IPTABLES linux command manual

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
       table.


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

       ACCEPT means to let the packet through.  DROP means to drop the packet
       on the floor.  QUEUE means to pass the packet to  userspace  (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 tar-
       get RETURN is matched, the target specified by the chain policy deter-
       mines 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
              appropriate module for that table if it is not already there.

              The tables are as follows:

              filter:
                  This is the default table (if no -t option is passed).   It
                  contains the built-in chains INPUT (for packets coming into
                  the box itself), 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
                  supported:  INPUT (for packets coming into the box itself),
                  FORWARD (for altering  packets  being  routed  through  the
                  box),  and  POSTROUTING  (for  altering packets as they are
                  about to go out).

OPTIONS
       The options that are recognized by iptables can be divided  into  sev-
       eral 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  differen-
       tiate it from all other options.

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

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

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

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

       -L, --list [chain]
              List all rules in the selected chain.  If no chain is selected,
              all  chains  are  listed.   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
              counters immediately before they are cleared. (See above.)

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

       -X, --delete-chain [chain]
              Delete  the  optional user-defined chain specified.  There must
              be no references to the chain.  If there are, you  must  delete
              or replace the referring rules before the chain can be deleted.
              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 ta-
              ble.

       -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
              different  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 net-
              work 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  immedi-
              ately,  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 is going to be 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
              fragments 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  opera-
              tions).

   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.

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

       --ahspi [!] spi[:spi]

   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  sup-
       porting 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
              packets  in  both  directions,  NEW meaning that the packet has
              started a new connection, or otherwise associated with  a  con-
              nection  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.  SNAT A virtual state,  match-
              ing  if the original source address differs from the reply des-
              tination.  DNAT A virtual state, matching if the original  des-
              tination 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.

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

       --espspi [!] spi[:spi]

   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

   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,
              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).

   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.

   owner
       This  module  attempts  to match various characteristics of the packet
       creator, for locally-generated packets.  It is only valid in the  OUT-
       PUT  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
              session 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
              compiled under a kernel supporting this feature)

   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  use-
       ful 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
              output 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]

   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 asso-
              ciated with a connection which has seen packets in both  direc-
              tions,  NEW  meaning  that the packet has started a new connec-
              tion, 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.

   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 ttl
              Matches 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.

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

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

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

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

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

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

   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 tar-
       get.  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).  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,
       connection tracking or NAT, to avoid loops and other problems.

   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

   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 multi-
              ple  --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   fire-
       wall/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.

   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
       subscribe to various multicast groups and receive the  packets.   Like
       LOG, this is a "non-terminating target", i.e. rule traversal continues
       at the next rule.

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

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

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

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

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' ta-
       ble, with optional extension modules.  This should  simplify  much  of
       the  previous  confusion  over  the combination of IP masquerading and
       packet filtering seen previously.  So the following options  are  han-
       dled differently:
        -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  filter-
       ing, the NAT-HOWTO details NAT, the netfilter-extensions-HOWTO details
       the extensions that are not in the standard distribution, and the net-
       filter-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
       everywhere.

       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 tar-
       gets.

       The Netfilter Core Team is: Marc  Boucher,  Martin  Josefsson,  Jozsef
       Kadlecsik, James Morris, Harald Welte and Rusty Russell.

       Man page written by Herve Eychenne .



                                 Mar 09, 2002                     IPTABLES(8)