iptables



IPTABLES(8)                                                        IPTABLES(8)




NAME

       iptables - administration tool for IPv4 packet filtering and NAT


SYNOPSIS

       iptables [-t table] -[AD] chain rule-specification [options]
       iptables [-t table] -I chain [rulenum] rule-specification [options]
       iptables [-t table] -R chain rulenum rule-specification [options]
       iptables [-t table] -D chain rulenum [options]
       iptables [-t table] -[LFZ] [chain] [options]
       iptables [-t table] -N chain
       iptables [-t table] -X [chain]
       iptables [-t table] -P chain target [options]
       iptables [-t table] -E old-chain-name new-chain-name


DESCRIPTION

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

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



TARGETS

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

       ACCEPT  means to let the packet through.  DROP means to drop the packet
       on the floor.  QUEUE means to pass the packet  to  userspace  (if  sup-
       ported  by  the  kernel).   RETURN means stop traversing this chain and
       resume at the next rule in the previous (calling) chain.  If the end of
       a  built-in  chain is reached or a rule in a built-in chain with target
       RETURN is matched, the target specified by the chain policy  determines
       the fate of the packet.


TABLES

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

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

              The tables are as follows:

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

              nat:
                  This  table  is  consulted  when a packet that creates a new
                  connection is encountered.  It consists of three  built-ins:
                  PREROUTING  (for  altering packets as soon as they come in),
                  OUTPUT (for altering locally-generated packets before  rout-
                  ing),  and  POSTROUTING  (for  altering  packets as they are
                  about to go out).

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

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


OPTIONS

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

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

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

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

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

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

       -L, --list [chain]
              List all rules in the selected chain.  If no chain is  selected,
              all  chains  are  listed.   As  every other iptables command, it
              applies to the specified table (filter is the default),  so  NAT
              rules get listed by
               iptables -t nat -n -L
              Please  note  that it is often used with the -n option, in order
              to avoid long reverse DNS lookups.  It is legal to  specify  the
              -Z  (zero)  option  as  well, in which case the chain(s) will be
              atomically listed and zeroed.  The exact output is  affected  by
              the  other arguments given. The exact rules are suppressed until
              you use
               iptables -L -v

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

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

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

       -X, --delete-chain [chain]
              Delete the optional user-defined chain specified.  There must be
              no references to the chain.  If there are, you  must  delete  or
              replace the referring rules before the chain can be deleted.  If
              no argument is given, it  will  attempt  to  delete  every  non-
              builtin chain in the table.

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

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

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

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

       -p, --protocol [!] protocol
              The  protocol of the rule or of the packet to check.  The speci-
              fied protocol can be one of tcp, udp, icmp, or all, or it can be
              a  numeric  value, representing one of these protocols or a dif-
              ferent  one.   A  protocol  name  from  /etc/protocols  is  also
              allowed.   A  "!" argument before the protocol inverts the test.
              The number zero is equivalent to all.  Protocol all  will  match
              with  all  protocols and is taken as default when this option is
              omitted.

       -s, --source [!] address[/mask]
              Source specification.  Address can be either a network  name,  a
              hostname  (please  note  that specifying any name to be resolved
              with a remote query such as DNS is a really bad idea), a network
              IP address (with /mask), or a plain IP address.  The mask can be
              either a network mask or a plain number, specifying  the  number
              of 1’s at the left side of the network mask.  Thus, a mask of 24
              is equivalent to  255.255.255.0.   A  "!"  argument  before  the
              address specification inverts the sense of the address. The flag
              --src is an alias for this option.

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

       -j, --jump target
              This specifies the target of the rule; i.e., what to do  if  the
              packet  matches  it.   The  target  can  be a user-defined chain
              (other than the one this rule is in), one of the special builtin
              targets  which  decide the fate of the packet immediately, or an
              extension (see EXTENSIONS below).  If this option is omitted  in
              a  rule,  then  matching  the  rule  will  have no effect on the
              packet’s fate, but the counters on the rule will be incremented.

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

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

       [!]  -f, --fragment
              This means that the rule only refers to second and further frag-
              ments of fragmented packets.  Since there is no way to tell  the
              source  or  destination  ports  of such a packet (or ICMP type),
              such a packet will not match any rules which specify them.  When
              the  "!"  argument  precedes  the  "-f" flag, the rule will only
              match head fragments, or unfragmented packets.

