Read form POST variables with nginx - nginx

Question

I have a client program I cannot modify. It makes large POST (x-www-form-urlencoded) requests containing hundreds of variables across WAN links, but I only need 5 of them. I'm inserting nginx as a reverse proxy on the local client system. What's the easiest to get nginx to strip out the extra data?
Two ways I see so far:
1. Use Lua (If I did, should I do content_by_lua, rewrite the body, and then make a subrequest? Or is there a simpler way?)
2. Use form-input-nginx-module and proxy_set_body to parse and grab a few variables out.
I'm already using OpenResty, so Lua means no extra modules. But, it probably means writing more locations and so on to do subrequests.

Answer 1

In my opinion the easiest way will be using lua. The choice between content_by_lua, rewrite_by_lua, access_by_lua or any combination of them; will depend on how you use the response body of your subrequest. That decision will also determine if you would need additional locations or not.
Here are a couple of examples:
1. with content_by_lua targeting a local location.
(This approach requires the definition of the sub request location)
location /original/url {
lua_need_request_body on;
content_by_lua '
--Lots of params but I only need 5 for the subrequest
local limited_post_args, err = ngx.req.get_post_args(5)
if not limited_post_args then
ngx.say("failed to get post args: ", err)
return
end
local subreq_uri = "/test/local"
local subreq_response = ngx.location.capture(subreq_uri, {method=ngx.HTTP_POST,
body = ngx.encode_args(limited_post_args)})
ngx.print(subreq_response.body)
';
}
location ~/test/local {
lua_need_request_body on;
proxy_set_header Accept-Encoding "";
proxy_pass http://echo.200please.com;
}
2. with rewrite_by_lua to remote target
(No additional location is needed)
location /original/url/to/remote {
lua_need_request_body on;
rewrite_by_lua '
--Lost of params but I only need 5 for the subrequest
local limited_post_args, err = ngx.req.get_post_args(5)
if not limited_post_args then
ngx.say("failed to get post args: ", err)
return
end
--setting limited number of params
ngx.req.set_body_data(ngx.encode_args(limited_post_args))
--rewriting url
local subreq_path = "/test"
ngx.req.set_uri(subreq_path)
';
proxy_pass http://echo.200please.com;
}
Sample post request with 7 args limited to 5:
curl 'http://localhost/original/url/to/remote' --data 'param1=test&param2=2&param3=3&param4=4&param5=5&param6=6&param7=7' --compressed
response:
POST /test HTTP/1.0
Host: echo.200please.com
Connection: close
Content-Length: 47
User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.14.0.0 zlib/1.2.3 libidn/1.18 libssh2/1.4.2
Accept: */*
Accept-Encoding: deflate, gzip
Content-Type: application/x-www-form-urlencoded
param3=3&param4=4&param1=test&param2=2&param5=5