Running Linux, 5th Edition - Matthias Kalle Dalheimer [245]
Routing tables can contain entries for specific machines as well as for networks. In addition, each machine has a routing table entry for itself.
Let's examine the routing table for eggplant. Using the command netstat -rn, we see the following:
eggplant:$ netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
128.17.75.0 128.17.75.20 255.255.255.0 UN 1500 0 0 eth0
default 128.17.75.98 0.0.0.0 UGN 1500 0 0 eth0
127.0.0.1 127.0.0.1 255.0.0.0 UH 3584 0 0 lo
128.17.75.20 127.0.0.1 255.255.255.0 UH 3584 0 0 lo
The first column displays the destination networks (and hosts) that the routing table includes. The first entry is for the network 128.17.75 (note that the host address is 0 for network entries), which is the network that eggplant lives on. Any packets sent to this network should be routed through 128.17.75.20, which is the IP address of eggplant. In general, a machine's route to its own network is through itself.
The Flags column of the routing table gives information on the destination address for this entry; U specifies that the route is "up," N that the destination is a network, and so on. The MSS field shows how many bytes are transferred at a time over the respective connection, Window indicates how many frames may be sent ahead before a confirmation must be made, irtt gives statistics on the use of this route, and Iface lists the network device used for the route. On Linux systems, Ethernet interfaces are named eth0, eth1, and so on. lo is the loopback device, which we'll discuss shortly.
The second entry in the routing table is the default route, which applies to all packets destined for networks or hosts for which there is no entry in the table. In this case, the default route is through papaya, which can be considered the door to the outside world. Every machine on the 128.17.75 subnet must go through papaya to talk to machines on any other network.
The third entry in the table is for the address 127.0.0.1, which is the loopback address. This address is used when a machine wants to make a TCP/IP connection to itself. It uses the lo device as its interface, which prevents loopback connections from using the Ethernet (via the eth0 interface). In this way, network bandwidth is not wasted when a machine wishes to talk to itself.
The last entry in the routing table is for the IP address 128.17.75.20, which is the eggplant host's own address. As we can see, it uses 127.0.0.1 as its gateway. This way, any time eggplant makes a TCP/IP connection to itself, the loopback address is used as the gateway, and the lo network device is used.
Let's say that eggplant wants to send a packet to zucchini. The IP datagram contains a source address of 128.17.75.20 and a destination address of 128.17.75.37. IP determines that the network portion of the destination address is 128.17.75 and uses the routing table entry for 128.17.75.0 accordingly. The packet is sent directly to the network, which zucchini receives and is able to process.
What happens if eggplant wants to send packets to a machine not on the local network, such as pear? The destination address is 128.17.112.21. IP attempts to find a route for the 128.17.112 network in the routing tables, but none exists, so it selects the default route through papaya. papaya receives the packet and looks up the destination address in its own routing tables. The routing table for papaya might look like this:
Destination Gateway Genmask Flags MSS Window irtt Iface
128.17.75.0 128.17.75.98 255.255.255.0 UN 1500 0 0 eth0
128.17.112.0 128.17.112.3 255.255.255.0 UN 1500 0 0 eth1
default 128.17.112.40