Number of additional records in queries

[Tony Finch]

Petr Špaček via Unbound-users <unbound-users at lists.nlnetlabs.nl> wrote:
>
> If any future protocol extension makes use of new RR type in additional
> section and these are silently ignored then we have no way of detecting
> if the new extension is supported or not.

I don't think you can design a DNS protocol extension on the assumption
that servers will reject extra records in a query! Many servers won't!
The extension would have to say that the server must indicate that it
knows what is going on by putting an extension response record in the
answer (like TKEY, TSIG, EDNS do).

There's a downside to eagerly rejecting the entire query if it contains
anything unexpected: a client probing for a new feature will have to retry
with a simpler request, which is slower and more complicated than getting
a useful response that omits some of the parts that the client wanted. If
you have multiple extensions you can end up with a combinatorial explosion
of retry attempts :-(

But that's basically an argument for using a well-defined extension
framework, i.e. EDNS, that is designed so that extensions can coexist.
There's also the perennial discussion about whether it's best to design
protocol extensions as must-understand or must-ignore: for the basic DNS
it's unspecified; for EDNS and DSO it's must-ignore; for TSIG it's
must-understand.

Tony.
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