Source address selection

When a host creates an IP packet, it must select some source address. Correct source address selection is a critical procedure, because it gives the receiver the information needed to deliver a reply. If the source is selected incorrectly, in the best case, the backward path may appear different to the forward one which is harmful for performance. In the worst case, when the addresses are administratively scoped, the reply may be lost entirely.

Linux-2.2 selects source addresses using the following algorithm:

• The application may select a source address explicitly with bind(2) syscall or supplying it to sendmsg(2) via the ancillary data object IP_PKTINFO. In this case the kernel only checks the validity of the address and never tries to ``improve'' an incorrect user choice, generating an error instead. 1cm NB. Never say ``Never''. The sysctl option ip_dynaddr breaks this axiom. It has been made deliberately with the purpose of automatically reselecting the address on hosts with dynamic dial-out interfaces. However, this hack must not be used on multihomed hosts and especially on routers: it would break them.

• Otherwise, IP routing tables can contain an explicit source address hint for this destination. The hint is set with the src parameter to the ip route command, sec.7, p..

• Otherwise, the kernel searches through the list of addresses attached to the interface through which the packets will be routed. The search strategies are different for IP and IPv6. Namely:
  • IPv6 searches for the first valid, not deprecated address with the same scope as the destination.

  • IP searches for the first valid address with a scope wider than the scope of the destination but it prefers addresses which fall to the same subnet as the nexthop of the route to the destination. Unlike IPv6, the scopes of IPv4 destinations are not encoded in their addresses but are supplied in routing tables instead (the scope parameter to the ip route command, sec.7, p.).

• Otherwise, if the scope of the destination is link or host, the algorithm fails and returns a zero source address.

• Otherwise, all interfaces are scanned to search for an address with an appropriate scope. The loopback device lo is always the first in the search list, so that if an address with global scope (not 127.0.0.1!) is configured on loopback, it is always preferred.