Logging POST data from $request_body

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.

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.

Try echo_read_request_body.

"echo_read_request_body ... Explicitly reads request body so that the $request_body variable will always have non-empty values (unless the body is so big that it has been saved by Nginx to a local temporary file)."

location /log {
  log_format postdata $request_body;
  access_log /mnt/logs/nginx/my_tracking.access.log postdata;
  echo_read_request_body;
}

Ok. So finally I was able to log the post data and return a 200. It's kind of a hacky solution that I'm not too proud of which basically overrides the natural behavior for error_page, but my inexperience of nginx plus timelines lead me to this solution:

location /bk {
  if ($request_method != POST) {
    return 405;
  }
  proxy_set_header  X-Forwarded-For $proxy_add_x_forwarded_for;
  proxy_set_header Host $host;
  proxy_set_header X-Real-IP $remote_addr;
  proxy_redirect off;
  proxy_pass $scheme://127.0.0.1:$server_port/success;
  log_format my_tracking $request_body;
  access_log  /mnt/logs/nginx/my_tracking.access.log my_tracking;
}
location /success {
  return 200;
}
error_page   500 502 503 504  /50x.html;
location = /50x.html {
  root   /var/www/nginx-default;
  log_format my_tracking $request_body;
  access_log  /mnt/logs/nginx/my_tracking.access.log my_tracking_2;
}

Now according to that config, it would seem that the proxy pass would return a 200 all the time. Occasionally I would get 500 but when I threw in an error_log to see what was going on, all of my request_body data was in there and I couldn't see a problem. So I caught that and wrote to the same log. Since nginx doesn't like the same name for the tracking variable, I just used my_tracking_2 and wrote to the same log as when it returns a 200. Definitely not the most elegant solution and I welcome any better solution. I've seen the post module, but in my scenario, I couldn't recompile from source.

The solution below was the best format I found.

log_format postdata escape=json '$remote_addr - $remote_user [$time_local] '
                       '"$request" $status $bytes_sent '
                       '"$http_referer" "$http_user_agent" "$request_body"';
server {
        listen 80;

        server_name api.some.com;

        location / {
         access_log  /var/log/nginx/postdata.log  postdata;
         proxy_pass      http://127.0.0.1:8080;
        }

}

For this input

curl -d '{"key1":"value1", "key2":"value2"}' -H "Content-Type: application/json" -X POST http://api.deprod.com/postEndpoint

Generate that great result

201.23.89.149 -  [22/Aug/2019:15:58:40 +0000] "POST /postEndpoint HTTP/1.1" 200 265 "" "curl/7.64.0" "{\"key1\":\"value1\", \"key2\":\"value2\"}"