Investigate if its safe to reuse request for recursive DNS

[tomwilkie]

I'm not sure you can just send the same request we get from the client to the upstream server. This could lead to a conflict if two clients send a request with the same message ID. I would make a copy...

[tomwilkie]

Just checked the dns code, and we should be making a copy and giving it a fresh id. We can use dns.Id() to generate fresh ids.

[rade]

What would the user-visible impact of an ID clash be?

[tomwilkie]

If two clients happened to send requests to us with the same id but for different hostnames, and we sent the request upstream as is, there is a chance we might mix up the replies from upstream and send the wrong reply back to the wrong client.

[rade]

@tomwilkie AFAICT the DNS code does not correlate replies by ID. A fresh UDP/TCP "connection" is established for every request, so correlation happens by virtue of receiving replies on distinct ephemeral ports. The library checks that the reply ID matches the request but only to error when it doesn't.