drop-ssh-key.rb

#!/usr/bin/env ruby

require 'tempfile'

KNOWN_HOSTS = ENV['HOME'] + '/.ssh/known_hosts'

read_fh, write_fh = IO.pipe

command = ['ssh', '-oStrictHostKeyChecking=yes'] + ARGV + ['echo', 'Successful connection.']

fork do
  read_fh.close
  STDERR.reopen(write_fh)
  exec(*command)
end

write_fh.close
lines = read_fh.readlines

lines.each do |line|
  if line =~ /^Offending (?:(?:RSA|DSA|ECDSA) key|key for IP) in (.*):(\d+)\r?$/
    file, line = $1, $2
    raise "Unexpected file: #{file}" unless file == KNOWN_HOSTS

    File.open(KNOWN_HOSTS) do |in_fh|
      Tempfile.open('known_hosts', File.dirname(KNOWN_HOSTS)) do |out_fh|
        lines = in_fh.readlines
        lines.delete_at(line.to_i - 1)
        out_fh.puts(*lines)

        File.rename(KNOWN_HOSTS, KNOWN_HOSTS + '~')
        File.rename(out_fh.path, KNOWN_HOSTS)
      end
    end

    puts "Removed key from #{file}, line #{line}."
    exec(__FILE__, *ARGV)
  end
end

$stderr.puts *lines