Setting the Default Route

Adding a default route with route

Ex:

[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.99.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
[root@morgan]# route add default gw 192.168.99.254
[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.99.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.99.254 0.0.0.0 UG 0 0 0 eth0

These changes to the routing table on morgan will stay in effect until they are manually changed, the network is restarted, or the machine reboots. With knowledge of the addressing scheme of a network, and the use of ifconfig and route it's simple to readdress a machine on just about any Ethernet you can attach to. The benefits of familiarity with these commands extend to non-Ethernet IP networks as well, because these commands operate on the IP layer, independent of the link layer.


Adding and removing a static route

A static route is any route entered into a routing table which specifies at least a destination address and a gateway or device. Static routes are special instructions regarding the path a packet should take to reach a destination and are usually used to specify reachability of a destination through a router other than the default gateway.

the address of the destination (192.168.98.0)

the netmask of the destination (255.255.255.0)

EITHER the IP address of the router through which the destination (192.168.99.1) is reachable

OR the name of the link layer device to which the destination is directly connected

Adding a static route with route

[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.99.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.99.254 0.0.0.0 UG 0 0 0 eth0
[root@morgan]# route add -net 192.168.98.0 netmask 255.255.255.0 gw 192.168.99.1
[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.99.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.98.0 192.168.99.1 255.255.255.0 UG 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.99.254 0.0.0.0 UG 0 0 0 eth0


“Adding a static route with route” shows how to add a static route to the 192.168.98.0/24 network. In order to test the reachability of the remote network, ping any machine on the 192.168.98.0/24 network. Routers are usually a good choice, since they rarely have packet filters and are usually alive

Because a more specific route is always chosen over a less specific route, it is even possible to support host routes. These are routes for destinations which are single IP addresses. This can be accomplished with a manually added static route as below

Removing a static network route and adding a static host route

[root@morgan]# route del -net 192.168.98.0 netmask 255.255.255.0 gw 192.168.99.1
[root@morgan]# route add -net 192.168.98.42 netmask 255.255.255.255 gw 192.168.99.1
[root@morgan]# route add -host 192.168.98.42 gw 192.168.99.1
SIOCADDRT: File exists
[root@morgan]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.99.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.98.42 192.168.99.1 255.255.255.255 UGH 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.99.254 0.0.0.0 UG 0 0 0 eth0