Get IP in PassportJS / ExpressJS

I'm using PassportJS in an ExpressJS web app.

In app.js:

//passportjs google
passport.use(new GoogleStrategy({
    clientID: '...',
    clientSecret: '...',
    callbackURL: '...'
  }, user.oauth2Login
));

app.get('/auth/google',
  passport.authenticate('google', { scope: 'email' }));

app.get('/auth/google/callback', 
    passport.authenticate('google', { successReturnToOrRedirect: '/home', failureRedirect: '/login' }))

In my user modulue:

exports.oauth2Login = function(accessToken, refreshToken, profile, done) {
    //get ip here
}

How can I get the IP of the user in oauth2Login?