NAME¶
inetd
—
internet “super-server”
SYNOPSIS¶
inetd |
[ -d ]
[-R
rate ]
[--environment ]
[--resolve ]
[configuration_files
... ] |
WARNING¶
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
info inetd
The online copy of the documentation is available at the following address:
https://www.gnu.org/software/inetutils/manual.
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.
-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.
--version
- 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:
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 reenabled 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.