Logging POST data from $request_body

Question

I have my config setup to handle a bunch of GET requests which render pixels that work fine to handle analytics and parse query strings for logging. With an additional third party data stream, I need to handle a POST request to a given url that has JSON in an expected loggable format inside of it's request body. I don't want to use a secondary server with proxy_pass and just want to log the whole response into an associated log file like what it does with GET requests. A snippet of some code that I'm using looks like the following:

GET request (which works great):

location ^~ /rl.gif {
  set $rl_lcid $arg_lcid;
  if ($http_cookie ~* "lcid=(.*\S)")
  {
    set $rl_lcid $cookie_lcid;
  }
  empty_gif;
  log_format my_tracking '{ "guid" : "$rl_lcid", "data" : "$arg__rlcdnsegs" }';
  access_log  /mnt/logs/nginx/my.access.log my_tracking;
  rewrite ^(.*)$ http://my/url?id=$cookie_lcid? redirect;
}

Here is kinda what I am trying to do: POST request (which does not work):

location /bk {
  log_format bk_tracking $request_body;
  access_log  /mnt/logs/nginx/bk.access.log bk_tracking;
}

Curling curl http://myurl/bk -d name=example gives me a 404 page not found.

Then I tried:

location /bk.gif {
  empty_gif;
  log_format bk_tracking $request_body;
  access_log  /mnt/logs/nginx/bk.access.log bk_tracking;
}

Curling curl http://myurl/bk.gif -d name=example gives me a 405 Not Allowed.

My current version is nginx/0.7.62. Any help in the right direction is very much appreciated! Thanks!

UPDATE So now my post looks like this:

location /bk {
  if ($request_method != POST) {
    return 405;
  }
  proxy_pass $scheme://127.0.0.1:$server_port/dummy;
  log_format my_tracking $request_body;
  access_log  /mnt/logs/nginx/my.access.log my_tracking;
}
location /dummy { set $test 0; }

It is logging the post data correctly, but returns a 404 on the requesters end. If I change the above code to return a 200 like so:

location /bk {
  if ($request_method != POST) {
    return 405;
  }
  proxy_pass $scheme://127.0.0.1:$server_port/dummy;
  log_format my_tracking $request_body;
  access_log  /mnt/logs/nginx/my.access.log my_tracking;
  return 200;
}
location /dummy { set $test 0; }

Then it return the 200 correctly, but no longer records the post data.

ANOTHER UPDATE Kinda found a working solution. Hopefully this can help other on their way.

Answer 1

This solution works like a charm (updated in 2017 to honor that log_format needs to be in the http part of the nginx config):

log_format postdata $request_body;

server {
    # (...)

    location = /post.php {
       access_log  /var/log/nginx/postdata.log  postdata;
       fastcgi_pass php_cgi;
    }
}

I think the trick is making nginx believe that you will call a cgi script.