Method to test RDLENGTH == 0 #866

For Dynamic DNS Updates (RFC 2136), the Server must perform the following test:

3.2.1. For RRs in this section whose CLASS is ANY, test to see that
TTL and RDLENGTH are both zero (0), else signal FORMERR to the
requestor.

This version of the DNS library does not expose the RDLENGTH value. It would be useful to expose a helper function that can be used to test if any arbitrary RDATA is empty. Right now I could only figure out how to test individual RDATAs. If there is already a generic method for this, let me know!

hmm, yes. This is indeed impossible to do. Adding a method just for this check seems overkill, although an Empty() method or function for both RRs, EDNS0s and RDATAs might make sense? For RDATA you can just compare to the empty struct, this code can be generated, even outside of the library (has to be a function then, but that's it)

Like dyn. update has other use cases to check for empty rdata? Outside of that area I don't know if this would useful?

In dynamic update, this check is just used for request validation; in theory there’s no security issue with choosing to ignore the RDATA in those cases.

I don’t think it’s always possible to detect empty RDATA by comparing with the default-constructed type: all-zeros could be a valid RDATA. I’m not sure what the right answer is.

all zeros would be something that is different than no rdata, comparing to an empty rdata.XXX is the only way. But if this is the only use-case (iirc there might be another in dyn updates...?) you might be better off just writing or generating your own