Inetd
inetd (internet service daemon) is a super-server daemon on many Unix systems that provides Internet services. For each configured service, it listens for requests from connecting clients. Requests are served by spawning a process which runs the appropriate executable, but simple services such as echo are served by inetd itself. External executables, which are run on request, can be single- or multi-threaded. First appearing in 4.3BSD, it is generally located at /usr/sbin/inetd. Often called a super-server, inetd listens on designated ports used by Internet services such as , POP3, and telnet. When a TCP packet or UDP packet arrives with a particular destination port number, inetd launches the appropriate server program to handle the connection. For services that are not expected to run with high loads, this method uses memory more efficiently, since the specific servers run only when needed. Furthermore, in inetd's "nowait" mode of service management, no network code is required in the service-specific programs, as inetd hooks the network stream directly to stdin and stdout of the spawned process. For protocols that have frequent traffic, such as HTTP and POP3, either inetd's "wait" mode of operation, or a dedicated server that intercepts the traffic directly may be preferable. The list of services that will be serviced is given in a configuration file, usually /etc/inetd.conf. A GUI for managing the configuration file is an optional accessory. The daemon may need a signal in order to re-read its configuration. For an example, telnet can be configured as follows (line taken from a machine running AIX version 5.1): telnet stream tcp6 nowait root /usr/sbin/telnetd telnetd -a The first word, telnet, is the official name of the service. It is resolved using the system database to map port numbers and protocols to service names. In this case, /etc/services should contain: telnet 23/tcp The second and third words describe the type of socket and underlying protocol respectively. The /etc/protocols database is consulted.