Understanding Linux Network Internals

Examples

Let's take the topology of Figure 28-21 as an example.

Figure 28-21. Example of network with proxy ARP configured on host RT

Let's make the following hypotheses:

• All the hosts use Ethernet cards.

• All the hosts of LAN1 and LAN2 are configured with a netmask of 255.255.255.0 (/24). They do not have any routes in their routing tables, nor do they have a default gateway configured. In other words, hosts in LAN1 and LAN2 can communicate only with other hosts within their same logical subnet.

• All neighbor caches are empty, which means that no one host knows any link layer address of any other host.

• Bridging is disabled everywhere. This excludes the top-right case of Figure 26-10 in Chapter 26. If the implications of this hypothesis are not clear, you should read Part IV.

Note that even if both hosts on LAN1 and LAN2 have been configured as belonging to the same logical subnet (network 10.0.0.0/24, netmask 255.255.255.0), they actually belong to different LANs. This means that as far as the configuration is concerned, they share the same subnet and can communicate with each other without the help of any router. However, by looking at the network topology, it is clear that they cannot do that without the help of RT.

For RT to give the hosts of LAN1 and LAN2 the illusion that they are on the same subnet, RT needs to have some extra knowledge: the real netmask of the LANs it wants to merge, ...