Bug#343140: libc6: resolver always checks search list in /etc/resolv.conf

[Edward Buck <ed@bashware.net>]

Hi Stephen,

Stephen Gran wrote:
> This one time, at band camp, Edward Buck said:
> 
>>If you do a 'strace telnet mx1.hotmail.com 25' on a woody machine,
>>you'll see that it works according to the documentation.  Sarge does
>>not.  I can forward you more strace output if it will help.  Maybe all
>>my woody machines are weird.  I don't know.  But as I said, this
>>functionality is new with sarge.
> 
> I took the easy way out, and turned on query logging in my local caching
> nameserver (since that also helps to see exactly what the load is on the
> nameserver, the point of this bug, really).  This is what I see:

Yes, you are correct in that this bug is all about DNS load (especially
extraneous load).

> Dec 16 00:07:27 hadrian named[17102]: client 127.0.0.1#41145: query: mx1.hotmail.com IN AAAA
> Dec 16 00:07:27 hadrian named[17102]: client 127.0.0.1#41145: query: mx1.hotmail.com.lobefin.net IN AAAA
> Dec 16 00:07:27 hadrian named[17102]: client 127.0.0.1#41145: query: mx1.hotmail.com IN A
> [ followed by normal PTR lookups for the records returned ]
> 
> Which is exactly what it should be.  This is a sarge system.

I just did the same test and got similar results.  With two domains in
the search path:

Dec 15 16:37:53 potato named[914]: client 192.168.0.40#43823: query:
mx1.hotmail.com IN AAAA
Dec 15 16:37:53 potato named[914]: client 192.168.0.40#43823: query:
mx1.hotmail.com.hn.aspextra.net IN AAAA
Dec 15 16:37:53 potato named[914]: client 192.168.0.40#43823: query:
mx1.hotmail.com.bashware.net IN AAAA
Dec 15 16:37:53 potato named[914]: client 192.168.0.40#43825: query:
mx1.hotmail.com IN A

I guess the problem then is in the ipv6 support and how it implements
domains in the search path.  Instead of doing ipv6, then ipv4 for
mx1.hotmail.com, it runs through all possible ipv6 queries, including
exhausting all domains in the search path, before ipv4 queries are
attempted.  That seems (is) really inefficient.  As a result of ipv6
supports, DNS queries have tripled on systems with two domains in their
search path.

Okay, perhaps this isn't a bug.  It's just ipv6 hell.

Thanks for your help.

Regards,
Ed


> 
> Maybe you didn't notice that the extra lookups were IPv6?  IIRC woody
> didn't have a working IPv6 stack out of the box, so this would explain
> the behavior you're seeing.