ROUTE(8) Linux Programmer’s Manual ROUTE(8)NAMEroute ‐ show / manipulate the IP routing table
SYNOPSISroute [‐CFvnee]
route [‐v] [‐A family] add [‐net|‐host] target [netmask
Nm] [gw Gw] [metric N] [mss M] [window W] [irtt I]
[reject] [mod] [dyn] [reinstate] [[dev] If]
route [‐v] [‐A family] del [‐net|‐host] target [gw Gw]
[netmask Nm] [metric N] [[dev] If]
route [‐V] [‐‐version] [‐h] [‐‐help]
DESCRIPTION
Route manipulates the kernel’s IP routing table. Its pri‐
mary use is to set up static routes to specific hosts or
networks via an interface after it has been configured
with the ifconfig(8) program.
OPTIONS
‐v select verbose operation.
‐A family
Use the specified address family (eg ‘inet’,
‘inet6’).
‐n show numerical addresses instead of trying to
determine symbolic host names. This is useful if
you are trying to determine why the route to your
nameserver has vanished.
‐e use netstat(8)‐format for displaying the routing
table. ‐ee will generate a very long line with all
parameters from the routing table.
‐net the target is a network.
‐host the target is a host.
‐F displays the kernel FIB routing table. The layout
can be changed with ‐e and ‐ee
‐C displays the kernel’s route cache.
net‐tools 8 August 1997 1
ROUTE(8) Linux Programmer’s Manual ROUTE(8)
del deletes a route.
add adds a route.
target The destination network or host. You can provide IP
addresses in dotted decimal or host/network names.
netmask Nm
modifier specifies the netmask of the route to be
added.
gw Gw Any IP packets for the target network/host will be
routed through the specified gateway. NOTE: The
specified gateway must be reachable first. This
usually means that you have to set up a static
route to the gateway beforehand. If you specify the
adress of one of your local interfaces, it will be
used to decide about the interface to which the
packets should be routed to. This is a BSDism com‐
patibility hack.
metric M
Set the metric field in the routing table (used by
routing daemons) to M.
mss M Set the TCP Maximum Segment Size (MSS) for connec‐
tions over this route to M bytes. This is normally
used only for fine optimisation of routing setups.
The default is 536.
window W
Set the TCP window size for connections over this
route to W bytes. This is typically only used on
AX.25 networks and with drivers unable to handle
back to back frames.
irtt I Set the initial round trip time (irtt) for TCP con‐
nections over this route to I milliseconds
(1‐12000). This is typically only used on AX.25
networks. If omitted the RFC 1122 default of 300ms
is used.
reject Install a blocking route, which will force a route
lookup to fail. This is for example used to mask
out networks before using the default route. This
net‐tools 8 August 1997 2
ROUTE(8) Linux Programmer’s Manual ROUTE(8)
is NOT for firewalling.
mod, dyn, reinstate
Install a dynamic or modified route. Both flags are
generally only set by a routing daemon. This is
only for diagnostic purpose.
dev If Forces the route to be associated with the speci‐
fied device, as the kernel will otherwise try to
determine the device on its own (by checking
already existing routes and device specifications,
and where the route is added to). In most normal
networks you won’t need this.
If dev If is the last option on the command line,
the word dev may be omitted, as it’s the default.
Otherwise the order of the route modifiers (metric
‐ netmask ‐ gw ‐ dev) doesn’t matter.
EXAMPLESroute add ‐net 127.0.0.0
adds the normal loopback entry, using netmask
255.0.0.0 (class A net, determined from the desti‐
nation address) and associated with the "lo" device
(assuming this device was prviously set up cor‐
rectly with ifconfig(8)).
route add ‐net 192.56.76.0 netmask 255.255.255.0 dev eth0
adds a route to the network 192.56.76.x via "eth0".
The Class C netmask modifier is not really neces‐
sary here because 192.* is a Class C IP address.
The word "dev" can be omitted here.
route add default gw mango‐gw
adds a default route (which will be used if no
other route matches). All packets using this route
will be gatewayed through "mango‐gw". The device
which will actually be used for that route depends
on how we can reach "mango‐gw" ‐ the static route
to "mango‐gw" will have to be set up before.
route add ipx4 sl0
Adds the route to the "ipx4" host via the SLIP
interface (assuming that "ipx4" is the SLIP host).
route add ‐net 192.57.66.0 netmask 255.255.255.0 gw ipx4
This command adds the net "192.57.66.x" to be
net‐tools 8 August 1997 3
ROUTE(8) Linux Programmer’s Manual ROUTE(8)
gatewayed through the former route to the SLIP
interface.
route add 224.0.0.0 netmask 240.0.0.0 dev eth0
This is an obscure one documented so people know
how to do it. This sets all of the class D (multi‐
cast) IP routes to go via "eth0". This is the cor‐
rect normal configuration line with a multicasting
kernel.
route add 10.0.0.0 netmask 255.0.0.0 reject
This installs a rejecting route for the private
network "10.x.x.x."
OUTPUT
The output of the kernel routing table is organized in the
following columns
Destination
The destination network or destination host.
Gateway
The gateway address or ’*’ if none set.
Genmask
The netmask for the destination net;
’255.255.255.255’ for a host destination and
’0.0.0.0’ for the default route.
Flags Possible flags are
U (route is up)
H (target is a host)
G (use gateway)
R (reinstate route for dynamic routing)
D (dynamically installed by daemon or redirect)
M (modified from routing daemon or rederict)
! (reject route)
Metric The ’distance’ to the target (usually counted in
hops). It is not used by recent kernels, but may be
needed by routing daemons.
Ref Number of references to this route. (Not used in
the Linux kernel.)
Use Count of lookups for the route. Depending on the
use of ‐F and ‐C this will be either route cache
misses (‐F) or hits (‐C).
Iface Interface to which packets for this route will be
net‐tools 8 August 1997 4
ROUTE(8) Linux Programmer’s Manual ROUTE(8)
sent.
MSS Default maximum segement size for TCP connections
over this route.
Window Default window size for TCP connections over this
route.
irtt Initial RTT (Round Trip Time). The kernel uses this
to guess about the best TCP protocol parameters
without waiting on (possible slow) answers.
HH (cached only)
The number of ARP entries and cached routes that
refer to the hardware header cache for the cached
route. This will be ‐1 if a hardware address is not
needed for the interface of the cached route (e.g.
lo).
Arp (cached only)
Whether or not the hardware address for the cached
route is up to date.
FILES
/proc/net/ipv6_route
/proc/net/route
/proc/net/rt_cache
SEE ALSOifconfig(8), netstat(8), arp(8), rarp(8)HISTORY
Route for Linux was originally written by Fred N. van
Kempen, <waltje@uwalt.nl.mugnet.org> and then modified by
Johannes Stille and Linus Torvalds for pl15. Alan Cox
added the mss and window options for Linux 1.1.22. irtt
support and merged with netstat from Bernd Eckenfels.
AUTHOR
Currently maintained by Phil Blundell <Philip.Blun‐
dell@pobox.com>.
net‐tools 8 August 1997 5
