Survey of Common Services

There are a variety of network services available on UNIX machines. Most are independent of UNIX, and may be used across the Internet. Below is a listing of common services: You can use the following commands as per your UNIX operating systems to start or stop networking service.

# /sbin/init.d/net stop
# /sbin/init.d/net start
# /sbin/init.d/hostname start

TCP/IP offers a host of very simple services that are useful for debugging. Often, no server processes are associated to these servers. Instead, the kernel handles the service directly. Two of the most useful trivial services are:

1. echo, which simply repeats back whatever you type to it
2. daytime, which reports, in human-readable form, the machine's idea of the current date and time

(TCP/IP) is a protocol system, a collection of protocols that supports network communications. The answer to the question

What is a protocol?

must begin with the question

What is a network?

A network is a collection of computers or computer-like devices that can communicate across a common transmission medium.
Often the transmission medium is an insulated metal wire that carries electrical pulses between the computers, but the transmission medium could also be a phone line, or even no line at all in the case of a wireless network. Regardless of how the computers are connected, the communication process requires that data from one computer pass across the transmission medium to another computer. Computer A must be able to send a message or request to computer B. Computer B must be able to understand the message of computer A and respond to it by sending a message back to computer A.

The (NNTP) Network News Transfer Protocol is an application protocol used for transporting Usenet news articles between news servers and for reading and posting articles by end user client applications. Brian Kantor of the University of California, San Diego and Phil Lapsley of the University of California, Berkeley authored RFC 977, the specification for the Network News Transfer Protocol, in March 1986.
Usenet was originally designed based on the UUCP network, with most article transfers taking place over direct point-to-point telephone links between news servers, which were powerful time-sharing systems. Readers and posters logged into these computers reading the articles directly from the local disk. As local area networks and Internet participation proliferated, it became desirable to allow newsreaders to be run on personal computers connected to local networks. The resulting protocol was NNTP, which resembled the Simple Mail Transfer Protocol (SMTP) but was tailored for exchanging newsgroup articles. A newsreader, also known as a news client, is a software application that reads articles on Usenet, either directly from the news server's disks or via the NNTP. The well-known TCP port 119 is reserved for NNTP and TCP port 433 (NNSP) may be used when doing a bulk transfer of articles from one server to another. When clients connect to a news server with Transport Layer Security (TLS), TCP port 563 is often used. This is sometimes referred to as NNTPS. Alternatively, a plain-text connection over port 119 may be changed to use TLS via the STARTTLS command.

Here are the steps you needed to follow to successfully complete this exercise:

1. You can use your telnet client to talk directly to many different services. In this exercise, you will work with several trivial TCP/IP services (the echo and daytime ports), then with port 80, which is the one generally used by the Hypertext Transfer Protocol (HTTP). First, telnet to Acme's Web server, using the actual port number for the echo port on that box. Remember: use the number for the port, not the name.
   Solution: telnet www.acmecorp.com 7
2. Now, enter the following sentence: This is the echo port.
   Solution: This is the echo port.
3. Press Enter.
   Solution: Enter
   
4. Notice that the echo port returns exactly the same sentence you entered. Normally, you would end this telnet session by entering Ctrl + ].
   For this simulation, however, the telnet session is closed automatically for you. Note that the terminal reports a ^] signal upon exit. Now, learn the time of day on the West Coast of the United States by telnetting to the daytime port of Acme's Web server. Use the port number, not the name.
   Solution: telnet www.acmecorp.com 13
5. You should see that the Web server gives the current time. Also, notice that the server drops the connection right after giving you the time. This behavior is standard for the daytime port. You need not issue a termination sequence (e.g., Ctrl + ]) in this case. In addition to the TCP/IP trivial services, you can use telnet to access various servers, including FTP and HTTP. Use telnet to open a connection with the www.acmecorp.com Web server's HTTP port. Use the port number.
   Solution: telnet www.acmecorp.com 80
6. HTTP servers are not configured to expect human interaction, and therefore do not generate prompts. However, you can still get this server to give you information. For example, you can tell the server to send its default document, as specified by /. To get this information, issue the following command: GET / HTTP/1.0.
   Note: you must enter this command exactly as shown, so include the proper spacing.
   Solution: GET / HTTP/1.0
7. Press Enter twice to create two blank lines that direct the Web server to transfer its information.
   Solution: Enter, Enter
8. Note that the server will terminate the telnet connection immediately, because the server initiates a connection, then ends it as soon as the information is sent.