Unix: Getting from here to there (routing basics)

August 03, 2013, 5:21 PM — What is routing? It's the set of rules that govern how you make connections to other systems. Any time you make a connection from one system to another system -- whether you're sending email, transferring a set of files or logging in with ssh -- you're routing. And, since most connections aren't direct (in other words, they're travelling through one or more system en route to the target), most of the time you're going to be crossing a router -- or maybe a long series of routers -- to get there.

To view the routing table on a Linux system, use the netstat -rn command. The output of this command will tell you how connections you initiate are going to be handled. The routing table on most Linux systems will look something like this:

$ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
192.168.0.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U         0 0          0 eth0
0.0.0.0         192.168.0.1     0.0.0.0         UG        0 0          0 eth0

The fields in this output are:

Destination -- where the connections are headed. This can be a specific network, one particular system or everything not covered by some other routing entry (i.e., the default).

Gateway -- where those connections first have to go before being sent to the ultimate destination. This can be a local router or a "0.0.0.0" (no router involved) kind of entry.

Genmask -- the network mask that determines what systems are covered by your destination.

Flags -- indicators that tell you more about each routing table entry (e.g., whether it's a gateway).

MSS -- maximum segment size

Window -- size of packet that can be transmitted

irtt -- initial round trip time

Iface -- the network interface that is involved

For several of these settings, a size of 0 means that the default value is being used.

Now, let's examine this output line by line.

Line 1

First, 192.168.0.0 is the local network. How do you know this? Well, with a gateway of 0.0.0.0, connections clearly aren't going through another system.