README.md

Usage

$ ruby fake_smtp_server

require "net/smtp"

smtp = Net::SMTP.new("localhost", 3555)
# smtp.enable_starttls_auto
smtp.read_timeout = 15
smtp.open_timeout = 15
smtp.set_debug_output $stderr
smtp.start("localhost", "usernam", "pass", :plain) { status = smtp.helo("localhost") }

fake_smtp_server

#!/usr/bin/env ruby

require 'socket'
require 'openssl'

class FakeSmtpServer
  def initialize(port)
    @port = port
    @socket = TCPServer.new(@port)
    @client = nil
    @ssl_client = nil
  end

  def start
    return unless @client.nil?

    puts "fake_smtp_server: Waiting for one connection to port #{@port} ..."
    @client = @socket.accept

    send "220 dummy-smtp.example.com SMTP"
    cmd = receive

    while cmd !~ /^QUIT\r/
      if cmd =~ /^HELO(.*)\r/
        send "250-Welcome to a dummy smtp server"
        send "250-STARTTLS"
        send "250-AUTH PLAIN LOGIN"
        send "250 Ok"
      elsif cmd =~ /^AUTH(.*)\r/
        send "235 2.7.0 Authentication successful"
      elsif cmd =~ /^STARTTLS\r/
        send "220 Ready to start TLS"
        @socket, @ssl_client = tlsconnect(@socket)
      else
        send "502 I am so dumb I only understand HELO, AUTH, STARTTLS and QUIT"
      end

      cmd = receive
    end
    send "221 Bye Bye"

    close
  end

  private

  def client
    @ssl_client || @client
  end

  def close
    @client.close unless @client.nil?
    @ssl_client.close unless @ssl_client.nil?
  end

  def send(line)
    client.puts line
    puts "-> #{line}"
  end

  def receive
    line = client.gets
    puts "<- #{line}"
    line
  end

  def ssl_socket(socket, context)
    OpenSSL::SSL::SSLServer.new(socket, context)
  end

  def ssl_context
    @_ssl_context ||= begin
      key, cert = generate_certificate

      c = OpenSSL::SSL::SSLContext.new
      c.key = key
      c.cert = cert
      c
    end
  end

  def tlsconnect(s)
    s = ssl_socket(s, ssl_context)
    puts "=> TLS connection started"
    client = s.accept
    puts "=> TLS connection accepted"

    [s, client]
  end

  def generate_certificate
    key = OpenSSL::PKey::RSA.new(2048)
    name = OpenSSL::X509::Name.parse('CN=nobody/DC=example')

    cert = OpenSSL::X509::Certificate.new
    cert.version = 2
    cert.serial = 0
    cert.not_before = Time.now
    cert.not_after = Time.now + 3600

    cert.public_key = key.public_key
    cert.subject = name

    extension_factory = OpenSSL::X509::ExtensionFactory.new nil, cert

    cert.add_extension extension_factory.create_extension('basicConstraints', 'CA:FALSE', true)
    cert.add_extension extension_factory.create_extension('keyUsage', 'keyEncipherment,dataEncipherment,digitalSignature')
    cert.add_extension extension_factory.create_extension('subjectKeyIdentifier', 'hash')

    cert.issuer = name
    cert.sign key, OpenSSL::Digest::SHA256.new

    [key, cert]
  end
end

FakeSmtpServer.new(3555).start

puts "fake_smtp_server: Exiting now the conversation has finished."
exit 0