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 @SYSCONFDIR@/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. 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 "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 "@SYSCONFDIR@/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 "@SYSCONFDIR@/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 " [ FAMILY ] ADDRESS" the address of the nexthop router, in the address family FAMILY. 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 "@SYSCONFDIR@/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 compatibility 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 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 " "(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.
features " FEATURES " (3.18+ only) Enable or disable per-route features. Only available feature at this time is ecn to enable explicit congestion notification when initiating connections to the given destination network. When responding to a connection request from the given network, ecn will also be used even if the net.ipv4.tcp_ecn sysctl is set to 0.
quickack " BOOL " "(3.11+ only)" Enable or disable quick ack for connections to this destination.
congctl " NAME " "(3.20+ only)"
"congctl lock" " NAME " "(3.20+ only)" Sets a specific TCP congestion control algorithm only for a given destination. If not specified, Linux keeps the current global default TCP congestion control algorithm, or the one set from the application. If the modifier lock is not used, an application may nevertheless overwrite the suggested congestion control algorithm for that destination. If the modifier lock is used, then an application is not allowed to overwrite the specified congestion control algorithm for that destination, thus it will be enforced/guaranteed to use the proposed algorithm.
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 " [ FAMILY ] 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 "@SYSCONFDIR@/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 "@SYSCONFDIR@/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.
pref " PREF" the IPv6 route preference. PREF is a string specifying the route preference as defined in RFC4191 for Router Discovery messages. Namely: low - the route has a lowest priority medium - the route has a default priority high - the route has a highest priority
encap " ENCAPTYPE ENCAPHDR" attach tunnel encapsulation attributes to this route. ENCAPTYPE is a string specifying the supported encapsulation type. Namely: mpls - encapsulation type MPLS ip - IP encapsulation (Geneve, GRE, VXLAN, ...) ENCAPHDR is a set of encapsulation attributes specific to the ENCAPTYPE. mpls MPLSLABEL - mpls label stack with labels separated by "/" ip id TUNNEL_ID dst REMOTE_IP " [ " tos TOS " ] [" ttl TTL " ]"
expires " TIME " "(4.4+ only)" the route will be deleted after the expires time. Only support IPv6 at present.
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 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 " [ FAMILY ] 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.
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.
ip route add 10.1.1.0/30 encap mpls 200/300 via 10.1.1.1 dev eth0 Adds an ipv4 route with mpls encapsulation attributes attached to it.
