Comment 8 for bug 403411

User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1b1) Gecko/20081007 Firefox/3.1b1
Build Identifier: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1b1) Gecko/20081007 Firefox/3.1b1

Firefox (3.0.1-4 and 3.1b1-4) all freeze on websites which have their DNS servers incorrectly setup which going through a proxy server.

Facebook does not have its DNS records correctly set for AAAA and therefore returns SERVFAIL but not instantly, but after 4-5 seconds. Firefox is trying to look up the domain's IPv6 address and hangs while it is waiting.

The bug is two-fold:

1) The JavaScript engine should not be looking up the domain in the first place since it is using a proxy and therefore that will look it up itself.

2) Firefox should not hang while doing DNS queries such that none of the other tabs are available and the window cannot be rendered.

This does not occur on the same network and version on Windows.

Utilising gdb I have found the issue seems to be in the library libxul when it calls pthread_wait_cond. But strace showed the lagging to occur during a DNS query.

We have managed to stop the browser freezing by disabling IPv6 lookups, but it should not be looking the IP up anyway.

Reproducible: Always

Steps to Reproduce:
1. Have IPv6 lookups turned on (network.dns.disableIPv6 = false)
2. Use a proxy server (preferably squid-based to resemble conditions)
3. Navigate to www.facebook.com (from uk)
4. Watch Firefox (3.0.1-3.0.4 and 3.1b1-4) freeze while it tries to get the non-existent AAAA records.
Actual Results:
The browser freezes and the window does not render. It also utilises 100% CPU.

Expected Results:
The browser should remain operable while the DNS query is executing.