linux ip-route - routing table management

linux:~$ man ip-route
IP-ROUTE(8)                                     Linux                                    IP-ROUTE(8)


NAME
       ip-route - routing table management


SYNOPSIS
       ip [ ip-OPTIONS ] route  { COMMAND | help }


       ip route { list | flush } SELECTOR


       ip route save SELECTOR


       ip route restore


       ip route get ADDRESS [ from ADDRESS iif STRING  ] [ oif STRING ] [ tos TOS ]


       ip route { add | del | change | append | replace } ROUTE


       SELECTOR := [ root PREFIX ] [ match PREFIX ] [ exact PREFIX ] [ table TABLE_ID ] [ proto
               RTPROTO ] [ type TYPE ] [ scope SCOPE ]


       ROUTE := NODE_SPEC [ INFO_SPEC ]


       NODE_SPEC := [ TYPE ] PREFIX [ tos TOS ] [ table TABLE_ID ] [ proto RTPROTO ] [ scope SCOPE ]
               [ metric METRIC ]


       INFO_SPEC := NH OPTIONS FLAGS [ nexthop NH ] ...


       NH := [ via ADDRESS ] [ dev STRING ] [ weight NUMBER ] NHFLAGS


       OPTIONS := FLAGS [ mtu NUMBER ] [ advmss NUMBER ] [ rtt TIME ] [ rttvar TIME ] [ reordering
               NUMBER ] [ window NUMBER ] [ cwnd NUMBER ] [ ssthresh REALM ] [ realms REALM ] [
               rto_min TIME ] [ initcwnd NUMBER ] [ initrwnd NUMBER ] [ quickack BOOL ]


       TYPE := [ unicast | local | broadcast | multicast | throw | unreachable | prohibit | black-
               hole | nat ]


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


       SCOPE := [ host | link | global | NUMBER ]


       NHFLAGS := [ onlink | pervasive ]


       RTPROTO := [ kernel | boot | static | NUMBER ]


DESCRIPTION
       ip route is used to manipulate entries in the kernel routing tables.


       Route types:


               unicast - the route entry describes real paths to the destinations covered by the
               route prefix.


               unreachable - these destinations are unreachable.  Packets are discarded and the ICMP
               message host unreachable is generated.  The local senders get an EHOSTUNREACH error.


               blackhole - these destinations are unreachable.  Packets are discarded silently.  The
               local senders get an EINVAL error.


               prohibit - these destinations are unreachable.  Packets are discarded and the ICMP
               message communication administratively prohibited is generated.  The local senders
               get an EACCES error.


               local - the destinations are assigned to this host.  The packets are looped back and
               delivered locally.


               broadcast - the destinations are broadcast addresses.  The packets are sent as link
               broadcasts.


               throw - a special control route used together with policy rules. If such a route is
               selected, lookup in this table is terminated pretending that no route was found.
               Without policy routing it is equivalent to the absence of the route in the routing
               table.  The packets are dropped and the ICMP message net unreachable is generated.
               The local senders get an ENETUNREACH error.


               nat - a special NAT route.  Destinations covered by the prefix are considered to be
               dummy (or external) addresses which require translation to real (or internal) ones
               before forwarding.  The addresses to translate to are selected with the attribute
               via.  Warning: Route NAT is no longer supported in Linux 2.6.


               anycast - not implemented the destinations are anycast addresses assigned to this
               host.  They are mainly equivalent to local with one difference: such addresses are
               invalid when used as the source address of any packet.


               multicast - a special type used for multicast routing.  It is not present in normal
               routing tables.


       Route tables: Linux-2.x can pack routes into several routing tables identified by a number in
       the range from 1 to 2^31 or by name from the file /etc/iproute2/rt_tables By default all nor-
       mal routes are inserted into the main table (ID 254) and the kernel only uses this table when
       calculating routes.  Values (0, 253, 254, and 255) are reserved for built-in use.


       Actually, one other table always exists, which is invisible but even more important.  It is
       the local table (ID 255).  This table consists of routes for local and broadcast addresses.
       The kernel maintains this table automatically and the administrator usually need not modify
       it or even look at it.


       The multiple routing tables enter the game when policy routing is used.


       ip route add
              add new route


       ip route change
              change route


       ip route replace
              change or add new one


              to TYPE PREFIX (default)
                     the destination prefix of the route.  If TYPE is omitted, ip assumes type uni-
                     cast.  Other values of TYPE are listed above.  PREFIX is an IP or IPv6 address
                     optionally followed by a slash and the prefix length.  If the length of the
                     prefix is missing, ip assumes a full-length host route.  There is also a spe-
                     cial PREFIX default - which is equivalent to IP 0/0 or to IPv6 ::/0.


