bionic (8) iptables-extensions.8.gz

Provided by: iptables_1.6.1-2ubuntu2.1_amd64 bug

NAME

       iptables-extensions — list of extensions in the standard iptables distribution

SYNOPSIS

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

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

MATCH EXTENSIONS

       iptables  can  use  extended  packet  matching  modules  with  the -m or --match options, followed by the
       matching module name; after these, various extra command line options become available, depending on  the
       specific  module.  You can specify multiple extended match modules in one line, and you can use the -h or
       --help options after the module has been specified to receive help specific to that module.  The extended
       match modules are evaluated in the order they are specified in the rule.

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

   addrtype
       This module matches packets based on their address type.   Address  types  are  used  within  the  kernel
       networking  stack  and  categorize  addresses  into  various  groups.  The exact definition of that group
       depends on the specific layer three protocol.

       The following address types are possible:

       UNSPEC an unspecified address (i.e. 0.0.0.0)

       UNICAST
              an unicast address

       LOCAL  a local address

       BROADCAST
              a broadcast address

       ANYCAST
              an anycast packet

       MULTICAST
              a multicast address

       BLACKHOLE
              a blackhole address

       UNREACHABLE
              an unreachable address

       PROHIBIT
              a prohibited address

       THROW  FIXME

       NAT    FIXME

       XRESOLVE

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

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

       --limit-iface-in
              The address type checking can be limited to the interface the packet is coming in. This option  is
              only  valid  in  the  PREROUTING,  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  POSTROUTING,  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 path to an eBPF object or a cBPF program in decimal format.

       --object-pinned path
              Pass a path to a pinned eBPF object.

       Applications load eBPF programs into the kernel with the bpf() system call and BPF_PROG_LOAD command  and
       can pin them in a virtual filesystem with BPF_OBJ_PIN.  To use a pinned object in iptables, mount the bpf
       filesystem using

              mount -t bpf bpf ${BPF_MOUNT}

       then insert the filter in iptables by path:

              iptables -A OUTPUT -m bpf --object-pinned ${BPF_MOUNT}/{PINNED_PATH} -j ACCEPT

       --bytecode code
              Pass the BPF byte code format as generated by the nfbpf_compile utility.

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

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

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

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

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

       Or instead, you can invoke the nfbpf_compile utility.

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

       Or use tcpdump -ddd. In that case, generate BPF targeting a device with the same data link  type  as  the
       xtables match. Iptables passes packets from the network layer up, without mac layer. Select a device with
       data link type RAW, such as a tun device:

              ip tuntap add tun0 mode tun
              ip link set tun0 up
              tcpdump -ddd -i tun0 ip proto 6

       See tcpdump -L -i $dev for a list of known data link types for a given device.

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

   cgroup
       [!] --path path
              Match cgroup2 membership.

              Each socket is associated with the v2 cgroup of the creating process.  This matches packets coming
              from  or  going  to  all  sockets  in the sub-hierarchy of the specified path.  The path should be
              relative to the root of the cgroup2 hierarchy.

       [!] --cgroup classid
              Match cgroup net_cls classid.

              classid is the marker set through the cgroup net_cls controller.  This option and --path can't  be
              used together.

       Example:

              iptables -A OUTPUT -p tcp --sport 80 -m cgroup ! --path service/http-server -j DROP

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

       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 not advised unless the implications are
       well understood.

       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 --cluster-total-nodes 2 --cluster-local-node 1
              --cluster-hash-seed 0xdeadbeef -j MARK --set-mark 0xffff

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

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

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

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

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

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

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

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

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

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

       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 average bytes per packet.

       The counters are 64-bit and are thus not expected to overflow ;)

       The  primary  use  is to detect long-lived downloads and mark them to be scheduled using a lower priority
       band in traffic control.

       The transferred bytes per connection  can  also  be  viewed  through  `conntrack  -L`  and  accessed  via
       ctnetlink.

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

       [!] --connbytes from[:to]
              match packets from a connection whose packets/bytes/average packet size is more than FROM and less
              than  TO bytes/packets. if TO is omitted only FROM check is done. "!" is used to match packets not
              falling in the range.

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

       --connbytes-mode {packets|bytes|avgpkt}
              whether to check the amount of packets, number of bytes transferred or the average size (in bytes)
              of  all packets received so far. Note that when "both" is used together with "avgpkt", and data is
              going (mainly) only in one direction (for example HTTP), the average packet  size  will  be  about
              half of the actual data packets.

