xinetd—The extended Internet services daemon
xinetd performs the same function as inetd: it starts programs that provide Internet services. Instead of having such servers started at system initialization time, and be dormant until a connection request arrives, xinetd is the only daemon process started and it listens on all service ports for the services listed in its configuration file. When a request comes in, xinetd starts the appropriate server. Because of the way it operates, xinetd (as well as inetd) is also referred to as a super-server.
The services listed in xinetd’s configuration file can be separated into two groups. Services in the first group are called multithreaded and they require the forking of a new server process for each new connection request. The new server then handles that connection. For such services, xinetd keeps listening for new requests so that it can spawn new servers. On the other hand, the second group includes services for which the service daemon is responsible for handling all new connection requests. Such services are called single-threaded and xinetd will stop handling new requests for them until the server dies. Services in this group are usually datagram based.
So far, the only reason for the existence of a super-server was to conserve system resources by avoiding to fork a lot of processes who might be dormant for most of their lifetime. While fulfilling this function, xinetd takes advantage of the idea of a super-server to provide features such as access control and logging. Furthermore, xinetd is not limited to services listed in /etc/services. Therefore, anybody can use xinetd to start special-purpose servers.
|–d||Enables debug mode. This produces a lot of debugging output, and it makes it possible to use a debugger on xinetd.|
|–syslog syslog_facility||This option enables syslog logging of xinetd-produced messages using the specified syslog facility. The following facility names are supported: daemon, auth, user, local[0-7] (check syslog.conf(5) for their meanings). This option is ineffective in debug mode because all relevant messages are sent to the terminal.|
|–filelog logfile||xinetd-produced messages will be placed in the specified file. Messages are always appended to the file. If the file does not exist, it will be created. This option is ineffective in debug mode because all relevant messages are sent to the terminal.|
|–f config_file||Determines the file that xinetd uses for configuration. The default is /etc/xinetd.conf.|
|–pid||The process pid is written to standard error. This option is ineffective in debug mode.|
|–loop rate||This option sets the loop rate beyond which a service is considered in error and is deactivated. The loop rate is specified in terms of the number of servers per second that can be forked for a process. The speed of your machine determines the correct value for this option. The default rate is 10.|
|–reuse||If this option is used, xinetd will set the socket option SO_REUSEADDR before binding the service socket to an Internet address. This allows binding of the address even if there are programs that use it, which happens when a previous instance of xinetd has started some servers that are still running. This option has no effect on RPC services.|
|–limit proc_limit||This option places a limit on the number of concurrently running processes that can be started by xinetd. Its purpose is to prevent process table overflows.|
|–logprocs limit||This option places a limit on the number of concurrently running servers for remote user ID acquisition.|
|–shutdownprocs limit||This option places a limit on the number of concurrently running servers for service shutdown (forked when the RECORD option is used).|
The syslog and filelog options are mutually exclusive. If none is specified, the default is syslog using the daemon facility. You should not confuse xinetd messages with messages related to service logging. The latter are logged only if this is specified via the configuration file.