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iptables-extensions(8)		iptables 1.4.21		iptables-extensions(8)

NAME
       iptables-extensions  — list of extensions in the standard iptables dis‐
       tribution

SYNOPSIS
       ip6tables  [-m  name  [module-options...]]   [-j	 target-name  [target-
       options...]

       iptables	  [-m  name  [module-options...]]   [-j	 target-name  [target-
       options...]

MATCH EXTENSIONS
       iptables can use extended  packet  matching  modules  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 modules 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 extended
       match modules are evaluated in the order	 they  are  specified  in  the
       rule.

       If  the	-p  or	--protocol was specified and if and only if an unknown
       option is encountered, iptables will try load a	match  module  of  the
       same name as the protocol, to try making the option available.

   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 (IPv6-specific)
       This module matches the parameters in Authentication  header  of	 IPsec
       packets.

       [!] --ahspi spi[:spi]
	      Matches SPI.

       [!] --ahlen length
	      Total length of this header in octets.

       --ahres
	      Matches if the reserved field is filled with zero.

   ah (IPv4-specific)
       This module matches the SPIs in Authentication header of IPsec packets.

       [!] --ahspi spi[:spi]

   bpf
       Match  using Linux Socket Filter. Expects a BPF program in decimal for‐
       mat. This is the format generated by the nfbpf_compile utility.

       --bytecode code
	      Pass the BPF byte code format (described in the example below).

       The code format is similar to the output of the tcpdump	-ddd  command:
       one  line  that stores the number of instructions, followed by one line
       for each instruction. Instruction lines follow the pattern 'u16	u8  u8
       u32'  in	 decimal notation. Fields encode the operation, jump offset if
       true, jump offset if false and generic multiuse field 'K'. Comments are
       not supported.

       For  example,  to  read	only packets matching 'ip proto 6', insert the
       following, without the comments or trailing whitespace:

	      4		      # number of instructions
	      48 0 0 9	      # load byte  ip->proto
	      21 0 1 6	      # jump equal IPPROTO_TCP
	      6 0 0 1	      # return	   pass (non-zero)
	      6 0 0 0	      # return	   fail (zero)

       You can pass this filter to the bpf match with the following command:

	      iptables -A OUTPUT -m bpf --bytecode '4,48 0 0 9,21 0 1 6,6 0  0
	      1,6 0 0 0' -j ACCEPT

       Or instead, you can invoke the nfbpf_compile utility.

	      iptables	-A  OUTPUT  -m	bpf --bytecode "`nfbpf_compile RAW 'ip
	      proto 6'`" -j ACCEPT

       You may want to learn more about BPF from FreeBSD's bpf(4) manpage.

   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

       NOTE: the arptables commands above use mainstream syntax.  If  you  are
       using arptables-jf included in some RedHat, CentOS and Fedora versions,
       you will hit syntax errors. Therefore, you'll have to  adapt  these  to
       the arptables-jf syntax to get them working.

       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 -i eth1 -m comment --comment "my local LAN"

   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 ...

   connlabel
       Module matches or adds connlabels to a connection.  connlabels are sim‐
       ilar to connmarks, except labels are bit-based; i.e.  all labels may be
       attached	 to a flow at the same time.  Up to 128 unique labels are cur‐
       rently supported.

       [!] --label name
	      matches if label name has been set on a connection.  Instead  of
	      a	 name  (which  will  be	 translated  to	 a number, see EXAMPLE
	      below), a number may be used instead.   Using  a	number	always
	      overrides connlabel.conf.

       --set  if  the  label has not been set on the connection, set it.  Note
	      that setting a label can fail.  This is because the kernel allo‐
	      cates  the  conntrack  label storage area when the connection is
	      created, and it only reserves the amount of memory  required  by
	      the  ruleset  that exists at the time the connection is created.
	      In this case, the match will fail (or succeed, in	 case  --label
	      option was negated).

       This  match  depends  on	 libnetfilter_conntrack 1.0.4 or later.	 Label
       translation is done via the  /etc/xtables/connlabel.conf	 configuration
       file.

       Example:

	      0	   eth0-in
	      1	   eth0-out
	      2	   ppp-in
	      3	   ppp-out
	      4	   bulk-traffic
	      5	   interactive

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

       --connlimit-upto n
	      Match if the number of existing connections is below or equal n.

       --connlimit-above n
	      Match if the number of existing connections is 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. If not specified, the maximum prefix length for the	appli‐
	      cable protocol is used.

       --connlimit-saddr
	      Apply  the  limit	 onto the source group. This is the default if
	      --connlimit-daddr is not specified.

       --connlimit-daddr
	      Apply the limit onto the destination group.

       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-upto 2 -j ACCEPT

       # limit the number of parallel HTTP requests to 16 per  class  C	 sized
       source 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

       # Limit the number of connections to a particular host:
	      ip6tables -p tcp --syn --dport  49152:65535  -d  2001:db8::1  -m
	      connlimit --connlimit-above 100 -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[:port]

       [!] --ctorigdstport port[:port]

       [!] --ctreplsrcport port[:port]

       [!] --ctrepldstport port[:port]
	      Match    against	  original/reply    source/destination	  port
	      (TCP/UDP/etc.) or GRE key.  Matching against port ranges is only
	      supported in kernel versions above 2.6.38.

       [!] --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
	      The packet is associated with no known connection.

       NEW    The packet has started a new connection or otherwise  associated
	      with a connection which has not seen packets in both directions.