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

   connlabel
       Module  matches  or  adds connlabels to a connection.  connlabels are similar 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
       currently 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 allocates 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  applicable
              protocol is used.

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

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

       Examples:

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

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

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

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

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

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

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

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

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

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

       [!] --ctorigsrc address[/mask]

       [!] --ctorigdst address[/mask]

       [!] --ctreplsrc address[/mask]

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

       [!] --ctorigsrcport port[:port]

       [!] --ctorigdstport port[:port]

       [!] --ctreplsrcport port[:port]

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

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

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

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

       States for --ctstate:

       INVALID
              The packet is associated with no known connection.

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

       ESTABLISHED
              The packet is associated with a connection which has seen packets 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  explicitly  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  different
              queues.

       Example:

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

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

       Available since Linux 2.6.36.

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

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

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

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

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

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

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

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

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

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

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

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

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

   ecn
       This  allows  you  to match the ECN bits of the IPv4/IPv6 and TCP header.  ECN is the Explicit Congestion
       Notification mechanism as specified in RFC3168

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

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

       [!] --ecn-ip-ect num
              This matches a particular IPv4/IPv6 ECT (ECN-Capable Transport). You  have  to  specify  a  number
              between `0' and `3'.

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

       [!] --espspi spi[:spi]

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

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

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

       --hashlimit-burst amount
              Maximum initial number of packets to match: this number gets recharged by one every time the limit
              specified  above  is  not  reached,  up  to  this  number; the default is 5.  When byte-based rate
              matching is requested, this option specifies the amount of bytes that can exceed the  given  rate.
              This option should be used with caution -- if the entry expires, the burst value is reset too.

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

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

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

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

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

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

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

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

       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 without matching" --hashlimit-mode
              dstip --hashlimit-above 512kb/s --hashlimit-burst 1mb

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

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

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

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

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

              string can be "ftp" for packets related to a ftp-session on default port.  For other ports  append
              -portnr to the value, ie. "ftp-2121".

              Same rules apply for other conntrack-helpers.

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

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

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

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

   icmp (IPv4-specific)
       This extension can be used if `--protocol icmp' is specified. It provides the following option:

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

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

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

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

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

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

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

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

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

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

       dst    Destination Options header

       route  Routing header

       frag   Fragment header

       auth   Authentication header

       esp    Encapsulating Security Payload header

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

       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 optional `/second', `/minute',
              `/hour', or `/day' suffix; the default is 3/hour.

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

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

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

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

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

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

   multiport
       This module matches a set of source or destination ports.  Up to 15 ports can be specified.  A port range
       (port:port) counts as two ports.  It can only be used in conjunction with one of the following protocols:
       tcp, udp, udplite, dccp and sctp.

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

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

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

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

       The only option available for this match is the following:

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

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

              nfacct add http-traffic

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

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

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

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

              nfacct get http-traffic

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

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

   osf
       The osf module does passive operating system fingerprinting. This  modules  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 fingerprinting.

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

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

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

       •   2 - Do not compare the TTL at all.

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

       •   0 - Log all matched or unknown signatures

       •   1 - Log only the first one

       •   2 - Log all known matched signatures

       You may find something like this in syslog:

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

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

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

       To remove them again,

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

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

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

       [!] --uid-owner username

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

       [!] --gid-owner groupname

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

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

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

       [!] --physdev-in name
              Name of a bridge port via which a packet is received (only for packets entering the INPUT, FORWARD
              and PREROUTING chains). If the interface name ends in a "+", then any interface which begins  with
              this name will match. If the packet didn't arrive through a bridge device, this packet won't match
              this option, unless '!' is used.

       [!] --physdev-out name
              Name of a bridge port via which a packet is going to be sent (for  bridged  packets  entering  the
              FORWARD  and  POSTROUTING  chains).  If the interface name ends in a "+", then any interface 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 POSTROUTING chains.

   pkttype
       This module matches the link-layer packet type.

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

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

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

       --pol {none|ipsec}
              Matches if the packet is subject to IPsec processing. --pol none cannot be combined with --strict.

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

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

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

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

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

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

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

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

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

   quota
       Implements network quotas by decrementing a byte counter with each packet. The  condition  matches  until
       the  byte  counter  reaches  zero.  Behavior is reversed with negation (i.e. the condition does not match
       until the byte counter reaches zero).

       [!] --quota bytes
              The quota in bytes.

