Ask HN: Simulate or run a corporate proxy and firewall locally for debugging

Some large companies have staff use computers running behind a corporate proxy and firewall without free access to the internet.

When making requests to an external service they need to supply a host, port and in some cases a username and password to authenticate.

As a library and service developer, for testing purposes I'd like to simulate such a proxy on my local development machine (Mac). Ideally I click a button / hit a command and it's enabled, click again, it's disabled. I can then test requests with the credentials provided and not provided in http requests to test whether my library handles them correctly.

Anyone found a good way to do this on a Mac?