Why my Http Proxy doesn't reply [SYN,ACK] after received a [SYN] packet from the client?

Question

I have a http proxy running on Linux (RHEL 5.5), intermittently it doesn't reply [SYN,ACK] packet after received a [SYN] packet from the client. This issue can be recovered by restarting the proxy. I ran the gstack against the proxy process, however the stack of each thread doesn't indicate any problems(hanging). And there were less than 100 connections staying at the "SYN_RECV" state during this issue.

Any ideas? For such issue, where should i start to debug?

Any suggestions would be greatly appreciated.

Answer 1

Lack of SYN,ACK usually indicates that the server software doesn't accept the connection. This might be due to a number of reasons, most likely exhaustion of resources of some kind or another. You could try to strace the server process to see whether it is in the accept syscall, or equivalently doing some select or poll with the server socket among the file descriptors to select from.

Answer 2

Looks like a firewall issue. If a TCP connection on receiving the first SYN + ACK response, it's most likely your packet wasn't sent to the proxy server.

Check on both server side and client side.