mirror_iproute2/man/man8/ip-route.8

.TH IP\-ROUTE 8 "20 Dec 2011" "iproute2" "Linux"
.SH "NAME"
ip route \- routing table management
.SH "SYNOPSIS"
.sp
.ad l
.in +8
.ti -8
.B ip
.RI "[ " OPTIONS " ]" 
.B route 
.RI " { " COMMAND " | "
.BR help " }"
.sp
.ti -8

.ti -8
.BR "ip route" " { "
.BR list " | " flush " } "
.I  SELECTOR

.ti -8
.BR "ip route save"
.I SELECTOR

.ti -8
.BR "ip route restore"

.ti -8
.B  ip route get
.IR ADDRESS " [ "
.BI from " ADDRESS " iif " STRING"
.RB " ] [ " oif
.IR STRING " ] [ "
.B  tos
.IR TOS " ]"

.ti -8
.BR "ip route" " { " add " | " del " | " change " | " append " | "\
replace " } "
.I  ROUTE

.ti -8
.IR SELECTOR " := "
.RB "[ " root
.IR PREFIX " ] [ "
.B  match
.IR PREFIX " ] [ "
.B  exact
.IR PREFIX " ] [ "
.B  table
.IR TABLE_ID " ] [ "
.B  proto
.IR RTPROTO " ] [ "
.B  type
.IR TYPE " ] [ "
.B  scope
.IR SCOPE " ]"

.ti -8
.IR ROUTE " := " NODE_SPEC " [ " INFO_SPEC " ]"

.ti -8
.IR NODE_SPEC " := [ " TYPE " ] " PREFIX " ["
.B  tos
.IR TOS " ] [ "
.B  table
.IR TABLE_ID " ] [ "
.B  proto
.IR RTPROTO " ] [ "
.B  scope
.IR SCOPE " ] [ "
.B  metric
.IR METRIC " ]"

.ti -8
.IR INFO_SPEC " := " "NH OPTIONS FLAGS" " ["
.B  nexthop
.IR NH " ] ..."

.ti -8
.IR NH " := [ "
.B  via
.IR ADDRESS " ] [ "
.B  dev
.IR STRING " ] [ "
.B  weight
.IR NUMBER " ] " NHFLAGS

.ti -8
.IR OPTIONS " := " FLAGS " [ "
.B  mtu
.IR NUMBER " ] [ "
.B  advmss
.IR NUMBER " ] [ "
.B  rtt
.IR TIME " ] [ "
.B  rttvar
.IR TIME " ] [ "
.B  window
.IR NUMBER " ] [ "
.B  cwnd
.IR NUMBER " ] [ "
.B  ssthresh
.IR REALM " ] [ "
.B  realms
.IR REALM " ] [ "
.B  rto_min
.IR TIME " ] [ "
.B  initcwnd
.IR NUMBER " ] [ "
.B  initrwnd
.IR NUMBER " ]"

.ti -8
.IR TYPE " := [ "
.BR unicast " | " local " | " broadcast " | " multicast " | "\
throw " | " unreachable " | " prohibit " | " blackhole " | " nat " ]"

.ti -8
.IR TABLE_ID " := [ "
.BR local "| " main " | " default " | " all " |"
.IR NUMBER " ]"

.ti -8
.IR SCOPE " := [ "
.BR host " | " link " | " global " |"
.IR NUMBER " ]"

.ti -8
.IR NHFLAGS " := [ "
.BR onlink " | " pervasive " ]"

.ti -8
.IR RTPROTO " := [ "
.BR kernel " | " boot " | " static " |"
.IR NUMBER " ]"


.SH DESCRIPTION
.B ip route
is used to manipulate entries in the kernel routing tables.
.sp
.B Route types:

.in +8
.B unicast
- the route entry describes real paths to the destinations covered
by the route prefix.

.sp
.B unreachable
- these destinations are unreachable.  Packets are discarded and the
ICMP message
.I host unreachable
is generated.
The local senders get an
.I EHOSTUNREACH
error.

.sp
.B blackhole
- these destinations are unreachable.  Packets are discarded silently.
The local senders get an
.I EINVAL
error.

.sp
.B prohibit
- these destinations are unreachable.  Packets are discarded and the
ICMP message
.I communication administratively prohibited
is generated.  The local senders get an
.I EACCES
error.

.sp
.B local
- the destinations are assigned to this host.  The packets are looped
back and delivered locally.

.sp
.B broadcast
- the destinations are broadcast addresses.  The packets are sent as
link broadcasts.

.sp
.B 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
.I net unreachable
is generated.  The local senders get an
.I ENETUNREACH
error.

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


.BR "via" .
.sp
.B anycast
.RI "- " "not implemented"
the destinations are
.I anycast
addresses assigned to this host.  They are mainly equivalent
to
.B local
with one difference: such addresses are invalid when used
as the source address of any packet.

