[gstaro]

Can't you do that with DNS records where there are multiple IPs on a A record?

Basically, we're already doing this for fault tolerance and load balancing within a single CDN. Except that currently we randomize the IPs. To enforce priorities, you'd want the IPs in the A record at least partially ordered by provider.

[toast0]

Multiple IPs on an A record works to some extent, most (many?) browsers will silently retry another ip from the list if some of the IPs don't accept a connection; I don't know if they'll try another IP on timeout though.

But you can't actually expect any ordering to make it through to the client. Your authoritative server may reorder the records, their recursive server may reorder the records, and the client resolution library may also reorder the records. There's actually an RFC advocating reordering records in client libraries; it's fairly misguided, but it exists in the wild. Reordering is also likely to happen in OS dns caches where those are used.

[toast0]

For reference, RFC 3484 [1] is the misguided RFC that tells people they should sort their DNS responses to maximize the common prefix between the source and destination address. This is probably helpful when the common prefix is meaningful, but when numbering agencies give out neighboring /24's to unrelated networks, and related networks often have ips widely distributed across the overall address space, it's not actually useful.

[1] https://www.ietf.org/rfc/rfc3484.txt

[gstaro]

Thanks for clarifying!

[nostrebored]

That's not quite what an anycast to those addresses does. It's more like an approximation of the nearest server.