[sf-lug] multiple dns lookups for one page?

Nathan Hoover nathan at foo-o-rama.com
Thu Apr 15 12:12:25 PDT 2010

I think I have something here, although I am still not sure why Firefox is
taking so long to do what should be a local (in-process to FF) cache lookup
for DNS.

I compared the HTTP headers from my site that's behaving more sensibly to
the site you mentioned, and I noticed in all the subsequent requests that
are doing an additional DNS lookup, the "Connection:" HTTP header is being
set to "close" rather than "keep-alive" - thus initiating a new TCP
connection, an expensive prospect DNS lookup or no. (In fact, as I write
this, it might be that Firebug incorrectly reports some of the connection
overhead time as DNS lookup even if no actual lookup is being done).
Certainly, on my system, a remote lookup (even on our corporate DNS system)
didn't happen - I observed the whole network stream with Wireshark.

I hope this helps!


On Thu, Apr 15, 2010 at 11:52 AM, Nathan Hoover <nathan at foo-o-rama.com>wrote:

> Interestingly, I am seeing the same behavior on my machine. My first
> inclination was that Firebug was erroneously reporting DNS lookup time, but
> I discounted that quickly by looking at one of my own sites, and not
> observing a similar pattern. My next inclination was that it was a DNS
> configuration problem (ridiculously low TTL?) but a quick look doesn't seem
> to bear that out either. Now my curiosity is piqued! If I find anything,
> I'll let everyone know.
> Nathan
> On Thu, Apr 15, 2010 at 11:18 AM, Rick Moen <rick at linuxmafia.com> wrote:
>> Quoting Andrew E (andrewevansc at gmail.com):
>> [RM: snip server URL and related text, which I'm pretty sure were the
>> only confidential parts]
>> > Firebug shows me a whole bunch of DNS lookups. Does it look like that
>> IPv6
>> > vs IPv4 issue? I'm unfamiliar with how to diagnose issues with these
>> > protocols or DNS in general. Here is a phpinfo() about the server
>> > http://native-instinct.com/portfolio/info.php if that helps.
>> As Nathan said, and I was implying, Firebug is reporting what's going on
>> at the machine where _Firebug_ is running, which is where your Web
>> browser is -- which Web browser I believe would be Firefox, right?
>> So, the real question you should be asking yourself is, where's your
>> Web-browsing machine (the machine you're running your -browser- on)
>> consulting for DNS?  Is the Web-browsing machine attempting IPv6-enabled
>> DNS and falling back to IPv4-based queries when those fail?
>> One quick way to check is to install Unbound, an IP6-enabled open-source
>> recursive nameserver available for Unixes (including Linux and OS X) and
>> MS-Windows, pointing your machine at it for nameservice, and seeing if
>> the problem goes away.
>> The URL I posted earlier details how.
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