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

       For a better understanding of the available options, these are all possible combinations:

       •   rateest operator rateest-bpsrateest operator rateest-pps

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

       •   (rateest minus rateest-pps1) operator rateest-pps2rateest1 operator rateest2 rateest-bps(without rate!)

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

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

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

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

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

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

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

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

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

       --rateest1 name

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

       --rateest-bps [value]

       --rateest-pps [value]

       --rateest-bps1 [value]

       --rateest-bps2 [value]

       --rateest-pps1 [value]

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

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

       # Estimate outgoing rates

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

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

       # Mark based on available bandwidth

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

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

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

   realm (IPv4-specific)
       This  matches  the  routing  realm.   Routing realms are used in complex routing setups involving dynamic
       routing protocols like BGP.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       --hitcount hits
              This option must be used in conjunction with one of --rcheck or --update.  When  used,  this  will
              narrow  the  match  to  only  happen when the address is in the list and packets had been received
              greater than or equal to the given value. This option may be used along with --seconds  to  create
              an  even  narrower  match  requiring  a  certain  number of hits within a specific time frame. The
              maximum value for the hitcount parameter is  given  by  the  "ip_pkt_list_tot"  parameter  of  the
              xt_recent  kernel  module.  Exceeding  this  value  on  the command line will cause the rule to be
              rejected.

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

       Examples:

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

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

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

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

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

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

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

       The module itself accepts parameters, defaults shown:

       ip_list_tot=100
              Number of addresses remembered per table.

       ip_pkt_list_tot=20
              Number of packets per address remembered.

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

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

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

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

   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 specify that the reverse path filter test should match even if the selected output device
              is not the expected one.

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

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

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

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

       iptables -t raw -N RPFILTER

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

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

       iptables -t raw -A RPFILTER -j DROP

       iptables -t raw -A PREROUTING -j RPFILTER

       Example to drop failed packets, without logging:

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

   rt (IPv6-specific)
       Match on IPv6 routing header

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

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

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

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

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

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

   sctp
       This module matches Stream Control Transmission Protocol headers.

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

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

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

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

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

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

       Examples:

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

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

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

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

       [!] --match-set setname flag[,flag]...
              where flags are the comma separated list of src and/or dst specifications 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 packets for which the source address
              can be found in the specified set.

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

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

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

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

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

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

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

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

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

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

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

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

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

       --transparent
              Ignore non-transparent sockets.

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

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

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

       --restore-skmark
              Set the packet mark to the matching socket's mark. Can be  combined  with  the  --transparent  and
              --nowildcard options to 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 manpage.

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

       Supported options:

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

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

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

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

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

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

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

       --to offset
              Set the offset up to which should be scanned. That is, byte offset-1 (counting from 0) is the last
              one that is scanned.  If not passed, default is the packet size.

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

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

       --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 characters, 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 provides the following options:

       [!] --source-port,--sport port[:port]
              Source port or port range specification. This can either be a service name or a  port  number.  An
              inclusive range can also be specified, using the format first:last.  If the first port is omitted,
              "0" is assumed; if the last is omitted, "65535" is assumed.  The  flag  --sport  is  a  convenient
              alias for this option.

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

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

       [!] --syn
              Only match TCP packets with the SYN bit set and the ACK,RST and FIN bits  cleared.   Such  packets
              are  used  to  request  TCP connection initiation; for example, blocking such packets coming in an
              interface will prevent incoming TCP connections, but outgoing TCP connections will be  unaffected.
              It  is  equivalent  to --tcp-flags SYN,RST,ACK,FIN SYN.  If the "!" flag precedes the "--syn", the
              sense of the option is inverted.

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

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

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

   time
       This matches if the packet arrival time/date is within a given range. All options are optional,  but  are
       ANDed when specified. 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
       variable.  The  TZ  userspace  environment variable specifies how the UTC-based system time is displayed,
       e.g. when you run date(1), or what you see on your desktop clock.  The TZ string may resolve to different
       offsets  at  different  dates,  which  is  what enables the automatic time-jumping in userspace. when DST
       changes. The kernel's timezone offset variable is used when it has to convert  between  non-UTC  sources,
       such as FAT filesystems, to UTC (since the latter is what the rest of the system uses).

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

       EXAMPLES. To match on weekends, use:

              -m time --weekdays Sa,Su

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

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

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

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

       During lunch hour:

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

       The fourth Friday in the month:

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

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

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

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

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

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

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

   ttl (IPv4-specific)
       This module matches the time to live field in the IP header.

       [!] --ttl-eq ttl
              Matches the given TTL value.

