Provided by: iptables_1.4.4-2ubuntu2_i386 bug

NAME

       ip6tables - IPv6 packet filter administration

SYNOPSIS

       ip6tables [-t table] {-A|-D} chain rule-specification [options...]

       ip6tables [-t table] -I chain [rulenum] rule-specification [options...]

       ip6tables [-t table] -R chain rulenum rule-specification [options...]

       ip6tables [-t table] -D chain rulenum [options...]

       ip6tables [-t table] -S [chain [rulenum]]

       ip6tables [-t table] {-F|-L|-Z} [chain] [options...]

       ip6tables [-t table] -N chain

       ip6tables [-t table] -X [chain]

       ip6tables [-t table] -P chain target [options...]

       ip6tables [-t table] -E old-chain-name new-chain-name

DESCRIPTION

       Ip6tables is used to set up, maintain, and inspect the tables  of  IPv6
       packet  filter rules in the Linux kernel.  Several different tables may
       be defined.  Each table contains a number of built-in  chains  and  may
       also contain user-defined chains.

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

TARGETS

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

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

TABLES

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

       -t, --table table
              This  option  specifies  the  packet  matching  table  which the
              command should operate on.  If the  kernel  is  configured  with
              automatic  module  loading,  an attempt will be made to load the
              appropriate module for that table if it is not already there.

              The tables are as follows:

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

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

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

OPTIONS

       The  options  that  are  recognized  by  ip6tables  can be divided into
       several different groups.

   COMMANDS
       These options specify the specific action to perform.  Only one of them
       can  be specified on the command line unless otherwise specified below.
       For all the long versions of the command and option names, you need  to
       use  only  enough letters to ensure that ip6tables can differentiate it
       from all other options.

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

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

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

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

       -L, --list [chain]
              List  all rules in the selected chain.  If no chain is selected,
              all chains are listed. Like every other  ip6tables  command,  it
              applies to the specified table (filter is the default).

              Please  note  that it is often used with the -n option, in order
              to avoid long reverse DNS lookups.  It is legal to  specify  the
              -Z  (zero)  option  as  well, in which case the chain(s) will be
              atomically listed and zeroed.  The exact output is  affected  by
              the  other arguments given. The exact rules are suppressed until
              you use
               ip6tables -L -v

       -S, --list-rules [chain]
              Print all rules in the selected chain.  If no chain is selected,
              all  chains  are  printed  like ip6tables-save. Like every other
              ip6tables command, it applies to the specified table (filter  is
              the default).

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

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

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

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

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

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

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

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

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

       [!] -p, --protocol protocol
              The protocol of the  rule  or  of  the  packet  to  check.   The
              specified protocol can be one of tcp, udp, udplite, icmpv6, esp,
              mh or all, or it can be a numeric  value,  representing  one  of
              these  protocols  or  a  different  one.  A  protocol  name from
              /etc/protocols is also  allowed.   But  IPv6  extension  headers
              except  esp  are  not  allowed.  esp and ipv6-nonext can be used
              with Kernel version 2.6.11 or later.  A "!" argument before  the
              protocol  inverts  the  test.   The number zero is equivalent to
              all.  Protocol all will match with all protocols and is taken as
              default when this option is omitted.

       [!] -s, --source address[/mask]
              Source  specification.  Address can be either a hostname (please
              note that specifying any name to be resolved with a remote query
              such  as DNS is a really bad idea), a network IPv6 address (with
              /mask), or a  plain  IPv6  address.   (the  network  name  isn’t
              supported  now).   The  mask  is  a plain number, specifying the
              number of 1’s at the left side  of  the  network  mask.   A  "!"
              argument  before  the address specification inverts the sense of
              the address. The flag --src is an alias for this option.

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

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

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

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

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

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

   OTHER OPTIONS
       The following additional options can be specified:

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

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

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

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

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

MATCH EXTENSIONS

       ip6tables can use extended packet matching modules.  These  are  loaded
       in  two  ways:  implicitly, when -p or --protocol is specified, or with
       the -m or --match options, followed by the matching module name;  after
       these,  various  extra command line options become available, depending
       on the specific  module.   You  can  specify  multiple  extended  match
       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 following are included in the base package, and most of  these  can
       be preceded by a "!" to invert the sense of the match.

   ah
       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.

