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 ] SUPPRESSOR

       SUPPRESSOR := [ suppress_prefixlength NUMBER ] [ suppress_ifgroup GROUP ]

       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 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.

SEE ALSO

       ip(8)

AUTHOR

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