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Lesson 12

Port Number and Unix Service Conclusion

Review what you have learned in this module.

This module discussed:
  1. The role of port numbers in addressing particular services on a UNIX machine (or any other machine running TCP/IP)
  2. The difference between TCP and UDP
  3. The central role of the inet daemon in starting server processes
  4. Common network services

Key terms

In this module, we introduced the following terms:
  1. Client: (1) a process running on a host computer which requests services; (2) a computer running a client process.
  2. Concurrent server: A concurrent server operates in this fashion: 1. The server receives the incoming connection. 2. The server calls fork() to split itself into two processes, a parent and a child. 3. The child process handles the connection, while the parent returns to listen on the original port. 4. When the child process is finished with the connection, it terminates. A server which operates like this is called a concurrent server. A concurrent server is always available for incoming connections. For example, a system in which the telnet server is set up as a concurrent server can handle multiple telnet connections, each of which is managed by a different child of the listening server process.
  3. Daemon: On UNIX systems, a process which runs independently of any login session and performs system maintenance or functions as a server.
  4. DNS: Domain Name Service--resolves domain names to IP addresses
  5. Domain: On the Internet, domain is most commonly used to refer to a group of computers whose hostnames share a common suffix, the domain name. The last component of this is the top-level domain.
  6. File Transfer Protocol (FTP): Protocol used to transfer data over a network.
  7. Hypertext Transfer Protocol (HTTP): Hypertext Transport Protocol defines how messages are formatted and transmitted over the Web and how Web browsers should respond to those messages.
  8. IP address: Server or Host address. Usually represented in dotted decimal notation, eg.,'
  9. Iterative server: An iterative server operates in this way: 1. The server receives the incoming connection. 2. The server handles the connection. 3. The server closes the connection. 4. The server returns to listening on its well-known port. A server that operates like this is called an iterative server. When an iterative server is handling a request, other connections to that port are blocked. The incoming connections must be handled one after another. For example, a system in which the telnet server is set up as an iterative server can handle only one incoming telnet connection at a time.
  10. kernel: The kernel is the core of the UNIX operating system. The kernel remains hidden from typical users.
  11. NFS: Network File System (NFS) is a Unix-based file system that allows remote access to shared disk resources.
  12. Network Information Service (NIS): NIS is Sun Microsystems' Yellow Pages (yp) client-server protocol for distributing system configuration data such as user and host names between computers on a network.
  13. NNTP: Network News Transfer Protocol (NNTP) is a protocol that allows the browsing and downloading of messages from discussion forums on a TCP/IP network. NNTP also defines the procedures used by NNTP servers for replication messages between them.
  14. process: A UNIX process is a running program.
  15. Server: a process running on a host computer which supplies services in response to client requests. (2) a computer which runs server processes.
  16. Service Access Facility: The Service Access Facility is a Solaris mechanism for controlling access to server processes.
  17. SMTP: Simple Mail Transfer Protocol (SMTP) is a protocol that allows the sending of messages over a TCP/IP-based network from one computer to another. It is generally used to send messages from one Internet mail server to another and to send messages from clients to servers. Messages are typically retrieved from servers by clients using POP3 or IMAP4.
  18. TCP/IP: TCP/IP is the network protocol suite used by the Internet and most local area networks.
  19. Transport layer: Provides communication sessions between computers.
  20. UDP (User Datagram Protocol): A connectionless datagram service in the Transport layer used by applications that typically transmit small quantities of data.
  21. 'X': The X protocol is used for sharing of graphical display resources across a network. Most UNIX systems use X for their graphical user interface. The X protocol allows a process to run on one machine and open a window for graphical output on another machine. In the X protocol, an X server usually corresponds to a display, and an X client is a program that shows data on that display. The X server is typically called X, and a sample X client is xterm, which opens a terminal window. X clients usually use port numbers starting at 6000 to make connections with X servers.


In this module, we discussed the following commands:
Command Purpose
telnet Used to open a terminal session on a remote machine
ftp Used to retrieve files from a remote machine
rpcinfo Used to query the portmapper process regarding registered RPC services

TCP/IP Conclusion - Quiz

Click the Quiz link below to take a short multiple-choice quiz on the material covered in this module.
TCP/IP Conclusion - Quiz

TCP/IP Illustrated