   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
              --cluster-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
              --cluster-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
              --mangle-mac-s 01:00:5e:00:01:02

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

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

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

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

       --comment comment

       Example:
              iptables  -A  INPUT  -s  192.168.0.0/16  -m comment --comment "A
              privatized IP block"

   connbytes
       Match by how many bytes or packets a connection  (or  one  of  the  two
       flows  constituting  the  connection)  has  transferred  so  far, or by
       average 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
       ‘conntrack -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
              transferred or the  average  size  (in  bytes)  of  all  packets
              received  so  far.  Note  that when "both" is used together with
              "avgpkt", and data is going (mainly) only in one direction  (for
              example HTTP), the average packet size will be about half of the
              actual data packets.

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

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

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

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

       Examples:

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

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

       # limit the number of parallel HTTP requests to 16 per  class  C  sized
       network (24 bit netmask)
              iptables -p tcp --syn --dport 80 -m connlimit  --connlimit-above
              16 --connlimit-mask 24 -j REJECT

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

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

       [!] --mark value[/mask]
              Matches  packets  in connections with the given mark value (if a
              mask is specified, this is logically ANDed with the mark  before
              the comparison).

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

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

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

       [!] --ctorigsrc address[/mask]

       [!] --ctorigdst address[/mask]

       [!] --ctreplsrc address[/mask]

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

       [!] --ctorigsrcport port

       [!] --ctorigdstport port

       [!] --ctreplsrcport port

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

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

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

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

       States for --ctstate:

       INVALID
              meaning that the packet is associated with no known connection

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

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

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

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

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

       Statuses for --ctstatus:

       NONE   None of the below.

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

       SEEN_REPLY
              Conntrack has seen packets in both directions.

       ASSURED
              Conntrack entry should never be early-expired.

       CONFIRMED
              Connection is confirmed: originating packet has left box.

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

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

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

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

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

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

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

   dst
       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.

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

       [!] --espspi spi[:spi]

   eui64
       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
       FORWARD chains.

   frag
       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":

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

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

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

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

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

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

       --hashlimit-burst amount
              Maximum  initial  number  of  packets to match: this number gets
              recharged by one every time the limit  specified  above  is  not
              reached, up to this number; the default is 5.

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

       --hashlimit-srcmask prefix
              When  --hashlimit-mode  srcip  is  used,  all  source  addresses
              encountered will be grouped according to the given prefix length
              and  the  so-created subnet will be subject to hashlimit. prefix
              must   be   between   (inclusive)   0   and   32.   Note    that
              --hashlimit-srcmask  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.

   hbh
       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
       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.

   icmp6
       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
       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.
              Possible 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.

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

       [!] --length length[:length]

   limit
       This module matches at a limited rate using a token bucket  filter.   A
       rule  using  this  extension  will  match  until  this limit is reached
       (unless the ‘!’ flag is used).  It can be used in combination with  the
       LOG target to give limited logging, for example.

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

       --limit-burst number
              Maximum  initial  number  of  packets to match: this number gets
              recharged by one every time the limit  specified  above  is  not
              reached, up to this number; the default is 5.

   mac
       [!] --mac-source address
              Match   source   MAC   address.    It   must   be  of  the  form
              XX:XX:XX:XX:XX:XX.  Note that this only makes sense for  packets
              coming  from  an  Ethernet  device  and entering the PREROUTING,
              FORWARD 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
              comparison).

   mh
       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 ipv6-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 -p tcp or -p udp.

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

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

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

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

       [!] --uid-owner username

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

       [!] --gid-owner groupname

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

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

   physdev
       This module matches  on  the  bridge  port  input  and  output  devices
       enslaved   to   a   bridge  device.  This  module  is  a  part  of  the
       infrastructure 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
              interface  which  begins with this name will match. Note that in
              the nat and mangle OUTPUT chains one cannot match on the  bridge
              output  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
              POSTROUTING 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
              decapsulation or the policy that will be used for encapsulation.
              in is valid in the PREROUTING, INPUT and FORWARD chains, out  is
              valid in the POSTROUTING, OUTPUT and FORWARD chains.

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

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

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

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

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

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

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

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

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

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

       --quota bytes
              The quota in bytes.

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

       --rateest1 name
              Name of the first rate estimator.

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

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

       --rateest1-bps value

       --rateest2-bps value
              Compare bytes per second.

       --rateest1-pps value

       --rateest2-pps value
              Compare packets per second.

