net/dns: handle all possible translations of high-level DNS config.

With this change, all OSes can sort-of do split DNS, except that the
default upstream is hardcoded to 8.8.8.8 pending further plumbing.
Additionally, Windows 8-10 can do split DNS fully correctly, without
the 8.8.8.8 hack.

Part of #953.

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

net/dns/manager.go

@@ -5,6 +5,7 @@
 package dns
 
 import (
+	"strings"
 	"time"
 
 	"inet.af/netaddr"
@@ -50,49 +51,151 @@ func NewManager(logf logger.Logf, oscfg OSConfigurator, linkMon *monitor.Mon) *M
 	return m
 }
 
+// forceSplitDNSForTesting alters cfg to be a split DNS configuration
+// that only captures search paths. It's intended for testing split
+// DNS until the functionality is linked up in the admin panel.
+func forceSplitDNSForTesting(cfg *Config) {
+	if len(cfg.DefaultResolvers) == 0 {
+		return
+	}
+
+	if cfg.Routes == nil {
+		cfg.Routes = map[string][]netaddr.IPPort{}
+	}
+	for _, search := range cfg.SearchDomains {
+		cfg.Routes[search] = cfg.DefaultResolvers
+	}
+	cfg.DefaultResolvers = nil
+}
+
 func (m *Manager) Set(cfg Config) error {
 	m.logf("Set: %+v", cfg)
 
-	if len(cfg.DefaultResolvers) == 0 {
-		// TODO: make other settings work even if you didn't set a
-		// default resolver. For now, no default resolvers == no
-		// managed DNS config.
-		cfg = Config{}
+	if false {
+		// Temporary, for danderson to test things.
+		forceSplitDNSForTesting(&cfg)
 	}
 
-	resolverCfg := resolver.Config{
-		Hosts:        cfg.Hosts,
-		LocalDomains: cfg.AuthoritativeSuffixes,
-		Routes:       map[string][]netaddr.IPPort{},
-	}
-	osCfg := OSConfig{
-		SearchDomains: cfg.SearchDomains,
-	}
-	// We must proxy through quad-100 if MagicDNS hosts are in
-	// use, or there are any per-domain routes.
-	mustProxy := len(cfg.Hosts) > 0 || len(cfg.Routes) > 0
-	if mustProxy {
-		osCfg.Nameservers = []netaddr.IP{tsaddr.TailscaleServiceIP()}
-		resolverCfg.Routes["."] = cfg.DefaultResolvers
-		for suffix, resolvers := range cfg.Routes {
-			resolverCfg.Routes[suffix] = resolvers
-		}
-	} else {
-		for _, resolver := range cfg.DefaultResolvers {
-			osCfg.Nameservers = append(osCfg.Nameservers, resolver.IP)
-		}
-	}
+	rcfg, ocfg := m.compileConfig(cfg)
 
-	if err := m.resolver.SetConfig(resolverCfg); err != nil {
+	m.logf("Resolvercfg: %+v", rcfg)
+	m.logf("OScfg: %+v", ocfg)
+
+	if err := m.resolver.SetConfig(rcfg); err != nil {
 		return err
 	}
-	if err := m.os.SetDNS(osCfg); err != nil {
+	if err := m.os.SetDNS(ocfg); err != nil {
 		return err
 	}
 
