28.4. Responding from Multiple Interfaces
Linux has a rather unusual design: it considers an IP address as belonging to a host rather than an interface, even though administrators always assign IP addresses to particular interfaces.[*] This has impacts that some administrators complain about:
Imagine you have a host with two NICs on the same LAN, and that another host sends an ARP request for one of the addresses. The request is received by both interfaces, as shown in Figure 28-5, and both interfaces reply.
Figure 28-5. The ARP flux problem
The host sending the solicitation therefore receives two replies to its request. One comes from the NIC with the correct L2 address (eth0) but the other bears the other NIC's address (eth1). Which address is entered by the correspondent in its ARP cache depends on the order in which the requests happen to be received and the host's way of handing duplicate repliesin short, it's nondeterministic.
The ARP flux problem can be solved with the features described in the section "Tunable ARP Options."