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

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

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

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

       # Estimate outgoing rates

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

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

       # Mark based on available bandwidth

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

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

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

   recent
       Allows  you to dynamically create a list of IP addresses and then match
       against that list in a few different ways.

       For example, you can create a "badguy" list out of people attempting to
       connect  to  port 139 on your firewall and then DROP all future packets
       from them without considering them.

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

       [!] --set
              This  will  add the source address of the packet to the list. If
              the source address is already in the list, this will update  the
              existing entry. This will always return success (or failure if !
              is passed in).

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

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

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

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

       [!] --remove
              Check if the source address of the packet is  currently  in  the
              list  and  if  so that address will be removed from the list and
              the rule will return true. If the address is not found, false is
              returned.

       [!] --seconds seconds
              This  option must be used in conjunction with one of --rcheck or
              --update. When used, this will narrow the match to  only  happen
              when  the  address  is  in the list and was seen within the last
              given number of seconds.

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

       [!] --hitcount hits
              This option must be used in conjunction with one of --rcheck  or
              --update.  When  used, this will narrow the match to only happen
              when the address is in the list and packets  had  been  received
              greater  than  or  equal  to the given value. This option may be
              used along with --seconds  to  create  an  even  narrower  match
              requiring a certain number of hits within a specific time frame.

       --rttl This option may only be used in conjunction with one of --rcheck
              or  --update.  When  used,  this  will  narrow the match to only
              happen 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
              disallowing others access to your site by sending bogus  packets
              to you.

       Examples:

              iptables  -A  FORWARD -m recent --name badguy --rcheck --seconds
              60 -j DROP

              iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent  --name
              badguy --set -j DROP

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

       /proc/net/xt_recent/*  are  the  current   lists   of   addresses   and
       information 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.

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

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

       [!] --chunk-types {all|any|only} chunktype[:flags] [...]
              The  flag  letter  in  upper  case indicates that the flag is to
              match if set, in the lower case indicates to match if unset.

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

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

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

       Examples:

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

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

       iptables -A INPUT -p sctp --chunk-types any DATA:Be -j ACCEPT

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

       [!] --state state
              Where  state  is a comma separated list of the connection states
              to match.  Possible states are INVALID meaning that  the  packet
              could  not  be identified for some reason which includes running
              out of memory and ICMP errors  which  don’t  correspond  to  any
              known   connection,  ESTABLISHED  meaning  that  the  packet  is
              associated with a connection which  has  seen  packets  in  both
              directions,  NEW  meaning  that  the  packet  has  started a new
              connection, or otherwise associated with a connection which  has
              not  seen  packets  in both directions, and RELATED meaning that
              the packet is starting a new connection, but is associated  with
              an existing connection, such as an FTP data transfer, or an ICMP
              error.

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

       Supported options:

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

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

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

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

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

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

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

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

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

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

   tcp
       These  extensions  can  be  used  if  ‘--protocol tcp’ is specified. It
       provides 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-
              separated 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
              connection initiation; for example, blocking such packets coming
              in  an  interface  will  prevent  incoming  TCP connections, but
              outgoing TCP connections will be unaffected.  It  is  equivalent
              to  --tcp-flags  SYN,RST,ACK,FIN  SYN.  If the "!" flag precedes
              the "--syn", the sense of the option is inverted.

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

   tcpmss
       This matches the TCP MSS  (maximum  segment  size)  field  of  the  TCP
       header.  You can only use this on TCP SYN or SYN/ACK packets, since the
       MSS is only negotiated during the TCP handshake at  connection  startup
       time.

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

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

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

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

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

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

       --timestart hh:mm[:ss]

       --timestop hh:mm[:ss]

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

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

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

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

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

       --utc

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

       --localtz

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

       EXAMPLES. To match on weekends, use:

              -m time --weekdays Sa,Su

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

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

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

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

       During lunch hour:

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

       The fourth Friday in the month:

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

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

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

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

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

   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
       further 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
       characters  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).
              Alternatively, 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
       provides 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
              description  of  the  --destination-port  option  of   the   TCP
              extension for details.

TARGET EXTENSIONS

       ip6tables  can  use extended target modules: the following are included
       in the standard distribution.

   CLASSIFY
       This module allows  you  to  set  the  skb->priority  value  (and  thus
       classify 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.