       ESTABLISHED
	      The  packet is associated with a connection which has seen pack‐
	      ets in both directions.

       RELATED
	      The packet is starting a new connection, but is associated  with
	      an  existing connection, such as an FTP data transfer or an ICMP
	      error.

       UNTRACKED
	      The packet is not tracked at all, which happens if  you  explic‐
	      itly untrack it by using -j CT --notrack in the raw table.

       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.

   cpu
       [!] --cpu number
	      Match cpu handling this packet. cpus  are	 numbered  from	 0  to
	      NR_CPUS-1	 Can  be  used	in combination with RPS (Remote Packet
	      Steering) or multiqueue NICs to spread network traffic  on  dif‐
	      ferent queues.

       Example:

       iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 0 -j REDI‐
       RECT --to-port 8080

       iptables -t nat -A PREROUTING -p tcp --dport 80 -m cpu --cpu 1 -j REDI‐
       RECT --to-port 8081

       Available since Linux 2.6.36.

   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 DCCP option set.

   devgroup
       Match device group of a packets incoming/outgoing interface.

       [!] --src-group name
	      Match device group of incoming device

       [!] --dst-group name
	      Match device group of outgoing device

   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.

   dst (IPv6-specific)
       This module matches the parameters in Destination Options header

       [!] --dst-len length
	      Total length of this header in octets.

       --dst-opts type[:length][,type[:length]...]
	      numeric  type  of	 option	 and  the length of the option data in
	      octets.

   ecn
       This allows you to match the ECN bits of the IPv4/IPv6 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/IPv6 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]

   eui64 (IPv6-specific)
       This module matches the EUI-64 part of a stateless autoconfigured  IPv6
       address.	 It compares the EUI-64 derived from the source MAC address in
       Ethernet frame with the lower 64 bits of the IPv6 source	 address.  But
       "Universal/Local"  bit is not compared. This module doesn't match other
       link layer frame, and is only valid in the PREROUTING, INPUT  and  FOR‐
       WARD chains.

   frag (IPv6-specific)
       This module matches the parameters in Fragment header.

       [!] --fragid id[:id]
	      Matches the given Identification or range of it.

       [!] --fraglen length
	      This  option cannot be used with kernel version 2.6.10 or later.
	      The length of Fragment header is static and this option  doesn't
	      make sense.

       --fragres
	      Matches if the reserved fields are filled with zero.

       --fragfirst
	      Matches on the first fragment.

       --fragmore
	      Matches if there are more fragments.

       --fraglast
	      Matches if this is the last fragment.

   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" or "N bytes per seconds" (see below for some
       examples).

       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 either as a number, with an optional time quantum suf‐
	      fix  (the	 default  is  3/hour), or as amountb/second (number of
	      bytes per second).

       --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.   When  byte-based
	      rate  matching is requested, this option specifies the amount of
	      bytes that can exceed the given rate.   This  option  should  be
	      used  with  caution  -- if the entry expires, the burst value is
	      reset too.

       --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.

       Examples:

       matching on source host
	      "1000 packets per second for every host in 192.168.0.0/16" => -s
	      192.168.0.0/16 --hashlimit-mode srcip --hashlimit-upto 1000/sec

       matching on source port
	      "100 packets per second for every service of 192.168.1.1" =>  -s
	      192.168.1.1 --hashlimit-mode srcport --hashlimit-upto 100/sec

       matching on subnet
	      "10000  packets  per  minute  for	 every /28 subnet (groups of 8
	      addresses) in 10.0.0.0/8" => -s 10.0.0.0/8  --hashlimit-mask  28
	      --hashlimit-upto 10000/min

       matching bytes per second
	      "flows	 exceeding     512kbyte/s"     =>     --hashlimit-mode
	      srcip,dstip,srcport,dstport --hashlimit-above 512kb/s

       matching bytes per second
	      "hosts that exceed 512kbyte/s, but permit up to 1Megabytes with‐
	      out  matching"  --hashlimit-mode dstip --hashlimit-above 512kb/s
	      --hashlimit-burst 1mb

   hbh (IPv6-specific)
       This module matches the parameters in Hop-by-Hop Options header

       [!] --hbh-len length
	      Total length of this header in octets.

       --hbh-opts type[:length][,type[:length]...]
	      numeric type of option and the length  of	 the  option  data  in
	      octets.

   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.

   hl (IPv6-specific)
       This module matches the Hop Limit field in the IPv6 header.

       [!] --hl-eq value
	      Matches if Hop Limit equals value.

       --hl-lt value
	      Matches if Hop Limit is less than value.

       --hl-gt value
	      Matches if Hop Limit is greater than value.

   icmp (IPv4-specific)
       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

   icmp6 (IPv6-specific)
       This extension can be used if  `--protocol  ipv6-icmp'  or  `--protocol
       icmpv6' is specified. It provides the following option:

       [!] --icmpv6-type type[/code]|typename
	      This  allows  specification  of  the ICMPv6 type, which can be a
	      numeric ICMPv6 type, type and code, or one of  the  ICMPv6  type
	      names shown by the command
	       ip6tables -p ipv6-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.

   ipv6header (IPv6-specific)
       This module matches IPv6 extension headers and/or upper layer header.

       --soft Matches if the packet includes any of the headers specified with
	      --header.

