flag[,flag...]
	      delete the address(es)/port(s) of the packet from the sets

	      where  flags  are src and/or dst specifications and there can be
	      no more than six of them.

   SNAT
       This target is only valid in the nat table, in the  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:

       --to-source ipaddr[-ipaddr][:port[-port]]
	      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 -p tcp or -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 alter‐
	      ation will

	      In Kernels  up  to  2.6.10,  you	can  add  several  --to-source
	      options.	For those kernels, 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 addresses.  Later  Kernels  (>=  2.6.11-rc1)
	      don't have the ability to NAT to multiple ranges anymore.

       --random
	      If  option --random is used then port mapping will be randomized
	      (kernel >= 2.6.21).

       --persistent
	      Gives a client the  same	source-/destination-address  for  each
	      connection.   This  supersedes the SAME target. Support for per‐
	      sistent mappings is available from 2.6.29-rc2.

   TCPMSS
       This target allows to alter the MSS value of TCP SYN packets,  to  con‐
       trol  the maximum size for that connection (usually limiting it to your
       outgoing interface's MTU minus 40 for IPv4  or  60  for	IPv6,  respec‐
       tively).	  Of  course,  it can only be used in conjunction with -p tcp.
       It is only valid in the mangle table.
       This target is used to overcome criminally braindead  ISPs  or  servers
       which  block  "ICMP  Fragmentation  Needed"  or "ICMPv6 Packet Too Big"
       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:
	1) Web browsers connect, then hang with no data received.
	2) Small mail works fine, but large emails hang.
	3) ssh works fine, but scp hangs after initial handshaking.
       Workaround: activate this option and add a rule to your	firewall  con‐
       figuration like:

	       iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN
			   -j TCPMSS --clamp-mss-to-pmtu

       --set-mss value
	      Explicitly sets MSS option to specified value. If the MSS of the
	      packet is already lower than value, it  will  not	 be  increased
	      (from  Linux  2.6.25  onwards) to avoid more problems with hosts
	      relying on a proper MSS.

       --clamp-mss-to-pmtu
	      Automatically clamp MSS value to (path_MTU - 40  for  IPv4;  -60
	      for  IPv6).   This  may not function as desired where asymmetric
	      routes with differing path MTU exist — the kernel uses the  path
	      MTU which it would use to send packets from itself to the source
	      and destination IP addresses. Prior to Linux  2.6.25,  only  the
	      path  MTU	 to  the destination IP address was considered by this
	      option; subsequent kernels also consider the  path  MTU  to  the
	      source IP address.

       These options are mutually exclusive.

   TCPOPTSTRIP
       This  target will strip TCP options off a TCP packet. (It will actually
       replace them by NO-OPs.) As such, you will  need	 to  add  the  -p  tcp
       parameters.

       --strip-options option[,option...]
	      Strip  the  given option(s). The options may be specified by TCP
	      option number or	by  symbolic  name.  The  list	of  recognized
	      options  can be obtained by calling iptables with -j TCPOPTSTRIP
	      -h.

   TOS
       This module sets the Type of Service field in the IPv4 header  (includ‐
       ing  the	 "precedence"  bits) or the Priority field in the IPv6 header.
       Note that TOS shares the same bits as DSCP and ECN. The TOS  target  is
       only valid in the mangle table.

       --set-tos value[/mask]
	      Zeroes  out  the	bits  given  by	 mask  and XORs value into the
	      TOS/Priority field. If mask is omitted, 0xFF is assumed.

       --set-tos symbol
	      You can specify a symbolic name when using the  TOS  target  for
	      IPv4.  It	 implies  a  mask  of 0xFF. The list of recognized TOS
	      names can be obtained by calling iptables with -j TOS -h.

       The following mnemonics are available:

       --and-tos bits
	      Binary AND the TOS value	with  bits.  (Mnemonic	for  --set-tos
	      0/invbits, where invbits is the binary negation of bits.)

       --or-tos bits
	      Binary  OR  the  TOS  value  with	 bits. (Mnemonic for --set-tos
	      bits/bits.)

       --xor-tos bits
	      Binary XOR the TOS value	with  bits.  (Mnemonic	for  --set-tos
	      bits/0.)

   TPROXY
       This  target is only valid in the mangle table, in the PREROUTING chain
       and user-defined chains which are only called from this chain. It redi‐
       rects  the  packet to a local socket without changing the packet header
       in any way. It can also change the mark value which can then be used in
       advanced routing rules.	It takes three options:

       --on-port port
	      This  specifies  a  destination  port  to	 use. It is a required
	      option, 0 means the new destination port	is  the	 same  as  the
	      original.	 This  is only valid if the rule also specifies -p tcp
	      or -p udp.

