Are there DNS servers that support versioning? The best solution I could imagine would simply be to set Tarsnap to normally use the current DNS records for S3, but be able to rollback to a valid zone record if they encounter an update that makes the servers stop resolving.
The Tarsnap client only talks to the Tarsnap server, but I do cache that lookup in order to avoid problems with glitchy DNS resolution.
I could have the Tarsnap server cache DNS lookups if I was only worried about working around DNS outages -- but as I said, that wasn't something I was considering at all when I made the decision to eschew DNS.