       [!] --header header[,header...]
	      Matches the packet which EXACTLY includes all specified headers.
	      The headers encapsulated with ESP header are out of scope.  Pos‐
	      sible header types can be:

       hop|hop-by-hop
	      Hop-by-Hop Options header

       dst    Destination Options header

       route  Routing header

       frag   Fragment header

       auth   Authentication header

       esp    Encapsulating Security Payload header

       none   No Next header which matches 59 in the 'Next  Header  field'  of
	      IPv6 header or any IPv6 extension headers

       proto  which  matches  any upper layer protocol header. A protocol name
	      from /etc/protocols and numeric value also allowed.  The	number
	      255 is equivalent to proto.

   ipvs
       Match IPVS connection properties.

       [!] --ipvs
	      packet belongs to an IPVS connection

       Any of the following options implies --ipvs (even negated)

       [!] --vproto protocol
	      VIP protocol to match; by number or name, e.g. "tcp"

       [!] --vaddr address[/mask]
	      VIP address to match

       [!] --vport port
	      VIP port to match; by number or name, e.g. "http"

       --vdir {ORIGINAL|REPLY}
	      flow direction of packet

       [!] --vmethod {GATE|IPIP|MASQ}
	      IPVS forwarding method used

       [!] --vportctl port
	      VIP port of the controlling connection to match, e.g. 21 for FTP

   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.  It
       can be used in combination with the LOG target to give limited logging,
       for example.

       xt_limit	 has no negation support - you will have to use -m hashlimit !
       --hashlimit rate in this case whilst omitting --hashlimit-mode.

       --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).

   mh (IPv6-specific)
       This extension is loaded if `--protocol ipv6-mh' or `--protocol mh'  is
       specified. It provides the following option:

       [!] --mh-type type[:type]
	      This allows specification of the Mobility Header(MH) type, which
	      can be a numeric MH type, type or one of the MH type names shown
	      by the command
	       ip6tables -p mh -h

   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 one of the following protocols:
       tcp, udp, udplite, dccp and sctp.

       [!] --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.

   nfacct
       The nfacct match provides the extended  accounting  infrastructure  for
       iptables.   You	have  to  use  this match together with the standalone
       user-space utility nfacct(8)

       The only option available for this match is the following:

       --nfacct-name name
	      This allows you to specify the existing object name that will be
	      use for accounting the traffic that this rule-set is matching.

       To use this extension, you have to create an accounting object:

	      nfacct add http-traffic

       Then, you have to attach it to the accounting object via iptables:

	      iptables	-I  INPUT  -p  tcp  --sport 80 -m nfacct --nfacct-name
	      http-traffic

	      iptables -I OUTPUT -p tcp --dport	 80  -m	 nfacct	 --nfacct-name
	      http-traffic

       Then, you can check for the amount of traffic that the rules match:

	      nfacct get http-traffic

	      {	 pkts = 00000000000000000156, bytes = 00000000000000151786 } =
	      http-traffic;

       You can obtain nfacct(8)	 from  http://www.netfilter.org	 or,  alterna‐
       tively, from the git.netfilter.org repository.

   osf
       The  osf module does passive operating system fingerprinting. This mod‐
       ules compares some data (Window Size, MSS,  options  and	 their	order,
       TTL, DF, and others) from packets with the SYN bit set.

       [!] --genre string
	      Match  an operating system genre by using a passive fingerprint‐
	      ing.

       --ttl level
	      Do additional TTL checks on the packet to determine the  operat‐
	      ing system.  level can be one of the following values:

       ·   0  - True IP address and fingerprint TTL comparison. This generally
	   works for LANs.

       ·   1 - Check if the IP header's TTL is less than the fingerprint  one.
	   Works for globally-routable addresses.

       ·   2 - Do not compare the TTL at all.

       --log level
	   Log	determined  genres  into  dmesg	 even if they do not match the
	   desired one.	 level can be one of the following values:

       ·   0 - Log all matched or unknown signatures

       ·   1 - Log only the first one

       ·   2 - Log all known matched signatures

       You may find something like this in syslog:

       Windows [2000:SP3:Windows XP Pro SP1, 2000  SP3]:  11.22.33.55:4024  ->
       11.22.33.44:139	hops=3	Linux [2.5-2.6:] : 1.2.3.4:42624 -> 1.2.3.5:22
       hops=4

       OS fingerprints are loadable using the nfnl_osf program. To  load  fin‐
       gerprints from a file, use:

       nfnl_osf -f /usr/share/xtables/pf.os

       To remove them again,

       nfnl_osf -f /usr/share/xtables/pf.os -d

       The  fingerprint	 database  can	be  downlaoded	from  http://www.open‐
       bsd.org/cgi-bin/cvsweb/src/etc/pf.os .

   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. --pol none
	      cannot be combined with --strict.

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

       For  each  policy  element  that is to be described, one can use one or
       more of the following options. When --strict is in effect, at least one
       must be used per element.

       [!] --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. The condition matches until  the	 byte  counter	reaches	 zero.
       Behavior	 is  reversed with negation (i.e. the condition does not match
       until the byte counter reaches zero).

       [!] --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.

       For a better understanding of the available options, these are all pos‐
       sible combinations:

       ·   rateest operator rateest-bps

       ·   rateest operator rateest-pps

       ·   (rateest minus rateest-bps1) operator rateest-bps2

       ·   (rateest minus rateest-pps1) operator rateest-pps2

       ·   rateest1 operator rateest2 rateest-bps(without rate!)

       ·   rateest1 operator rateest2 rateest-pps(without rate!)

