Print TCP connection traffic to stdout.

tcp_tap.rb

#!/usr/bin/env ruby
require 'socket'
require 'thwait'

unless ARGV.size == 2
  STDERR.puts "Usage: #{$PROGRAM_NAME} listen_address:listen_port target_address:target_port"
  exit 1
end

listen_address, listen_port = ARGV.shift.split(':')
target_address, target_port = ARGV.shift.split(':')

def read_and_write prefix, from, to
  Thread.new do
    from.each do |line|
      puts "#{prefix} #{line}"
      STDOUT.flush
      to.write(line)
      to.flush
    end
  end
end

puts "Listening on #{listen_address}:#{listen_port}..."
tcp_server = TCPServer.new(listen_address, listen_port)
loop do
  client = tcp_server.accept
  addr_family, port, hostname, numeric_addr = *client.addr(true)
  puts "New connection from #{hostname} (#{numeric_addr}:#{port})."
  puts "Connecting to #{target_address}:#{target_port}."
  target_socket = TCPSocket.new(target_address, target_port)

  client_thread = read_and_write("<", client,        target_socket)
  target_thread = read_and_write(">", target_socket, client)

  client_thread.join
  puts "Closing connection from #{hostname} (#{numeric_addr}:#{port})."
  client.close
  target_socket.close
  target_thread.join rescue IOError
end