# When does nginx return a Bad gateway (502)? **URL:** \ **Category:** NGINX\ **Created:** [March 19, 2013, 9:29pm UTC](https://www.ruby-forum.com/t/when-does-nginx-return-a-bad-gateway-502/227572 "2013-03-19T21:29:35Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![fastcatch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/db5fbb/32.png) [@fastcatch](https://www.ruby-forum.com/u/fastcatch)\ **Post date:** [March 19, 2013, 9:29pm UTC](https://www.ruby-forum.com/t/when-does-nginx-return-a-bad-gateway-502/227572/1 "2013-03-19T21:29:35Z")
I have an application behind nginx (Rails on Unicorn if it matters) which listens on a UNIX socket. ## It all works nice, especially when load is low. When there is some load on the server though (say 70%), I randomly(?) get a bunch of 502 responses usually in batches. Thus the question: when (under what conditions) does nginx send a 502 response? Thanks in advance, fastcatch PS: The app’s worker processes do not seem to be irresponsive, slow, blocked for IO or anything else I have been able to identify as a possible cause. Thus I want to understand more the nginx side so as to find the root cause – which I believe is on the other side (or some config tweak needed soemwhere). Posted at Nginx Forum: > **[When does nginx return a Bad gateway (502)?](https://forum.nginx.org/read.php?2,237573,237573#msg-237573)** > > I have an application behind nginx (Rails on Unicorn if it matters) which listens on a UNIX socket. It all works nice, especially when load is low. When there is some load on the server though (say 70%), I randomly(?) get a bunch of 502 responses --... ---
**Author:** ![fastcatch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/db5fbb/32.png) [@fastcatch](https://www.ruby-forum.com/u/fastcatch)\ **Post date:** [March 19, 2013, 9:48pm UTC](https://www.ruby-forum.com/t/when-does-nginx-return-a-bad-gateway-502/227572/2 "2013-03-19T21:48:16Z")
In my experience, Nginx returns 502 when the upstream server (Unicorn in your case) doesn’t respond or terminates the connection unexpectedly. \_Nik ---
**Author:** ![fastcatch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/db5fbb/32.png) [@fastcatch](https://www.ruby-forum.com/u/fastcatch)\ **Post date:** [March 20, 2013, 11:04am UTC](https://www.ruby-forum.com/t/when-does-nginx-return-a-bad-gateway-502/227572/3 "2013-03-20T11:04:02Z")
Thank you. BTW: having googled it for different expressions I’ve found that it is a well known issue and that it has nothing to do with nginx or the Rails app itself. Unicorn in and of itself can neither be blamed (albeit IMHO there should be some official documentation on this issue, esp. since nginx is the preferred/“official” web server for unicorn). It looks like a socket backlog setting issue to be handled at OS level but I’m still unsure if that’s a resolution or not. Posted at Nginx Forum: > **[Re: When does nginx return a Bad gateway (502)?](https://forum.nginx.org/read.php?2,237573,237590#msg-237590)** > > I have an application behind nginx (Rails on Unicorn if it matters) which listens on a UNIX socket. It all works nice, especially when load is low. When there is some load on the server though (say 70%), I randomly(?) get a bunch of 502 responses --... ---
**Author:** ![fastcatch](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/db5fbb/32.png) [@fastcatch](https://www.ruby-forum.com/u/fastcatch)\ **Post date:** [March 21, 2013, 11:03am UTC](https://www.ruby-forum.com/t/when-does-nginx-return-a-bad-gateway-502/227572/4 "2013-03-21T11:03:51Z")
I can confirm that allowing a longer backlog queue (in unicorn config + net.core.somaxconn and net.core.netdev\_max\_backlog) did solve the issue (for me). I posted a bit more details at [Bad gateway errors at load on nginx + Unicorn (Rails 3 app) - Stack Overflow](http://stackoverflow.com/a/15544229/1061997). Posted at Nginx Forum: > **[Re: When does nginx return a Bad gateway (502)?](https://forum.nginx.org/read.php?2,237573,237622#msg-237622)** > > I have an application behind nginx (Rails on Unicorn if it matters) which listens on a UNIX socket. It all works nice, especially when load is low. When there is some load on the server though (say 70%), I randomly(?) get a bunch of 502 responses --...