bionic (8) inetutils-inetd.8.gz

NAME

     inetd — internet “super-server”

SYNOPSIS

     inetd [-d] [-R rate] [-p [filename]] [--environment] [--resolve] [configuration_files ...]

DESCRIPTION

     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.

     --environment
             Pass local and remote address data via environment variables. See ENVIRONMENT below.

     -p, --pidfile [filename]
             Specifies the pidfile to use instead of the default.

     -R, --rate rate
             Specifies the maximum number of times a service can be invoked in one minute; the default is 40.

     --resolve
             Resolve local and remote IP addresses and pass them to the server program via TCPLOCALHOST and
             TCPREMOTEHOST environment variables. See ENVIRONMENT below. This option implies --environment.

     -V, --version
             Shows the version.

     -?, --help
             Shows the help.

     --usage
             Shows the usage message.

     Upon execution, inetd reads its configuration information from a configuration file on the command line, by
     default, /etc/inetd.conf and /etc/inetd.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.  The fields of the
     configuration file are as follows:

           service name
           socket type
           protocol
           wait/nowait[.max]
           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 “tcpmux”
     well-known port and specifies the service name.  This feature is useful for adding locally-developed
     servers.

     The service-name entry is the name of a valid service in the file /etc/services.  For “internal” services
     (discussed below), the service name must be the official name of the service (that is, the first entry in
     /etc/services).  For TCPMUX services, the value of the service-name field consists of the string “tcpmux”
     followed by a slash and the locally-chosen service name.  The service names listed in /etc/services and the
     name “help” 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 socket-type should be one of “stream”, “dgram”, “raw”, “rdm”, or “seqpacket”, depending on whether the
     socket is a stream, datagram, raw, reliably delivered message, or sequenced packet socket.  TCPMUX services
     must use “stream”.

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

     The wait/nowait[.max] 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 “wait”, 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 “multi-threaded” 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
     “single-threaded”.  Comsat(8), (biff(1)) and talkd(8) are both examples of the latter type of datagram
     server.  Tftpd(8) is an example of a multi-threaded datagram server.  The optional “max” suffix (separated
     from “wait” or “nowait” by a dot) specifies the maximum number of times a service can be invoked in one
     minute; the default is 40.  If a service exceeds this limit, inetd will log the problem and stop servicing
     requests for the specific service for ten minutes.  See also the -R option above.

     Servers using stream sockets generally are multi-threaded and use the “nowait” 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 “wait” 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 “nowait”.

     The optional “max” suffix (separated from “wait” or “nowait” by a dot) is a decimal number that specifies
     the maximum number of server instances that may be spawned from inetd within an interval of 60 seconds.  It
     overrides the settings of the -R command line option.

     The user 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 server-program 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
     “internal”.

     The server program arguments 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 “internal” should take the place of
     this entry.

     The inetd program provides several “trivial” services internally by use of routines within itself.  These
     services are “echo”, “discard”, “chargen” (character generator), “daytime” (human readable time), and
     “time” (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.

TCPMUX

     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 “help” causes inetd to list TCPMUX services in inetd.conf.

ENVIRONMENT

     If a connection is made with a streaming protocol (TCP) and if --environment option has been given, inetd
     will set the following environment variables before starting the program:

     PROTO: always "TCP".

     TCPLOCALIP: the local IP address of the interface which accepted the connection.

     TCPLOCALPORT: the port number on which the TCP connection was established.

     TCPREMOTEIP: the IP address of the remote client.

     TCPREMOTEPORT: the port number on the client side of the TCP connection.

     In addition, if given the --remote option, inetd will set the following environment variables:

     TCPLOCALHOST: the DNS name of TCPLOCALIP.

     TCPREMOTEHOST: the DNS name of TCPREMOTEIP.

EXAMPLES

     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

ERROR MESSAGES

     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 re-enabled automatically in 10 minutes.

     service/protocol: No such user 'user', service ignored
     service/protocol: getpwnam: user: No such user
     No entry for user 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 user is invalid.

SEE ALSO

     comsat(8), fingerd(8), ftpd(8), rexecd(8), rlogind(8), rshd(8), telnetd(8), tftpd(8)

BUGS

     The environment variables (see ENVIRONMENT) are set only for TCP IPv4 nowait connections.

HISTORY

     The inetd command appeared in 4.3BSD.  TCPMUX is based on code and documentation by Mark Lottor.