What is the HTTP Host request header?

The HTTP Host request header is a required part of an HTTP request in HTTP/1.1 and later. It specifies the domain name (and optionally, the port number) of the server that the client is attempting to access. This is crucial because a single server may host multiple websites, each identified by a different domain name. Without the Host header, the server wouldn't know which site the user intends to reach.

Essentially, the `Host` header is formatted as "Host: domain_name[:port]", where `domain_name` is the hostname or IP address of the server and `port` is the optional port number (if a non-default port such as 80 for HTTP or 443 for HTTPS is used). For example, a request to `www.example.com` on the default HTTPS port would include the header `Host: www.example.com`. A request to `api.example.com:8080` would include `Host: api.example.com:8080`.

The `Host` header plays a vital role in web server functionalities like virtual hosting, where multiple websites share the same IP address. By examining this header, the web server can determine the appropriate configuration, file system location, and content to return to the user. If the `Host` header is missing or invalid, the server might return an error (such as HTTP error 400 - Bad Request), or it may serve the default website configured on that server.