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       ip [ OPTIONS ] rule { COMMAND | help }

       ip rule [ list ]

       ip rule { add | del } SELECTOR ACTION

       ip rule { flush }

       SELECTOR := [ not ] [ from PREFIX ] [ to PREFIX ] [ tos TOS ] [ fwmark
               FWMARK[/MASK] ] [ iif STRING ] [ oif STRING ] [ pref NUMBER ]

       ACTION := [ table TABLE_ID ] [ nat ADDRESS ] [ realms
               [SRCREALM/]DSTREALM ] [ goto NUMBER ]

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


       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 'rout-
       ing policy database' (or RPDB), which selects routes by executing some
       set of rules.

       Each policy routing rule consists of a selector and an action predi-
       cate.  The RPDB is scanned in order of decreasing priority. The selec-
       tor 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 indica-
       tion and the RPDB lookup is terminated. Otherwise, the RPDB program
       continues with the next rule.

       Semantically, the natural action is to select the nexthop and the out-
              ing 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 rout-
              ing 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 masquerading 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 val-
       ues 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.

              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 pref-
                     erence and order are synonyms with priority.

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

              realms FROM/TO
                     Realms to select if the rule matched and the routing ta-
                     ble 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 rout-
                     ing 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 syn-
              onyms with show.


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