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I'm wondering what it would take to make a DNS caching service with updates based on reliable notifications rather than polling? After all, every cellphone does it.

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This already exists (NOTIFY), but it's only used for master-slave setups (ie. a bunch of DNS servers serving some authoritative zone who want changes to be transmitted to all slaves ASAP)

It would be interesting to (ab)use this mechanism in the way you suggest. A recursive DNS server could ask to be NOTIFY'ed of changes in the zone they are querying...it would, of course, add load to the server, and it would need strict limits to avoid DoS, but it seems an interesting idea.

The big problem, to the extent there is one, is between the client and the recursive server. Not as much the recursive and the authoritative. Cost is highly amortized between recursive and authoritative for busy names.

DNS Push Notifications[1] could be tacked on to resolvers and clients.

1: https://tools.ietf.org/html/draft-ietf-dnssd-push

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