10.1 The Basics of Services
TCP services are among the easiest to understand because they are built upon simple, uninterrupted two-way data streams. Perhaps the best way to see how they work is to talk directly to an unencrypted web server on TCP port 80 to get an idea of how data moves across the connection. For example, run the following command to connect to the IANA documentation example web server:
$ telnet example.org 80
You should get a response like this, indicating a successful connection to the server:
Trying some address...
Connected to example.org.
Escape character is '^]'.
Now enter these two lines:
GET / HTTP/1.1
Host: example.org
HTTP 1.1, like its predecessor HTTP 1.0, shows its age; there are newer protocols in use such as HTTP/2, QUIC, and the emerging HTTP/3.
Press ENTER twice after the last line. The server should send a bunch of HTML text as a response. To terminate the connection, press CTRL-D.
This exercise demonstrates that:
- The remote host has a web server process listening on TCP port 80.
telnetwas the client that initiated the connection.
The reason you have to terminate the connection with CTRL-D is that, because most web pages need several requests to load, it makes sense to keep the connection open. If you explore web servers at the protocol level, you may find that this behavior varies. For example, many servers disconnect quickly if they do not receive a request soon after a connection opens.
telnet was originally meant to enable logins to remote hosts. The client program may not be installed on your distribution by default, but is easily installed as an extra package. Although the telnet remote login server is completely insecure (as you will learn later), the telnet client can be useful for debugging remote services. telnet does not work with UDP or any transport layer other than TCP. If you’re looking for a general-purpose network client, consider netcat, described in Section 10.5.3.