iptables -- administration tool for IPv4 packet filtering and NAT

linux:~$ man iptables

IPTABLES(8)                                iptables 1.4.20                               IPTABLES(8)

NAME

       iptables -- administration tool for IPv4 packet filtering and NAT

SYNOPSIS

       iptables [-t table] {-A|-C|-D} chain rule-specification

       iptables [-t table] -I chain [rulenum] rule-specification

       iptables [-t table] -R chain rulenum rule-specification

       iptables [-t table] -D chain rulenum

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

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

       iptables [-t table] -N chain

       iptables [-t table] -X [chain]

       iptables [-t table] -P chain target

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

       rule-specification = [matches...] [target]

       match = -m matchname [per-match-options]

       target = -j targetname [per-target-options]

DESCRIPTION

       Iptables  is  used to set up, maintain, and inspect the tables of IPv4 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 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.   (How the packet can be received by a userspace

       process differs by the particular queue handler.   2.4.x  and  2.6.x  kernels  up  to  2.6.13

       include  the  ip_queue  queue  handler.   Kernels  2.6.14  and later additionally include the

       nfnetlink_queue queue handler.  Packets with a target of QUEUE will be sent to  queue  number

       '0'  in  this  case. Please also see the NFQUEUE target as described later in this man page.)

       RETURN means stop traversing this chain and resume at the next rule in the previous (calling)

       chain.   If  the end of a built-in chain is reached or a rule in a built-in chain with target

       RETURN is matched, the target specified by the  chain  policy  determines  the  fate  of  the

       packet.

TABLES

       There  are currently five independent tables (which tables are present at any time depends on

       the kernel configuration options and which modules are present).

       -t, --table table

              This option specifies the packet matching table which the command should  operate  on.

              If  the kernel is configured with automatic module loading, an attempt will be made to

              load the appropriate module for that table if it is not already there.

              The tables are as follows:

              filter:

                  This is the default table (if no -t option is passed). It  contains  the  built-in

                  chains  INPUT  (for packets destined to local sockets), FORWARD (for packets being

                  routed through the box), and OUTPUT (for locally-generated packets).

              nat:

                  This table is consulted when a packet that creates a  new  connection  is  encoun-

                  tered.   It  consists of three built-ins: PREROUTING (for altering packets as soon

                  as they come in), OUTPUT (for altering locally-generated packets before  routing),

                  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).

              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)

              security:

                  This table is used for Mandatory Access Control (MAC) networking  rules,  such  as

                  those enabled by the SECMARK and CONNSECMARK targets.  Mandatory Access Control is

                  implemented by Linux Security Modules such as  SELinux.   The  security  table  is

                  called  after  the  filter  table, allowing any Discretionary Access Control (DAC)

                  rules in the filter table to take effect before MAC rules.   This  table  provides

                  the  following  built-in  chains:  INPUT (for packets coming into the box itself),

                  OUTPUT (for altering locally-generated packets before routing), and  FORWARD  (for

                  altering packets being routed through the box).

OPTIONS

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

   COMMANDS

       These options specify the desired action to perform. Only one of them can be specified on the

       command line unless otherwise stated below. For 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 pos-

              sible address combination.

       -C, --check chain rule-specification

              Check whether a rule matching the specification does exist in the selected chain. This

              command  uses  the  same  logic as -D to find a matching entry, but does not alter the

              existing iptables configuration and uses its exit code to indicate success or failure.

       -D, --delete chain rule-specification

       -D, --delete chain rulenum

              Delete one or more rules from the selected chain.  There are two versions of this com-

              mand:  the rule can be specified as a number in the chain (starting at 1 for the first

              rule) or a rule to match.

       -I, --insert chain [rulenum] rule-specification

              Insert one or more rules in the selected chain as the given rule number.  So,  if  the

              rule  number  is  1, the rule or rules are inserted at the head of the chain.  This is

              also the default if no rule number is specified.

