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IP-RULE(8)                           Linux                          IP-RULE(8)



NAME
       ip-rule - routing policy database management

SYNOPSIS
       ip [ OPTIONS ] rule  { COMMAND | help }


       ip rule  [ list | add | del | flush ] SELECTOR ACTION

       SELECTOR := [ from PREFIX ] [ to PREFIX ] [ tos TOS ] [ fwmark FWMARK[/MASK] ] [ iif STRING ] [ oif STRING ] [
               pref NUMBER ]

       ACTION := [ table TABLE_ID ] [ nat ADDRESS ] [ prohibit | reject | unreachable ] [ realms [SRCREALM/]DSTREALM ]

       TABLE_ID := [ local | main | default | NUMBER ]


DESCRIPTION
       ip rule manipulates rules in the routing policy database control the route selection algorithm.


       Classic routing algorithms used in the Internet make routing decisions based only on the destination address of
       packets (and in theory, but not in practice, on the TOS field).


       In some circumstances we want to route packets differently depending not only on destination addresses, but
       also on other packet fields: source address, IP protocol, transport protocol ports or even packet payload.
       This task is called 'policy routing'.


       To solve this task, the conventional destination based routing table, ordered according to the longest match
       rule, is replaced with a 'routing policy database' (or RPDB), which selects routes by executing some set of
       rules.


       Each policy routing rule consists of a selector and an action predicate.  The RPDB is scanned in the order of
       increasing priority. The selector of each rule is applied to {source address, destination address, incoming
       interface, tos, fwmark} and, if the selector matches the packet, the action is performed.  The action predicate
       may return with success.  In this case, it will either give a route or failure indication and the RPDB lookup
       is terminated. Otherwise, the RPDB program continues on the next rule.


       Semantically, natural action is to select the nexthop and the output device.


       At startup time the kernel configures the default RPDB consisting of three rules:


       1.     Priority: 0, Selector: match anything, Action: lookup routing table local (ID 255).  The local table is
              a special routing table containing high priority control routes for local and broadcast addresses.

              Rule 0 is special. It cannot be deleted or overridden.


       2.     Priority: 32766, Selector: match anything, Action: lookup routing table main (ID 254).  The main table
              is the normal routing table containing all non-policy routes. This rule may be deleted and/or overridden
              with other ones by the administrator.


       3.     Priority: 32767, Selector: match anything, Action: lookup routing table default (ID 253).  The default
              table is empty.  It is reserved for some post-processing if no previous default rules selected the
              packet.  This rule may also be deleted.


       Each RPDB entry has additional attributes.  F.e. each rule has a pointer to some routing table.  NAT and mas-
       querading rules have an attribute to select new IP address to translate/masquerade.  Besides that, rules have
       some optional attributes, which routes have, namely realms.  These values do not override those contained in
       the routing tables.  They are only used if the route did not select any attributes.


       The RPDB may contain rules of the following types:

               unicast - the rule prescribes to return the route found in the routing table referenced by the rule.

               blackhole - the rule prescribes to silently drop the packet.

               unreachable - the rule prescribes to generate a 'Network is unreachable' error.

               prohibit - the rule prescribes to generate 'Communication is administratively prohibited' error.

               nat - the rule prescribes to translate the source address of the IP packet into some other value.


   ip rule add - insert a new rule
   ip rule delete - delete a rule
       type TYPE (default)
              the type of this rule.  The list of valid types was given in the previous subsection.


       from PREFIX
              select the source prefix to match.


       to PREFIX
              select the destination prefix to match.


       iif NAME
              select the incoming device to match.  If the interface is loopback, the rule only matches packets origi-
              nating from this host.  This means that you may create separate routing tables for forwarded and local
              packets and, hence, completely segregate them.


       oif NAME
              select the outgoing device to match.  The outgoing interface is only available for packets originating
              from local sockets that are bound to a device.


       tos TOS

       dsfield TOS
              select the TOS value to match.


       fwmark MARK
              select the fwmark value to match.


       priority PREFERENCE
              the priority of this rule.  Each rule should have an explicitly set unique priority value.  The options
              preference and order are synonyms with priority.


       table TABLEID
              the routing table identifier to lookup if the rule selector matches.  It is also possible to use lookup
              instead of table.


       realms FROM/TO
              Realms to select if the rule matched and the routing table lookup succeeded.  Realm TO is only used if
              the route did not select any realm.


       nat ADDRESS
              The base of the IP address block to translate (for source addresses).  The ADDRESS may be either the
              start of the block of NAT addresses (selected by NAT routes) or a local host address (or even zero).  In
              the last case the router does not translate the packets, but masquerades them to this address.  Using
              map-to instead of nat means the same thing.

              Warning: Changes to the RPDB made with these commands do not become active immediately.  It is assumed
              that after a script finishes a batch of updates, it flushes the routing cache with ip route flush cache.


   ip rule flush - also dumps all the deleted rules.
       This command has no arguments.


   ip rule show - list rules
       This command has no arguments.  The options list or lst are synonyms with show.


SEE ALSO
       ip(8)


AUTHOR
       Original Manpage by Michail Litvak <mciATowl.com>



iproute2                          20 Dec 2011                       IP-RULE(8)