What's a good default TTL for DNS keys in the kernel

[David Howells <dhowells@redhat.com>]

Since key.dns_resolver isn't given a TTL for the address information obtained
for getaddrinfo(), no expiry is set on dns_resolver keys in the kernel for
NFS, CIFS or Ceph.  AFS gets one if it looks up a cell SRV or AFSDB record
because that is looked up in the DNS directly, but it doesn't look up A or
AAAA records, so doesn't get an expiry for the addresses themselves.

I've previously asked the libc folks if there's a way to get this information
exposed in struct addrinfo, but I don't think that ended up going anywhere -
and, in any case, would take a few years to work through the system.

For the moment, I think I should put a default on any dns_resolver keys and
have it applied either by the kernel (configurable with a /proc/sys/ setting)
or by the key.dnf_resolver program (configurable with an /etc file).

Any suggestion as to the preferred default TTL?  10 minutes?

David