The inetd program should be run at boot time by /etc/rc (see rc(8) ) . It then listens for connections on certain internet sockets. When a connection is found on one of its sockets, it decides what service the socket corresponds to, and invokes a program to service the request. The server program is invoked with the service socket as its standard input, output and error descriptors. After the program is finished, inetd continues to listen on the socket (except in some cases which will be described below). Essentially, inetd allows running one daemon to invoke several others, reducing load on the system.

The options available for inetd:

-d, --debug

Turns on debugging.

-R, --rate rate

Specifies the maximum number of times a service can be invoked in one minute; the default is 1000.

---version

Shows the version.

---help

Shows the help.

Upon execution, inetd reads its configuration information from a configuration pathnames on the command line, by default, /etc/inetd.conf and /etc/initd.d . If the configuration pathname is a directory, all the files in the directory are read like a configuration file. All of the configuration files are read and merged. There must be an entry for each field in the configuration file, with entries for each field separated by a tab or a space. Comments are denoted by a ``#'' at the beginning of a line. There must be an entry for each field. The fields of the configuration file are as follows:

service name socket type protocol wait/nowait user server program server program arguments

There are two types of services that inetd can start: standard and TCPMUX. A standard service has a well-known port assigned to it; it may be a service that implements an official Internet standard or is a BSD-specific service. As described in RFC 1078 , TCPMUX services are nonstandard services that do not have a well-known port assigned to them. They are invoked from inetd when a program connects to the well-known port and specifies the service name. This feature is useful for adding locally-developed servers.

The entry is the name of a valid service in the file /etc/services . For services (discussed below), the service name be the official name of the service (that is, the first entry in /etc/services ) . For TCPMUX services, the value of the field consists of the string followed by a slash and the locally-chosen service name. The service names listed in /etc/services and the name are reserved. Try to choose unique names for your TCPMUX services by prefixing them with your organization's name and suffixing them with a version number.

The should be one of or depending on whether the socket is a stream, datagram, raw, reliably delivered message, or sequenced packet socket. TCPMUX services must use

The must be a valid protocol as given in /etc/protocols . Examples might be or TCPMUX services must use

The entry specifies whether the server that is invoked by inetd will take over the socket associated with the service access point, and thus whether inetd should wait for the server to exit before listening for new service requests. Datagram servers must use as they are always invoked with the original datagram socket bound to the specified service address. These servers must read at least one datagram from the socket before exiting. If a datagram server connects to its peer, freeing the socket so inetd can received further messages on the socket, it is said to be a server; it should read one datagram from the socket and create a new socket connected to the peer. It should fork, and the parent should then exit to allow inetd to check for new service requests to spawn new servers. Datagram servers which process all incoming datagrams on a socket and eventually time out are said to be Comsat(8) , and talkd() are both examples of the latter type of datagram server. Tftpd() is an example of a multi-threaded datagram server.

Servers using stream sockets generally are multi-threaded and use the entry. Connection requests for these services are accepted by inetd , and the server is given only the newly-accepted socket connected to a client of the service. Most stream-based services operate in this manner. Stream-based servers that use are started with the listening service socket, and must accept at least one connection request before exiting. Such a server would normally accept and process incoming connection requests until a timeout. TCPMUX services must use

The entry should contain the user name of the user as whom the server should run. This allows for servers to be given less permission than root.

The entry should contain the pathname of the program which is to be executed by inetd when a request is found on its socket. If inetd provides this service internally, this entry should be

The should be just as arguments normally are, starting with argv[0], which is the name of the program. If the service is provided internally, the word should take the place of this entry.

The inetd program provides several services internally by use of routines within itself. These services are (character generator), (human readable time), and (machine readable time, in the form of the number of seconds since midnight, January 1, 1900). All of these services are tcp based. For details of these services, consult the appropriate RFC from the Network Information Center.

The inetd program rereads its configuration file when it receives a hangup signal, SIGHUP . Services may be added, deleted or modified when the configuration file is reread.

RFC 1078 describes the TCPMUX protocol: ``A TCP client connects to a foreign host on TCP port 1. It sends the service name followed by a carriage-return line-feed <CRLF>. The service name is never case sensitive. The server replies with a single character indicating positive (+) or negative (-) acknowledgment, immediately followed by an optional message of explanation, terminated with a <CRLF>. If the reply was positive, the selected protocol begins; otherwise the connection is closed.'' The program is passed the TCP connection as file descriptors 0 and 1.

If the TCPMUX service name begins with a ``+'', inetd returns the positive reply for the program. This allows you to invoke programs that use stdin/stdout without putting any special server code in them.

The special service name causes inetd to list TCPMUX services in inetd.conf .

Here are several example service entries for the various types of services: ftp stream tcp nowait root /usr/libexec/ftpd ftpd -l ntalk dgram udp wait root /usr/libexec/ntalkd ntalkd tcpmux/+date stream tcp nowait guest /bin/date date tcpmux/phonebook stream tcp nowait guest /usr/local/bin/phonebook phonebook

The inetd server logs error messages using syslog(3) . Important error messages and their explanations are: service/protocol server failing (looping), service terminated. The number of requests for the specified service in the past minute exceeded the limit. The limit exists to prevent a broken program or a malicious user from swamping the system. This message may occur for several reasons: 1) there are lots of hosts requesting the service within a short time period, 2) a 'broken' client program is requesting the service too frequently, 3) a malicious user is running a program to invoke the service in a 'denial of service' attack, or 4) the invoked service program has an error that causes clients to retry quickly. Use the [-R] option, as described above, to change the rate limit. Once the limit is reached, the service will be reenabled automatically in 10 minutes. service/protocol: No such user 'user', service ignored service/protocol: getpwnam: user: No such user No entry for exists in the passwd file. The first message occurs when inetd (re)reads the configuration file. The second message occurs when the service is invoked. service: can't set uid number service: can't set gid number The user or group ID for the entry's is invalid.

The command appeared in TCPMUX is based on code and documentation by Mark Lottor.