       ·   (rateest1 minus rateest-bps1)  operator  (rateest2  minus  rateest-
	   bps2)

       ·   (rateest1  minus  rateest-pps1)  operator  (rateest2 minus rateest-
	   pps2)

       --rateest-delta
	   For each estimator (either absolute or  relative  mode),  calculate
	   the	difference  between the estimator-determined flow rate and the
	   static value chosen with the BPS/PPS options. If the flow  rate  is
	   higher than the specified BPS/PPS, 0 will be used instead of a neg‐
	   ative value. In other words, "max(0, rateest#_rate - rateest#_bps)"
	   is used.

       [!] --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.

       In  the	so-called "absolute mode", only one rate estimator is used and
       compared against a static value, while in  "relative  mode",  two  rate
       estimators are compared against another.

       --rateest name
	      Name of the one rate estimator for absolute mode.

       --rateest1 name

       --rateest2 name
	      The names of the two rate estimators for relative mode.

       --rateest-bps [value]

       --rateest-pps [value]

       --rateest-bps1 [value]

       --rateest-bps2 [value]

       --rateest-pps1 [value]

       --rateest-pps2 [value]
	      Compare  the  estimator(s)  by  bytes or packets per second, and
	      compare against the chosen value. See the above bullet list  for
	      which  option  is to be used in which case. A unit suffix may be
	      used - available ones  are:  bit,	 [kmgt]bit,  [KMGT]ibit,  Bps,
	      [KMGT]Bps, [KMGT]iBps.

       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 (IPv4-specific)
       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.

       --set, --rcheck, --update and --remove are mutually exclusive.

       --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.

       --mask netmask
	      Netmask that will be applied to this recent list.

       [!] --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 This  option  can	 only  be  used in conjunction with --seconds.
	      When used, this will cause entries older	than  the  last	 given
	      number of seconds to be purged.

       --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

       /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.

   rpfilter
       Performs	 a  reverse  path  filter test on a packet.  If a reply to the
       packet would be sent via the same interface that the packet arrived on,
       the  packet  will  match.   Note	 that, unlike the in-kernel rp_filter,
       packets protected by IPSec are not  treated  specially.	 Combine  this
       match  with  the policy match if you want this.	Also, packets arriving
       via the loopback interface are always permitted.	 This match  can  only
       be used in the PREROUTING chain of the raw or mangle table.

       --loose
	      Used  to specifiy that the reverse path filter test should match
	      even if the selected output device is not the expected one.

       --validmark
	      Also use the packets' nfmark value when performing  the  reverse
	      path route lookup.

       --accept-local
	      This will permit packets arriving from the network with a source
	      address that is also assigned to the local machine.

       --invert
	      This will invert the sense of the match.	 Instead  of  matching
	      packets  that  passed  the reverse path filter test, match those
	      that have failed it.

       Example to log and drop packets failing the reverse path filter test:

       iptables -t raw -N RPFILTER

       iptables -t raw -A RPFILTER -m rpfilter -j RETURN

       iptables -t raw	-A  RPFILTER  -m  limit	 --limit  10/minute  -j	 NFLOG
       --nflog-prefix "rpfilter drop"

       iptables -t raw -A RPFILTER -j DROP

       iptables -t raw -A PREROUTING -j RPFILTER

       Example to drop failed packets, without logging:

       iptables -t raw -A RPFILTER -m rpfilter --invert -j DROP

   rt (IPv6-specific)
       Match on IPv6 routing header

       [!] --rt-type type
	      Match the type (numeric).

       [!] --rt-segsleft num[:num]
	      Match the `segments left' field (range).

       [!] --rt-len length
	      Match the length of this header.

       --rt-0-res
	      Match the reserved field, too (type=0)

       --rt-0-addrs addr[,addr...]
	      Match type=0 addresses (list).

       --rt-0-not-strict
	      List of type=0 addresses is not a strict list.

   sctp
       This module matches Stream Control Transmission Protocol headers.

       [!] --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 FORWARD_TSN

	      chunk type	    available flags
	      DATA		    I U B E i 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.

       --return-nomatch
	      If  the  --return-nomatch	 option	 is specified and the set type
	      supports the nomatch flag, then  the  matching  is  reversed:  a
	      match with an element flagged with nomatch returns true, while a
	      match with a plain element returns false.

       ! --update-counters
	      If the --update-counters flag is negated, then  the  packet  and
	      byte  counters  of  the  matching	 element  in  the set won't be
	      updated. Default the packet and byte counters are updated.

       ! --update-subcounters
	      If the --update-subcounters flag is negated, then the packet and
	      byte  counters  of  the  matching element in the member set of a
	      list type of set won't be updated. Default the packet  and  byte
	      counters are updated.

       [!] --packets-eq value
	      If  the  packet  is matched an element in the set, match only if
	      the packet counter of the element matches the given value too.

       --packets-lt value
	      If the packet is matched an element in the set,  match  only  if
	      the  packet  counter of the element is less than the given value
	      as well.

       --packets-gt value
	      If the packet is matched an element in the set,  match  only  if
	      the  packet  counter  of	the  element is greater than the given
	      value as well.

       [!] -bytes-eq value
	      If the packet is matched an element in the set,  match  only  if
	      the byte counter of the element matches the given value too.

       --bytes-lt value
	      If  the  packet  is matched an element in the set, match only if
	      the byte counter of the element is less than the given value  as
	      well.

       --bytes-gt value
	      If  the  packet  is matched an element in the set, match only if
	      the byte counter of the element is greater than the given	 value
	      as well.

       The packet and byte counters related options and flags are ignored when
       the set was defined without counter support.

