ip route

Manipulate route entries in the kernel routing tables keep information about paths to other networked nodes.
route can be abbreviated to r

ip route add - add new route
ip route change - change route
ip route replace - change or add new route
ip route delete - delete route
ip route show - list routes
ip route flush - flush routing tables
ip route get - get a single route

Syntax
      ip route { list | flush } SELECTOR

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

      ip route { add | del | change | append | replace | monitor } 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 ]
                       [ window NUMBER ] [ cwnd NUMBER ] [ initcwnd NUMBER ] [ ssthresh REALM ]
                          [ realms REALM ] [ rto_min TIME ] [ initrwnd NUMBER ]

         TYPE := [ unicast | local | broadcast | multicast | throw | unreachable | prohibit | blackhole | nat ]

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

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

         FLAGS := [ equalize ]

         NHFLAGS := [ onlink | pervasive ]

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

Key
   to TYPE PREFIX (default)
      the destination prefix of the route. If TYPE is omitted, ip assumes type unicast.
      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 special 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 modifier 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 compatability with
      previous releases. Otherwise if a suffix of s, sec or secs is used to specify
      seconds; ms, msec or msecs to specify milliseconds; us, usec or usecs to specify microseconds;
      ns, nsec or nsecs to specify nanoseconds; j, hz or jiffies to specify jiffies,
      the value is converted to what the routing code expects.

   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 destination.
      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
      the maximum initial congestion window (cwnd) size in MSS of a TCP connection.

   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.

   advmss NUMBER (2.3.15+ only)
      the MSS ('Maximal Segment Size') to advertise to these destinations when establishing
      TCP connections. If it is not given, Linux uses a default value calculated
      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 syntax 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 autoconfiguration.

         boot - the route was installed during the bootup sequence. If a routing 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.

   equalize
      allow packet by packet randomization on multipath routes. Without this modifier, the route will
      be frozen to one selected nexthop, so that load splitting will only occur on per-flow base.
      equalize only works if the kernel is patched.

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.

   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 tablemain.
    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 equivalent 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. 

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 Warning: Route NAT is no longer supported in Linux 2.6.

via.

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 255 or by name from the file /etc/iproute2/rt_tables By default all normal routes are inserted into the main table (ID 254) and the kernel only uses this table when calculating routes.

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.

Examples:

Delete the default gateway:
$ ip route del default

“No camel route is long, with good company” ~ Turkish Proverb

Related linux commands:

ip - Routing, devices and tunnels.
Equivalent Windows command: ROUTE - Manipulate network routing tables.


 
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