       --on-ip address
	      This specifies a destination address  to	use.  By  default  the
	      address  is  the	IP  address of the incoming interface. This is
	      only valid if the rule also specifies -p tcp or -p udp.

       --tproxy-mark value[/mask]
	      Marks packets with the given value/mask. The  fwmark  value  set
	      here  can be used by advanced routing. (Required for transparent
	      proxying to work: otherwise these packets	 will  get  forwarded,
	      which is probably not what you want.)

   TRACE
       This  target  marks packes so that the kernel will log every rule which
       match the packets as those traverse the	tables,	 chains,  rules.  (The
       ipt_LOG	or  ip6t_LOG  module is required for the logging.) The packets
       are  logged  with   the	 string	  prefix:   "TRACE:   tablename:chain‐
       name:type:rulenum  "  where type can be "rule" for plain rule, "return"
       for implicit rule at the end of a user defined chain and	 "policy"  for
       the policy of the built in chains.
       It can only be used in the raw table.

   TTL
       This is used to modify the IPv4 TTL header field.  The TTL field deter‐
       mines how many hops (routers) a packet can traverse until it's time  to
       live is exceeded.

       Setting	or  incrementing the TTL field can potentially be very danger‐
       ous, so it should be avoided at any cost.

       Don't ever set or increment the value on packets that leave your	 local
       network!	 mangle table.

       --ttl-set value
	      Set the TTL value to `value'.

       --ttl-dec value
	      Decrement the TTL value `value' times.

       --ttl-inc value
	      Increment the TTL value `value' times.

   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 netlink socket. One or more userspace processes may then sub‐
       scribe 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.

       --ulog-nlgroup nlgroup
	      This specifies the netlink group (1-32) to which the  packet  is
	      sent.  Default value is 1.

       --ulog-prefix prefix
	      Prefix  log messages with the specified prefix; up to 32 charac‐
	      ters long, and useful for distinguishing messages in the logs.

       --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.

       --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).

DIAGNOSTICS
       Various error messages are printed to standard error.  The exit code is
       0 for correct functioning.  Errors which appear to be caused by invalid
       or abused command line parameters cause an exit code of	2,  and	 other
       errors cause an exit code of 1.

BUGS
       Bugs?   What's  this?  ;-)  Well,  you  might  want  to	have a look at
       http://bugzilla.netfilter.org/

COMPATIBILITY WITH IPCHAINS
       This iptables is very similar to ipchains by Rusty Russell.   The  main
       difference  is  that the chains INPUT and OUTPUT are only traversed for
       packets coming into the local host and originating from the local  host
       respectively.   Hence every packet only passes through one of the three
       chains (except loopback traffic, which involves both INPUT  and	OUTPUT
       chains); previously a forwarded packet would pass through all three.

       The  other main difference is that -i refers to the input interface; -o
       refers to the output interface, and  both  are  available  for  packets
       entering the FORWARD chain.

       The  various  forms  of NAT have been separated out; iptables is a pure
       packet filter when using the  default  `filter'	table,	with  optional
       extension modules.  This should simplify much of the previous confusion
       over the combination of IP masquerading and packet filtering seen  pre‐
       viously.	 So the following options are handled differently:
	-j MASQ
	-M -S
	-M -L
       There are several other changes in iptables.

SEE ALSO
       iptables-save(8), iptables-restore(8), ip6tables(8), ip6tables-save(8),
       ip6tables-restore(8), libipq(3).

       The packet-filtering-HOWTO details iptables usage for packet filtering,
       the  NAT-HOWTO  details NAT, the netfilter-extensions-HOWTO details the
       extensions that are not in the standard distribution, and  the  netfil‐
       ter-hacking-HOWTO details the netfilter internals.
       See http://www.netfilter.org/.

AUTHORS
       Rusty  Russell  originally  wrote  iptables, in early consultation with
       Michael Neuling.

       Marc Boucher made Rusty abandon ipnatctl	 by  lobbying  for  a  generic
       packet  selection  framework  in iptables, then wrote the mangle table,
       the owner match, the mark stuff, and ran around doing cool stuff every‐
       where.

       James Morris wrote the TOS target, and tos match.

       Jozsef Kadlecsik wrote the REJECT target.

       Harald  Welte  wrote  the  ULOG and NFQUEUE target, the new libiptc, as
       well as the TTL, DSCP, ECN matches and targets.

       The Netfilter Core Team is: Marc Boucher,  Martin  Josefsson,  Yasuyuki
       Kozakai,	 Jozsef	 Kadlecsik, Patrick McHardy, James Morris, Pablo Neira
       Ayuso, Harald Welte and Rusty Russell.

       Man page originally written by Herve Eychenne <rv@wallfire.org>.

iptables 1.4.6							   IPTABLES(8)
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