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

   OTHER OPTIONS
       The following additional options can be specified:

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

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

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

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

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


MATCH EXTENSIONS

       iptables can use extended packet matching modules.  These are loaded in
       two  ways:  implicitly, when -p or --protocol is specified, or with the
       -m or --match options, followed by  the  matching  module  name;  after
       these,  various  extra command line options become available, depending
       on the specific module.  You can specify multiple extended  match  mod-
       ules  in  one  line, and you can use the -h or --help options after the
       module has been specified to receive help specific to that module.

       The following are included in the base package, and most of  these  can
       be preceded by a !  to invert the sense of the match.

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

       The following address types are possible:

       UNSPEC an unspecified address (i.e. 0.0.0.0) UNICAST an unicast address
              LOCAL  a  local address BROADCAST a broadcast address ANYCAST an
              anycast packet MULTICAST a multicast address BLACKHOLE a  black-
              hole  address UNREACHABLE an unreachable address PROHIBIT a pro-
              hibited address THROW FIXME NAT FIXME XRESOLVE FIXME

       --src-type type
              Matches if the source address is of given type

       --dst-type type
              Matches if the destination address is of given type

   ah
       This module matches the SPIs in AH header of IPSec packets.

       --ahspi [!] spi[:spi]

   childlevel
       This is an experimental module.  It matches on whether  the  packet  is
       part  of  a master connection or one of its children (or grandchildren,
       etc).  For instance, most packets are level 0.  FTP  data  transfer  is
       level 1.

       --childlevel [!] level

   condition
       This matches if a specific /proc filename is ’0’ or ’1’.

       --condition [!] filename
              Match  on  boolean value stored in /proc/net/ipt_condition/file-
              name file

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

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

   connrate
       This module matches the current transfer rate in a connection.

       --connrate [!] [from]:[to]
              Match  against the current connection transfer rate being within
              ’from’ and ’to’ bytes per second. When the "!" argument is  used
              before the range, the sense of the match is inverted.

   conntrack
       This  module,  when combined with connection tracking, allows access to
       more connection tracking information than  the  "state"  match.   (this
       module is present only if iptables was compiled under a kernel support-
       ing this feature)

       --ctstate state
              Where state is a comma separated list of the  connection  states
              to  match.   Possible states are INVALID meaning that the packet
              is associated with no known connection, ESTABLISHED meaning that
              the  packet is associated with a connection which has seen pack-
              ets in both directions, NEW meaning that the packet has  started
              a  new  connection,  or  otherwise  associated with a connection
              which has not seen packets in both directions, and RELATED mean-
              ing that the packet is starting a new connection, but is associ-
              ated with an existing connection, such as an FTP data  transfer,
              or  an ICMP error.  SNAT A virtual state, matching if the origi-
              nal source address differs from the reply destination.   DNAT  A
              virtual state, matching if the original destination differs from
              the reply source.

       --ctproto proto
              Protocol to match (by number or name)

       --ctorigsrc [!] address[/mask]
              Match against original source address

       --ctorigdst [!] address[/mask]
              Match against original destination address

       --ctreplsrc [!] address[/mask]
              Match against reply source address

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

       --ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,...]
              Match against internal conntrack states

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

   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 [0-32].

       --dscp-class DiffServ Class
              Match the DiffServ class. This value may be any of the  BE,  EF,
              AFxx  or  CSx  classes.   It  will  then  be converted into it’s
              according numeric value.

   dstlimit
       This module allows you to limit the packet per second (pps) rate  on  a
       per  destination  IP  or  per destination port base.  As opposed to the
       ‘limit’ match, every destination ip / destination  port  has  it’s  own
       limit.

       --dstlimit avg
              Maximum  average  match rate (packets per second unless followed
              by /sec /minute /hour /day postfixes).

       --dstlimit-mode mode
              The limiting hashmode.  Is the specified limit per dstip, dstip-
              dstport  tuple,  srcip-dstip  tuple,  or  per srcipdstip-dstport
              tuple.

       --dstlimit-name name
              Name for /proc/net/ipt_dstlimit/* file entry

       [--dstlimit-burst burst]
              Number of packets to match in a burst.  Default: 5

       [--dstlimit-htable-size size]
              Number of buckets in the hashtable

       [--dstlimit-htable-max max]
              Maximum number of entries in the hashtable

       [--dstlimit-htable-gcinterval interval]
              Interval between garbage collection runs of  the  hashtable  (in
              miliseconds).  Default is 1000 (1 second).