 	return nil
 }
 
+// compileConfig converts cfg into a quad-100 resolver configuration
+// and an OS-level configuration.
+func (m *Manager) compileConfig(cfg Config) (resolver.Config, OSConfig) {
+	// Deal with trivial configs first.
+	switch {
+	case !cfg.needsOSResolver():
+		// Set search domains, but nothing else. This also covers the
+		// case where cfg is entirely zero, in which case these
+		// configs clear all Tailscale DNS settings.
+		return resolver.Config{}, OSConfig{
+			SearchDomains: cfg.SearchDomains,
+		}
+	case cfg.hasDefaultResolversOnly():
+		// Trivial CorpDNS configuration, just override the OS
+		// resolver.
+		return resolver.Config{}, OSConfig{
+			Nameservers:   toIPsOnly(cfg.DefaultResolvers),
+			SearchDomains: cfg.SearchDomains,
+		}
+	case cfg.hasDefaultResolvers():
+		// Default resolvers plus other stuff always ends up proxying
+		// through quad-100.
+		rcfg := resolver.Config{
+			Routes: map[string][]netaddr.IPPort{
+				".": cfg.DefaultResolvers,
+			},
+			Hosts:        cfg.Hosts,
+			LocalDomains: addFQDNDots(cfg.AuthoritativeSuffixes),
+		}
+		for suffix, resolvers := range cfg.Routes {
+			rcfg.Routes[suffix+"."] = resolvers
+		}
+		ocfg := OSConfig{
+			Nameservers:   []netaddr.IP{tsaddr.TailscaleServiceIP()},
+			SearchDomains: cfg.SearchDomains,
+		}
+		return rcfg, ocfg
+	}
+
+	// From this point on, we're figuring out split DNS
+	// configurations. The possible cases don't return directly any
+	// more, because as a final step we have to handle the case where
+	// the OS can't do split DNS.
+	var rcfg resolver.Config
+	var ocfg OSConfig
+
+	if !cfg.hasHosts() && cfg.singleResolverSet() != nil && m.os.SupportsSplitDNS() {
+		// Split DNS configuration requested, where all split domains
+		// go to the same resolvers. We can let the OS do it.
+		return resolver.Config{}, OSConfig{
+			Nameservers:   toIPsOnly(cfg.singleResolverSet()),
+			SearchDomains: cfg.SearchDomains,
+			MatchDomains:  cfg.matchDomains(),
+		}
+	}
+
+	// Split DNS configuration with either multiple upstream routes,
+	// or routes + MagicDNS, or just MagicDNS, or on an OS that cannot
+	// split-DNS. Install a split config pointing at quad-100.
+	rcfg = resolver.Config{
+		Routes:       map[string][]netaddr.IPPort{},
+		Hosts:        cfg.Hosts,
+		LocalDomains: addFQDNDots(cfg.AuthoritativeSuffixes),
+	}
+	for suffix, resolvers := range cfg.Routes {
+		rcfg.Routes[suffix+"."] = resolvers
+	}
+	ocfg = OSConfig{
+		Nameservers:   []netaddr.IP{tsaddr.TailscaleServiceIP()},
+		SearchDomains: cfg.SearchDomains,
+	}
+
+	// If the OS can't do native split-dns, read out the underlying
+	// resolver config and blend it into our config.
+	// TODO: for now, use quad-8 as the upstream until more plumbing
+	// is done.
+	if m.os.SupportsSplitDNS() {
+		ocfg.MatchDomains = cfg.matchDomains()
+	} else {
+		rcfg.Routes["."] = []netaddr.IPPort{netaddr.MustParseIPPort("8.8.8.8:53")}
+	}
+
+	return rcfg, ocfg
+}
+
+func addFQDNDots(domains []string) []string {
+	ret := make([]string, 0, len(domains))
+	for _, dom := range domains {
+		ret = append(ret, strings.TrimSuffix(dom, ".")+".")
+	}
+	return ret
+}
+
+// toIPsOnly returns only the IP portion of ipps.
+// TODO: this discards port information on the assumption that we're
+// always pointing at port 53.
+// https://github.com/tailscale/tailscale/issues/1666 tracks making
+// that not true, if we ever want to.
+func toIPsOnly(ipps []netaddr.IPPort) (ret []netaddr.IP) {
+	for _, ipp := range ipps {
+		ret = append(ret, ipp.IP)
+	}
+	return ret
+}
+
 func (m *Manager) EnqueueRequest(bs []byte, from netaddr.IPPort) error {
 	return m.resolver.EnqueueRequest(bs, from)
 }