HTTP protocol basics

(This assumes you have read the Network and protocols section
or are otherwise already familiar with protocols.)

HTTP is a protocol that is easy to learn the basics of. A client connects to a
server—and it is always the client that takes the initiative—sends a
request and receives a response. Both the request and the response consist of
headers and a body. There can be little or a lot of information going in both
directions.

An HTTP request sent by a client starts with a request line, followed by
headers and then optionally a body. The most common HTTP request is probably
the GET request which asks the server to return a specific resource, and this
request does not contain a body.

When a client connects to ‘example.com’ and asks for the ‘/‘ resource, it
sends a GET without a request body:

GET / HTTP/1.1
User-agent: curl/2000
Host: example.com

…the server could respond with something like below, with response headers
and a response body (‘hello’). The first line in the response also contains
the response code and the specific version the server supports:

HTTP/1.1 200 OK
Server: example-server/1.1
Content-Length: 5
Content-Type: plain/text
hello

If the client would instead send a request with a small request body
(‘hello’), it could look like this:

POST / HTTP/1.1
Host: example.com
User-agent: curl/2000
Content-Length: 5
hello

A server always responds to an HTTP request unless something is wrong.

The URL converted to a request

So when a HTTP client is given a URL to operate on, that URL is then used,
picked apart and those parts are used in various places in the outgoing
request to the server. Let’s take the an example URL:

https://www.example.com/path/to/file

• https means that curl will use TLS to the remote port 443 (which is the
  default port number when no specified is used in the URL).

• www.example.com is the host name that curl will resolve to one or more IP
  address to connect to. This host name will also be used in the HTTP request in
  the Host: header.

• /path/to/file is used in the HTTP request to tell the server which exact
  document/resources curl wants to fetch

—path-as-is

The path part of the URL is the part that starts with the first slash after
the host name and ends either at the end of the URL or at a ‘?’ or ‘#’
(roughly speaking).

If you include substrings including /../ or /./ in the path, curl will
automatically squash them before the path is sent to the server, as is
dictated by standards and how such strings tend to work in local file
systems. The /../ sequence will remove the previous section so that
/hello/sir/../ ends up just /hello/ and /./ is simply removed so that
/hello/./sir/ becomes /hello/sir/.

To prevent curl from squashing those magic sequences before they are sent to
the server and thus allow them through, the --path-as-is option exists.