       [--dstlimit-htable-expire time
              After  which  time  are  idle entries expired from hashtable (in
              miliseconds)?  Default is 10000 (10 seconds).

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

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

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

       --ecn-ip-ect num
              This  matches a particular IPv4 ECT (ECN-Capable Transport). You
              have to specify a number between ‘0’ and ‘3’.

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

       --espspi [!] spi[:spi]

   fuzzy
       This module matches a rate limit based  on  a  fuzzy  logic  controller
       [FLC]

       --lower-limit  number"
              Specifies the lower limit (in packets per second).

       --upper-limit number
              Specifies the upper limit (in packets per second).

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

       --helper string
              Matches packets related to the specified conntrack-helper.

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

              Same rules apply for other conntrack-helpers.

   icmp
       This  extension  is  loaded if ‘--protocol icmp’ is specified.  It pro-
       vides the following option:

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

   iprange
       This matches on a given arbitrary range of IPv4 addresses

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

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

   length
       This module matches the length 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
       (unless  the ‘!’ flag is used).  It can be used in combination with the
       LOG target to give limited logging, for example.

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

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

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

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

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

   mport
       This module matches a set of source or destination  ports.   Up  to  15
       ports can be specified.  It can only be used in conjunction with -p tcp
       or -p udp.

       --source-ports port[,port[,port...]]
              Match if the source port is one of the given  ports.   The  flag
              --sports is a convenient alias for this option.

       --destination-ports 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...]]
              Match if the both the source and destination ports are equal  to
              each other and to one of the given ports.

   multiport
       This  module  matches  a  set of source or destination ports.  Up to 15
       ports can be specified.  It can only be used in conjunction with -p tcp
       or -p udp.

       --source-ports port[,port[,port...]]
              Match  if  the  source port is one of the given ports.  The flag
              --sports is a convenient alias for this option.

       --destination-ports 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...]]
              Match  if the both the source and destination ports are equal to
              each other and to one of the given ports.

   nth
       This module matches every ‘n’th packet

       --every value
              Match every ‘value’ packet

       [--counter num]
              Use internal counter number ‘num’.  Default is ‘0’.

       [--start num]
              Initialize the counter at the number ‘num’ insetad of ‘0’.  Most
              between ‘0’ and ‘value’-1.

       [--packet num]
              Match on ‘num’ packet.  Most be between ‘0’ and ‘value’-1.

   owner
       This  module  attempts  to  match various characteristics of the packet
       creator, for locally-generated packets.  It is only valid in the OUTPUT
       chain,  and  even  this  some packets (such as ICMP ping responses) may
       have no owner, and hence never match.

       --uid-owner userid
              Matches if the packet was created by a process  with  the  given
              effective user id.

       --gid-owner groupid
              Matches  if  the  packet was created by a process with the given
              effective group id.

       --pid-owner processid
              Matches if the packet was created by a process  with  the  given
              process id.

       --sid-owner sessionid
              Matches if the packet was created by a process in the given ses-
              sion group.

       --cmd-owner name
              Matches if the packet was created by a process  with  the  given
              command name.  (this option is present only if iptables was com-
              piled under a kernel supporting this feature)

       NOTE: pid, sid and command matching are broken on SMP

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

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

       --physdev-out name
              Name of a bridge port via which a packet is  going  to  be  sent
              (for  packets  entering  the  FORWARD,  OUTPUT  and  POSTROUTING
              chains).  If the interface name ends in a "+", then  any  inter-
              face  which  begins  with this name will match. Note that in the
              nat and mangle OUTPUT chains one cannot match on the bridge out-
              put  port,  however  one  can in the filter OUTPUT chain. If the
              packet won’t leave by a bridge device or it is yet unknown  what
              the  output  device  will  be,  then the packet won’t match this
              option, unless

       --physdev-is-in
              Matches if the packet has entered through a bridge interface.

       --physdev-is-out
              Matches if the packet will leave through a bridge interface.

       --physdev-is-bridged
              Matches if the packet is being  bridged  and  therefore  is  not
              being  routed.  This is only useful in the FORWARD and POSTROUT-
              ING chains.

   pkttype
       This module matches the link-layer packet type.

       --pkt-type [unicast|broadcast|multicast]

   random
       This module randomly matches a certain percentage of all packets.

       --average percent
              Matches the given percentage.  If omitted, a probability of  50%
              is set.

   realm
       This  matches  the  routing  realm.  Routing realms are used in complex
       routing setups involving dynamic routing protocols like BGP.

