iptables(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | TARGETS | TABLES | OPTIONS | LOCK FILE | MATCH AND TARGET EXTENSIONS | DIAGNOSTICS | BUGS | COMPATIBILITY WITH IPCHAINS | SEE ALSO | AUTHORS | VERSION | COLOPHON

IPTABLES(8)                  iptables 1.8.7                  IPTABLES(8)

NAME         top

       iptables/ip6tables — administration tool for IPv4/IPv6 packet
       filtering and NAT

SYNOPSIS         top

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

       ip6tables [-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         top

       Iptables and ip6tables are used to set up, maintain, and inspect
       the tables of IPv4 and IPv6 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         top

       A firewall rule specifies criteria for a packet and a target.  If
       the packet does not match, the next rule in the chain is
       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, one of the targets described in iptables-extensions(8), or
       one of the special values ACCEPT, DROP or RETURN.

       ACCEPT means to let the packet through.  DROP means to drop the
       packet on the floor.  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         top

       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 encountered.  It consists of four
                  built-ins: PREROUTING (for altering packets as soon as
                  they come in), INPUT (for altering packets destined
                  for local sockets), OUTPUT (for altering locally-
                  generated packets before routing), and POSTROUTING
                  (for altering packets as they are about to go out).
                  IPv6 NAT support is available since kernel 3.7.

              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         top

       The options that are recognized by iptables and ip6tables 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 possible
              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 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. 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
              or iptables-save(8).

       -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 specified
              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 built-in (non-user-defined) chain
              to the given target.  The policy target must be either
              ACCEPT or DROP.

       -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).

       -4, --ipv4
              This option has no effect in iptables and iptables-
              restore.  If a rule using the -4 option is inserted with
              (and only with) ip6tables-restore, it will be silently
              ignored. Any other uses will throw an error. This option
              allows IPv4 and IPv6 rules in a single rule file for use
              with both iptables-restore and ip6tables-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 IPv4
              and IPv6 rules in a single rule file for use with both
              iptables-restore and ip6tables-restore.  This option has
              no effect in ip6tables 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,
              icmpv6,esp, ah, sctp, mh 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.  Note that, in
              ip6tables, IPv6 extension headers except esp are not
              allowed.  esp and ipv6-nonext can be used with Kernel
              version 2.6.11 or later.  The number zero is equivalent to
              all, which means that you cannot test the protocol field
              for the value 0 directly. To match on a HBH header, even
              if it were the last, you cannot use -p 0, but always need
              -m hbh.

       [!] -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 an ipv4 network mask (for iptables) or a plain
              number, specifying the number of 1's at the left side of
              the network mask.  Thus, an iptables 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.  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 property. 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 extension (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 counters 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
              IPv4 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. This option is IPv4 specific, it is
              not available in ip6tables.

       -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 [seconds]
              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
              (indefinitely or for optional seconds) until the exclusive
              lock can be obtained.

       -W, --wait-interval microseconds
              Interval to wait per each iteration.  When running latency
              sensitive applications, waiting for the xtables lock for
              extended durations may not be acceptable. This option will
              make each iteration take the amount of time specified. The
              default interval is 1 second. This option only works with
              -w.

       -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 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).

LOCK FILE         top

       iptables uses the /run/xtables.lock file to take an exclusive
       lock at launch.

       The XTABLES_LOCKFILE environment variable can be used to override
       the default setting.

MATCH AND TARGET EXTENSIONS         top

       iptables can use extended packet matching and target modules.  A
       list of these is available in the iptables-extensions(8) manpage.

DIAGNOSTICS         top

       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         top

       Bugs?  What's this? ;-) Well, you might want to have a look at
       http://bugzilla.netfilter.org/

COMPATIBILITY WITH IPCHAINS         top

       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.

       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 previously.  So the following options are
       handled differently:
        -j MASQ
        -M -S
        -M -L
       There are several other changes in iptables.

SEE ALSO         top

       iptables-apply(8), iptables-save(8), iptables-restore(8),
       iptables-extensions(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 netfilter-hacking-HOWTO details the
       netfilter internals.
       See http://www.netfilter.org/ .

AUTHORS         top

       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: Jozsef Kadlecsik, Pablo Neira Ayuso,
       Eric Leblond, Florian Westphal and  Arturo Borrero Gonzalez.
       Emeritus Core Team members are: Marc Boucher, Martin Josefsson,
       Yasuyuki Kozakai, James Morris, Harald Welte and Rusty Russell.

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

VERSION         top

       This manual page applies to iptables/ip6tables 1.8.7.

COLOPHON         top

       This page is part of the iptables (administer and maintain packet
       filter rules) project.  Information about the project can be
       found at ⟨http://www.netfilter.org/⟩.  If you have a bug report
       for this manual page, see ⟨http://bugzilla.netfilter.org/⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨git://git.netfilter.org/iptables⟩ on 2021-04-01.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2021-03-09.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

iptables 1.8.7                                               IPTABLES(8)

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