Enforce bracket notation for IPv6 bootstrap resolvers

The isIPAndPort validation function was accepting ambiguous IPv6
addresses without brackets (e.g., "2606:4700:4700::1113:53"). The
ExtractHostAndPort function would treat the last colon as a port
separator, incorrectly splitting the IPv6 address.

This fix enforces that IPv6 addresses must use bracket notation
(e.g., "[2606:4700:4700::1113]:53") to prevent ambiguity and ensure
correct parsing.

Author: jedisct1

dnscrypt-proxy/config.go

@@ -774,6 +774,11 @@ func isIPAndPort(addrStr string) error {
 		return fmt.Errorf("Port missing '%s'", addrStr)
 	} else if _, err := strconv.ParseUint(strconv.Itoa(port), 10, 16); err != nil {
 		return fmt.Errorf("Port does not parse '%s' [%v]", addrStr, err)
+	} else if ip.To4() == nil {
+		// IPv6 address must use bracket notation to avoid ambiguity
+		if !strings.HasPrefix(host, "[") || !strings.HasSuffix(host, "]") {
+			return fmt.Errorf("IPv6 addresses must use bracket notation, e.g., [%s]:%d", ip.String(), port)
+		}
 	}
 	return nil
 }