Proxies of certain providers may leak your actual DNS server IP address. Likewise, accessing certain services through their affiliate DNS servers (e.g. accessing Google through Google DNS) may reveal your real IP address.
DNS leaks can be checked on the following test websites:
To prevent this kind of leaks, you may define your own DNS server in the settings of your browser profiles. Currently this setting is located in the "Other" section but will later be moved to a connection type configuration view. Note that custom DNS configuration can only work with SOCKS4/5 proxies, VPN, or direct connection!
In order to define a custom DNS values, you need to provide an actual existing DNS server. You may use the ones, which are publicly available. It's a good practice to add a local DNS from a location that corresponds to the location of your external IP. However, some worldwide DNS servers are widely used too. For example, CloudFlare DNS running on 220.127.116.11 address.
To define multiple DNS values, use a semi-colon ";' as a delimiter. Example: