net/dns: add custom scheme resolvers

If another part of the client code registers a custom scheme with the
forwarder, the forwarder will check resolver addresses to see if they
match the scheme. If they do, the corresponding custom scheme handler
will be called to find the actual address for the resolver at this
moment. If the handler returns the empty string then that resolver will
be ignored.

This is useful if you want to dynamically determine where to send
certain DNS requests. It is being added to support new app connector
(conn25) work that would like to make sure it sends DNS requests to the
current connector peer in a high availability configuration.

Updates tailscale/corp#39858

Signed-off-by: Fran Bull <fran@tailscale.com>

net/dns/resolver/tsdns.go

@@ -293,6 +293,18 @@ func (r *Resolver) SetConfig(cfg Config) error {
 	return nil
 }
 
+// CustomSchemeHandler takes a URI (retrieved from [dnstype.Resolver.Addr]) and
+// returns an updated URI to use for the current query. The result is only valid
+// for right now and may change over time.
+type CustomSchemeHandler func(addr string) (newAddr string, err error)
+
+// RegisterCustomScheme adds a [CustomSchemaHandler] that is called to provide
+// an updated address to the forwarder when a [dnstype.Resolver.Addr] uses that
+// scheme.
+func (r *Resolver) RegisterCustomScheme(scheme string, h CustomSchemeHandler) error {
+	return r.forwarder.RegisterCustomScheme(scheme, h)
+}
+
 // Close shuts down the resolver and ensures poll goroutines have exited.
 // The Resolver cannot be used again after Close is called.
 func (r *Resolver) Close() {