INETD(8) System Manager's Manual INETD(8)
inetd - internet ``super-server''
inetd [-d] [-R rate] [configuration file]
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 Turns on debugging.
-R rate Specifies the maximum number of times a service can be invoked in one minute; the default is 1000.
Upon execution, inetd reads its configuration information from a configuration file which, by default, is /etc/inetd.conf. There must be
an entry for each field of 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:
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 organi-
zation'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''.
NOTE: ``rdm'' and ``seqpacket'' are not supported in 2.11BSD.
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 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 examples of the latter type of datagram server. Tftpd(8) is an example of a multi-threaded datagram
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 ser-
vices 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 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 entry.
The inetd program provides several ``trivial'' services internally by use of routines within itself. These services are ``echo'', ``dis-
card'', ``chargen'' (character generator), ``daytime'' (human readable time), and ``time'' (machine readable time, in the form of the num-
ber of seconds since midnight, January 1, 1900). All of these services are tcp based. For details of these services, consult the appro-
priate 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 posi-
tive (+) 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.
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/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
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), telnetd(8), tftpd(8)
The inetd command appeared in 4.3BSD. TCPMUX is based on code and documentation by Mark Lottor.
4.4 Berkeley Distribution November 7, 1996 INETD(8)