DNSSD based queries fail to provide TTL

[gmilos]

On macOS (where DNSSD is being used by default), queries such as:

let resolver = try AsyncDNSResolver()
let out = try await resolver.queryAAAA(name: "iadsdk.apple.com.akadns.net")
print("\(out)")

fail to provide the TTL:

[AAAARecord(address=2a02:26f0:5d00:f91::12c5, ttl=), AAAARecord(address=2a02:26f0:5d00:f9b::12c5, ttl=)]

Despite the fact TTL is certainly available:

$ dig AAAA iadsdk.apple.com.akadns.net
...
e4805.dsca.akamaiedge.net. 3	IN	AAAA	2a02:26f0:5d00:f9b::12c5
                           ↑
                          TTL
...

The TTL is available, if the c-ares underlying implementation is used:

let resolver = try AsyncDNSResolver(CAresDNSResolver())
let out = try await resolver.queryAAAA(name: "e4805.dsca.akamaiedge.net")
print("\(out)") 

results in:

[AAAARecord(address=2a02:26f0:5d00:f91::12c5, ttl=13), AAAARecord(address=2a02:26f0:5d00:f9b::12c5, ttl=13)]

Tested on: https://github.com/apple/swift-async-dns-resolver/releases/tag/0.4.0

[danieleggert]

1. Generally, all result records should have TTL info, not just A / AAAA records. E.g. the MX record currently has no TTL, even though it’s in the DNS response.

2. I’m not entirely sure, but it seems odd to have the TTL be optional. Don’t the RFCs make TTL a mandatory part of any DNS response?

3. Generally, from an API perspective, it is odd to have the TTL be part of the record type (e.g. ARecord) since in the DNS world, the TTL is part of the response, but not the record. It seems like it would have been a better design to have something like

Response<A> {
    var record: A
    var ttl: Int
}

or similar, where A would then be ARecord, MXRecord, etc. Just a thought.