iptables man page on Ubuntu

Man page or keyword search:  
man Server   6591 pages
apropos Keyword Search (all sections)
Output format
Ubuntu logo
[printable version]

IPTABLES(8)			iptables 1.4.4			   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] [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]
	      Zero the packet and byte counters in all chains.	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 (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 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 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.

       [!] -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) f 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.

       --reap reap
	      This  option  must  be  used in conjunction with --seconds. When
	      used, this will remove entries with the  most  recent  timestamp
	      older then --seconds since the last packet was received.

       [!] --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.

       --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 modules macthes 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.

   unclean
       This  module takes no options, but attempts to match packets which seem
       malformed or unusual.  This is regarded as experimental.

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

   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.

       --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).

       --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.

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

       It can only be used with Kernel versions	 2.6.14	 or  later,  since  it
       requires the nfnetlink_queue kernel support.

   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.

       --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 set MSS option to specified value.

       --clamp-mss-to-pmtu
	      Automatically clamp MSS value to (path_MTU - 40  for  IPv4;  -60
	      for IPv6).

       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 1.4.4							   IPTABLES(8)
[top]

List of man pages available for Ubuntu

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Tweet
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
....................................................................
Vote for polarhome
Free Shell Accounts :: the biggest list on the net