       --realm [!]value[/mask]
              Matches a given realm number (and optionally mask).

   set
       This modules macthes IP sets which can be defined by ipset(8).

       --set setname flag[,flag...]
              where flags are src and/or dst and there can be no more than six
              of them. Hence the command
               iptables -A FORWARD -m set --set test src,dst
              will match packets, for which (depending on the type of the set)
              the source address or port number of the packet can be found  in
              the specified set. If there is a binding belonging to the mached
              set element or there is a default binding  for  the  given  set,
              then  the  rule  will  match  the  packet  only  if additionally
              (depending on the type of the set) the  destination  address  or
              port  number  of the packet can be found in the set according to
              the binding.

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

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

   tcp
       These extensions are loaded if ‘--protocol tcp’ is specified.  It  pro-
       vides the following options:

       --source-port [!] 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  port:port.  If the first port is
              omitted, "0" is assumed; if the  last  is  omitted,  "65535"  is
              assumed.  If the second port greater then the first they will be
              swapped.  The flag  --sport  is  a  convenient  alias  for  this
              option.

       --destination-port [!] 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
              is  the  flags which we should examine, written as a comma-sepa-
              rated list, and the second argument 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 and RST
              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 SYN.  If the "!" flag  precedes  the  "--syn",
              the sense of the option is inverted.

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

       --mss value[:value]
              Match  TCP  SYN  or SYN/ACK packets with the specified MSS value
              (or range), which control the maximum packet size for that  con-
              nection.

   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 facultative.

        --timestart value
              Match  only  if  it is after ‘value’ (Inclusive, format: HH:MM ;
              default 00:00).

       --timestop  value
              Match only if it is before ‘value’ (Inclusive, format:  HH:MM  ;
              default 23:59).

       --days listofdays
              Match  only  if  today  is  one  of  the  given  days.  (format:
              Mon,Tue,Wed,Thu,Fri,Sat,Sun ; default everyday)

       --datestart date
              Match  only  if  it  is   after   ‘date’   (Inclusive,   format:
              YYYY[:MM[:DD[:hh[:mm[:ss]]]]]  ; h,m,s start from 0 ; default to
              1970)

       --datestop date
              Match  only  if  it  is  before   ‘date’   (Inclusive,   format:
              YYYY[:MM[:DD[:hh[:mm[:ss]]]]]  ; h,m,s start from 0 ; default to
              2037)

   tos
       This module matches the 8 bits of Type  of  Service  field  in  the  IP
       header (ie. including the precedence bits).

       --tos tos
              The argument is either a standard name, (use
               iptables -m tos -h
              to see the list), or a numeric value to match.

   ttl
       This module matches the time to live field in the IP header.

       --ttl-eq ttl
              Matches the given TTL value.

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

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

   udp
       These  extensions are loaded if ‘--protocol udp’ is specified.  It pro-
       vides the following options:

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

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

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


TARGET EXTENSIONS

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

   BALANCE
       This  allows  you to DNAT connections in a round-robin way over a given
       range of destination addresses.

       --to-destination ipaddr-ipaddr
              Address range to round-robin over.

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

       --set-class MAJOR:MINOR
              Set the major and minor class value.

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

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

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

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

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

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

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

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

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

       --save-mark [--mask mask]
              Copy the netfilter packet mark value to the connection mark.  If
              a mask is specified then only those bits are copied.

       --restore-mark [--mask mask]
              Copy the connection mark value to the packet. If a mask is spec-
              ified then only those bits are copied. This is only valid in the
              mangle table.

   DNAT
       This  target is only valid in the nat table, in the PREROUTING and OUT-
       PUT chains, and user-defined chains which are only  called  from  those
       chains.  It specifies that the destination address of the packet should
       be modified (and all future packets in this  connection  will  also  be
       mangled),  and rules should cease being examined.  It takes one type of
       option:

       --to-destination ipaddr[-ipaddr][:port-port]
              which can specify a single new destination IP address, an inclu-
              sive  range of IP addresses, and optionally, a port range (which
              is only valid if the rule also specifies -p tcp or -p udp).   If
              no port range is specified, then the destination port will never
              be modified.

              You can add several --to-destination options.   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
              adresses.

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

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

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

   ECN
       This target allows to selectively work around known ECN blackholes.  It
       can only be used in the mangle table.

