How can I read and manipulate POST request variables with nginx?

Question

I'm using nginx (v1.4.1) to proxy an external service, but would like to be able to inspect and possibly modify the body of a POST request before proxying it. The problem is, I'm currently unable to access the POST request body while handling the request, whether through the $request_body variable or otherwise.

I've read a number of posts and SO questions, and have implemented this suggested strategy, which is intended to allow the logging of the POST body (when using a proxy_pass directive). However, while this works for me I'm still unable to read anything from $request_body during the processing of the request.

For clarity, here's the relevant section of my configuration:

location /proxy-this/ {
    client_max_body_size 8k;
    client_body_buffer_size 16k;
    client_body_in_single_buffer on;
    proxy_pass https://example.com/external-endpoint/;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header Host $http_host;
    proxy_set_header Content-Length '';
}

And what I'd like to be able to do:

location /proxy-this/ {
    ... (same configuration as above) ...
    set $request_body $request_body&extra_param=1;
}

I'm aware that this is possible using the nginx_lua module, and have looked at the HttpFormInputModule, but the first seems overkill and the latter hasn't been updated in a while.

Answer 1

Have you tried using proxy_set_body?

That would be like

location /proxy-this/ {
  ... (same configuration as above) ...
  proxy_set_body $request_body&extra_param=1;
}