inetd - internet “super-server”
inetd [-d] [-R rate] [--environment] [--resolve]
The information in this man page may be inaccurate or incomplete. The
authoritative documentation for the inetd utility is contained in
inetutils.info document. To access it from your command line, type
The online copy of the documentation is available at the following
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:
Turns on debugging.
Pass local and remote address data via environment variables. See
-R, --rate rate
Specifies the maximum number of times a service can be invoked in
one minute; the default is 40.
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.
Shows the version.
--help Shows the help.
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:
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
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
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, which is the name of the program. If the service
is provided internally, the word “internal” should take the place of this
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
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
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
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
TCPLOCALPORT: the port number on which the TCP connection was
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
TCPLOCALHOST: the DNS name of TCPLOCALIP.
TCPREMOTEHOST: the DNS name of TCPREMOTEIP.
Here are several example service entries for the various types of
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
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.
comsat(8), fingerd(8), ftpd(8), rexecd(8), rlogind(8), rshd(8),
The environment variables (see ENVIRONMENT) are set only for TCP IPv4
The inetd command appeared in 4.3BSD. TCPMUX is based on code and
documentation by Mark Lottor.