iptables-extensions

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

       --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:
       [!] --cgroup fwid
              Match corresponding cgroup for this packet.

              Can be used in the OUTPUT chain to  assign  particular  firewall
              policies for aggregated task/jobs on the system. This allows for
              more fine-grained firewall policies that only match for a subset
              of  the  system's  processes.  fwid is the maker set through the
              net_cls cgroup's id.

              IMPORTANT: when being  used  in  the  INPUT  chain,  the  cgroup
              matcher  is  currently only of limited functionality, meaning it
              will only match on packets that are processed for local  sockets
              through  early  socket demuxing. Therefore, general usage on the
              INPUT chain is  disadviced  unless  the  implications  are  well
              understood.

       Example:

              iptables  -A  OUTPUT -p tcp --sport 80 -m cgroup ! --cgroup 1 -j
              DROP

       Available since Linux 3.14.

   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

              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.

       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
       # 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]
       --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-

   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.

   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
              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.
              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.

       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

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

   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
              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.

       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:

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

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

       o   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:

       o   0 - Log all matched or unknown signatures

       o   1 - Log only the first one

       o   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,

       [!] --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  bridged  packets  entering  the  FORWARD  and  POSTROUTING
              chains).  If the interface name ends in a "+", then  any  inter-
              face which begins with this name will match.

       [!] --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
       --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:

       o   rateest operator rateest-bps

       o   rateest operator rateest-pps

           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  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
              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_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]
              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.

       [!] --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
              restrict the sockets to be matched  when  restoring  the  packet
              mark.

       Example:  An  application  opens 2 transparent (IP_TRANSPARENT) sockets
       and sets a mark on them with  SO_MARK  socket  option.  We  can  filter
       matching packets:

              -t mangle -I PREROUTING -m socket --transparent --restore-skmark
              -j action

              -t mangle -A action -m mark --mark 10 -j action2

              -t mangle -A action -m mark --mark 11 -j action3

   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  =
              Matches the given pattern.

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

       --icase
              Ignore case when searching.

       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.  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.

   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
       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.

   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
              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:

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

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

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

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

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

   AUDIT
       This target allows 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).
              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
              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-orig {id|mark}
              For traffic coming from ORIGINAL direction, assign  this  packet
              to  zone  id and only have lookups done in that zone. If mark is
              used instead of id, the zone is derived from the packet nfmark.

              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:

       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
              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-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

       --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.

   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.
              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
              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:

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

   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

       --map-set setname flag[,flag...]
              [--map-mark] [--map-prio] [--map-queue]  map  packet  properties
              (firewall mark, tc priority, hardware queue)

              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

       --map-set set-name
              the  set-name should be created with --skbinfo option --map-mark
              map firewall mark to packet  by  lookup  of  value  in  the  set
              --map-prio  map  traffic control priority to packet by lookup of
              value in the set --map-queue map hardware NIC queue to packet by
              lookup of value in the set

              The --map-set option can be used from the mangle table only. The
              --map-prio and --map-queue flags can be used in the OUTPUT, FOR-
              WARD and POSTROUTING chains.

       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
              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
              through a hash-based algorithm (kernel >= 2.6.21).

       --random-fully
              If option --random-fully is used then port mapping will be fully
              randomized through a PRNG (kernel >= 3.14).

       --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

       --wscale window scale
              Window scale announced to clients. This must match the backend.

       --sack-perm
              Pass client selective acknowledgement option to backend (will be
              disabled if not present).

       --timestamps
              Pass client timestamp option to backend (will be disabled if not
              present,  also  needed  for selective acknowledgement and window
              scaling).

       Example:

       Determine tcp options used by backend, from an external system

              tcpdump -pni eth0 -c 1 'tcp[tcpflags] == (tcp-syn|tcp-ack)'
                  port 80 &
              telnet 192.0.2.42 80
              18:57:24.693307 IP 192.0.2.42.80 > 192.0.2.43.48757:
                  Flags [S.], seq 360414582, ack 788841994, win 14480,
                  options [mss 1460,sackOK,
                  TS val 1409056151 ecr 9690221,
                  nop,wscale 9],
                  length 0

       Switch tcp_loose mode off, so conntrack will mark  out-of-flow  packets
       as state INVALID.

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

       Make SYN packets untracked

              iptables -t raw -A PREROUTING -i eth0 -p tcp --dport 80
                  --syn -j CT --notrack

       Catch UNTRACKED (SYN packets) and INVALID (3WHS ACK packets) states and
       send them to SYNPROXY. This rule  will  respond  to  SYN  packets  with
       SYN+ACK  syncookies, create ESTABLISHED for valid client response (3WHS
       ACK  packets)  and  drop  incorrect  cookies.  Flags  combinations  not
       expected  during  3WHS  will  not  match  and  continue  (e.g. SYN+FIN,
       SYN+ACK).

              iptables -A INPUT -i eth0 -p tcp --dport 80
                  -m state --state UNTRACKED,INVALID -j SYNPROXY
                  --sack-perm --timestamp --mss 1460 --wscale 9

       Drop invalid packets, this will be out-of-flow packets  that  were  not
       matched by SYNPROXY.

              iptables  -A  INPUT  -i  eth0 -p tcp --dport 80 -m state --state
              INVALID -j DROP

   TCPMSS

       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:


       --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.)

       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 1.6.0                                          iptables-extensions(8)
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