Tuesday, July 22, 2014

Troubleshooting haproxy 502 errors related to malformed/large HTTP headers

We had a situation recently where our web application started to behave strangely. First nginx (which sits in front of the application) started to error out with messages of this type:

upstream sent too big header while reading response header from upstream

A quick Google search revealed that a fix for this is to bump up proxy_buffer_size in nginx.conf, for both http and https traffic, along these lines:

proxy_buffer_size   256k;
proxy_buffers   4 256k;
proxy_busy_buffers_size   256k;

Now nginx was happy when hit directly. However, haproxy was still erroring out with a 502 'bad gateway' return code, followed by PH. Here is a snippet from the haproxy log file:

Jul 22 21:27:13 haproxy[14317]: [22/Jul/2014:21:27:12.776] www-frontend www-backend/www2:80 1/0/1/-1/898 502 8396 - - PH-- 0/0/0/0/0 0/0 "GET /someurl HTTP/1.1"

Another Google search revealed that PH means that haproxy rejected the header from the backend because it was malformed.

At this point, an investigation into the web app did discover a loop in the code that kept adding elements to a cookie included in the response header.

Anyway, I leave this here in the hope that somebody will stumble on it and benefit from it.


Anonymous said...

Came across something similar with the header size being too large for HAProxy. Added this to the config:

# Descrease the size of the rewrite buffer so that
# there is more room to handle large (>8K) headers. See the HAProxy
# manual around tune.bufsize and tune.maxrewrite.
tune.maxrewrite 4096

Shivali Bari said...

I was facing 502 bad gateway issue. This one line - "tune.maxrewrite 4096" resolved my issue :)