What was the motivation for adding the IPV6_V6ONLY flag?

Question

In IPv6 networking, the IPV6_V6ONLY flag is used to ensure that a socket will only use IPv6, and in particular that IPv4-to-IPv6 mapping won\'t be used for that socket. On many

Answer 1

The reason most often mentioned is for the case where the server has some form of ACL (Access Control List). For instance, imagine a server with rules like:

Allow 192.0.2.4
Deny all

It runs on IPv4. Now, someone runs it on a machine with IPv6 and, depending on some parameters, IPv4 requests are accepted on the IPv6 socket, mapped as ::192.0.2.4 and then no longer matched by the first ACL. Suddenly, access would be denied.

Being explicit in your application (using IPV6_V6ONLY) would solve the problem, whatever default the operating system has.