       -R, --replace chain rulenum rule-specification

              Replace a rule in the selected chain.  If the source and/or destination names  resolve

              to multiple addresses, the command will fail.  Rules are numbered starting at 1.

       -L, --list [chain]

              List all rules in the selected chain.  If no chain is selected, all chains are listed.

              Like every other 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

       -S, --list-rules [chain]

              Print all rules in the selected chain.  If  no  chain  is  selected,  all  chains  are

              printed like iptables-save. Like every other iptables command, it applies to the spec-

              ified table (filter is the default).

       -F, --flush [chain]

              Flush the selected chain (all the chains in the table if  none  is  given).   This  is

              equivalent to deleting all the rules one by one.

       -Z, --zero [chain [rulenum]]

              Zero  the packet and byte counters in all chains, or only the given chain, or only the

              given rule in a chain. It is legal to specify the -L, --list (list) option as well, to

              see the counters immediately before they are cleared. (See above.)

       -N, --new-chain chain

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

              name already.

       -X, --delete-chain [chain]

              Delete the optional user-defined chain specified.  There must be no references to  the

              chain.   If there are, you must delete or replace the referring rules before the chain

              can be deleted.  The chain must be empty, i.e. not contain any rules.  If no  argument

              is given, it will attempt to delete every non-builtin chain in the table.

       -P, --policy chain target

              Set  the  policy  for  the chain to the given target.  See the section TARGETS for the

              legal targets.  Only built-in (non-user-defined) chains can have policies, and neither

              built-in nor user-defined chains can be policy targets.

       -E, --rename-chain old-chain new-chain

              Rename  the user specified chain to the user supplied name.  This is cosmetic, and has

              no effect on the structure of the table.

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

   PARAMETERS

       The following parameters make up
1062b
a rule specification (as used in the  add,  delete,  insert,

       replace and append commands).

       -4, --ipv4

              This option has no effect in iptables and iptables-restore.

       -6, --ipv6

              If  a  rule  using the -6 option is inserted with (and only with) iptables-restore, it

              will be silently ignored. Any other uses will throw an error. This  option  allows  to

              put  both IPv4 and IPv6 rules in a single rule file for use with both iptables-restore

              and ip6tables-restore.

       [!] -p, --protocol protocol

              The protocol of the rule or of the packet to check.  The specified protocol can be one

              of tcp, udp, udplite, icmp, esp, ah, sctp or the special keyword "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. "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, a network IP

              address (with /mask), or a plain IP address. Hostnames will  be  resolved  once  only,

              before  the  rule is submitted to the kernel.  Please note that specifying any name to

              be resolved with a remote query such as DNS is a really bad idea.   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 "!"  argu-

              ment before the address specification inverts the sense of the address. The flag --src

              is an alias for this option.  Multiple addresses  can  be  specified,  but  this  will

              expand  to  multiple  rules  (when adding with -A), or will cause multiple rules to be

              deleted (with -D).

       [!] -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.

       -m, --match match

              Specifies a match to use, that is, an extension module that tests for a specific prop-

              erty. The set of matches make up the  condition  under  which  a  target  is  invoked.

              Matches  are  evaluated  first  to  last  as specified on the command line and work in

              short-circuit fashion, i.e. if one extension yields false, evaluation will stop.

       -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  exten-

              sion  (see  EXTENSIONS  below).   If  this  option is omitted in a rule (and -g is not

              used), then matching the rule will have no effect on the packet's fate, but the  coun-

              ters on the rule will be incremented.

       -g, --goto chain

              This  specifies  that the processing should continue in a user specified chain. Unlike

              the --jump option return will not continue processing in this chain but instead in the

              chain that called us via --jump.

       [!] -i, --in-interface name

              Name  of  an  interface via which a packet was received (only for packets entering the

              INPUT, FORWARD and PREROUTING chains).  When the  "!"  argument  is  used  before  the

              interface  name, the sense is inverted.  If the interface name ends in a "+", then any

              interface which begins with this name will match.  If  this  option  is  omitted,  any

              interface name will match.

