Practical UNIX and Internet Security, 3rd Edition

Understanding Unix Internet Servers and Services

Most Unix network services are provided by individual programs called servers. For a server to operate, it must be assigned a protocol (e.g., TCP or UDP), be assigned a port number, and somehow be started.

The /etc/services File

As we saw in the last chapter, most Internet services are assigned a specific port for their exclusive use. When a client opens a connection across the network to a server, the client uses the port to specify which service it wishes to use. These ports are called well-known ports because they need to be known in advance by both the client and the server. Unix uses the /etc/services file as a small local database; for each service this file specifies the service’s well-known port number and notes whether the service is available as a TCP or UDP service. The /etc/services file is distributed as part of the Unix operating system.

The information in the /etc/services file is derived from Internet RFCs^[132] and other sources. Some of the services listed in the /etc/services file are no longer in widespread use; nevertheless, their names still appear in the file.

The following is an excerpt from the /etc/services file that specifies the ports for the Telnet, SMTP, and Network Time Protocol (NTP) services:

# /etc/services
#
 . . . 
telnet 23/tcp
smtp   25/tcp mail
time   37/udp timeserver
 . . .

Each line gives the canonical name of the service, the port number and protocol, and any aliases for the service name. As ...