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

       Use  of	-m  set requires that ipset kernel support is provided, which,
       for standard kernels, is the case since Linux 2.6.39.

   socket
       This matches if an open TCP/UDP socket can be found by doing  a	socket
       lookup on the packet. It matches if there is an established or non-zero
       bound listening socket (possibly with a non-local address). The	lookup
       is performed using the packet tuple of TCP/UDP packets, or the original
       TCP/UDP header embedded in an ICMP/ICPMv6 error packet.

       --transparent
	      Ignore non-transparent sockets.

       --nowildcard
	      Do not ignore sockets bound to 'any' address.  The socket	 match
	      won't  accept  zero-bound listeners by default, since then local
	      services could intercept traffic that would  otherwise  be  for‐
	      warded.	This  option  therefore has security implications when
	      used to match traffic being forwarded to redirect	 such  packets
	      to  local	 machine  with	policy routing.	 When using the socket
	      match to implement fully transparent proxies bound to  non-local
	      addresses	 it  is	 recommended  to  use the --transparent option
	      instead.

       Example (assuming packets with mark 1 are delivered locally):

	      -t  mangle  -A  PREROUTING  -m  socket  --transparent  -j	  MARK
	      --set-mark 1

   state
       The  "state"  extension is a subset of the "conntrack" module.  "state"
       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. Only a subset of the states unterstood by "conntrack"
	      are recognized: INVALID, ESTABLISHED, NEW, RELATED or UNTRACKED.
	      For  their description, see the "conntrack" heading in this man‐
	      page.

   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 for a packet to be randomly matched. It only
	      works  with  the	random mode. p must be within 0.0 and 1.0. The
	      supported granularity is in 1/2147483648th increments.

       [!] --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 up to which should be scanned. That is, byte off‐
	      set-1 (counting from 0) is the last one that is scanned.	If not
	      passed, default is the packet size.

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

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

       Examples:

	      # The string pattern can be used for simple text characters.
	      iptables -A INPUT -p tcp --dport 80 -m string --algo bm --string
	      'GET /index.html' -j LOG

	      # The hex string pattern can be used for	non-printable  charac‐
	      ters, like |0D 0A| or |0D0A|.
	      iptables -p udp --dport 53 -m string --algo bm --from 40 --to 57
	      --hex-string '|03|www|09|netfilter|03|org|00|'

   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. All times are
       interpreted as UTC by default.

       --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.).

       --contiguous
	      When --timestop is smaller than --timestart value, match this as
	      a single time period instead distinct intervals.	See EXAMPLES.

       --kerneltz
	      Use  the	kernel	timezone instead of UTC to determine whether a
	      packet meets the time regulations.

       About kernel timezones: Linux keeps the system time in UTC, and	always
       does  so.   On boot, system time is initialized from a referential time
       source. Where this time source has no timezone information, such as the
       x86 CMOS RTC, UTC will be assumed. If the time source is however not in
       UTC, userspace should provide the correct system time and  timezone  to
       the kernel once it has the information.

       Local  time  is	a  feature on top of the (timezone independent) system
       time. Each process has its own idea of local time, specified via the TZ
       environment variable. The kernel also has its own timezone offset vari‐
       able. The TZ userspace environment variable specifies how the UTC-based
       system time is displayed, e.g. when you run date(1), or what you see on
       your desktop clock.  The TZ string may resolve to different offsets  at
       different  dates,  which	 is what enables the automatic time-jumping in
       userspace. when DST changes. The kernel's timezone offset  variable  is
       used  when  it  has  to	convert	 between  non-UTC sources, such as FAT
       filesystems, to UTC (since the latter is what the rest  of  the	system
       uses).

       The  caveat  with  the  kernel timezone is that Linux distributions may
       ignore to set the kernel timezone, and  instead	only  set  the	system
       time.  Even if a particular distribution does set the timezone at boot,
       it is usually does not keep the kernel timezone offset - which is  what
       changes	on DST - up to date.  ntpd will not touch the kernel timezone,
       so running it will not resolve the issue. As such, one may encounter  a
       timezone that is always +0000, or one that is wrong half of the time of
       the year. As such, using --kerneltz is highly discouraged.

       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.)

       Matching across days might not do what is expected.  For instance,

	      -m time --weekdays Mo --timestart 23:00  --timestop  01:00  Will
	      match  Monday,  for  one	hour from midnight to 1 a.m., and then
	      again for another hour from 23:00 onwards.  If this is unwanted,
	      e.g.  if	you  would like 'match for two hours from Montay 23:00
	      onwards' you need to also specify the --contiguous option in the
	      example above.

   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 (IPv4-specific)
       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 (IPv4-specific)
       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.

   AUDIT
       This  target  allows creates audit records for packets hitting the tar‐
       get.  It can be used to record accepted, dropped, and rejected packets.
       See auditd(8) for additional details.

       --type {accept|drop|reject}
	      Set type of audit record.

       Example:

	      iptables -N AUDIT_DROP

	      iptables -A AUDIT_DROP -j AUDIT --type drop

	      iptables -A AUDIT_DROP -j DROP

   CHECKSUM
       This  target  selectively works around broken/old applications.	It can
       only be used in the mangle table.

       --checksum-fill
	      Compute and fill in the checksum in a packet that lacks a check‐
	      sum.   This  is  particularly useful, if you need to work around
	      old applications such as dhcp clients, that  do  not  work  well
	      with  checksum offloads, but don't want to disable checksum off‐
	      load in your device.