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

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

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

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

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

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

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

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

   MARK
       This is used to set  the  netfilter  mark  value  associated  with  the
       packet.   It  is only valid in the mangle table.  It can for example be
       used in conjunction with iproute2.

       --set-mark mark

   MASQUERADE
       This target is only valid in the nat table, in the  POSTROUTING  chain.
       It  should  only  be used with dynamically assigned IP (dialup) connec-
       tions: if you have a static IP address, you should use the SNAT target.
       Masquerading is equivalent to specifying a mapping to the IP address of
       the interface the packet is going out, but also  has  the  effect  that
       connections  are  forgotten  when the interface goes down.  This is the
       correct behavior when the next dialup is  unlikely  to  have  the  same
       interface  address (and hence any established connections are lost any-
       way).  It takes one option:

       --to-ports port[-port]
              This specifies a range of source ports to  use,  overriding  the
              default SNAT source port-selection heuristics (see above).  This
              is only valid if the rule also specifies -p tcp or -p udp.

   MIRROR
       This is an experimental demonstration target which inverts  the  source
       and destination fields in the IP header and retransmits the packet.  It
       is only valid in the INPUT, FORWARD and PREROUTING  chains,  and  user-
       defined  chains which are only called from those chains.  Note that the
       outgoing packets are NOT seen by any packet filtering  chains,  connec-
       tion tracking or NAT, to avoid loops and other problems.

   NETMAP
       This  target  allows you to statically map a whole network of addresses
       onto another network of addresses.  It can only be used from  rules  in
       the nat table.

       --to address[/mask]
              Network  address  to map to.  The resulting address will be con-
              structed in the following way: All ’one’ bits in  the  mask  are
              filled in from the new ‘address’.  All bits that are zero in the
              mask are filled in from the original address.

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

       It can only be used in the
              raw table.

   REDIRECT
       This  target is only valid in the nat table, in the PREROUTING and OUT-
       PUT chains, and user-defined chains which are only  called  from  those
       chains.  It alters the destination IP address to send the packet to the
       machine itself (locally-generated packets are mapped to  the  127.0.0.1
       address).  It takes one option:

       --to-ports port[-port]
              This  specifies  a  destination  port  or range of ports to use:
              without this, the destination port is never  altered.   This  is
              only valid if the rule also specifies -p tcp or -p udp.

   REJECT
       This  is  used  to send back an error packet in response to the matched
       packet: otherwise it is equivalent to DROP so it is a terminating  TAR-
       GET,  ending  rule  traversal.  This target is only valid in the INPUT,
       FORWARD and OUTPUT chains,  and  user-defined  chains  which  are  only
       called  from those chains.  The following option controls the nature of
       the error packet returned:

       --reject-with type
              The type given can be
               icmp-net-unreachable
               icmp-host-unreachable
               icmp-port-unreachable
               icmp-proto-unreachable
               icmp-net-prohibited
               icmp-host-prohibited or
               icmp-admin-prohibited (*)
              which return the appropriate ICMP error  message  (port-unreach-
              able 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

   ROUTE
       This  is  used  to explicitly override the core network stack’s routing
       decision.  mangle table.

       --oif ifname
              Route the packet through ‘ifname’ network interface

       --iif ifname
              Change the packet’s incoming interface to ‘ifname’

       --gw IP_address
              Route the packet via this gateway

       --continue
              Behave like a non-terminating target and continue traversing the
              rules.  Not valid in combination with ‘--iif’ or ‘--tee’

       --tee  Make a copy of the packet, and route that copy to the given des-
              tination. For the original, uncopied packet, behave like a  non-
              terminating target and continue traversing the rules.  Not valid
              in combination with ‘--iif’ or ‘--continue’

   SET
       This modules adds and/or deletes entries from  IP  sets  which  can  be
       defined by ipset(8).

       --add-set setname flag[,flag...]
              add the address(es)/port(s) of the packet to the sets

       --del-set setname flag[,flag...]
              delete  the  address(es)/port(s)  of  the  packet from the sets,
              where flags are src and/or dst and there can be no more than six
              of them.

       The bindings to follow must previously be defined in order to use
              multilevel adding/deleting by the SET target.

