Unix: Getting from here to there (routing basics)

- reject route

Line 2

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

The 169.254.0.0 entry requires some explanation. This is a link-local address -- a special address defined in RFC 5735 for link-local addressing. Its appearance in your netstat output doesn't mean it's being used. It just shows up unless you take steps to remove it. A link-local address is an Internet Protocol address that is intended only for communications within the segment of a local network (a link) or a point-to-point connection that a host is connected to. Routers do not forward packets with link-local addresses.

You can add NOZEROCONF=yes at the end of your /etc/sysconfig/network file to remove this additional route, though it does no harm being there.

$ cat /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=vader.aacc.edu

Line 3

Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
...
0.0.0.0         192.168.0.1     0.0.0.0         UG        0 0          0 eth0

0.0.0.0 is your default route. This is where connections are routed whenever those connections aren't headed for the local network segment or other specific routes. If you use the command netstat -r (without the -n option) , the word "default" will appear in place of 0.0.0.0. The -n option suppresses translation of addresses to symbolic names.

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

This also shows the name of the gateway -- appearently a Cisco PIX router.

Think of the default route as "everywhere else". In this case, we can see that to connect to systems anywhere other than the local network, we have to go through 192.168.0.1. Most network admins will use the .1 address of each LAN for its router -- a very is a sensible convention.

So, if your connection is headed anywhere else, you need to go through the gateway listed in the second column -- generally your default router.

The flags for the default route line clearly include G, confirming that this is a router or "gateway".

Using traceroute

If you want to see the specific route that a connection might take and get an idea how well that route performs, then traceroute is the command to use. This command will display each hop that a connection might take and will show you how long each hop takes.