DNS server example

[jschlyter]

I've written a simple DNS server example. Would it be useful to include as an example or perhaps even a start of a server class?

Features:

• Fully async using anyio (supporting both asyncio and trio backends)
• Support for UDP, TCP and TLS
• Pluggable query handling

[pspacek]

Just my point of view. I'm sure other people will have different opinions.

I can see the value because I had to write something similar several times already, but at the same time it is too much and too little because use-cases vary.

Perhaps we could start very minimal. Perhaps provide a class which can easily listen on UDP/TCP/DoT/DoH and calls a method every time a message comes in and nothing else? I guess that might be a minimal useful base which can be reused often. In other words, the logic how to handle packets is very different in different use-cases.

Example where IMHO it is going too far:

• DNSQueryException seems like going too far - the handler could just return arbitrary message no matter what happens
• a mechanism to return bytes instead of Message is often handy for exercising weird cases
• QR handling

[jschlyter]

I agree with the DNSQueryException going to far, I've removed it. I've also added support for returning bytes.

You think QR handling should done in query()? Would that also apply to multiple questions? (which might be something people like to experiment with)? Moving both of them to the ExampleDNSServer might make sense and keep things slim.

[pspacek]

I think it can be argued both ways. For my use-cases

        # Silently ignore messages that are themselves responses (QR flag
        # set); answering them could create a reflection loop between servers
        if query.flags & dns.flags.QR:
            self.logger.warning("Ignoring message with QR flag set")
            return None

        if len(query.question) != 1:
            self.logger.warning("Refusing query with %d questions", len(query.question))
            response = dns.message.make_response(query)
            response.set_rcode(dns.rcode.FORMERR)
            return [response]

is annoying because I would not want it most of the time. On the other hand other people might feel differently.

Perhaps we can find middle ground and move these into separate method which would be easy to override when needed?

[jschlyter]

I've now moved these checks to a separate function, validate_query, that can be replaced by pass if you want to drive without a seatbelt :-)

[jschlyter]

I've submitted a draft PR #1292