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.\"     @(#)inetd.8	8.4 (Berkeley) 6/1/94
.\"
.Dd August 7, 2023
.Dt inetd 8 SMM
.Os "GNU Network Utilities"
.Sh NAME
.Nm inetd
.Nd internet
.Dq super-server
.Sh SYNOPSIS
.Nm inetd
.Op Va option ...
.Oo
.Ar conf-file Op Ar conf-dir
.Oc Ar ...
.Sh DESCRIPTION
The
.Nm inetd
program
should be run at boot time by
.Pa /etc/rc
(see
.Xr 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,
.Nm inetd
continues to listen on the socket (except in some cases which
will be described below).  Essentially,
.Nm inetd
allows running one daemon to invoke several others,
reducing load on the system.
.Sh OPTIONS
The options available for
.Nm inetd :
.Bl -tag -width Ds
.It Fl -foreground
Runs in foreground mode.
.It Fl d, -debug
Turns on debugging, and runs in foreground mode.
.It Fl -environment
Pass local and remote address data via environment variables. See
\fBENVIRONMENT\fP below.
.It Fl p, -pidfile Op Ar filename
Specifies the pidfile to use instead of the default.
When a
.Ar filename
is not specified, that disables writing a pidfile.
.It Fl R, -rate Ar rate
Specifies the maximum number of times a service can be invoked
in one minute; the default is 40.
.It Fl -resolve
Resolve local and remote IP addresses and pass them to the server program
via \fBTCPLOCALHOST\fP and \fBTCPREMOTEHOST\fP environment variables. See
\fBENVIRONMENT\fP below. This option implies \fB--environment\fP.
.It Fl V, -version
Shows the version.
.It Fl ?, -help
Shows the help.
.It Fl -usage
Shows the usage message.
.El
.Sh OPERATION
Upon execution,
.Nm inetd
reads its configuration information from a configuration
file on the command line, by default,
.Pa /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:
.Pp
.Bd -unfilled -offset indent -compact
service name
socket type
protocol
wait/nowait[.max]
user[:group]
server program
server program arguments
.Ed
.Pp
There are two types of services that
.Nm 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
.Tn RFC 1078 ,
TCPMUX services are nonstandard services that do not have a
well-known port assigned to them.
They are invoked from
.Nm inetd
when a program connects to the
.Dq tcpmux
well-known port and specifies
the service name.
This feature is useful for adding locally-developed servers.
.Pp
The
.Em service-name
entry is the name of a valid service in
the file
.Pa /etc/services .
For
.Dq internal
services (discussed below), the service
name
.Em must
be the official name of the service (that is, the first entry in
.Pa /etc/services ) .
For TCPMUX services, the value of the
.Em service-name
field consists of the string
.Dq tcpmux
followed by a slash and the
locally-chosen service name.
The service names listed in
.Pa /etc/services
and the name
.Dq 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.
.Pp
The
.Em socket-type
should be one of
.Dq stream ,
.Dq dgram ,
.Dq raw ,
.Dq rdm ,
or
.Dq seqpacket ,
depending on whether the socket is a stream, datagram, raw,
reliably delivered message, or sequenced packet socket.
TCPMUX services must use
.Dq stream .
.Pp
The
.Em protocol
must be a valid protocol as given in
.Pa /etc/protocols .
Examples might be
.Dq tcp
or
.Dq udp .
TCPMUX services must use
.Dq tcp .
By default the protocols will only listen on IPv4 connections.
To modify this behavior per service, either
.Dq 4
or
.Dq 6
can be appended to the protocol name, such as
.Dq udp4
or
.Dq tcp6
to accept only IPv4 UDP connections or both IPv4 and IPv6 TCP connections
respectively.
To restrict to IPv6-only connections,
.Dq 6only
can be appended to the protocol name, such as
.Dq tcp6only .
.Pp
The
.Em 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
.Nm inetd
should wait for the server to exit before listening for new service
requests.
Datagram servers must use
.Dq 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
.Nm inetd
can received further messages on the socket, it is said to be
a
.Dq 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
.Nm 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
.Dq single-threaded .
.Xr Comsat 8 ,
.Pq Xr biff 1
and
.Xr talkd 8
are both examples of the latter type of
datagram server.
.Xr Tftpd 8
is an example of a multi-threaded datagram server.
