ipn: delete domainsForProxying, require explicit DNS search domains (mapver 9) (#1078)

Previously the client had heuristics to calculate which DNS search domains
to set, based on the peers' names. Unfortunately that prevented us from
doing some things we wanted to do server-side related to node sharing.

So, bump MapRequest.Version to 9 to signal that the client only uses the
explicitly configured DNS search domains and doesn't augment it with its own
list.

Updates tailscale/corp#1026

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

wgengine/tsdns/tsdns.go

@@ -194,8 +194,8 @@ func (r *Resolver) Resolve(domain string, tp dns.Type) (netaddr.IP, dns.RCode, e
 	}
 
 	anyHasSuffix := false
-	for _, rootDomain := range dnsMap.rootDomains {
-		if strings.HasSuffix(domain, rootDomain) {
+	for _, suffix := range dnsMap.rootDomains {
+		if NameHasSuffix(domain, suffix) {
 			anyHasSuffix = true
 			break
 		}
@@ -611,3 +611,12 @@ func (r *Resolver) respond(query []byte) ([]byte, error) {
 
 	return marshalResponse(resp)
 }
+
+// NameHasSuffix reports whether the provided DNS name ends with the
+// component(s) in suffix, ignoring any trailing dots.
+func NameHasSuffix(name, suffix string) bool {
+	name = strings.TrimSuffix(name, ".")
+	suffix = strings.TrimSuffix(suffix, ".")
+	nameBase := strings.TrimSuffix(name, suffix)
+	return len(nameBase) < len(name) && strings.HasSuffix(nameBase, ".")
+}