Linux route command description and examples

Besides Linux ifconfig command, the Linux route command is another important network commands every Linux user should know. The route command can be used to add or modify a static route and a default gateway in the Linux or Unix system. As specify in the Linux Programmer's Manual, Linux route command shows or manipulate the IP routing table. The IP stated is the Internet Protocol (ip address, netmask, gateway) which has been set during configuring network interface card (NIC) in the Linux ifconfig tutorial earlier.

The Linux route command comes with many options but most network administrators familiar with add and del options. The route add command is used to add a new route while the route del command is to delete a route.

The route add syntax:
route add -net < ip address > netmask < netmask ip > dev < interface >

< ip address > is network interface card ip address
< netmask ip > ip netmask such
Interface = eth0, eth1, eth2, etc

The route del syntax:
route del -net < ip address > netmask < netmask ip > dev <interface >

<ip address > is network interface card ip address
<netmask ip > ip netmask such
Interface = eth0, eth1, eth2, etc

Here is an example of Linux route command in action: Linux basic network configurations.

Below is a complete manual when issuing the man route command in the command line terminal. Do remember that in order to use route command you must have root privilege.

ROUTE(8) Linux Programmer's Manual ROUTE(8)

route - show / manipulate the IP routing table

route [-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]

Route manipulates the kernel's IP routing tables. Its primary 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.

When the add or del options are used, route modifies the routing
tables. Without these options, route displays the current contents of
the routing tables.

-A family
use the specified address family (eg `inet'; use `route --help'
for a full list).

operate on the kernel's FIB (Forwarding Information Base)
routing table. This is the default.

-C operate on the kernel's routing cache.

-v select verbose operation.

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.

del delete a route.

add add a new route.

the destination network or host. You can provide IP addresses
in dotted decimal or host/network names.

-net the target is a network.

-host the target is a host.

netmask NM
when adding a network route, the netmask to be used.

gw GW route packets via a 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
address 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 compatibility hack.

metric M
set the metric field in the routing table (used by routing dae-
mons) to M.

mss M
set the TCP Maximum Segment Size (MSS) for connections over
this route to M bytes. The default is the device MTU minus
headers, or a lower MTU when path mtu discovery occured.
This setting can be used to force smaller TCP packets on the
other end when path mtu discovery does not work (usually
because of misconfigured firewalls that block ICMP
Fragmentation Needed)

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 connections 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 is NOT for firewalling.

mod, dyn, reinstate
install a dynamic or modified route. These flags are for diag-
nostic purposes, and are generally only set by routing

dev If force 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.

route add -net
adds the normal loopback entry, using netmask (class
A net, determined from the destination address) and associated
with the "lo" device (assuming this device was prviously set up
correctly with ifconfig(8)).

route add -net netmask dev eth0
adds a route to the network 192.56.76.x via "eth0". The Class C
netmask modifier is not really necessary 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 (assum-
ing that "ipx4" is the SLIP host).

route add -net netmask gw ipx4
This command adds the net "192.57.66.x" to be gatewayed
through the former route to the SLIP interface.

route add -net netmask dev eth0
This is an obscure one documented so people know how to do
it. This sets all of the class D (multicast) IP routes to go via
"eth0". This is the correct normal configuration line with a
multicasting kernel.

route add -net netmask reject
This installs a rejecting route for the private network

The output of the kernel routing table is organized in the following

The destination network or destination host.

The gateway address or '*' if none set.

The netmask for the destination net; '' for a
host destination and '' for the default route.

Flags Possible flags include
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 redirect)
A (installed by addrconf)
C (cache entry)
! (reject route)

The 'distance' to the target (usually counted in hops). It is
not used by recent kernels, but may be needed by routing

Ref Number of references to this route. (Not used in the Linux ker-

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 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 (pos-
sibly 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.


ifconfig(8), netstat(8), arp(8), rarp(8)

Route for Linux was originally written by Fred N. van Kempen,
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
Bern Eckenfels.

Currently maintained by Phil Blundell .

net-tools 2 January 2000 ROUTE(8)

Back to Linux network command tutorial page.

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