gistfile1.txt

begin
  puts resolver.getaddress('foo.example.org')
rescue Exception => e
  puts e.class
  puts e.message
end

# DNS server not available
$ ruby foo.rb 
Resolv::ResolvError
DNS result has no information for foo.example.org

# DNS server available but no record for foo.example.org
$ ruby foo.rb 
Resolv::ResolvError
DNS result has no information for foo.example.org