              tos TOS


              dsfield TOS
                     the Type Of Service (TOS) key.  This key has no associated mask and the longest
                     match is understood as: First, compare the TOS of the route and of the packet.
                     If they are not equal, then the packet may still match a route with a zero TOS.
                     TOS is either an 8 bit hexadecimal number or an identifier from
                     /etc/iproute2/rt_dsfield.


              metric NUMBER


              preference NUMBER
                     the preference value of the route.  NUMBER is an arbitrary 32bit number.


              table TABLEID
                     the table to add this route to.  TABLEID may be a number or a string from the
                     file /etc/iproute2/rt_tables.  If this parameter is omitted, ip assumes the
                     main table, with the exception of local, broadcast and nat routes, which are
                     put into the local table by default.


              dev NAME
                     the output device name.


              via ADDRESS
                     the address of the nexthop router.  Actually, the sense of this field depends
                     on the route type.  For normal unicast routes it is either the true next hop
                     router or, if it is a direct route installed in BSD compatibility mode, it can
                     be a local address of the interface.  For NAT routes it is the first address of
                     the block of translated IP destinations.


              src ADDRESS
                     the source address to prefer when sending to the destinations covered by the
                     route prefix.


              realm REALMID
                     the realm to which this route is assigned.  REALMID may be a number or a string
                     from the file /etc/iproute2/rt_realms.


              mtu MTU


              mtu lock MTU
                     the MTU along the path to the destination.  If the modifier lock is not used,
                     the MTU may be updated by the kernel due to Path MTU Discovery.  If the modi-
                     fier lock is used, no path MTU discovery will be tried, all packets will be
                     sent without the DF bit in IPv4 case or fragmented to MTU for IPv6.


              window NUMBER
                     the maximal window for TCP to advertise to these destinations, measured in
                     bytes.  It limits maximal data bursts that our TCP peers are allowed to send to
                     us.


              rtt TIME
                     the initial RTT ('Round Trip Time') estimate. If no suffix is specified the
                     units are raw values passed directly to the routing code to maintain compati-
                     bility with previous releases.  Otherwise if a suffix of s, sec or secs is used
                     to specify seconds and ms, msec or msecs to specify milliseconds.


              rttvar TIME (2.3.15+ only)
                     the initial RTT variance estimate. Values are specified as with rtt above.


              rto_min TIME (2.6.23+ only)
                     the minimum TCP Retransmission TimeOut to use when communicating with this des-
                     tination.  Values are specified as with rtt above.


              ssthresh NUMBER (2.3.15+ only)
                     an estimate for the initial slow start threshold.


              cwnd NUMBER (2.3.15+ only)
                     the clamp for congestion window.  It is ignored if the lock flag is not used.


              initcwnd NUMBER (2.5.70+ only)
                     the initial congestion window size for connections to this destination.  Actual
                     window size is this value multiplied by the MSS (``Maximal Segment Size'') for
                     same connection. The default is zero, meaning to use the values specified in
                     RFC2414.


              initrwnd NUMBER (2.6.33+ only)
                     the initial receive window size for connections to this destination.  Actual
                     window size is this value multiplied by the MSS of the connection.  The default
                     value is zero, meaning to use Slow Start value.


              quickack BOOL (3.11+ only)
                     Enable or disable quick ack for connections to this destination.


              advmss NUMBER (2.3.15+ only)
                     the MSS ('Maximal Segment Size') to advertise to these destinations when estab-
                     lishing TCP connections.  If it is not given, Linux uses a default value calcu-
                     lated from the first hop device MTU.  (If the path to these destination is
                     asymmetric, this guess may be wrong.)


              reordering NUMBER (2.3.15+ only)
                     Maximal reordering on the path to this destination.  If it is not given, Linux
                     uses the value selected with sysctl variable net/ipv4/tcp_reordering.


              nexthop NEXTHOP
                     the nexthop of a multipath route.  NEXTHOP is a complex value with its own syn-
                     tax similar to the top level argument lists:


                             via ADDRESS - is the nexthop router.


                             dev NAME - is the output device.


                             weight NUMBER - is a weight for this element of a multipath route
                             reflecting its relative bandwidth or quality.


              scope SCOPE_VAL
                     the scope of the destinations covered by the route prefix.  SCOPE_VAL may be a
                     number or a string from the file /etc/iproute2/rt_scopes.  If this parameter is
                     omitted, ip assumes scope global for all gatewayed unicast routes, scope link
                     for direct unicast and broadcast routes and scope host for local routes.


              protocol RTPROTO
                     the routing protocol identifier of this route.  RTPROTO may be a number or a
                     string from the file /etc/iproute2/rt_protos.  If the routing protocol ID is
                     not given, ip assumes protocol boot (i.e. it assumes the route was added by
                     someone who doesn't understand what they are doing).  Several protocol values
                     have a fixed interpretation.  Namely:


                             redirect - the route was installed due to an ICMP redirect.


                             kernel - the route was installed by the kernel during autoconfigura-
                             tion.


                             boot - the route was installed during the bootup sequence.  If a rout-
                             ing daemon starts, it will purge all of them.


