# iptables

## 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 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) 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
                      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** 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 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 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 **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
                  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: 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
                  (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).
    
           **--compat**(nft-variants only)
                  When creating a rule, attach compatibility data to the
                  rule's userdata section for use as aid in parsing the rule
                  by an older version of the program. The old version
                  obviously needs to support this, though.  Specifying this
                  option a second time instructs the program to default to
                  the rule's compatibility data when parsing, which is mostly
                  useful for debugging or testing purposes.
    
                  The **XTABLES_COMPAT** environment variable can be used to
                  override the default setting. The expected value is a
                  natural number representing the number of times **--compat**
                  was specified.
    


## 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, failing 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.
    


## VERSION



           This manual page applies to iptables/ip6tables 1.8.11.
    


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