We have 7 squids in use. 3 of them are squid 3.1.11, 4 of them are 3.1.12. The squids often crash. The following is cited from the cache.log of squid 3.1.11 2012/07/28 11:28:54| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/28 13:08:03| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/28 14:47:05| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/29 13:37:34| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/29 15:19:55| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/29 16:26:18| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/07/30 13:07:23| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... The active connections on squid are about 2000. We have upgraded one 3.1.12 to 3.1.20, but the problem still exists. 2012/07/28 09:50:37| Starting Squid Cache version 3.1.20 for x86_64-redhat-linux-gnu... 2012/07/28 09:50:54| Starting Squid Cache version 3.1.20 for x86_64-redhat-linux-gnu... 2012/07/30 11:27:04| Starting Squid Cache version 3.1.20 for x86_64-redhat-linux-gnu... 2012/07/30 11:27:11| Starting Squid Cache version 3.1.20 for x86_64-redhat-linux-gnu... How can I find out the reason ? It seems there is no clue in the cache.log 2012/07/31 11:23:55| WARNING: unparseable HTTP header field {: 211.137.119.10} 2012/07/31 11:23:55| WARNING: unparseable HTTP header field {: 211.137.119.10} 2012/07/31 11:26:46| WARNING: HTTP: Invalid Response: No object data received for http://mydomain.com/ajax/book.getUserBookFans Score.do AKA mydomain.com/ajax/book.getUserBookFansScore.do 2012/07/31 11:28:15| WARNING: HTTP: Invalid Response: No object data received for http://mydomain.com/ajax/book.getUserBookFans Score.do AKA mydomain.com/ajax/book.getUserBookFansScore.do 2012/07/31 11:43:10| NETDB state saved; 0 entries, 0 msec 2012/07/31 11:49:09| Starting Squid Cache version 3.1.20 for x86_64-redhat-linux-gnu... 2012/07/31 11:49:09| Process ID 19635 Another problem is the squids in the same cluster almost crash at the same time. The 7 squids are in 2 clusters, 3 squids are in one cluster and 4 squids are in another cluster. The squids are load-balanced using DNS round robin. squid1 cache.log 2012/08/01 12:09:19| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/01 15:26:36| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 11:05:27| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 13:43:20| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/03 13:29:41| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 09:33:05| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 11:13:05| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 12:40:19| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... squid2 cache.log 2012/08/01 12:09:17| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/01 15:26:39| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 11:05:25| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 13:43:18| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 21:08:27| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/03 13:29:39| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 09:33:08| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 12:40:17| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... squid3 cache.log 2012/08/01 12:09:20| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/01 13:43:17| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/01 15:26:38| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 11:05:23| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/02 13:43:21| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/03 12:14:33| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/03 13:29:51| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 09:33:06| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu... 2012/08/04 12:40:20| Starting Squid Cache version 3.1.11 for x86_64-koji-linux-gnu...