.sp
.B multicast
- a special type used for multicast routing.  It is not present in
normal routing tables.
.in -8

.P
.B 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
.B /etc/iproute2/rt_tables
By default all normal routes are inserted into the
.B 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.

.sp
Actually, one other table always exists, which is invisible but
even more important.  It is the
.B 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
.I policy routing
is used.

.SS ip route add - add new route
.SS ip route change - change route
.SS ip route replace - change or add new one

.TP
.BI to " TYPE PREFIX " (default)
the destination prefix of the route.  If
.I TYPE
is omitted,
.B ip
assumes type
.BR "unicast" .
Other values of
.I TYPE
are listed above.
.I PREFIX
is an IP or IPv6 address optionally followed by a slash and the
prefix length.  If the length of the prefix is missing,
.B ip
assumes a full-length host route.  There is also a special
.I PREFIX
.B default
- which is equivalent to IP
.B 0/0
or to IPv6
.BR "::/0" .

.TP
.BI tos " TOS"
.TP
.BI 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.
.I TOS
is either an 8 bit hexadecimal number or an identifier
from
.BR "/etc/iproute2/rt_dsfield" .

.TP
.BI metric " NUMBER"
.TP
.BI preference " NUMBER"
the preference value of the route.
.I NUMBER
is an arbitrary 32bit number.

.TP
.BI table " TABLEID"
the table to add this route to.
.I TABLEID
may be a number or a string from the file
.BR "/etc/iproute2/rt_tables" .
If this parameter is omitted,
.B ip
assumes the
.B main
table, with the exception of
.BR local " , " broadcast " and " nat
routes, which are put into the
.B local
table by default.

.TP
.BI dev " NAME"
the output device name.

.TP
.BI via " ADDRESS"
the address of the nexthop router.  Actually, the sense of this field
depends on the route type.  For normal
.B 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.

.TP
.BI src " ADDRESS"
the source address to prefer when sending to the destinations
covered by the route prefix.

.TP
.BI realm " REALMID"
the realm to which this route is assigned.
.I REALMID
may be a number or a string from the file
.BR "/etc/iproute2/rt_realms" .

.TP
.BI mtu " MTU"
.TP
.BI "mtu lock" " MTU"
the MTU along the path to the destination.  If the modifier
.B lock
is not used, the MTU may be updated by the kernel due to
Path MTU Discovery.  If the modifier
.B 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.

.TP
.BI 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.

.TP
.BI 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.


.TP
.BI rttvar " TIME " "(2.3.15+ only)"
the initial RTT variance estimate. Values are specified as with
.BI rtt
above.

.TP
.BI rto_min " TIME " "(2.6.23+ only)"
the minimum TCP Retransmission TimeOut to use when communicating with this
destination.  Values are specified as with
.BI rtt
above.

.TP
.BI ssthresh " NUMBER " "(2.3.15+ only)"
an estimate for the initial slow start threshold.

.TP
.BI cwnd " NUMBER " "(2.3.15+ only)"
the clamp for congestion window.  It is ignored if the
.B lock
flag is not used.

.TP
.BI 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.

.TP
.BI 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.

.TP
.BI 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.)

.TP
.BI 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
.B sysctl
variable
.BR "net/ipv4/tcp_reordering" .

.TP
.BI nexthop " NEXTHOP"
the nexthop of a multipath route.
.I NEXTHOP
is a complex value with its own syntax similar to the top level
argument lists:

.in +8
.BI via " ADDRESS"
- is the nexthop router.
.sp

.BI dev " NAME"
- is the output device.
.sp

.BI weight " NUMBER"
- is a weight for this element of a multipath
route reflecting its relative bandwidth or quality.
.in -8

.TP
.BI scope " SCOPE_VAL"
the scope of the destinations covered by the route prefix.
.I SCOPE_VAL
may be a number or a string from the file
.BR "/etc/iproute2/rt_scopes" .
If this parameter is omitted,
.B ip
assumes scope
.B global
for all gatewayed
.B unicast
routes, scope
.B link
for direct
.BR unicast " and " broadcast
routes and scope
.BR host " for " local
routes.

.TP
.BI protocol " RTPROTO"
the routing protocol identifier of this route.
.I RTPROTO
may be a number or a string from the file
.BR "/etc/iproute2/rt_protos" .
If the routing protocol ID is not given,
.B ip assumes protocol
.B 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:

.in +8
.B redirect
- the route was installed due to an ICMP redirect.
.sp

.B kernel
- the route was installed by the kernel during autoconfiguration.
.sp

.B boot
- the route was installed during the bootup sequence.
If a routing daemon starts, it will purge all of them.
.sp

.B 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.
.sp

.B ra
- the route was installed by Router Discovery protocol.
.in -8

.sp
The rest of the values are not reserved and the administrator is free
to assign (or not to assign) protocol tags.

