> Is there a spectacular downside to doing so? Since the last-known-good resolution would only be used if a TTL-specified refresh failed, I don't see much downside.
Because you would keep old DNS records around forever if a server goes away for good. So you need to have a timeout for that anyways.
1) Memory and disk are cheap. My caching DNS resolver can handle some stale records.
2) I suggested above that this behavior would continue until an administrator-specified and potentially quite generous maximum TTL expires. That is, I could configure my caching DNS resolver to fully purge expired records after, for example, 2 weeks.
> 1) Memory and disk are cheap. My caching DNS resolver can handle some stale records.
The problem is not that it would require storage but that stale records can be outright wrong. That timeout would require configuration and DNS does not provide that.
So sure, a new timeout could be introduced but that currently does not exist in DNS.
> The problem is not that it would require storage but that stale records can be outright wrong.
Again, the scenario is that the authoritative/upstream resolver cannot be reached in order to refresh after the authority-provided TTL expires. Are you saying that in the case of a service having been intentionally removed from the Internet (the domain is deactivated; the service is simply no more), my caching resolver will continue to resolve the domain for a time? Yes, it would. What's the downside though?
> That timeout would require configuration and DNS does not provide that.
Yes. This would be a configurable option in my caching DNS resolver, in the same vein as specifying the forwarders, roots, and so on. But to be clear, this would not be a change to the DNS protocol, merely a configuration change to control the cache expiration behavior of my resolver. I'm not wanting to sound flippant, I'm not sure I understand the point you're trying to make here.
If a server goes away for good, at some point NS records will stop pointing to it. We could serve stale records as long as all of the stale record's authority chain is either still there or unreachable.
I've had an IP address from a certain cloud provider for a month. Some abandoned domain still has its nameserver and glue records pointing to the IP, and i get DNS queries all the time.
The domain expires in January. I hope it's not set to auto-renew. :-)
Note that this is already happening. The only thing my proposal would change is that it would also affect servers that used to be authoritative for subdomains of such abandoned domains. I would expect there to be very few of them: very few domains have delegations of subdomains to a different DNS server and they are larger and thus less likely to be abandoned.
Because you would keep old DNS records around forever if a server goes away for good. So you need to have a timeout for that anyways.