SSH local/remote proxy

ssh-proxy.rb

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

$opts = OpenStruct.new
parser = OptionParser.new do |parser|
  parser.banner = <<-EOS
#$0:
  SSH into a host via a proxy, but only if both have an IP address in common.
  This is typical of hosts hidden behind a NAT, and allows efficient connections when inside the NAT network.

Example for .ssh/config
  ProxyCommand ~/.ssh/#$0 <proxy.example.com> %h %p

Usage:
  #$0 [options] proxy-host host port
  EOS
  parser.on("-v", "--[no-]verbose", "Run verbosely"){|v| $opts.verbose = v}
end
parser.parse!

proxy_host = ARGV.shift
host = ARGV.shift
port = ARGV.shift || 22

unless host
  puts parser
  exit 1
end

dns = Resolv::DNS.new

$stderr << "#$0: ... #{proxy_host} ... #{host}:#{port}\n" if $opts.verbose

proxy_addrs = dns.getaddresses(proxy_host)
host_addrs = dns.getaddresses(host)

if (proxy_addrs & host_addrs).empty? or proxy_host == host
  # proxy and host have the different addresses -- contact host directly
  cmd = "nc #{host} #{port}"
else
  # proxy and host have the same address -- we're likely outside the network, so proxy in
  cmd = "ssh -W #{host}:#{port} #{proxy_host} 2> /dev/null"
end

$stderr << "#$0: exec #{cmd}\n" if $opts.verbose
exec cmd