linux man ip-rule

linux:~$ man ip-rule 
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 ]
               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 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
                     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 origi-
                     nating 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>


iproute2                                              20 Dec 2011                                           IP-RULE(8)
linux:~$ 
linux:~$ 
linux:~$ man ip-rule