Starcraft – What do I do when an opponent builds a refinery in the base

If he's got cannons + gateways just outside your ramp, chances are he has minimal forces at his main base. Get a few mutalisks and/or Nydus worms to harass.

You might want to build a second hatchery to facilitate unit production. The reason a contain (not a 'cannon rush', as you labeled it) works so well against Zerg is because they typically need that second hatchery to keep up with (or surpass, usually) their opponent's unit production.

If you can, use your overlords to scout -- a protoss cannon contain might be annoying, but until he expands, you're just as well off economically as him.

It sounds like your problem is less, "I need a counter" and more "How could I have known?"

First off, you said you scouted him but he proxy'd his Barracks (hid it outside of his base). Let's go over the ways you could have still seen this coming:

• Usually not seeing a Barracks is a good sign he's Proxying.
• Watching for a very early Vespean Refinery is usually a sign of some sort of early Marauder/Reaper play
• Finally, many top Protoss players default into 1 Gate -> Cybernetics Core to set themselves up to stop this.

What else could you have done?

Some guide will suggest skipping that first Zealot, but if you can squeeze him out (Zealot has a 33 sec build time compared to the Cybernetics Core's 60) he serves as a good distraction to keep the reapers off your probes.

Make sure to save some extra chrono boosts to boost out a Stalker. While early game you should be using these on Probes, once you start building the Cybernetics Core stop using them and save them for the Stalker.

Another problem you had was a Bunker slipping in. There are a couple ways to stop this, but most prominently walling off your ramp. You can do this with your Gateway/Cybernetics Core, or leave space and place that first Zealot there. Alternatively, you can build an extra pylon closing off that small space with plans to kill it later.

Once he does start building the Bunker, you can use either your Stalker or even your Probes to try and kill it during production (or even better the SCV building it). The follow up will usually be a large number of Marauders so you can either push out Zealots to deal with this (its unlikely he'll have concussive shell this early), or a Sentry. With the Sentry you'll want to use force field at the bottom of your ramp to keep him from moving up and gaining sight. You can then use your Stalker to prevent Reapers from giving sight and to target the helpless Marauders.

In any case you should consider your next move. Are you going to go Robo? get out an early Immortal. Or do you want to throw up 2-3 Gate and out produce him (as he's seriously cut production).

One of the things you'll notice is that as you continue to play, the time it takes you to get out that first stalker will rapidly decrease. A Reaper rush really trades on the opponent not seeing it, and not being very experienced.