.TP
.B onlink
pretend that the nexthop is directly attached to this link,
even if it does not match any interface prefix.

.SS ip route delete - delete route

.B ip route del
has the same arguments as
.BR "ip route add" ,
but their semantics are a bit different.

Key values
.RB "(" to ", " tos ", " preference " and " table ")"
select the route to delete.  If optional attributes are present,
.B ip
verifies that they coincide with the attributes of the route to delete.
If no route with the given key and attributes was found,
.B ip route del
fails.

.SS ip route show - list routes
the command displays the contents of the routing tables or the route(s)
selected by some criteria.

.TP
.BI to " SELECTOR " (default)
only select routes from the given range of destinations.
.I SELECTOR
consists of an optional modifier
.RB "(" root ", " match " or " exact ")"
and a prefix.
.BI root " PREFIX"
selects routes with prefixes not shorter than
.IR PREFIX "."
F.e.
.BI root " 0/0"
selects the entire routing table.
.BI match " PREFIX"
selects routes with prefixes not longer than
.IR PREFIX "."
F.e.
.BI match " 10.0/16"
selects
.IR 10.0/16 ","
.IR 10/8 " and " 0/0 ,
but it does not select
.IR 10.1/16 " and " 10.0.0/24 .
And
.BI exact " PREFIX"
(or just
.IR PREFIX ")"
selects routes with this exact prefix. If neither of these options
are present,
.B ip
assumes
.BI root " 0/0"
i.e. it lists the entire table.

.TP
.BI tos " TOS"
.BI dsfield " TOS"
only select routes with the given TOS.

.TP
.BI table " TABLEID"
show the routes from this table(s).  The default setting is to show
.BR table main "."
.I TABLEID
may either be the ID of a real table or one of the special values:
.sp
.in +8
.B all
- list all of the tables.
.sp
.B cache
- dump the routing cache.
.in -8

.TP
.B cloned
.TP
.B 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
.BR "table cache" "."

.TP
.BI from " SELECTOR"
the same syntax as for
.BR to ","
but it binds the source address range rather than destinations.
Note that the
.B from
option only works with cloned routes.

.TP
.BI protocol " RTPROTO"
only list routes of this protocol.

.TP
.BI scope " SCOPE_VAL"
only list routes with this scope.

.TP
.BI type " TYPE"
only list routes of this type.

.TP
.BI dev " NAME"
only list routes going via this device.

.TP
.BI via " PREFIX"
only list routes going via the nexthop routers selected by
.IR PREFIX "."

.TP
.BI src " PREFIX"
only list routes with preferred source addresses selected
by
.IR PREFIX "."

.TP
.BI realm " REALMID"
.TP
.BI realms " FROMREALM/TOREALM"
only list routes with these realms.

.SS ip route flush - flush routing tables
this command flushes routes selected by some criteria.

.sp
The arguments have the same syntax and semantics as the arguments of
.BR "ip route show" ,
but routing tables are not listed but purged.  The only difference is
the default action:
.B show
dumps all the IP main routing table but
.B flush
prints the helper page.

.sp
With the
.B -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,
.B ip route flush
also dumps all the deleted routes in the format described in the
previous subsection.

.SS 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.

.TP
.BI to " ADDRESS " (default)
the destination address.

.TP
.BI from " ADDRESS"
the source address.

.TP
.BI tos " TOS"
.TP
.BI dsfield " TOS"
the Type Of Service.

.TP
.BI iif " NAME"
the device from which this packet is expected to arrive.

.TP
.BI oif " NAME"
force the output device on which this packet will be routed.

.TP
.B connected
if no source address
.RB "(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.

.P
Note that this operation is not equivalent to
.BR "ip route show" .
.B show
shows existing routes.
.B get
resolves them and creates new clones if necessary.  Essentially,
.B get
is equivalent to sending a packet along this path.
If the
.B 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
.BR "ip route ls cache" ,
however, no packets are actually sent.  With the
.B iif
argument, the kernel pretends that a packet arrived from this interface
and searches for a path to forward the packet.

.SS ip route save - save routing table information to stdout
this command behaves like
.BR "ip route show"
except that the output is raw data suitable for passing to
.BR "ip route restore" .

.SS ip route restore - restore routing table information from stdin
this command expects to read a data stream as returned from
.BR "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.

.SH EXAMPLES
.PP
ip ro
.RS 4
Show all route entries in the kernel.
.RE
.PP
ip route add default via 192.168.1.1 dev eth0
.RS 4
Adds a default route (for all addresses) via the local gateway 192.168.1.1 that can
be reached on device eth0.
.RE

.SH SEE ALSO
.br
.BR ip (8)

.SH AUTHOR
Original Manpage by Michail Litvak <mci@owl.openwall.com>