   CONNMARK
       This module sets the netfilter mark value associated with a connection.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   DSCP
       This  target  allows to alter the value of the DSCP bits within the TOS
       header of the IPv4 packet.  As this manipulates a packet, it  can  only
       be used in the mangle table.

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

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

   HL
       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
       mangle table.

       Dont  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.

   LOG
       Turn on kernel logging of matching packets.  When this  option  is  set
       for  a  rule,  the  Linux  kernel  will  print  some information on all
       matching packets (like most IPv6 IPv6-header fields) via the kernel log
       (where  it  can  be  read  with  dmesg or syslogd(8)).  This is a "non-
       terminating target", i.e. rule traversal continues at  the  next  rule.
       So  if  you  want to LOG the packets you refuse, use two separate rules
       with the same matching criteria, first using target LOG then  DROP  (or
       REJECT).

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

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

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

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

       --log-ip-options
              Log options from the 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.  The target can only be used in the mangle table. It  can,  for
       example,  be  used  in  conjunction with routing based on fwmark (needs
       iproute2).

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

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

       The following mnemonics are available:

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

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

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

   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
       continues at the next rule.

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

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

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

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

   NFQUEUE
       This target is an extension of the QUEUE target. As opposed  to  QUEUE,
       it  allows  you  to put a packet into any specific queue, identified by
       its 16-bit queue number.

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

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

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

       It can only be used in the raw table.

   RATEEST
       The  RATEEST  target  collects  statistics,  performs  rate  estimation
       calculation 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
              microseconds.

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

   REJECT
       This  is  used  to send back an error packet in response to the matched
       packet: otherwise it is equivalent to  DROP  so  it  is  a  terminating
       TARGET, 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-prohibited,   adm-prohibited,  icmp6-addr-unreachable,
              addr-unreach,  icmp6-port-unreachable  or   port-unreach   which
              return the appropriate ICMPv6 error message (port-unreach 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.

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

       --selctx security_context

   TCPMSS
       This target allows to alter the  MSS  value  of  TCP  SYN  packets,  to
       control  the  maximum  size for that connection (usually limiting it to
       your outgoing interface’s MTU  minus  40  for  IPv4  or  60  for  IPv6,
       respectively).   Of  course, it can only be used in conjunction with -p
       tcp.  It is only valid in the mangle table.
       This target is used to overcome criminally braindead  ISPs  or  servers
       which  block  "ICMP  Fragmentation  Needed"  or "ICMPv6 Packet Too Big"
       packets.  The symptoms of this problem are that everything  works  fine
       from  your  Linux  firewall/router,  but  machines  behind it can never
       exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround: activate this option  and  add  a  rule  to  your  firewall
       configuration like:

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

       --set-mss value
              Explicitly set MSS option to specified value.

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

       These options are mutually exclusive.

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

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

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

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

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

       The following mnemonics are available:

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

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

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

   TRACE
       This target marks packes so that the kernel will log every  rule  which
       match  the  packets  as  those traverse the tables, chains, rules. (The
       ipt_LOG or ip6t_LOG module is required for the  logging.)  The  packets
       are      logged      with      the      string      prefix:     "TRACE:
       tablename: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.

DIAGNOSTICS

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

BUGS

       Bugs?   What’s  this?  ;-)  Well...  the  counters  are not reliable on
       sparc64.

COMPATIBILITY WITH IPCHAINS

       This ip6tables is very similar to ipchains by Rusty Russell.  The  main
       difference  is  that the chains INPUT and OUTPUT are only traversed for
       packets coming into the local host and originating from the local  host
       respectively.   Hence every packet only passes through one of the three
       chains (except loopback traffic, which involves both INPUT  and  OUTPUT
       chains); previously a forwarded packet would pass through all three.

       The  other main difference is that -i refers to the input interface; -o
       refers to the output interface, and  both  are  available  for  packets
       entering  the  FORWARD  chain.   There  are  several  other  changes in
       ip6tables.

SEE ALSO

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

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

AUTHORS

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

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

       James Morris wrote the TOS target, and tos match.

       Jozsef Kadlecsik wrote the REJECT target.

       Harald Welte wrote the ULOG and NFQUEUE target,  the  new  libiptc,  as
       well as TTL match+target and libipulog.

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

       ip6tables  man  page created by Andras Kis-Szabo, based on iptables man
       page written by Herve Eychenne <rv@wallfire.org>.