ipn/ipnlocal, net/dns/resolver: use exit node's DoH proxy when available

Updates #1713

Change-Id: I3695a40ec12d2b4e6dac41cf4559daca6dddd68e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1974,12 +1974,32 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 		return dcfg
 	}
 
+	for _, dom := range nm.DNS.Domains {
+		fqdn, err := dnsname.ToFQDN(dom)
+		if err != nil {
+			logf("[unexpected] non-FQDN search domain %q", dom)
+		}
+		dcfg.SearchDomains = append(dcfg.SearchDomains, fqdn)
+	}
+	if nm.DNS.Proxied { // actually means "enable MagicDNS"
+		for _, dom := range magicDNSRootDomains(nm) {
+			dcfg.Routes[dom] = nil // resolve internally with dcfg.Hosts
+		}
+	}
+
 	addDefault := func(resolvers []dnstype.Resolver) {
 		for _, r := range resolvers {
 			dcfg.DefaultResolvers = append(dcfg.DefaultResolvers, normalizeResolver(r))
 		}
 	}
 
+	// If we're using an exit node and that exit node is new enough (1.19.x+)
+	// to run a DoH DNS proxy, then send all our DNS traffic through it.
+	if dohURL, ok := exitNodeCanProxyDNS(nm, prefs.ExitNodeID); ok {
+		addDefault([]dnstype.Resolver{{Addr: dohURL}})
+		return dcfg
+	}
+
 	addDefault(nm.DNS.Resolvers)
 	for suffix, resolvers := range nm.DNS.Routes {
 		fqdn, err := dnsname.ToFQDN(suffix)
@@ -2001,18 +2021,6 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 			dcfg.Routes[fqdn] = append(dcfg.Routes[fqdn], normalizeResolver(r))
 		}
 	}
-	for _, dom := range nm.DNS.Domains {
-		fqdn, err := dnsname.ToFQDN(dom)
-		if err != nil {
-			logf("[unexpected] non-FQDN search domain %q", dom)
-		}
-		dcfg.SearchDomains = append(dcfg.SearchDomains, fqdn)
-	}
-	if nm.DNS.Proxied { // actually means "enable MagicDNS"
-		for _, dom := range magicDNSRootDomains(nm) {
-			dcfg.Routes[dom] = nil // resolve internally with dcfg.Hosts
-		}
-	}
 
 	// Set FallbackResolvers as the default resolvers in the
 	// scenarios that can't handle a purely split-DNS config. See
@@ -3098,3 +3106,24 @@ func (b *LocalBackend) allowExitNodeDNSProxyToServeName(name string) bool {
 	}
 	return true
 }
+
+// exitNodeCanProxyDNS reports the DoH base URL ("http://foo/dns-query") without query parameters
+// to exitNodeID's DoH service, if available.
+//
+// If exitNodeID is the zero valid, it returns "", false.
+func exitNodeCanProxyDNS(nm *netmap.NetworkMap, exitNodeID tailcfg.StableNodeID) (dohURL string, ok bool) {
+	if exitNodeID.IsZero() {
+		return "", false
+	}
+	for _, p := range nm.Peers {
+		if p.StableID != exitNodeID {
+			continue
+		}
+		for _, s := range p.Hostinfo.Services {
+			if s.Proto == tailcfg.PeerAPIDNS && s.Port >= 1 {
+				return peerAPIBase(nm, p) + "/dns-query", true
+			}
+		}
+	}
+	return "", false
+}

net/dns/resolver/forwarder.go

@@ -423,8 +423,10 @@ func (f *forwarder) sendDoH(ctx context.Context, urlBase string, c *http.Client,
 // send expects the reply to have the same txid as txidOut.
 func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDelay) ([]byte, error) {
 	if strings.HasPrefix(rr.name.Addr, "http://") {
-		metricDNSFwdErrorType.Add(1)
-		return nil, fmt.Errorf("http:// resolvers not supported yet")
+		// TODO(bradfitz): this only work for TUN mode right now; plumb a universal dialer
+		// that can handle the dozen special cases for modes/platforms/routes.
+		TODOHTTPClient := http.DefaultClient
+		return f.sendDoH(ctx, rr.name.Addr, TODOHTTPClient, fq.packet)
 	}
 	if strings.HasPrefix(rr.name.Addr, "https://") {
 		metricDNSFwdErrorType.Add(1)