Fix bypass detection to correctly attribute encrypted DNS bypassing

firewall/bypassing.go

@@ -16,24 +16,44 @@ var resolverFilterLists = []string{"17-DNS"}
 // PreventBypassing checks if the connection should be denied or permitted
 // based on some bypass protection checks.
 func PreventBypassing(ctx context.Context, conn *network.Connection) (endpoints.EPResult, string, nsutil.Responder) {
+	// Exclude incoming connections.
+	if conn.Inbound {
+		return endpoints.NoMatch, "", nil
+	}
+
+	// Exclude ICMP.
+	switch packet.IPProtocol(conn.Entity.Protocol) { //nolint:exhaustive // Checking for specific values only.
+	case packet.ICMP, packet.ICMPv6:
+		return endpoints.NoMatch, "", nil
+	}
+
 	// Block firefox canary domain to disable DoH.
+	// This MUST also affect the System Resolver, because the return value must
+	// be correct for this to work.
 	if strings.ToLower(conn.Entity.Domain) == "use-application-dns.net." {
 		return endpoints.Denied,
 			"blocked canary domain to prevent enabling of DNS-over-HTTPs",
 			nsutil.NxDomain()
 	}
 
-	// Block direct connections to known DNS resolvers.
-	switch packet.IPProtocol(conn.Entity.Protocol) { //nolint:exhaustive // Checking for specific values only.
-	case packet.ICMP, packet.ICMPv6:
-		// Make an exception for ICMP, as these IPs are also often used for debugging.
-	default:
-		if conn.Entity.MatchLists(resolverFilterLists) {
-			compat.ReportSecureDNSBypassIssue(conn.Process())
-			return endpoints.Denied,
-				"blocked rogue connection to DNS resolver",
-				nsutil.BlockIP()
-		}
+	// Exclude DNS requests coming from the System Resolver.
+	// This MUST also affect entities in the secure dns filter list, else the
+	// System Resolver is wrongly accused of bypassing.
+	if conn.Type == network.DNSRequest && conn.Process().IsSystemResolver() {
+		return endpoints.NoMatch, "", nil
+	}
+
+	// Block bypass attempts using an encrypted DNS server.
+	switch {
+	case conn.Entity.Port == 853:
+		// Block connections to port 853 - DNS over TLS.
+		fallthrough
+	case conn.Entity.MatchLists(resolverFilterLists):
+		// Block connection entities in the secure dns filter list.
+		compat.ReportSecureDNSBypassIssue(conn.Process())
+		return endpoints.Denied,
+			"blocked rogue connection to DNS resolver",
+			nsutil.BlockIP()
 	}
 
 	return endpoints.NoMatch, "", nil