                             static - the route was installed by the administrator to override
                             dynamic routing. Routing daemon will respect them and, probably, even
                             advertise them to its peers.


                             ra - the route was installed by Router Discovery protocol.


                     The rest of the values are not reserved and the administrator is free to assign
                     (or not to assign) protocol tags.


              onlink pretend that the nexthop is directly attached to this link, even if it does not
                     match any interface prefix.


       ip route delete
              delete route
              ip route del has the same arguments as ip route add, but their semantics are a bit
              different.


              Key values (to, tos, preference and table) select the route to delete.  If optional
              attributes are present, ip verifies that they coincide with the attributes of the
              route to delete.  If no route with the given key and attributes was found, ip route
              del fails.


       ip route show
              list routes
              the command displays the contents of the routing tables or the route(s) selected by
              some criteria.


              to SELECTOR (default)
                     only select routes from the given range of destinations.  SELECTOR consists of
                     an optional modifier (root, match or exact) and a prefix.  root PREFIX selects
                     routes with prefixes not shorter than PREFIX.  F.e.  root 0/0 selects the
                     entire routing table.  match PREFIX selects routes with prefixes not longer
                     than PREFIX.  F.e.  match 10.0/16 selects 10.0/16, 10/8 and 0/0, but it does
                     not select 10.1/16 and 10.0.0/24.  And exact PREFIX (or just PREFIX) selects
                     routes with this exact prefix. If neither of these options are present, ip
                     assumes root 0/0 i.e. it lists the entire table.


              tos TOS


              dsfield TOS
                     only select routes with the given TOS.


              table TABLEID
                     show the routes from this table(s).  The default setting is to show table main.
                     TABLEID may either be the ID of a real table or one of the special values:


                             all - list all of the tables.


                             cache - dump the routing cache.


              cloned


              cached list cloned routes i.e. routes which were dynamically forked from other routes
                     because some route attribute (f.e. MTU) was updated.  Actually, it is equiva-
                     lent to table cache.


              from SELECTOR
                     the same syntax as for to, but it binds the source address range rather than
                     destinations.  Note that the from option only works with cloned routes.


              protocol RTPROTO
                     only list routes of this protocol.


              scope SCOPE_VAL
                     only list routes with this scope.


              type TYPE
                     only list routes of this type.


              dev NAME
                     only list routes going via this device.


              via PREFIX
                     only list routes going via the nexthop routers selected by PREFIX.


              src PREFIX
                     only list routes with preferred source addresses selected by PREFIX.


              realm REALMID


              realms FROMREALM/TOREALM
                     only list routes with these realms.


       ip route flush
              flush routing tables
              this command flushes routes selected by some criteria.


              The arguments have the same syntax and semantics as the arguments of ip route show,
              but routing tables are not listed but purged.  The only difference is the default
              action: show dumps all the IP main routing table but flush prints the helper page.


              With the -statistics option, the command becomes verbose. It prints out the number of
              deleted routes and the number of rounds made to flush the routing table. If the option
              is given twice, ip route flush also dumps all the deleted routes in the format
              described in the previous subsection.


       ip route get
              get a single route
              this command gets a single route to a destination and prints its contents exactly as
              the kernel sees it.


              to ADDRESS (default)
                     the destination address.


              from ADDRESS
                     the source address.


              tos TOS


              dsfield TOS
                     the Type Of Service.


              iif NAME
                     the device from which this packet is expected to arrive.


              oif NAME
                     force the output device on which this packet will be routed.


              connected
                     if no source address (option from) was given, relookup the route with the
                     source set to the preferred address received from the first lookup.  If policy
                     routing is used, it may be a different route.


              Note that this operation is not equivalent to ip route show.  show shows existing
              routes.  get resolves them and creates new clones if necessary.  Essentially, get is
              equivalent to sending a packet along this path.  If the iif argument is not given, the
              kernel creates a route to output packets towards the requested destination.  This is
              equivalent to pinging the destination with a subsequent ip route ls cache, however, no
              packets are actually sent.  With the iif argument, the kernel pretends that a packet
              arrived from this interface and searches for a path to forward the packet.


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


       ip route restore
              restore routing table information from stdin
              This command expects to read a data stream as returned from ip route save.  It will
              attempt to restore the routing table information exactly as it was at the time of the
              save, so any translation of information in the stream (such as device indexes) must be
              done first.  Any existing routes are left unchanged.  Any routes specified in the data
              stream that already exist in the table will be ignored.


EXAMPLES
       ip ro
           Show all route entries in the kernel.


       ip route add default via 192.168.1.1 dev eth0
           Adds a default route (for all addresses) via the local gateway 192.168.1.1 that can be
           reached on device eth0.


SEE ALSO
       ip(8)


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


iproute2                                     13 Dec 2012                                 IP-ROUTE(8)
linux:~$ 
linux:~$