Lesson 03 · Ports and defaults - Build an HTTP Client

Ports and defaults

A host names a machine, but a connection needs a port too. Today you read an explicit port when the URL gives one and fall back to the scheme's default when it does not - 80 for http, 443 for https.

The goal

Parse an explicit port from the host, or supply the scheme's default port when none is given.

Start here - the target

TO DO

Scenario: Resolving the port

Giventhe URL string "http://example.com:8080/a"

Whenit is parsed

Thenthe URL has host "example.com" and port 8080

Andparsing "http://example.com/a" gives port 80, and parsing "https://example.com/a" gives port 443

Background

To open a connection you need a host and a port. The URL may state one explicitly as host:port - example.com:8080 means port 8080 - and when it does, the host is just the part before the colon. When the URL gives no port, the scheme’s default applies: 80 for http, 443 for https. A browser hides this from you, but a client has to make the default concrete before it can dial.

Pin both cases now: an explicit port must be parsed as a number and split cleanly off the host, and each scheme must resolve to the right default. From here on every URL carries a real port, so nothing downstream has to guess.

Make it work

// the authority may be "host" or "host:port". if a ":" is present,
// the part after it is the port. otherwise pick the default:
//   http  -> 80
//   https -> 443
func defaultPort(scheme string) int {
  // return 80 or 443 based on the scheme
}

CheckpointDONE

Every parsed URL now has a concrete port, explicit or defaulted. Commit and stop here.