Proxy for mocking requests to remote servers

I'am testing my iOS app, that use internet for getting data. Sometimes internet is unavailable, so I need a way to redirect remote connection to file on my local computer. I'am only need to test api. I have unit-tests with mocks,but they do not solve the problem completely.

I can setup server, and redirect connections to localhost, but I think there is a less complex and more accurate solution. Do you have any ideas? Thank you.