   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 (IPv4-specific)
       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 valid in the
       security table (for backwards compatibility with older kernels,	it  is
       also 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.

   CT
       The CT target sets parameters for a packet or  its  associated  connec‐
       tion. The target attaches a "template" connection tracking entry to the
       packet, which is then used by the conntrack core	 when  initializing  a
       new ct entry. This target is thus only valid in the "raw" table.

       --notrack
	      Disables connection tracking for this packet.

       --helper name
	      Use  the	helper	identified by name for the connection. This is
	      more flexible than loading the  conntrack	 helper	 modules  with
	      preset ports.

       --ctevents event[,...]
	      Only  generate  the  specified conntrack events for this connec‐
	      tion. Possible event types are: new,  related,  destroy,	reply,
	      assured, protoinfo, helper, mark (this refers to the ctmark, not
	      nfmark), natseqinfo, secmark (ctsecmark).

       --expevents event[,...]
	      Only generate the specified expectation events for this  connec‐
	      tion.  Possible event types are: new.

       --zone id
	      Assign this packet to zone id and only have lookups done in that
	      zone.  By default, packets have zone 0.

       --timeout name
	      Use the timeout policy identified by name	 for  the  connection.
	      This  is	provides  more flexible timeout policy definition than
	      global  timeout  values	available   at	 /proc/sys/net/netfil‐
	      ter/nf_conntrack_*_timeout_*.

   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 the follow‐
       ing options:

       --to-destination [ipaddr[-ipaddr]][:port[-port]]
	      which can specify a single new destination IP address, an inclu‐
	      sive range of IP addresses. Optionally a port range, if the rule
	      also specifies one of the following protocols: tcp, udp, dccp or
	      sctp.   If no port range is specified, then the destination port
	      will never be modified. If no IP address is specified then  only
	      the  destination 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 destination address, either via an
	      address range or multiple	 --to-destination  options,  a	simple
	      round-robin  (one	 after	another in cycle) load balancing takes
	      place between these 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.

       IPv6 support available since Linux kernels >= 3.7.

   DNPT (IPv6-specific)
       Provides stateless destination IPv6-to-IPv6 Network Prefix  Translation
       (as described by RFC 6296).

       You  have to use this target in the mangle table, not in the nat table.
       It takes the following options:

       --src-pfx [prefix/length]
	      Set source prefix that you want to translate and length

       --dst-pfx [prefix/length]
	      Set destination prefix that you want to use in  the  translation
	      and length

       You have to use the SNPT target to undo the translation. Example:

	      ip6tables	 -t mangle -I POSTROUTING -s fd00::/64	-o vboxnet0 -j
	      SNPT --src-pfx fd00::/64 --dst-pfx 2001:e20:2000:40f::/64

	      ip6tables	  -t   mangle	 -I    PREROUTING    -i	   wlan0    -d
	      2001:e20:2000:40f::/64  -j DNPT --src-pfx 2001:e20:2000:40f::/64
	      --dst-pfx fd00::/64

       You may need to enable IPv6 neighbor proxy:

	      sysctl -w net.ipv6.conf.all.proxy_ndp=1

       You also have to use the NOTRACK target to disable connection  tracking
       for translated flows.

   DSCP
       This  target alters 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 (IPv4-specific)
       This target selectively works 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.

   HL (IPv6-specific)
       This  is	 used  to  modify  the Hop Limit field in IPv6 header. The Hop
       Limit field is similar to what is known as TTL value in IPv4.   Setting
       or  incrementing the Hop Limit field can potentially be very dangerous,
       so it should be avoided at any cost. This target is only valid in  man‐
       gle table.

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

       --hl-set value
	      Set the Hop Limit to `value'.

       --hl-dec value
	      Decrement the Hop Limit `value' times.

       --hl-inc value
	      Increment the Hop Limit `value' times.

   HMARK
       Like MARK, i.e. set the fwmark, but the mark is calculated from hashing
       packet selector at choice. You have also to specify the mark range and,
       optionally, the offset to start from. ICMP error messages are inspected
       and used to calculate the hashing.

       Existing options are:

       --hmark-tuple tuple
	      Possible	tuple  members	are: src meaning source address (IPv4,
	      IPv6 address),  dst  meaning  destination	 address  (IPv4,  IPv6
	      address),	 sport	meaning	 source port (TCP, UDP, UDPlite, SCTP,
	      DCCP), dport meaning destination port (TCP, UDP, UDPlite,	 SCTP,
	      DCCP),  spi  meaning  Security Parameter Index (AH, ESP), and ct
	      meaning the usage of the conntrack tuple instead of  the	packet
	      selectors.

       --hmark-mod value (must be > 0)
	      Modulus  for  hash  calculation  (to limit the range of possible
	      marks)

       --hmark-offset value
	      Offset to start marks from.

       For advanced usage, instead of using  --hmark-tuple,  you  can  specify
       custom
	      prefixes and masks:

       --hmark-src-prefix cidr
	      The source address mask in CIDR notation.

       --hmark-dst-prefix cidr
	      The destination address mask in CIDR notation.

       --hmark-sport-mask value
	      A 16 bit source port mask in hexadecimal.

       --hmark-dport-mask value
	      A 16 bit destination port mask in hexadecimal.

       --hmark-spi-mask value
	      A 32 bit field with spi mask.

       --hmark-proto-mask value
	      An 8 bit field with layer 4 protocol number.

       --hmark-rnd value
	      A 32 bit random custom value to feed hash calculation.

