I’ve submitted the following to the nginx mailing list, after dealing with errors caused by this hard to dig up bug: What I’d like to propose is having requests with headers with single lines larger than large_client_header_buffers respond with a status of 414 rather than 400. Additionally, large_client_header_buffers should default to a larger value, double the platform’s page size, to bring up it up to an 8k minimum to match the largest cookie size in a mainstream browser (IE 8) which maxes out at 5117 bytes by my calculations.

I recently ran into this where very large (4.7kib) cookies from IE were causing requests to my nginx server to fail with 400 Bad Request errors. The underlying problem–besides the erroneously large cookie–was a too small large_client_header_buffers value. According to the nginx docs: “The request line can not be bigger than the size of one buffer, if the client send a bigger header nginx returns error “Request URI too large” (414). The longest header line of request also must be not more than the size of one buffer, otherwise the client get the error “Bad request” (400).”

The fact that it was a 400 error made it quite hard to debug, as the cause was rather ambiguous, and only a small subset of clients, due to app logic, had the swollen cookies. I ended up finding a particularly cooperative user and getting a TCP dump to ascertain the problem. Obviously, getting a user to agree to letting someone remote in and run wireshark is highly unusual, and it is my guess that nginx users who encounter this problem in the future are more likely to abandon nginx than go to this length to figure out the issue. An error like this should really provide more helpful info to debug on the server side.

I’ve run some tests on Firefox 3.5, Chrome 3, and IE 8 to see what their largest cookie sizes are:

Firefox 3.5.5:          4096 Bytes
IE 8.0.6001.18828:  5117 Bytes
Chrome 4131 Bytes

These numbers were generated by this code, running on my home system, a Windows Vista 64 bit machine (all browsers are 32 bit):

var data = '';
var last_len = -1;
while (last_len < document.cookie.length) {
  data += 'x';
  last_len = document.cookie.length;
  document.cookie = data;
document.write('Max Size: ' + last_len + ' Bytes');

I’m temporarily hosting it here: http://www.andrewvc.com/cookietest.html for convenience if anyone else wants to test their browser.