Here is a situation until today morning,every thing has been working perfectly fine with me. From past 6 months many of my domains wer accessible as

http://site1.myserver.com
http://site2.myserver.com
http://site3.myserver.com
http://site4.myserver.com

All these were Reverse Proxy configurations. I have some applications on each of them.

until today morning some people reported me that
http://site1.myserver.com/app1 is not working but http://site1.myserver.com is accessible
but http://site2.myserver.com is accessible
but http://site3.myserver.com is accessible
but http://site4.myserver.com not accessible

In past 6 months I have not changed any of these Apache configurations
(things were working perfectly so)

The error which can be seen in browser are while accessing http://site1.myserver.com/app1

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /app1.

Reason: DNS lookup failure for: myserver.com

and same is the error for http://site4.myserver.com

So what should I check in I have checked all the apache logs to an extent which I could see and

    192.168.1.25 - - [10/Jan/2011:14:50:48 +0530] "GET /app1 HTTP/1.1" 502 531 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3"

Mon Jan 10 14:27:42 2011] [error] (113)No route to host: proxy: HTTP: attempt to connect to 192.168.1.3:80 (192.168.1.3) failed
[Mon Jan 10 14:27:42 2011] [error] ap_proxy_connect_backend disabling worker for (192.168.1.3)
[Mon Jan 10 14:27:44 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:44 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:44 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:45 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:45 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:45 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:45 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:46 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:47 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:48 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:48 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:27:48 2011] [error] proxy: HTTP: disabled connection for (192.168.1.3)
[Mon Jan 10 14:35:29 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: myserver.com returned by /app1
[Mon Jan 10 14:35:30 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: myserver.com returned by /app1
[Mon Jan 10 14:35:30 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: myserver.com returned by /app1
[Mon Jan 10 14:50:30 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: myserver.com returned by /app1
[Mon Jan 10 14:50:48 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: myserver.com returned by /app1

and for site4.myserver.com

I get

[Mon Jan 10 14:57:40 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /favicon.ico
[Mon Jan 10 14:57:40 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /favicon.ico
[Mon Jan 10 14:57:43 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /favicon.ico
[Mon Jan 10 15:02:38 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /
[Mon Jan 10 15:03:04 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /, referer: http://site4.myserver.com/
[Mon Jan 10 15:03:04 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /favicon.ico
[Mon Jan 10 15:03:08 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /, referer: http://site4.myserver.com/
[Mon Jan 10 15:03:08 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /favicon.ico
[Mon Jan 10 15:03:10 2011] [error] [client <some external IP>] proxy: DNS lookup failure for: site4.myserver.com returned by /, referer: http://site4.myserver.com/
[Mon Jan 10 15:06:21 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /
[Mon Jan 10 15:06:31 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /, referer: http://site4.myserver.com/
[Mon Jan 10 15:26:03 2011] [error] [client 192.168.1.25] proxy: DNS lookup failure for: site4.myserver.com returned by /