The optional
.Dq max
suffix (separated from
.Dq wait
or
.Dq 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,
.Nm
will log the problem
and stop servicing requests for the specific service for ten minutes.
See also the
.Fl R
option above.
.Pp
Servers using stream sockets generally are multi-threaded and
use the
.Dq nowait
entry.
Connection requests for these services are accepted by
.Nm 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
.Dq 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
.Dq nowait .
.Pp
The optional
.Dq max
suffix (separated from
.Dq wait
or
.Dq nowait
by a dot) is a decimal number that specifies the maximum number of server
instances that may be spawned from
.Nm inetd
within an interval of 60 seconds.  It overrides the settings of the
\fB-R\fP command line option. 
.Pp
The
.Em user
entry should contain the user name, and the optional
.Em group
name (separated from
.Dq user
by a semicolon or a dot) of the user (and group) as whom the server
should run.  This allows for servers to be given less permission
than root.
.Pp
The
.Em server-program
entry should contain the pathname of the program which is to be
executed by
.Nm inetd
when a request is found on its socket.  If
.Nm inetd
provides this service internally, this entry should
be
.Dq internal .
.Pp
The
.Em 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
.Dq internal
should take the place of this entry.
.Pp
The
.Nm inetd
program
provides several
.Dq trivial
services internally by use of
routines within itself.  These services are
.Dq echo ,
.Dq discard ,
.Dq chargen
(character generator),
.Dq daytime
(human readable time), and
.Dq 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
.Tn RFC
from the Network Information Center.
.Pp
The
.Nm inetd
program
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP .
Services may be added, deleted or modified when the configuration file
is reread.
.Sh TCPMUX
.Pp
.Tn 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.
.Pp
If the TCPMUX service name begins with a "+",
.Nm 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.
.Pp
The special service name
.Dq help
causes
.Nm inetd
to list TCPMUX services in
.Pa inetd.conf .
.ne 1i
.Sh "ENVIRONMENT"
If a connection is made with a streaming protocol (TCP) and if
\fB--environment\fP option has been given, inetd will set
the following environment variables before starting the program:
.Pp
\fBPROTO\fP: always "TCP".
.Pp
\fBTCPLOCALIP\fP: the local IP address of the interface which accepted the connection.
.Pp
\fBTCPLOCALPORT\fP: the port number on which the TCP connection was established.
.Pp
\fBTCPREMOTEIP\fP: the IP address of the remote client.
.Pp
\fBTCPREMOTEPORT\fP: the port number on the client side of the TCP connection.
.Pp
In addition, if given the \fB--environment\fP or \fB--resolve\fP options,
.Nm inetd
will set the following environment variables:
.Pp
\fBTCPLOCALHOST\fP: the DNS name of \fITCPLOCALIP\fR.
.Pp
\fBTCPREMOTEHOST\fP: the DNS name of \fITCPREMOTEIP\fR.
.Sh "EXAMPLES"
.Pp
Here are several example service entries for the various types of services:
.Bd -literal
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
.Ed
.Sh "ERROR MESSAGES"
The
.Nm inetd
server
logs error messages using
.Xr syslog 3 .
Important error messages and their explanations are:
.Bd -literal
\fIservice\fP/\fIprotocol\fP server failing (looping), service terminated.
.Ed
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
.Op Fl 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.
.sp
.Bd -literal
\fIservice\fP/\fIprotocol\fP: No such user '\fIuser\fP', service ignored
\fIservice\fP/\fIprotocol\fP: getpwnam: \fIuser\fP: No such user
.Ed
No entry for
.Em user
exists in the
.Pa passwd
file. The first message
occurs when
.Nm inetd
(re)reads the configuration file. The second message occurs when the
service is invoked.
.sp
.Bd -literal
\fIservice\fP: can't set uid \fInumber\fP
\fIservice\fP: can't set gid \fInumber\fP
.Ed
The user or group ID for the entry's
.Em user
is invalid.
.Sh SEE ALSO
.Xr comsat 8 ,
.Xr fingerd 8 ,
.Xr ftpd 8 ,
.Xr rexecd 8 ,
.Xr rlogind 8 ,
.Xr rshd 8 ,
.Xr telnetd 8 ,
.Xr tftpd 8
.Sh BUGS
The environment variables (see \fBENVIRONMENT\fP) are set only for
TCP IPv4 nowait connections.
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
TCPMUX is based on code and documentation by Mark Lottor.