Lesson 11 · The Host header - Build an HTTP Client

The Host header

HTTP/1.1 requires every request to name its host in a Host header, because one server can host many sites. Today you derive that header automatically from the URL, including the port when it is not the default.

The goal

Set the Host header on a request from its URL, appending the port only when it is not the scheme default.

Start here - the target

TO DO

Scenario: Deriving the Host header

Givena request whose URL is parsed from "http://example.com/a"

Whenits Host header is derived

Thenthe Host header is "example.com" (port 80 is the http default, so it is omitted)

Anda URL of "http://example.com:8080/a" gives Host "example.com:8080" (non-default port included)

Background

In HTTP/1.1 the Host header is mandatory - a single IP address can serve many different sites (virtual hosting), so the request must state which host it means. The value comes straight from the URL you parsed: the host name, and the port only when it differs from the scheme’s default. http://example.com sends Host: example.com because port 80 is implied; http://example.com:8080 sends Host: example.com:8080 because 8080 is not.

This is the first header the client sets on its own rather than the caller setting it, and the rule - omit the default port, include an explicit one - is exactly the port defaulting you built in chapter one, now running in reverse. With Host in the collection, a request is nearly ready to serialize whole.

Make it work

// Host is the URL host, plus ":port" ONLY when the port is not
// the scheme default (80 for http, 443 for https).
func (r *Request) setHost() {
  // if port == defaultPort(scheme): Host = host
  // else: Host = host + ":" + port
}

CheckpointDONE

A request derives a correct Host header, with the port shown only when non-default. Commit and stop here.