As stated in the 5.6.6 release notes, the default value of –bind-address has changed from IPv4-specific “0.0.0.0” to “*” – the latter value which allows MySQL to bind to IPv6 interfaces by default.  There are a few implications to this change.

First, make sure you’ve hardened both your IPv4 and your IPv6 access points to your MySQL server.  If you are running IPv4 interfaces only, you can also change the –bind-address value back to the previous default value of “0.0.0.0”.  Because MySQL now listens by default on both IPv4 and IPv6 interfaces, an installation that has only hardened IPv4 interfaces may find unaddressed vulnerabilities exposed via the IPv6 interface that is now used by default.

Second, the change in defaults can have an impact on startup speed for deployments where there is no IPv6 support.  On my Windows XP laptop, startup stalls for 45 seconds while MySQL lets Windows determine that IPv6 is not supported:

120813 10:17:16 [Note] Server hostname (bind-address): ‘*’; port: 3307
120813 10:18:01 [Note] IPv6 is not available.
120813 10:18:01 [Note]   – ‘0.0.0.0’ resolves to ‘0.0.0.0’;
120813 10:18:01 [Note] Server socket created on IP: ‘0.0.0.0’.

Why Windows takes 45 seconds to figure this out is anybody’s guess, but explicitly setting –bind-address to 0.0.0.0 eliminates this behavior, and server start-up speed is restored:

120813 10:27:39 [Note] Server hostname (bind-address): ‘0.0.0.0’; port: 3307
120813 10:27:39 [Note]   – ‘0.0.0.0’ resolves to ‘0.0.0.0’;
120813 10:27:39 [Note] Server socket created on IP: ‘0.0.0.0’.

Another option is to make sure your OS has IPv6 support enabled (Microsoft documents this procedure for XP in KB article 2478747).

Whether you have IPv6 support enabled or not, make sure you consider how the change in behavior introduced in 5.6.6 will affect you.