       Examples:

       iptables -t mangle -A PREROUTING -m conntrack --ctstate NEW
	-j   HMARK   --hmark-tuple   ct,src,dst,proto	--hmark-offset	 10000
       --hmark-mod 10 --hmark-rnd 0xfeedcafe

       iptables -t mangle -A PREROUTING -j HMARK --hmark-offset 10000 --hmark-
       tuple src,dst,proto --hmark-mod 10 --hmark-rnd 0xdeafbeef

   IDLETIMER
       This  target can be used to identify when interfaces have been idle for
       a certain period of time.  Timers are identified by labels and are cre‐
       ated  when a rule is set with a new label.  The rules also take a time‐
       out value (in seconds) as an option.  If more than one  rule  uses  the
       same timer label, the timer will be restarted whenever any of the rules
       get a hit.  One entry  for  each	 timer	is  created  in	 sysfs.	  This
       attribute  contains  the	 timer remaining for the timer to expire.  The
       attributes are located under the xt_idletimer class:

       /sys/class/xt_idletimer/timers/<label>

       When the timer expires, the target module sends a sysfs notification to
       the userspace, which can then decide what to do (eg. disconnect to save
       power).

       --timeout amount
	      This is the time in seconds that will trigger the notification.

       --label string
	      This is a unique identifier for the timer.  The  maximum	length
	      for the label string is 27 characters.

   LED
       This creates an LED-trigger that can then be attached to system indica‐
       tor lights, to blink or	illuminate  them  when	certain	 packets  pass
       through	the  system. One example might be to light up an LED for a few
       minutes every time an SSH connection is made to the local machine.  The
       following options control the trigger behavior:

       --led-trigger-id name
	      This  is	the  name given to the LED trigger. The actual name of
	      the trigger will be prefixed with "netfilter-".

       --led-delay ms
	      This indicates how long (in milliseconds) the LED should be left
	      illuminated  when	 a  packet  arrives  before being switched off
	      again. The default is 0 (blink as fast as possible.) The special
	      value  inf  can  be  given  to leave the LED on permanently once
	      activated. (In this case the trigger will need  to  be  manually
	      detached	and  reattached	 to  the  LED  device to switch it off
	      again.)

       --led-always-blink
	      Always make the LED blink on packet arrival, even if the LED  is
	      already  on.   This allows notification of new packets even with
	      long delay values (which otherwise would result in a silent pro‐
	      longing of the delay time.)

       Example:

       Create an LED trigger for incoming SSH traffic:
	      iptables -A INPUT -p tcp --dport 22 -j LED --led-trigger-id ssh

       Then attach the new trigger to an LED:
	      echo netfilter-ssh >/sys/class/leds/ledname/trigger

   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/IPv6 header fields) via the kernel log (where
       it can be read with dmesg(1) or read in the syslog).

       This is a "non-terminating target", i.e. rule  traversal	 continues  at
       the  next  rule.	 So if you want to LOG the packets you refuse, use two
       separate rules with the same matching criteria, first using target  LOG
       then DROP (or REJECT).

       --log-level level
	      Level  of	 logging,  which can be (system-specific) numeric or a
	      mnemonic.	 Possible values are (in decreasing  order  of	prior‐
	      ity): emerg, alert, crit, error, warning, notice, info or debug.

       --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/IPv6 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.	It can, for example, be used in conjunction with routing based
       on fwmark (needs iproute2). If you plan on doing so, note that the mark
       needs to be set in the PREROUTING chain of the mangle table  to	affect
       routing.	 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).

       --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 one  of	the  following
	      protocols: tcp, udp, dccp or sctp.

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

       IPv6 support available since Linux kernels >= 3.7.

   MIRROR (IPv4-specific)
       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.

       IPv6 support available since Linux kernels >= 3.7.

   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 (0 - 2^16-1) 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 passes the packet to userspace  using  the  nfnetlink_queue
       handler.	  The  packet  is  put into the queue identified by its 16-bit
       queue number.  Userspace can inspect and modify the packet if  desired.
       Userspace  must	then  drop  or	reinject  the  packet into the kernel.
       Please see libnetfilter_queue for details.  nfnetlink_queue  was	 added
       in  Linux  2.6.14.  The queue-balance option was added in Linux 2.6.31,
       queue-bypass in 2.6.39.

       --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.

       --queue-bypass
	      By  default, if no userspace program is listening on an NFQUEUE,
	      then all packets that are to be queued are dropped.   When  this
	      option  is  used,	 the NFQUEUE rule behaves like ACCEPT instead,
	      and the packet will move on to the next table.

       --queue-cpu-fanout
	      Available starting Linux kernel 3.10. When  used	together  with
	      --queue-balance  this  will  use	the  CPU ID as an index to map
	      packets to the queues. The idea is that you can improve  perfor‐
	      mance  if there's a queue per CPU. This requires --queue-balance
	      to be specified.

   NOTRACK
       This extension disables connection tracking for	all  packets  matching
       that rule.  It is equivalent with -j CT --notrack. Like CT, NOTRACK 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  localhost   address,
       127.0.0.1 for IPv4 and ::1 for IPv6).

       --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 one of the following pro‐
	      tocols: tcp, udp, dccp or sctp.

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

       IPv6 support available starting Linux kernels >= 3.7.

