ip-route(8) - phpMan

IP-ROUTE(8) Linux IP-ROUTE(8)

NAME
ip-route - routing table management

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

ip route { show | flush } SELECTOR

ip route save SELECTOR

ip route restore

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

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

SELECTOR := [ root PREFIX ] [ match PREFIX ] [ exact PREFIX ] [ table TABLE_ID ] [ vrf
NAME ] [ 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 := [ encap ENCAP ] [ via [ FAMILY ] ADDRESS ] [ dev STRING ] [ weight NUMBER ] NHFLAGS

OPTIONS := FLAGS [ mtu NUMBER ] [ advmss NUMBER ] [ as [ to ] ADDRESS ] rtt TIME ] [
rttvar TIME ] [ reordering NUMBER ] [ window NUMBER ] [ cwnd NUMBER ] [ ssthresh
NUMBER ] [ realms REALM ] [ rto_min TIME ] [ initcwnd NUMBER ] [ initrwnd NUMBER ]
[ features FEATURES ] [ quickack BOOL ] [ congctl NAME ] [ pref PREF ] [ expires
TIME ]

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

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

NHFLAGS := [ onlink | pervasive ]

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

FEATURES := [ ecn | ]

PREF := [ low | medium | high ]

ENCAP := [ MPLS | IP| BPF ]

ENCAP_MPLS := mpls [ LABEL ]

ENCAP_IP := ip id TUNNEL_ID dst REMOTE_IP [ tos TOS ] [ ttl TTL ]

ENCAP_BPF := bpf [ in PROG ] [ out PROG ] [ xmit PROG ] [ headroom SIZE ]

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 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 mod-
ify 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 long-
est 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.

vrf NAME
the vrf name to add this route to. Implicitly means the table associated
with the VRF.

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 inter-
face. 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 compat-
ibility 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 destina-
tion. If not specified, Linux keeps the current global default TCP conges-
tion 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 conges-
tion control algorithm for that destination, thus it will be enforced/guar-
anteed 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.

The internal buffer used in iproute2 limits the maximum number of nexthops
that may be specified in one go. If only ADDRESS is given, the current buf-
fer size allows for 144 IPv6 nexthops and 253 IPv4 ones. For IPv4, this
effectively limits the number of nexthops possible per route. With IPv6,
further nexthops may be appended to the same route via ip route append com-
mand.

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 parame-
ter 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
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, ...)

bpf - Execution of BPF program

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 ]

bpf
in PROG - BPF program to execute for incoming packets

out PROG - BPF program to execute for outgoing packets

xmit PROG - BPF program to execute for transmitted packets

headroom SIZE - Size of header BPF program will attach (xmit)

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.

vrf NAME
show the routes for the table associated with the vrf name

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 for-
mat 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.

vrf NAME
force the vrf 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 pol-
icy 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.

NOTES
Starting with Linux kernel version 3.6, there is no routing cache for IPv4 anymore. Hence
ip route show cached will never print any entries on systems with this or newer kernel
versions.

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.

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.