Http error: 305 – Use Proxy
The 305 Use Proxy status code is deprecated and should no longer be used. It was originally intended to signify that the requested resource was only available through a proxy server.
The problem with using the 305 code is that it’s not very specific. It doesn’t say why the proxy is being used, or whether it’s even required. This can lead to confusion on the part of both users and search engines.
It’s better to use a more specific status code, such as 302 Found or 307 Temporary Redirect, if possible. If you must use 305, make sure to include information in the response headers that explains why a proxy is being used.
305 Use Proxy is no longer an official HTTP status code as of June 2014. Googlebot will no longer obey this code.
RFC#: 7231 Section 6.4.5