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

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

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

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

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

              value := range | value "," range

              range := number | number ":" number

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

           location := number | location operator number

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

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

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

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

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

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

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

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

              B and C are unsigned 32 bit integers, initially zero

       The instructions are:

              number B = number;

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

              &number C = C & number

              << number C = C << number

              >> number C = C >> number

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

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

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

       Example:

              match IP packets with total length >= 256

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

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

              read bytes 0-3

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

       Example: (more realistic, hence more complicated)

              match ICMP packets with icmp type 0

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

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

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

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

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

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

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

       Example:

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

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

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

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

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

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

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

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

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

TARGET EXTENSIONS

       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 target.  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 checksum.  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 offload in your device.

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

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

   CLUSTERIP (IPv4-specific)
       This module allows you to configure a simple cluster of nodes that share a certain  IP  and  MAC  address
       without  an  explicit load balancer in front of them.  Connections are statically distributed between the
       nodes in this cluster.

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

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

       --clustermac mac
              Specify the ClusterIP MAC address. Has to be a link-layer multicast address

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   CONNSECMARK
       This  module  copies  security  markings from packets to connections (if unlabeled), and from connections
       back to packets (also only if unlabeled).  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 connection does, copy the security marking
              from the connection to the packet.

   CT
       The CT target sets parameters for a packet or its associated connection. 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 connection. 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 connection.  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.

       --zone-reply {id|mark}
              For  traffic coming from REPLY 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.

       --zone {id|mark}
              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. By default, packets have zone 0. This option
              applies to both directions.

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

   DNAT
       This target is only valid in the nat table, in the PREROUTING and OUTPUT 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 following options:

       --to-destination [ipaddr[-ipaddr]][:port[-port]]
              which  can  specify  a  single  new  destination  IP  address, an inclusive range of IP addresses.
              Optionally a port range, if the rule also specifies one of the following protocols: tcp, udp, dccp
              or  sctp.   If no port range is specified, then 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 persistent mappings is available from 2.6.29-rc2.

       IPv6 support available since Linux kernels >= 3.7.

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

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

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

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

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

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

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

       You may need to enable IPv6 neighbor proxy:

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

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

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

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

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

   ECN (IPv4-specific)
       This target selectively works around known ECN blackholes.  It can only be used in the mangle table.

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

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

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

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

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

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

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

       Existing options are:

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

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

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

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

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

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

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

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

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

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

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

       Examples:

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

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

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

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

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

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

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

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

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

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

       --led-always-blink
              Always make the LED blink on packet  arrival,  even  if  the  LED  is  already  on.   This  allows
              notification  of new packets even with long delay values (which otherwise would result in a silent
              prolonging 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  matching  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 priority): emerg, alert, crit, error, warning, notice, info or debug.

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

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

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

       --log-ip-options
              Log options from the IP/IPv6 packet header.

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

   MARK
       This target is used to set the Netfilter mark value associated with the packet.  It can, for example,  be
       used in conjunction with routing based on fwmark (needs iproute2). If you plan on doing so, note that the
       mark needs to be set in the PREROUTING chain of the mangle table to affect routing.  The mark field is 32
       bits wide.

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

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

       The following mnemonics are available:

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

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

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

   MASQUERADE
       This target is only valid in the nat table, in the POSTROUTING  chain.   It  should  only  be  used  with
       dynamically  assigned  IP  (dialup) connections: 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 anyway).

       --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 constructed in the following way: All
              'one' bits in the mask are filled in from the new `address'.  All bits that are zero in  the  mask
              are filled in from the original address.

       IPv6 support available since Linux kernels >= 3.7.

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

       --nflog-group nlgroup
              The  netlink  group  (0  -  2^16-1)  to which packets are (only applicable for nfnetlink_log). The
              default value is 0.

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

       --nflog-range size
              This option has never worked, use --nflog-size instead

       --nflog-size 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 balanced 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 performance if
              there's a queue per CPU. This requires --queue-balance to be specified.

   NOTRACK
       This extension disables connection tracking for all packets matching that rule.  It is equivalent with -j
       CT --notrack. Like CT, NOTRACK can only be used in the raw table.

