How to find the local port a rails instance is running on?

You can call Rails::Server.new.options[:Port] to get the port that your Rails server is running on. This will parse the -p 3001 args from your rails server command, or default to port 3000.

From inside any controller action, check the content of request.port, thus:

class SomeController < ApplicationController
  def some_action
    raise "I'm running on port #{request.port}."
  end
end

Two ways.

If you're responding to a request in a controller or view, use the request object:

request.port

If you're in an initialiser and don't have access to the request object use the server options hash:

Rails::Server.new.options[:Port]

I played around with this a bit, and this might be the best solution for Rails 5.1:

Rails::Server::Options.new.parse!(ARGV)[:Port]

Building on the other answers (which saved my bacon!), I expanded this to give sane fallbacks:

In development:

port = Rails::Server::Options.new.parse!(ARGV)[:Port] || 3000 rescue 3000

In all other env's:

port = Rails::Server::Options.new.parse!(ARGV)[:Port] || 80 rescue 80

The rescue 80 covers you if you're running rails console. Otherwise, it raises NameError: uninitialized constant Rails::Server. (Maybe also early in initializers? I forget...)

The || 80 covers you if no -p option is given to server. Otherwise you get nil.