   SNAT
       This  target  is only valid in the nat table, in the POSTROUTING chain.
       It specifies that the source address of the packet should  be  modified
       (and  all  future packets in this connection will also be mangled), and
       rules should cease being examined.  It takes one type of option:

       --to-source  ipaddr[-ipaddr][:port-port]
              which can specify a single new source IP address,  an  inclusive
              range  of  IP  addresses, and optionally, a port range (which is
              only valid if the rule also specifies -p tcp or -p udp).  If  no
              port  range  is  specified,  then source ports below 512 will be
              mapped to other ports below 512:  those  between  512  and  1023
              inclusive  will  be  mapped to ports below 1024, and other ports
              will be mapped to 1024 or above. Where possible, no port  alter-
              ation will occur.

              You  can  add  several --to-source options.  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 adresses.

   TCPMSS
       This target allows to alter the MSS value of TCP SYN packets,  to  con-
       trol  the maximum size for that connection (usually limiting it to your
       outgoing interface’s MTU minus 40).  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 packets.  The  symptoms  of  this
       problem are that everything works fine from your Linux firewall/router,
       but machines behind it can never exchange large packets:
        1) Web browsers connect, then hang with no data received.
        2) Small mail works fine, but large emails hang.
        3) ssh works fine, but scp hangs after initial handshaking.
       Workaround: activate this option and add a rule to your  firewall  con-
       figuration like:
        iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
                    -j TCPMSS --clamp-mss-to-pmtu

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

       --clamp-mss-to-pmtu
              Automatically clamp MSS value to (path_MTU - 40).

       These options are mutually exclusive.

   TOS
       This  is  used to set the 8-bit Type of Service field in the IP header.
       It is only valid in the mangle table.

       --set-tos tos
              You can use a numeric TOS values, or use
               iptables -j TOS -h
              to see the list of valid TOS names.

   TRACE
       This target has no options.  It just turns on packet  tracing  for  all
       packets that match this rule.

   TTL
       This is used to modify the IPv4 TTL header field.  The TTL field deter-
       mines how many hops (routers) a packet can traverse until it’s time  to
       live is exceeded.

       Setting  or  incrementing the TTL field can potentially be very danger-
       ous,
              so it should be avoided at any cost.

       Dont  ever set or increment the value on packets that leave your local
       network!
              mangle table.

       --ttl-set value
              Set the TTL value to ‘value’.

       --ttl-dec value
              Decrement the TTL value ‘value’ times.

       --ttl-inc value
              Increment the TTL value ‘value’ times.

   ULOG
       This  target provides userspace logging of matching packets.  When this
       target is set for a rule, the Linux kernel will multicast  this  packet
       through a netlink socket. One or more userspace processes may then sub-
       scribe to various multicast groups and receive the packets.  Like  LOG,
       this  is  a  "non-terminating target", i.e. rule traversal continues at
       the next rule.

       --ulog-nlgroup nlgroup
              This specifies the netlink group (1-32) to which the  packet  is
              sent.  Default value is 1.

       --ulog-prefix prefix
              Prefix  log messages with the specified prefix; up to 32 charac-
              ters long, and useful for distinguishing messages in the logs.

       --ulog-cprange size
              Number of bytes to be copied to userspace.  A value of 0  always
              copies the entire packet, regardless of its size.  Default is 0.

       --ulog-qthreshold size
              Number of packet to queue inside kernel.  Setting this value to,
              e.g.  10 accumulates ten packets inside the kernel and transmits
              them as one netlink multipart message to userspace.  Default  is
              1 (for backwards compatibility).


DIAGNOSTICS

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


BUGS

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


COMPATIBILITY WITH IPCHAINS

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

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

       iptables is a pure packet filter when using the default ‘filter’ table,
       with optional extension modules.  This should simplify much of the pre-
       vious confusion over the combination of IP masquerading and packet fil-
       tering  seen  previously.  So the following options are handled differ-
       ently:
        -j MASQ
        -M -S
        -M -L
       There are several other changes in iptables.


SEE ALSO

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

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


AUTHORS

       Rusty Russell wrote iptables, in early consultation with Michael  Neul-
       ing.

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

       James Morris wrote the TOS target, and tos match.

       Jozsef Kadlecsik wrote the REJECT target.

       Harald Welte wrote the ULOG target, TTL, DSCP, ECN matches and targets.

       The  Netfilter  Core  Team  is:  Marc Boucher, Martin Josefsson, Jozsef
       Kadlecsik, Patrick McHardy, James Morris, Harald Welte and  Rusty  Rus-
       sell.

       Man page written by Herve Eychenne <rv@wallfire.org>.



                                 Mar 09, 2002                      IPTABLES(8)

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