   REJECT (IPv6-specific)
       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 icmp6-no-route,  no-route,	icmp6-adm-pro‐
	      hibited,	adm-prohibited,	 icmp6-addr-unreachable, addr-unreach,
	      or icmp6-port-unreachable, which return the  appropriate	ICMPv6
	      error  message (icmp6-port-unreachable is the default). Finally,
	      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).  tcp-reset can only  be  used
	      with kernel versions 2.6.14 or later.

   REJECT (IPv4-specific)
       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
	      (icmp-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 (IPv4-specific)
       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	valid  in  the
       security	 table	(for backwards compatibility with older kernels, it is
       also valid in the mangle table). The mark is 32 bits wide.

       --selctx security_context

   SET
       This module 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 set

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

	      where flag(s) are src and/or dst specifications and there can be
	      no more than six of them.

       --timeout value
	      when adding an entry, the timeout value to use  instead  of  the
	      default one from the set definition

       --exist
	      when  adding  an	entry  if it already exists, reset the timeout
	      value to the specified one or to the default from the set	 defi‐
	      nition

       Use  of	-j  SET requires that ipset kernel support is provided, which,
       for standard kernels, is the case since Linux 2.6.39.

   SNAT
       This target is only valid in the nat  table,  in	 the  POSTROUTING  and
       INPUT  chains, and user-defined chains which are only called from those
       chains.	It specifies that the source address of the packet  should  be
       modified	 (and  all future packets in this connection will also be man‐
       gled), and rules should cease being examined.  It takes	the  following
       options:

       --to-source [ipaddr[-ipaddr]][:port[-port]]
	      which  can  specify a single new source IP address, an inclusive
	      range of IP addresses. Optionally a port range, if the rule also
	      specifies	 one  of  the  following  protocols: tcp, udp, dccp or
	      sctp.  If no port range is specified, then  source  ports	 below
	      512  will	 be mapped to other ports below 512: those between 512
	      and 1023 inclusive will be mapped to ports below 1024, and other
	      ports  will  be mapped to 1024 or above. Where possible, no port
	      alteration will occur.  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 multi‐
	      ple 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.

       Kernels prior to 2.6.36-rc1 don't have the ability to SNAT in the INPUT
       chain.

       IPv6 support available since Linux kernels >= 3.7.

   SNPT (IPv6-specific)
       Provides stateless source IPv6-to-IPv6 Network Prefix  Translation  (as
       described by RFC 6296).

       You  have to use this target in the mangle table, not in the nat table.
       It takes the following options:

       --src-pfx [prefix/length]
	      Set source prefix that you want to translate and length

       --dst-pfx [prefix/length]
	      Set destination prefix that you want to use in  the  translation
	      and length

       You have to use the DNPT target to undo the translation. Example:

	      ip6tables	 -t mangle -I POSTROUTING -s fd00::/64	-o vboxnet0 -j
	      SNPT --src-pfx fd00::/64 --dst-pfx 2001:e20:2000:40f::/64

	      ip6tables	  -t   mangle	 -I    PREROUTING    -i	   wlan0    -d
	      2001:e20:2000:40f::/64  -j DNPT --src-pfx 2001:e20:2000:40f::/64
	      --dst-pfx fd00::/64

       You may need to enable IPv6 neighbor proxy:

	      sysctl -w net.ipv6.conf.all.proxy_ndp=1

       You also have to use the NOTRACK target to disable connection  tracking
       for translated flows.

   TCPMSS
       This  target  alters  the  MSS value of TCP SYN packets, to control the
       maximum size for that connection (usually limiting it to your  outgoing
       interface's  MTU	 minus	40 for IPv4 or 60 for IPv6, respectively).  Of
       course, it can only be used in conjunction with -p tcp.

       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.

   TEE
       The TEE target will clone a packet and redirect this clone  to  another
       machine	on the local network segment. In other words, the nexthop must
       be the target, or you will have to configure the nexthop to forward  it
       further if so desired.

       --gateway ipaddr
	      Send  the	 cloned	 packet	 to the host reachable at the given IP
	      address.	Use of 0.0.0.0 (for IPv4  packets)  or	::  (IPv6)  is
	      invalid.

       To  forward  all	 incoming  traffic on eth0 to an Network Layer logging
       box:

       -t mangle -A PREROUTING -i eth0 -j TEE --gateway 2001:db8::1

   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 (see NOTE below) 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 (see NOTE below).	 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.  See
	      NOTE below.)

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

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

       NOTE: In Linux kernels up to and including 2.6.38, with	the  exception
       of  longterm  releases  2.6.32  (>=.42),	 2.6.33	 (>=.15),  and	2.6.35
       (>=.14), there is a bug whereby IPv6 TOS mangling does  not  behave  as
       documented  and	differs	 from the IPv4 version. The TOS mask indicates
       the bits one wants to zero out, so  it  needs  to  be  inverted	before
       applying	 it to the original TOS field. However, the aformentioned ker‐
       nels forgo the inversion which breaks --set-tos and its mnemonics.

   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 packets so that the kernel will log every rule	 which
       match the packets as those traverse the tables, chains, rules.

       A  logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded
       for this to be visible.	The packets are logged with the string prefix:
       "TRACE: tablename:chainname: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 (IPv4-specific)
       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. This target is only valid  in
       mangle table.

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

       --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 (IPv4-specific)
       This is the deprecated ipv4-only predecessor of the NFLOG  target.   It
       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 subscribe to
       various multicast groups and receive the packets.  Like LOG, this is  a
       "non-terminating	 target",  i.e.	 rule  traversal continues at the next
       rule.

       --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).

iptables 1.4.21						iptables-extensions(8)
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