Displaying the Routing Table

When invoking netstat with the -r flag, it displays the kernel routing table in the way we've been doing this with route above. On vstout, it produces:

           # netstat -nr
           Kernel routing table
           Destination     Gateway         Genmask         Flags Metric Ref Use
           127.0.0.1       *               255.255.255.255 UH    1      0
           191.72.1.0      *               255.255.255.0   U     1      0
           191.72.2.0      191.72.1.1      255.255.255.0   UGN   1      0
 

The -n option makes netstat print addresses as dotted quad IP-numbers rather than the symbolic host and network names. This is especially useful when you want to avoid address lookups over the network (e.g. to a DNS or NIS server).

The second column of netstat's output shows the gateway the routing entry points to. If no gateway is used, an asterisk is printed instead. Column three shows the ``generality'' of the route. When given an IP-address to find a suitable route for, the kernel goes through all routing table entries, taking the bitwise AND of the address and the genmask before comparing it to the target of the route.

The fourth column displays various flags that describe the route:

G

The route uses a gateway.

U

The interface to be used is up.

H

Only a single host can be reached through the route. For example, this is the case for the loopback entry 127.0.0.1.

D

This is set if the table entry has been generated by an ICMP redirect message (see section 3.5).

M

This is set if the table entry was modified by an ICMP redi- rect message.

The Ref column of netstat's output shows the number of references to this route, that is, how many other routes (e.g. through gateways) rely on the presence of this route. The last two columns show the number of times the routing entry has been used, and the interface that datagrams are passed to for delivery.