net/dns: add new Config that captures tailscale+OS DNS config.

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

net/dns/config.go

@@ -8,6 +8,34 @@ import (
 	"inet.af/netaddr"
 )
 
+// Config is a DNS configuration.
+type Config struct {
+	// DefaultResolvers are the DNS resolvers to use for DNS names
+	// which aren't covered by more specific per-domain routes below.
+	// If empty, the OS's default resolvers (the ones that predate
+	// Tailscale altering the configuration) are used.
+	DefaultResolvers []netaddr.IPPort
+	// Routes maps a DNS suffix to the resolvers that should be used
+	// for queries that fall within that suffix.
+	// If a query doesn't match any entry in Routes, the
+	// DefaultResolvers are used.
+	Routes map[string][]netaddr.IPPort
+	// SearchDomains are DNS suffixes to try when expanding
+	// single-label queries.
+	SearchDomains []string
+	// Hosts maps DNS FQDNs to their IPs, which can be a mix of IPv4
+	// and IPv6.
+	// Queries matching entries in Hosts are resolved locally without
+	// recursing off-machine.
+	Hosts map[string][]netaddr.IP
+	// AuthoritativeSuffixes is a list of fully-qualified DNS suffixes
+	// for which the in-process Tailscale resolver is authoritative.
+	// Queries for names within AuthoritativeSuffixes can only be
+	// fulfilled by entries in Hosts. Queries with no match in Hosts
+	// return NXDOMAIN.
+	AuthoritativeSuffixes []string
+}
+
 // OSConfig is an OS DNS configuration.
 type OSConfig struct {
 	// Nameservers are the IP addresses of the nameservers to use.