10.2.5. Checking the host configuration with netstat

Apart from the ip command for displaying the network configuration, there's the common netstat command which has a lot of options and is generally useful on any UNIX system.

Routing information can be displayed with the -nr option to the netstat command:

bob:~> netstat -nr
Kernel IP routing table
Destination  Gateway      Genmask       Flags MSS Window irtt Iface
192.168.42.0 0.0.0.0      255.255.255.0 U      40 0         0 eth0
127.0.0.0    0.0.0.0      255.0.0.0     U      40 0         0 lo
0.0.0.0      192.168.42.1 0.0.0.0       UG     40 0         0 eth0

This is a typical client machine in an IP network. It only has one network device, eth0. The lo interface is the local loop.

	The modern way
	The novel way to get this info from your system is by using the ip command: ip route show

When this machine tries to contact a host that is on another network than its own, indicated by the line starting with 0.0.0.0, it will send the connection requests to the machine (router) with IP address 192.168.42.1, and it will use its primary interface, eth0, to do this.

Hosts that are on the same network, the line starting with 192.168.42.0, will also be contacted through the primary network interface, but no router is necessary, the data are just put on the network.

Machines can have much more complicated routing tables than this one, with lots of different "Destination-Gateway" pairs to connect to different networks. If you have the occasion to connect to an application server, for instance at work, it is most educating to check the routing information.