       [!] -o, --out-interface name

              Name  of an interface via which a packet is going to be sent (for packets entering the

              FORWARD, OUTPUT and POSTROUTING chains).  When the "!" argument  is  used  before  the

              interface  name, the sense is inverted.  If the interface name ends in a "+", then any

              interface which begins with this name will match.  If  this  option  is  omitted,  any

              interface name will match.

       [!] -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 packets bytes

              This  enables  the  administrator to initialize the packet and byte counters of a rule

              (during INSERT, APPEND, REPLACE operations).

   OTHER OPTIONS

       The following additional options can be specified:

       -v, --verbose

              Verbose output.  This option makes the list command show the interface name, the  rule

              options  (if  any),  and the TOS masks.  The packet and byte counters are also listed,

              with the suffix 'K', 'M' or 'G' for  1000,  1,000,000  and  1,000,000,000  multipliers

              respectively (but see the -x flag to change this).  For appending, insertion, deletion

              and replacement, this causes detailed information on the rule or rules to be  printed.

              -v may be specified multiple times to possibly emit more detailed debug statements.

       -w, --wait

              Wait  for the xtables lock.  To prevent multiple instances of the program from running

              concurrently, an attempt will be made to obtain  an  exclusive  lock  at  launch.   By

              default,  the program will exit if the lock cannot be obtained.  This option will make

              the program wait until the exclusive lock can be obtained.

       -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 ser-

              vices (whenever applicable).

       -x, --exact

              Expand numbers.  Display the exact value of the packet and byte counters,  instead  of

              only  the  rounded  number  in K's (multiples of 1000) M's (multiples of 1000K) or G's

              (multiples of 1000M).  This option is only relevant for the -L command.

       --line-numbers

              When listing rules, add line numbers to the beginning of each rule,  corresponding  to

              that rule's position in the chain.

       --modprobe=command

              When adding or inserting rules into a chain, use command to load any necessary modules

              (targets, match extensions, etc).

MATCH AND TARGET EXTENSIONS

       iptables can use extended packet matching and target modules.  A list of these  is  available

       in the iptables-extensions(8) manpage.

DIAGNOSTICS

       Various  error  messages are printed to standard error.  The exit code is 0 for correct func-

       tioning.  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, you might want to have a look at http://bugzilla.netfilter.org/
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  origi-

       nating  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);  previ-

       ously 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.

       The various forms of NAT have been separated out; iptables is a pure packet filter when using

       the  default  `filter'  table, with optional extension modules.  This should simplify much of

       the previous confusion over the combination of IP masquerading and packet filtering seen pre-

       viously.  So the following options are handled differently:

        -j MASQ

        -M -S

        -M -L

       There are several other changes in iptables.

SEE ALSO

       iptables-apply(8),     iptables-save(8),     iptables-restore(8),     iptables-extensions(8),

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

       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 dis-

       tribution, and the netfilter-hacking-HOWTO details the netfilter internals.

       See http://www.netfilter.org/.
AUTHORS

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

       Marc Boucher made Rusty abandon ipnatctl by lobbying for a generic packet selection framework

       in  iptables,  then  wrote  the mangle table, the owner match, the mark stuff, and ran around

       doing cool stuff everywhere.

       James Morris wrote the TOS target, and tos match.

       Jozsef Kadlecsik wrote the REJECT target.

       Harald Welte wrote the ULOG and NFQUEUE target, the new libiptc, as well as  the  TTL,  DSCP,

       ECN matches and targets.

       The  Netfilter Core Team is: Marc Boucher, Martin Josefsson, Yasuyuki Kozakai, Jozsef Kadlec-

       sik, Patrick McHardy, James Morris, Pablo Neira Ayuso, Harald Welte and Rusty Russell.

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

VERSION

       This manual page applies to iptables 1.4.20.

iptables 1.4.20                                                                          IPTABLES(8)

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