Nginx – How to debug nginx further than the error log

nginxreverse-proxy

I'm currently receiving a fairly large HTTP flood right now, and it's causing my nginx reverse proxy to produce a 502 Bad Gateway.

I have a frontend server running nginx as a proxy to my backend server, but it's just getting a bunch of connect() failed (110: Connection timed out) while connecting to upstream errors. Tons of them. If I bypass the proxy server to connect to the backend, I can run the site just fine, so I know it's in the reverse proxy somewhere. However, I have no idea how to determine why it's timing out.

Any help?

running nginx 1.2.3 on CentOS 6.2

Best Answer

I'm assuming you've already jacked your Nginx error logging level up to debug. If not, start there.

Your best bet is probably going to be using strace to view the system calls being made by Nginx. In particular, you'll want to pay attention to connect() calls, and keep an eye on the return codes of these (man 2 connect can be your friend here).

Once you have that information, you can better make an educated guess about whether the issue is confined to your frontend proxy, or has something to do with the interactions between the proxy and backend application server.

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