invalid response from Proxy dns lookup failure in my domains

[Tapas Mishra]

Hi all.
The problem I am reporting is not very clearly evident to me.
I have a virtualization setup where in I had configured in 4 virtual
machines (which serve 4 different websites)  available to internet via
a
Reverse Proxy on the Host OS (bare metal) I use Ubuntu 10.04 64 bit
server edition and kvm for this setup.

We have not changed any of the server configurations from past 6
months since things were working properly without any problem on this
server.
The sites are available to people as


http://site1.myserver.com <-- No problem
http://site1.myserver.com/app1 <-- Gives problem till weekend there
was no problem

http://site4.myserver.com   <--Is also giving problem till this Sunday
9 Jan there were no problem.

Here are the error messages which I see in Apache logs

Mon Jan 10 17:07:05 2011] [error] [client 172.21.100.250] proxy: DNS
lookup failure for: site4.myserver.com returned by /
[Mon Jan 10 17:07:06 2011] [error] [client 172.21.100.250] proxy: DNS
lookup failure for: site4.myserver.com returned by /

and for site1.myserver.com/app1 also I get same error

[Mon Jan 10 17:06:01 2011] [error] [client 172.21.100.250] proxy: DNS
lookup failure for: site1.myserver.com returned by /app1/
[Mon Jan 10 17:06:01 2011] [error] [client 172.21.100.250] proxy: DNS
lookup failure for: site1.myserver.com returned by /app1/
[Mon Jan 10 17:06:02 2011] [error] [client 172.21.100.250] proxy: DNS
lookup failure for: site1.myserver.com returned by /app1/


How ever when some one tries to access http://site1.myserver.com there
is no error at all.
Also if I access  http://site5.somebody_server.com (which is same as
http://site4.myserver.com pointing to same public IP) but on some body
elses server
then there is no problem at all and the site is very clearly visible.


Let me know what more should I look for I have no clue of this problem.
Because these error messages which I just report in this thread were
not at all present until today  morning.



--