HTTP proxy is primarily designed to proxy communication over the HTTP protocol, handling HTTP requests and responses. However, this does not mean that HTTP proxies can only proxy the HTTP protocol. In fact, some HTTP proxy servers can be configured to support other protocols such as FTP, SMTP, etc., but this is not their standard use.

HTTP proxy has limitations when dealing with HTTPS protocol, as HTTPS protocol uses encryption techniques such as SSL/TLS to protect the security of data transmission. Due to these encryption techniques, HTTP proxies are unable to see or modify encrypted data. Therefore, when users access HTTPS websites through HTTP proxies, the proxy server can only forward encrypted packets and cannot view or modify them. This limits the ability of HTTP proxies to handle the HTTPS protocol.

If you need to proxy encrypted HTTPS traffic, you usually need to use a proxy that supports SSL/TLS, such as HTTPS proxy or SSL proxy. These proxies are capable of decrypting and re encrypting data, allowing proxy servers to view and modify HTTPS traffic.

Overall, although HTTP proxies are primarily used to proxy the HTTP protocol, they can also be configured to support other protocols. However, for HTTPS protocols that require encryption protection, specialized HTTPS proxies or SSL proxies are usually needed to handle them.