Routing

We have managed to communicate between two machines connected on our network, but how did helium know where to send and receive information about hydrogen? The answer lies in a mechanism named “routing.”

The kernel maintains a structure known as a “routing table.” This contains information about where packets need to be sent, or routed, to get to their destination. We can view the routing table with netstat:

hydrogen# netstat -rn
Routing Table:
  Destination        Gateway       Flags  Ref   Use   Interface
----------------- ---------------- ----- ----- ------ ---------
192.168.22.0      192.168.22.1     U        3      5  hme0
127.0.0.1         127.0.0.1        UH       0      2  lo0
hydrogen#

This shows us that any packets destined for the 192.168.22.0 network will be routed through a gateway with an IP address of 192.168.22.1 (i.e., the network interface that we defined earlier). It also shows that the loopback interface is routed, again through itself.

This is a fairly simple implementation of a routing table, but we will see how it can be expanded later.