Feature: Cache DNS requests

[theblazehen]

When running Caddy as a reverse proxy with hostnames as the target, it appears to do a DNS request for each HTTP call.

It would be helpful it we could specify how long Caddy can cache a DNS result, something similar to valid as for example in http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver

I have found https://github.com/rs/dnscache which somewhat appears to be a drop in replacement for net.Resolver

[rumpelsepp]

The relevant DNS resolver should cache anyway, doesn't it?

[theblazehen]

I've seen higher-than-expected CPU usage when using systemd-resolved, but could change to dnsmasq perhaps if this feature doesn't get implemented.

[francislavoie]

Since we support srv://, it would need to not cache if srv is used. We will need to be careful with this.

[Forza-tng]

I've seen higher-than-expected CPU usage when using systemd-resolved, but could change to dnsmasq perhaps if this feature doesn't get implemented.

If DNS performance is needed I suggest to switch to Unbound or similar caching DNS server.

[francislavoie]

I think #4470 will essentially cover these requirements. A custom UpstreamSource module could be written which caches DNS queries. Or the AUpstreams implementation could be augmented to have optional caching.

But I feel like we might close this because this requirement doesn't affect the large majority of users. DNS is sufficiently fast for most everyone.

[mholt]

I don't understand why the system resolver isn't caching it, like why do we have to do the caching 🤔 More information / investigation / evidence would be useful here to be actionable.

Am OK with closing it, we can reopen this if it turns out to be an issue within Caddy.