program should be run at boot time by
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
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 ENVIRONMENT below.
- Specifies the pidfile to use instead of the default.
- 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
- Shows the version.
- Shows the help.
- 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
server program arguments
There are two types of services that inetd
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.
entry is the name of a valid
service in the file /etc/services
“internal” 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 service-name
consists of the string “tcpmux” followed by a slash and the
locally-chosen service name. The service names listed in
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.
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”.
must be a valid protocol as given in
. Examples might be
“tcp” or “udp”. TCPMUX services must use
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
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
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”.
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,
will log the problem and stop servicing
requests for the specific service for ten minutes. See also the
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
within an interval of 60 seconds. It
overrides the settings of the -R
command line option.
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.
entry should contain the
pathname of the program which is to be executed by
when a request is found on its socket. If
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.
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.
program rereads its configuration file
when it receives a hangup signal,
Services may be added, deleted or modified when the configuration file is
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 ``+'',
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
to list TCPMUX services in
If a connection is made with a streaming protocol (TCP) and if
option has been given, inetd will set the following
environment variables before starting the program:
: always "TCP".
: the local IP address of the interface which accepted the
: the port number on which the TCP connection was
: the IP address of the remote client.
: the port number on the client side of the TCP connection.
In addition, if given the --remote
will set the following environment
: the DNS name of TCPLOCALIP
: the DNS name of TCPREMOTEIP
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
server logs error messages using
. Important error messages and their
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
file. The first message occurs when
(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
The environment variables (see ENVIRONMENT
) are set only for TCP IPv4
command appeared in
. TCPMUX is based on code and documentation by