Logging POST data from $request_body

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.

This solution works like a charm:

http {

   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.

Edit 2022-03-15: there is some discussion on where log_format should be set. The documentation clearly says that it needs to be in the http context: http://nginx.org/en/docs/http/ngx_http_log_module.html#log_format

If you put log_format in the server context, nginx will fail to load the config: nginx: [emerg] "log_format" directive is not allowed here in <path>:<line> (tested with nginx 1.20 on ubuntu 18.04)