ROUTE(8)
NAME
route - show / manipulate the IP routing table
SYNOPSIS
route [-vnee]
route [-v] add [-net|-host] Target [netmask Nm] [gw Gw]
[metric N] [mss M] [window W] [irtt I] [reject]
[mod] [dyn] [reinstate] [[dev] If]
route [-v] 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 is a flag for verbose (not used).
-n shows 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
parametres from the routing table.
-net the Target is the address of a network (found in
/etc/networks by the getnetbyname(2) library func-
tion).
-host is the address of a host (found with gethostby-
name(2) library function).
(none) displays the kernel routing table. The layout can
be changed with -e and -ee
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. This only makes sense for a network route,
and when the address Target actually makes sense
with the specified netmask. If no netmask is given,
route guesses it instead, so for most normal setups
you won't need to specify a netmask.
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 specifie
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
Modifier sets the metric field in the roting table,
used from daemons for dynamic routing.
mss M Modifier specifies the TCP Maximum Segment Size in
Bytes (MSS) for TCP Connections over this route.
This is normally used only for fine optimisation of
routing setups. The default is 536.
window W
Modifier specifies the TCP window size for TCP Con-
nections over this route. This is typically only
used on AX.25 networks and with drivers unable to
handle back to back frames.
irtt I Modifier specifies the initial round trip time
(irtt) for TCP Connections over this route. This is
typically only used on AX.25 networks. The number
is given in milliseconds (1-12000). If ommited the
RFC 1122 default of 300ms is used.
reject Modifier installs 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 is NOT for firewalling.
mod, dyn, reinstate
Modifier installs a dynamic or modified route. Both
Flags are generally only set by a routing daemon.
This is only for diagnostic purpose.
dev If Modifier forces the route to be associated with the
specified 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.
EXAMPLES
route 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 gate-
wayed through the former route to the SLIP inter-
face.
route add -net 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 host 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 targed (usually counted in
hops). It is not used by recent kernels, only rout-
ing daemons may use it.
Ref Number of references to this route. Not used in the
Linux kernel, always 0.
Use Count of lookups for the route. With recent kernels
this numbers are very low, since the sockets have
its own cache and dont need to lookup routes.
Iface Interface to which the IP Packages will be send.
MSS Default maximum segement size for TCP Connections
over this route.
Window Default windowsize for TCP Connections over this
route.
irtt Innitial RTT (Round Trip Time). The kernels use
this to guess about the best TCP protocol parame-
ters without waiting on (possible slow) answers.
FILES
/proc/net/route
/etc/networks
/etc/hosts
/etc/init.d/network
SEE ALSO
ifconfig(8) netstat(8) arp(8)
HISTORY
Route for Linux was originally written by Fred N. van
Kempen, lt;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.
BUGS
none :)