Anonymous proxy

Unlike a transparent proxy, an anonymous proxy does not pass on your real address. But it may leave a Via header or other signs of a proxy.

For a site this is a signal: the address is not yours, although who you are is unknown. Some sites let such requests through, others check them more closely.

The anonymity level applies only to headers. A site can still detect a proxy by the network type, address reputation or browser fingerprint.

The next level, with no signs of a proxy in the headers at all, is called elite.

Which headers define the level

Checkers look at several headers at once: Via, X-Forwarded-For, Forwarded, X-Real-IP and sometimes Proxy-Connection. A transparent proxy writes your IP into one of them. An anonymous one either names only itself, for example in Via, or fills the field with something other than your address.

The standard Forwarded header (RFC 7239) even has a dedicated value for this: for=unknown. Such a request shows that a proxy is at work, but not who stands behind it.

Why the level matters mostly for HTTP

When a program opens an HTTPS site through a proxy, it first sends a CONNECT request and then sets up an encrypted connection with the site through the tunnel. The proxy cannot add headers inside this connection, so Via and X-Forwarded-For simply never reach the site. The exception is networks with HTTPS inspection: there an intermediate node decrypts the traffic and can technically add any headers.

That is why the anonymity level is checked over plain HTTP: only there can a proxy change the request. Since most sites now work over HTTPS, the difference between anonymous and elite proxies shows up less often in practice.

Header anonymity and the anonymity filter

The proxy checker shows the anonymity level of a specific address and which headers it passes on. Prokso also has an anonymity filter in the pool settings, but it solves a different task: it filters out IPs that sites have already flagged as proxy or VPN. Headers and address reputation are two separate checks, and it is worth looking at both. The filter narrows the choice of addresses, so turn it on where it is really needed.