400 Bad Request - request header or cookie too large

I am getting a 400 Bad Request request header or cookie too large from nginx with my Rails app. Restarting the browser fixes the issue. I am only storing a string id in my cookie so it should be tiny.

Where can I find the nginx error logs? I looked at nano /opt/nginx/logs/error.log, but it doesn't have anything related.

I tried to set following and no luck:

location / {
    large_client_header_buffers  4 32k;
    proxy_buffer_size  32k;
}

nginx.conf

#user  nobody;
worker_processes  1;
#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;
#pid        logs/nginx.pid;
events {
  worker_connections  1024;
}
http {
passenger_root /home/app/.rvm/gems/ruby-1.9.3-p392/gems/passenger-3.0.19;
passenger_ruby /home/app/.rvm/wrappers/ruby-1.9.3-p392/ruby;
include       mime.types;
default_type  application/octet-stream;
sendfile        on;
keepalive_timeout  65;
client_max_body_size 20M;
server {
    listen       80;
    server_name  localhost;
    root /home/app/myapp/current/public;
    passenger_enabled on;
    #charset koi8-r;
    #access_log  logs/host.access.log  main;

# location / {
#   large_client_header_buffers  4 32k;
#   proxy_buffer_size  32k;
# }

     #  location / {
     #   root   html;
     #   index  index.html index.htm;
     #   client_max_body_size 4M;
#   client_body_buffer_size 128k;
# }
    #error_page  404              /404.html;

    # redirect server error pages to the static page /50x.html
    #
    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   html;
    }

    # proxy the PHP scripts to Apache listening on 127.0.0.1:80
    #
    #location ~ \.php$ {
    #    proxy_pass   http://127.0.0.1;
    #}

    # pass the PHP scripts to FastCGI server listening on 127.0.0.1:9000
    #
    #location ~ \.php$ {
    #    root           html;
    #    fastcgi_pass   127.0.0.1:9000;
    #    fastcgi_index  index.php;
    #    fastcgi_param  SCRIPT_FILENAME  /scripts$fastcgi_script_name;
    #    include        fastcgi_params;
    #}

    # deny access to .htaccess files, if Apache's document root
    # concurs with nginx's one
    #
    #location ~ /\.ht {
    #    deny  all;
    #}
}


# another virtual host using mix of IP-, name-, and port-based configuration
#
#server {
#    listen       8000;
#    listen       somename:8080;
#    server_name  somename  alias  another.alias;

#    location / {
#        root   html;
#        index  index.html index.htm;
#    }
#}


# HTTPS server
#
#server {
#    listen       443;
#    server_name  localhost;

#    ssl                  on;
#    ssl_certificate      cert.pem;
#    ssl_certificate_key  cert.key;

#    ssl_session_timeout  5m;

#    ssl_protocols  SSLv2 SSLv3 TLSv1;
#    ssl_ciphers  HIGH:!aNULL:!MD5;
#    ssl_prefer_server_ciphers   on;

#    location / {
#        root   html;
#        index  index.html index.htm;
#    }
#}

}

Here's my code storing the cookies and a screenshot of the cookies in Firebug. I used firebug to check stored session and I found New Relic and jQuery are storing cookies too; could this be why the cookie size is exceeded?

def current_company
  return if current_user.nil?
  session[:current_company_id] = current_user.companies.first.id if session[:current_company_id].blank?
    @current_company ||= Company.find(session[:current_company_id])
end

It's just what the error says - Request Header Or Cookie Too Large. One of your headers is really big, and nginx is rejecting it.

You're on the right track with large_client_header_buffers. If you check the docs, you'll find it's only valid in http or server contexts. Bump it up to a server block and it will work.

server {
    # ...
    large_client_header_buffers 4 32k;
    # ...
}

By the way, the default buffer number and size is 4 and 8k, so your bad header must be the one that's over 8192 bytes. In your case, all those cookies (which combine to one header) are well over the limit. Those mixpanel cookies in particular get quite large.