tailcfg: add FallbackResolvers to DNSConfig.

Signed-off-by: David Anderson <danderson@tailscale.com>

tailcfg/tailcfg.go

@@ -849,6 +849,10 @@ type DNSConfig struct {
 	// Map keys must be fully-qualified DNS name suffixes, with a
 	// trailing dot but no leading dot.
 	Routes map[string][]DNSResolver `json:",omitempty"`
+	// FallbackResolvers is like Resolvers, but is only used if a
+	// split DNS configuration is requested in a configuration that
+	// doesn't work yet without explicit default resolvers.
+	FallbackResolvers []DNSResolver `json:",omitempty"`
 	// Domains are the search domains to use.
 	// Search domains must be FQDNs, but *without* the trailing dot.
 	Domains []string `json:",omitempty"`
@@ -865,8 +869,6 @@ type DNSConfig struct {
 	Nameservers []netaddr.IP `json:",omitempty"`
 
 	// PerDomain is not set by the control server, and does nothing.
-	// TODO(danderson): revise DNS configuration to make this useful
-	// again.
 	PerDomain bool `json:",omitempty"`
 }
 

tailcfg/tailcfg_clone.go

@@ -198,6 +198,10 @@ func (src *DNSConfig) Clone() *DNSConfig {
 			dst.Routes[k] = append([]DNSResolver{}, src.Routes[k]...)
 		}
 	}
+	dst.FallbackResolvers = make([]DNSResolver, len(src.FallbackResolvers))
+	for i := range dst.FallbackResolvers {
+		dst.FallbackResolvers[i] = *src.FallbackResolvers[i].Clone()
+	}
 	dst.Domains = append(src.Domains[:0:0], src.Domains...)
 	dst.Nameservers = append(src.Nameservers[:0:0], src.Nameservers...)
 	return dst
@@ -208,6 +212,7 @@ func (src *DNSConfig) Clone() *DNSConfig {
 var _DNSConfigNeedsRegeneration = DNSConfig(struct {
 	Resolvers         []DNSResolver
 	Routes            map[string][]DNSResolver
+	FallbackResolvers []DNSResolver
 	Domains           []string
 	Proxied           bool
 	Nameservers       []netaddr.IP