A small Ruby tool making it easy to establish an SSH tunnel

mole.rb

#!/usr/bin/env ruby
require 'optparse'
require 'ostruct'

# Extension for colored Strings
class String
  def red()     colored(:red);     end
  def green()   colored(:green);   end
  def yellow()  colored(:yellow);  end
  def blue()    colored(:blue);    end
  def magenta() colored(:magenta); end
  def cyan()    colored(:cyan);    end

  def colored(color=nil)
    code = case color
    when :red     then 31
    when :green   then 32
    when :yellow  then 33
    when :blue    then 34
    when :magenta then 35
    when :cyan    then 36
    end
    self.gsub!('<b>', "\033[1m")
    self.gsub!('</b>', "\033[22m")
    self.gsub!('<blink>', "\033[5m")
    self.gsub!('</blink>', "\033[25m")
    "\033[#{code}m#{self}\033[0m"
  end
end

# Setting default options
options = OpenStruct.new({
  :local => OpenStruct.new({
    :host => `hostname -f`.chomp,
    :port => 2000,
  }),
  :proxy => OpenStruct.new({
    :host => 'proxy.example.com',
    :ssh => OpenStruct.new({
      :user => nil,
      :port => nil,
    }),
  }),
  :target => OpenStruct.new({
    :host => 'target.example.com',
    :port => 3306,
  }),
})

# Draws a graph of the tunnel according to options
def graph(options)
  result = "Port <b>#{options.local.port}</b> of #{options.local.host}</b>".green
  result << " ➜  "
  if options.proxy.host
    result << "<b>#{options.proxy.ssh.user}@</b>".yellow if options.proxy.ssh.user
    result << "<b>#{options.proxy.host}</b>".yellow
    result << " (using port <b>#{options.proxy.ssh.port}</b> for SSH access)".yellow if options.proxy.ssh.port
    result << ' ➜  '
  end
  result << "Port <b>#{options.target.port}</b> of <b>#{options.target.host}</b>".red
  result
end

help = ''
OptionParser.new do |parser|
  parser.banner = ""
  parser.separator "  TUNNEL ENTRANCE".green
  parser.on("--local-port=PORT", "The tunnel begins on this port on <b>#{options.local.host}</b> ".colored) do |value|
    options.local.port = value
  end
  parser.separator ""
  parser.separator "  TUNNEL PROXY".yellow + " (optional)"
  parser.on("--proxy-host=HOST", "If this is set, the Tunnel will go via this server. ") do |value|
    options.proxy.host = value
  end
  parser.on("--proxy-user=USER", "The SSH user for connecting to the proxy server <b>#{options.proxy.host ? options.proxy.host + ' ' : ''}</b>".colored) do |value|
    options.proxy.ssh.user = value
  end
  parser.on("--proxy-port=PORT", "The SSH port for connecting to the proxy server <b>#{options.proxy.host ? options.proxy.host + ' ' : ''}</b>".colored) do |value|
    options.proxy.ssh.port = value
  end
  parser.separator ""
  parser.separator "  TUNNEL EXIT".red
  parser.on("--target-host=HOST", 'The server you\'re trying to establish a tunnel to. Use "localhost" if it\'s the proxy.'.colored) do |value|
    options.target.host = value
  end
  parser.on("--target-port=PORT", "The port on the target-host at the very end of the tunnel.".colored) do |value|
    options.target.port = value
  end
  parser.on('--go', 'Establishes the tunnel') do |value|
    options.go = true
  end
  parser.separator ""
  help = parser.help
end.order!

proxy_user_addon = options.proxy.ssh.user ? "#{options.proxy.ssh.user}@" : ''
proxy_port_addon = options.proxy.ssh.port ? "-p #{options.proxy.ssh.port} " : ''
command = %{ssh #{proxy_user_addon}#{options.proxy.host} #{proxy_port_addon}-N -L #{options.local.port}:#{options.target.host}:#{options.target.port}}

if options.go
  puts
  puts "  Establishing tunnel..."
  puts '  ' + graph(options)
  puts
  puts '  ' + command
  puts
  exec command
else
  puts
  puts "  USAGE".cyan
  puts "        Add the parameter " + '--go'.cyan + ' to establish the tunnel according to the "current configuration" below.'.colored
  puts
  puts "  CURRENT CONFIGURATION".cyan
  puts "        #{graph(options)}"
  puts help
  puts "  COMMAND USED".cyan
  puts "        #{command}"
  puts
  exit
end

Running this in bash, you'll get this help message.

  USAGE
        Add the parameter --go to establish the tunnel according to the "current configuration" below.

  CURRENT CONFIGURATION
        Port 2000 of mycomputer.local ➜  proxy.example.com ➜  Port 3306 of target.example.com

  TUNNEL ENTRANCE
        --local-port=PORT            The tunnel begins on this port on mycomputer.local 

  TUNNEL PROXY (optional)
        --proxy-host=HOST            If this is set, the Tunnel will go via this server. 
        --proxy-user=USER            The SSH user for connecting to the proxy server proxy.example.com 
        --proxy-port=PORT            The SSH port for connecting to the proxy server proxy.example.com 

  TUNNEL EXIT
        --target-host=HOST           The server you're trying to establish a tunnel to. Use "localhost" if it's the proxy.
        --target-port=PORT           The port on the target-host at the very end of the tunnel.
        --go                         Establishes the tunnel

  COMMAND USED
        ssh proxy.example.com -N -L 2000:target.example.com:3306

Note that it presents to you the ssh command it would use (the last line shown above)