Restrict login to specific domain using Node Passport with Google Auth

Here's an example:

// first make sure you have access to the proper scope on your login route
app.get("/login", passport.authenticate("google", {
    scope: ["profile", "email"]
}));

// set up your Google OAuth strategy elsewhere...
passport.use(new GoogleStrategy({
    clientID: "something",
    clientSecret: "something",
    callbackURL: "/something"
}, function(token, refreshToken, profile, done){
    if(profile._json.hd === "yourdomain.com"){
        // find or create user in database, etc
        User.find({ id: profile.id }).done(done);
    }else{
        // fail        
        done(new Error("Invalid host domain"));
    }
});

And for good measure here's a full variable dump of what the "profile" variable looks like.

{ 
    provider: 'google',
    id: '12345678987654321',
    displayName: 'Don Draper',
    name: { familyName: 'Whitman', givenName: 'Richard' },
    emails: [ { value: '[email protected]' } ],
    _raw: 'a bunch of stringified json',
    _json: { 
        id: '123456789',
        email: '[email protected]',
        verified_email: true,
        name: 'Don Draper',
        given_name: 'Don',
        family_name: 'Draper',
        link: 'https://plus.google.com/123456789',
        picture: 'https://lh3.googleusercontent.com/XdUIqdMkCWA/AAAAAAAAAAI/AAAAAAAAAAA/123456789/photo.jpg',
        gender: 'male',
        locale: 'en',
        hd: 'yourdomain.com' 
    } 
}

Here are some detailed tutorials that should answer your question about the theory behind all of this. You'll want some combination of the two.

1. Local authentication and basic setup
2. Google authentication

I recommend a 2 step approach to this. Would love to hear feedback if this is overcomplicating it.

1) Help your users pick the right account

passport.authenticate('google', {
    // Only show accounts that match the hosted domain.
    hd: 'example.com',
    // Ensure the user can always select an account when sent to Google.
    prompt: 'select_account',
    scope: [
        'https://www.googleapis.com/auth/plus.login',
        'https://www.googleapis.com/auth/plus.profile.emails.read'
    ]
})(req, res, next);

2) Validate their profile

When a user is sent to accounts.google.com to authenticate, there is a simple hd=example.com query parameter in the URL. You can remove this and authenticate with any account (Passport will successfully verify the Oauth code regardless of the domain of the chosen account), so it should only be considered sugar for the end user and not security for the server.

When Passport does resolve the authentication, just check the hosted domain as in aembke's answer:

passport.use(new google_strategy({
    clientID: ...
    clientSecret: ...
    callbackURL: ...
}, function(token, tokenSecret, profile, done) {

    if (profile._json.domain !== 'example.com') {
        done(new Error("Wrong domain!"));
    } else {
        done(null, profile);
    }

}));