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Re: DNS issues



On 04/23/2009 06:16 AM, Anne Wilson wrote:
On Wednesday 22 April 2009 23:56:38 Richard Körber wrote:
  
Hi!

I just installed F11 beta and updated to the latest packages. It was
working fine so far, but then suddenly I got issues with domain name
resolving.

When I use Firefox, Thunderbird, wget, whatever, I always get an error
message that the domain name could not be resolved. Anyhow the network is
up, DNS server IPs are set correctly and I can even use dig and ping to
successfully resolve domain names. When I enter a plain IP address at
Firefox, it also fetches the page correctly.

Configuration files seem to be correct, there are no hints in the log
files, I even checked that there is no proxy set, but I found nothing. I
created a clean user to make sure there is no configuration messed up. I
rebooted the system, but still got that issue. I'm totally clueless now...

Is this a F11 bug? I have had a look at bugzilla, but I wasn't sure what I
should search for.

    
This happens from time to time on my F10 netbook, too, although it's not a 
frequent event.  Rebooting is the only way I've found out of it.  Whatever 
causes it, it's common to both versions.

Anne
  

I've experienced the same ( or similar ) seemed to be domain specific.  

Had open a bug ( #496979 ) then closed it again cause we could duplicate it on
debian and f10 and thus deemed the fqdn in question had an broken dns setup.

Also on http://udrepper.livejournal.com/20948.html

"DNS NSS improvement

In glibc 2.9 I already implemented an improvement to the DNS NSS module which optimizes the lookup of IPv4 and IPv6 addresses for the same host. This can improve the response time of the lookup due to parallelism. It also fixes a bug in name lookup where the IPv4 and IPv6 addresses could be returned for different hosts.

The problem with this change was that there are broken DNS servers and broken firewall configurations which prevented the two results from being received successfully. Some broken DNS servers (especially those in cable modems etc) only send one reply. For this reason Fedora had this change disabled in F10.

For F11 I’ve added a work-around for broken servers. The default behavior is the same as described above. I.e., we get the improved performance for working DNS servers. In case the program detects a broken DNS server or firewall because it received only one reply the resolver switches into a mode where the second request is sent only after the first reply has been received. We still get the benefit of the bug fix described above, though.

The drawback is that a timeout is needed to detect the broken servers or firewalls. This delay is experienced once per process start and could be noticeable. But the broken setups of the few people affected must not prevent the far larger group of people with working setups to experience the advantage of the parallel lookup.

There are also ways to avoid the delays, some old, some new:

  • Install a caching name server on this machine or somewhere on the local network. bind is known to work correctly.
  • Run nscd on the local machine. In this case the delay is incurred once per system start (i.e., at the first lookup nscd performs).
  • Add “single-request” to the options in /etc/resolv.conf. This selects the compatibility mode from the start.

All of these work-arounds are easy to implement. Therefore there is no reason to not have the fast mode the default which in any case will work for 99% of the people."

However the end user will not blame the incompetent DNS admin but Fedora ( or what ever program he's using at the time.. firefox thunderbird network manager etc.. )
when the M$ machine next to him manages to reach the site successfully while the Fedora machine will not..

Are these "workarounds" in forums docs ?

JBG


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