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gotcha - A Debugging MiTM HTTP Proxy

gotcha is a small HTTP/HTTPS man-in-the-middle proxy, which you can use to troubleshoot HTTP traffic that occurs behind the veil of SSL/TLS encryption.

Man-in-the-Middle?!

Isn't that dangerous?

Yes, yes it is -- if it happens to you when you aren't expecting it. As a diagnostic method, it's pretty sweet.

It works like this:

+----------------+     +------------------------+     +----------+
|    CLIENT      | --> |        GOTCHA          | --> | UPSTREAM |
|                |     |                        |     +----------+
| connecting to  |     | binds 127.0.0.1:3128   |
| 127.0.0.1:3128 |     | connects to            |
| (plain HTTP)   |     | https://api.end.poi.nt |
+----------------+     +------------------------+
                                  |
                                  |
                                  V
                              +--------+
                              | stdout |
                              +--------+

So, instead of connecting directly to https://api.end.poi.nt, the client connects to the gotcha process, which is listening on some local port (https://127.0.0.1:3128 by default) and MitM-ing the requests/responses to the upstream.

The diagnostics part comes in because gotcha dumps the entire HTTP conversation to standard output, so you can see it in your terminal. The upstream is still talking HTTPS, but you can now see headers, bodies and response codes!

A Real Example!

It's a bit of a gimme, but here's gotcha being used to intercept HTTP traffic to www.google.com (over TLS) via curl:

$ /gotcha https://www.google.com &
targeting https://www.google.com
binding :3128

$ curl -X HEAD http://localhost:3128
HEAD / HTTP/1.1
Host: www.google.com
User-Agent: curl/7.43.0
Accept: */*


HTTP/1.1 200 OK
Transfer-Encoding: chunked
Accept-Ranges: none
Server: gws
...

Environment Variables

Some aspects of gotcha can be configured via environment variables, to facilitate running it as a Cloud Foundry application. Here they are:

  • PORT Specifies the port the app will listen on
  • GOTCHA_BACKEND Specifies the upstream endpoint gotcha will front
  • SSL_SKIP_VERIFY Specifies whether gotcha will care about invalid upstream SSL certificates