Routing table management
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 | blackhole | nat ]
TABLE_ID := [ local| main | default | all | NUMBER ]
SCOPE := [ host | link | global | NUMBER ]
NHFLAGS := [ onlink | pervasive ]
RTPROTO := [ kernel | boot | static | NUMBER ]
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 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 /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 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.
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 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.
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 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(8)
Original Manpage by Michail Litvak <[email protected]>