How can I observe all network traffic to and from a certain domain in OSX?

I'd like to observe requests from my machine to certain domains and optionally insert my own canned responses instead of the ones they provide.

eg: any GET request to api.googlemaps.com I intercept and retu { test: "foo" }

I'd like this to work for HTTP and HTTPS, though it'd be ok if I can't read the contents of the HTTPS messages, and just make up my own.

I'm not sure how to get started on this. Any ideas?

If it matters, I'm on OSX 10.11.4

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