I am having an issue with random timeouts with a Nginx proxy connecting to an IIS machine. I have been watching a packet capture between the two servers and it seems that the IIS machine is receiving a SYN packet but is not responding with what I think should be an ACK response. Before the timeout occurs there seems to be a slower response from the IIS server. There is no unusual memory or processor usage on the IIS or Nginx machine.

Some information on the servers and setup:

Nginx Machine:

• Ubuntu 10.04 64bit
• Nginx 0.7.65
• Amazon EC2

Windows Machine:

• Windows Server 2008
• IIS 7
• ASP.net Application in Integrated Mode

Nginx Error:

2011/01/10 17:57:40 [error] 8297#0: *30 connect() failed (110: Connection timed out) while connecting to upstream,
client: 209.***.***.***, server: secure.example.com, request:
"GET /a/path/deliver.aspx HTTP/1.1", upstream: "http://***.***.***.****:****//another/path/deliver.aspx", host: "secure.example.com"

WireShark Packets

6521.449528 10.***.***.*** -> 174.***.***.*** TCP 38695 > us-cli [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=477422103 TSER=0 WS=7
6524.443239 10.***.***.*** -> 174.***.***.*** TCP 38695 > us-cli [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=477422403 TSER=0 WS=7
6530.443241 10.***.***.*** -> 174.***.***.*** TCP 38695 > us-cli [SYN] Seq=0 Win=5840 Len=0 MSS=1460 TSV=477423003 TSER=0 WS=7