Add fallback protocol support

cmd/protoplex/protoplex.go

@@ -39,6 +39,7 @@ func main() {
 	socks5 := app.Flag("socks5", "The SOCKS5 server address").String()
 	socks4 := app.Flag("socks4", "The SOCKS4 server address").String()
 	// stRelay := flag.String("strelay", "", "The Syncthing Relay server address")
+	fallback := app.Flag("fallback", "The fallback address. If no protocol match, this address will be used.").String()
 
 	_, _ = app.Parse(os.Args[1:])
 
@@ -83,6 +84,9 @@ func main() {
 	if *http != "" {
 		p = append(p, protocols.NewHTTPProtocol(*http))
 	}
+	if *fallback != "" {
+		p = append(p, protocols.FallbackProtocol(*fallback))
+	}
 
 	protoplex.RunServer(*bind, p, logger)
 }

protoplex/protocols/fallback.go

@@ -0,0 +1,10 @@
+package protocols
+
+// FallbackProtocol initializes a Protocol with a SOCKS4 signature.
+func FallbackProtocol(targetAddress string) *Protocol {
+	return &Protocol{
+		Name:       "Fallback",
+		Target:     targetAddress,
+		MatchBytes: [][]byte{{}},
+	}
+}