iptables

IPTABLES(8) iptables 1.8.11 IPTABLES(8)

NAME

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

SYNOPSIS

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

   ip6tables [-t table] {-A|-C|-D|-V} 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 policy

   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  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

   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

   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  it‐
              self), 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 pri‐
              ority 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  inter‐
              face) and 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 imple‐
              mented 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-gen‐
              erated packets before routing), and FORWARD (for altering packets being routed through the box).

OPTIONS

   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 exist‐
          ing 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  de‐
          fault), 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 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 delete all empty chains in the table. Empty builtin chains can only be deleted with iptables-nft.

   -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  in‐
          verts  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 spe‐
          cial builtin targets which decide the fate of the packet immediately, or an extension (see MATCH AND TARGET 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 with 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  inter‐
          face  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  unfrag‐
          mented 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 replace‐
          ment, 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: Specified  twice,
          iptables-legacy will dump table info and entries in libiptc, iptables-nft dumps rules in netlink (VM code) presentation.  Specified three times, iptables-nft will also dump any
          netlink messages sent to kernel.

   -V, --version
          Show program version and the kernel API used.

   -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.

   -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 (multi‐
          ples 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

   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

   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 functioning.  Errors which appear to be caused by invalid or abused command line parameters cause
   an exit code of 2. Errors which indicate an incompatibility between kernel and user space cause an exit code of 3. Errors which indicate a resource problem, such as a busy lock, fail‐
   ing memory allocation or error messages from kernel cause an exit code of 4. Finally, other errors cause an exit code of 1.

BUGS

   Bugs?   What's this? ;-) Well, you might want to have a look at https://bugzilla.netfilter.org/ iptables will exit immediately with an error code of 111 if it finds that it was called
   as a setuid-to-root program.  iptables cannot be used safely in this manner because it trusts the shared libraries (matches, targets) loaded at run time, the search path  can  be  set
   using environment variables.

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.

   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 avoid much of
   the 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.