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

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

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

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

   REDIRECT
       This target is only valid in the nat table, in the PREROUTING and OUTPUT 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 protocols: 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 TARGET, ending rule traversal.  This target is only valid  in  the  INPUT,
       FORWARD  and  OUTPUT  chains,  and  user-defined  chains  which  are  only called from those chains.  The
       following option controls the nature of the error packet returned:

       --reject-with type
              The  type  given  can   be   icmp6-no-route,   no-route,   icmp6-adm-prohibited,   adm-prohibited,
              icmp6-addr-unreachable,  addr-unreach,  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 TARGET, ending rule traversal.  This target is only valid in the INPUT,
       FORWARD and OUTPUT chains, and user-defined  chains  which  are  only  called  from  those  chains.   The
       following option controls the nature of the error packet returned:

       --reject-with type
              The   type   given  can  be  icmp-net-unreachable,  icmp-host-unreachable,  icmp-port-unreachable,
              icmp-proto-unreachable, icmp-net-prohibited, icmp-host-prohibited, or  icmp-admin-prohibited  (*),
              which  return  the  appropriate  ICMP  error  message (icmp-port-unreachable is the default).  The
              option tcp-reset can be used on rules which only match the TCP protocol: this  causes  a  TCP  RST
              packet  to  be  sent  back.   This  is  mainly  useful  for  blocking ident (113/tcp) probes which
              frequently occur when sending mail to broken mail hosts (which won't accept your mail otherwise).

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

   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 definition

       --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, FORWARD 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 chains.  It specifies that the source address of the  packet  should  be
       modified  (and  all future packets in this connection will also be mangled), and rules should cease being
       examined.  It takes the following options:

       --to-source [ipaddr[-ipaddr]][:port[-port]]
              which can specify a single new source IP address, an inclusive range of IP addresses. Optionally a
              port range, if the rule also specifies one of the following protocols: tcp, udp, dccp or sctp.  If
              no port range is specified, then source ports below 512 will be mapped to other ports  below  512:
              those  between  512 and 1023 inclusive will be mapped to ports below 1024, and other ports will be
              mapped to 1024 or above. Where possible, no port alteration will occur.  In Kernels up to  2.6.10,
              you  can  add  several --to-source options. For those kernels, if you specify more than one source
              address, either via an address range or multiple --to-source options, a  simple  round-robin  (one
              after  another in cycle) takes place between these addresses.  Later Kernels (>= 2.6.11-rc1) don't
              have the ability to NAT to multiple 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 persistent mappings is available from 2.6.29-rc2.

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

       IPv6 support available since Linux kernels >= 3.7.

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

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

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

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

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

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

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

       You may need to enable IPv6 neighbor proxy:

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

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

   SYNPROXY
       This target will process TCP three-way-handshake parallel in netfilter context to protect either local or
       backend system. This target requires connection tracking because sequence numbers need to be translated.

       --mss maximum segment size
              Maximum segment size announced to clients. This must match the backend.

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

       This target is used to overcome criminally braindead ISPs or  servers  which  block  "ICMP  Fragmentation
       Needed"  or "ICMPv6 Packet Too Big" packets.  The symptoms of this problem are that everything works fine
       from your Linux firewall/router, but machines behind it can never exchange large packets:

       1.  Web browsers connect, then hang with no data received.

       2.  Small mail works fine, but large emails hang.

       3.  ssh works fine, but scp hangs after initial handshaking.

       Workaround: activate this option and add a rule to your firewall configuration like:

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

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

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

       These options are mutually exclusive.

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

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

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

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

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

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

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

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

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

       The following mnemonics are available:

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

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

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

       NOTE: In Linux kernels up to and including  2.6.38,  with  the  exception  of  longterm  releases  2.6.32
       (>=.42),  2.6.33 (>=.15), and 2.6.35 (>=.14), there is a bug whereby IPv6 TOS mangling does not behave as
       documented and differs from the IPv4 version. The TOS mask indicates the bits one wants to zero  out,  so
       it  needs to be inverted before applying it to the original TOS field. However, the aformentioned kernels
       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 redirects the packet to a local socket without changing the packet header
       in any way. It can also change the mark value which can then be used in advanced routing rules.  It takes
       three options:

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

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

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

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

       A logging backend, such as ip(6)t_LOG or nfnetlink_log, must be loaded  for  this  to  be  visible.   The
       packets  are  logged with the string prefix: "TRACE: tablename:chainname:type:rulenum " where type can be
       "rule" for plain rule, "return" for implicit rule at the end of a user defined chain and "policy" for the
       policy of the built in chains.
       It can only be used in the raw table.

   TTL (IPv4-specific)
       This  is  used  to  modify the IPv4 TTL header field.  The TTL field determines 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 dangerous, 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  characters  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).