ip-rule - routing policy database management


   ip [ OPTIONS ] rule  { COMMAND | help }

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

   ip rule  restore

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

   ACTION := [ table TABLE_ID ] [ nat ADDRESS ] [ realms

   SUPPRESSOR := [ suppress_prefixlength NUMBER ] [ suppress_ifgroup GROUP

   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
   '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 order of decreasing 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 with the next rule.

   Semantically, the 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 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
   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 originating 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.

          suppress_prefixlength NUMBER
                 reject routing decisions that have a prefix length of
                 NUMBER or less.

          suppress_ifgroup GROUP
                 reject routing decisions that use a device belonging to
                 the interface group GROUP.

          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.

   ip rule save
          save rules table information to stdout
          This command behaves like ip rule show except that the output is
          raw data suitable for passing to ip rule restore.

   ip rule restore
          restore rules table information from stdin
          This command expects to read a data stream as returned from ip
          rule save.  It will attempt to restore the rules table
          information exactly as it was at the time of the save. Any rules
          already in the table are left unchanged, and duplicates are not




   Original Manpage by Michail Litvak <mci@owl.openwall.com>


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