Why does the host address is included in HTTP 1.1 GET command?

Question

GET /calcuapp/calculator.jsp HTTP/1.1 
Host: 192.168.1.66:8080

I'm using PuTTy and the host destination is already set up on the settings. Why do I need again to type the host destination as you can see above?

Answer 1

Your local resolver (DNS etc) converts the host name on the command line to an IP address before connecting; there is no way for the remote server to know which host name you gave on the command line if there are multiple host names which resolve to the same IP address (this is what's called "virtual hosting"; with HTTP 1.0, you needed a separate IP address for each distinct HTTP host, which is extremely wasteful, but saves you from needing to transmit the Host: header).

Answer 2

The Host HTTP header is mandatory since HTTP/1.1 and it's used for virtual hosting.

It must include the domain name of the server, and the TCP port number on which the server is listening. The port number may be omitted if the port is the standard port for the service requested (80 for HTTP and 443 for HTTPS).

A HTTP/1.1 request that lacks the Host header should be responded with a 400 (Bad Request) status code.

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The RFC 7230, the current reference message syntax and routing in HTTP/1.1, tells the whole story about this header:

5.4. Host

The Host header field in a request provides the host and port information from the target URI, enabling the origin server to distinguish among resources while servicing requests for multiple host names on a single IP address.

Host = uri-host [ ":" port ]

A client MUST send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then a client MUST send a field-value for Host that is identical to that authority component, excluding any userinfo subcomponent and its @ delimiter. If the authority component is missing or undefined for the target URI, then a client MUST send a Host header field with an empty field-value.

Since the Host field-value is critical information for handling a request, a user agent SHOULD generate Host as the first header field following the request-line.

For example, a GET request to the origin server for http://www.example.org/pub/WWW/ would begin with:

GET /pub/WWW/ HTTP/1.1
Host: www.example.org

A client MUST send a Host header field in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.

When a proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. A proxy that forwards such a request MUST generate a new Host field-value based on the received request-target rather than forward the received Host field-value.

Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache, without first verifying that the intercepted connection is targeting a valid IP address for that host.

A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field or a Host header field with an invalid field-value.

Answer 3

The short answer is Virtual Hosts.

For many years now, it has been quite common to host multiple sites/domains from a single server. HTTP 1.1 supports this by requiring the host header. If you use HTTP 1.0 you may leave this out.