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Slow DNS lookups in Firefox due to IPv6 issues

Mon, 2012.01.09 - 02:11 — müzso
https://bugzilla.mozilla.org/show_bug.cgi?id=417689
I've found that in certain cases Firefox can "hang" (wait for something and not respond Smile ) while loading a site after a click. It turns out that another possible issue is with DNS lookups.

As described in the article of the network.dns.disableIPv6 setting:

One particular bug that has appeared exists not in Mozilla, but in IPv6-capable DNS servers: an IPv4 address may be returned when an IPv6 address is requested. It is possible for Mozilla to recover from this misinformation, but a significant delay is introduced.

Under certain versions of Mac OS X (those prior to 10.4), this bug is compounded by another bug wherein the OS still makes IPv6 DNS requests even if IPv6 support is disabled. A significant delay is introduced in all connections requiring DNS lookups while the OS and the DNS server exchange unnecessary (or redundant) queries and responses to resolve the address.

Two approaches were taken to work around these bugs: first, a preference to globally disable IPv6-based DNS lookups browser-wide (this preference), and a blacklist of domains which should never have IPv6-based DNS lookups performed.

So to avoid the problem in Mac OS X, you can disable IPv6 based DNS lookups altogether. It worked for me. Smile