The proxy string may be specified with a protocol:// prefix to specify alternative proxy protocols. (Added in 7.21.7)
If no protocol is specified in the proxy string or if the string does not match a supported one, the proxy is treated as an HTTP proxy.
The supported proxy protocol prefixes are as follows:
Makes it use it as an HTTP proxy. The default if no scheme prefix is used.
Makes it treated as an HTTPS proxy.
Makes it the equivalent of --socks4
Makes it the equivalent of --socks4a
Makes it the equivalent of --socks5
Makes it the equivalent of --socks5-hostname