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 valu
fbfa
es

                     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:~$