freebsd-skq/usr.sbin/inetd/inetd.8

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.\"     from: @(#)inetd.8	8.3 (Berkeley) 4/13/94
.\"	$Id: inetd.8,v 1.32 1999/07/02 15:58:32 sheldonh Exp $
.\"
.Dd February 7, 1996
.Dt INETD 8
.Os BSD 4.4
.Sh NAME
.Nm inetd
.Nd internet
.Dq super-server
.Sh SYNOPSIS
.Nm inetd
.Op Fl d
.Op Fl l
.Op Fl w Op Fl w
.Op Fl c Ar maximum
.Op Fl C Ar rate
.Op Fl a Ar address
.Op Fl p Ar filename
.Op Fl R Ar rate
.Op Ar configuration file
.Sh DESCRIPTION
The
.Nm
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
continues to listen on the socket (except in some cases which
will be described below).  Essentially,
.Nm
allows running one daemon to invoke several others,
reducing load on the system.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl d
Turn on debugging.
.It Fl l
Turn on logging.
.It Fl w
Turn on TCP Wrapping. If this option is specified twice, internal
services will also be wrapped. See the
.Sx "IMPLEMENTATION NOTES"
section for more information on TCP Wrappers support.
.It Fl c Ar maximum
Specify the default maximum number of services that can be invoked.
May be overridden on a per-service basis with the "max-child"
parameter.
.It Fl C Ar rate
Specify the default maximum number of times a service can be invoked
from a single IP address in one minute; the default is unlimited.
May be overridden on a per-service basis with the
"max-connections-per-ip-per-minute" parameter.
.It Fl R Ar rate
Specify the maximum number of times a service can be invoked
in one minute; the default is 256.
.It Fl a
Specify a specific IP address to bind to.
.It Fl p
Specify an alternate file in which to store the process ID.
.El
.Pp
Upon execution,
.Nm
reads its configuration information from a configuration
file which, by default, is
.Pa /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
.Dq #
at the beginning
of a line.  There must be an entry for each field.  The
fields of the configuration file are as follows:
.Pp
.Bd -unfilled -offset indent -compact
service name
socket type
protocol
{wait|nowait}[/max-child[/max-connections-per-ip-per-minute]]
user[:group][/login-class]
server program
server program arguments
.Ed
.Pp
To specify an
.No Tn "ONC RPC" Ns -based
service, the entry would contain these fields:
.Pp
.Bd -unfilled -offset indent -compact
service name/version
socket type
rpc/protocol
user[:group][/login-class]
server program
server program arguments
.Ed
.Pp
There are two types of services that 
.Nm
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
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.
TCPMUX requests are only accepted when the multiplexor service itself
is enabled, above and beyond and specific TCPMUX-based servers; see the
discussion of internal services below.
.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
should
be the official name of the service (that is, the first entry in
.Pa /etc/services ) .
When used to specify an
.No Tn "ONC RPC" Ns -based
service, this field is a valid RPC service name in
the file
.Pa /etc/rpc . 
The part on the right of the 
.Dq /
is the RPC version number. This
can simply be a single numeric argument or a range of versions.
A range is bounded by the low version to the high version - 
.Dq rusers/1-3 .
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 .
If it is desired that the service is reachable via T/TCP, one should
specify
.Dq tcp/ttcp .
Rpc based services are specified with the 
.Dq rpc/tcp
or 
.Dq rpc/udp 
service type.
TCPMUX services must use 
.Dq tcp .
.Pp
The
.Em wait/nowait
entry specifies whether the server that is invoked by
.Nm
will take over
the socket associated with the service access point, and thus whether
.Nm
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
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
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.
.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 maximum number of outstanding child processes (or
.Dq threads )
for a
.Dq nowait
service may be explicitly specified by appending a
.Dq /
followed by the number to the
.Dq nowait
keyword. Normally
(or if a value of zero is specified) there is no maximum. Otherwise,
once the maximum is reached, further connection attempts will be
queued up until an existing child process exits. This also works
in the case of
.Dq wait
mode, although a value other than one (the
default) might not make sense in some cases.
You can also specify the maximum number of connections per minute
for a given IP address by appending
a
.Dq /
followed by the number to the maximum number of
outstanding child processes. Once the maximum is reached, further
connections from this IP address will be dropped until the end of the
minute.
.Pp
The
.Em 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.
Optional
.Em group
part separated by
.Dq :
allows to specify group name different
than default group for this user.
Optional
.Em login-class
part separated by
.Dq /
allows to specify login class different
than default
.Dq daemon
login class.
.Pp
The
.Em server-program
entry should contain the pathname of the program which is to be
executed by
.Nm
when a request is found on its socket.  If
.Nm
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
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 available in
both TCP and UDP versions; the UDP versions will refuse service if the
request specifies a reply port corresponding to any internal service.
(This is done as a defense against looping attacks; the remote IP address
is logged.)
For details of these services, consult the
appropriate
.Tn RFC
document.
.Pp
The TCPMUX-demultiplexing service is also implemented as an internal service.
For any TCPMUX-based service to function, the following line must be included
in
.Pa inetd.conf :
.Bd -literal -offset indent
tcpmux	stream	tcp	nowait	root	internal
.Ed
.Pp
When given the
.Fl l
option
.Nm
will log an entry to syslog each time a connection is accepted, noting the
service selected and the IP-number of the remote requestor if available.
.Pp
The
.Nm
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.
Except when started in debugging mode,
.Nm
records its process ID in the file
.Pa /var/run/inetd.pid
to assist in reconfiguration.
.Sh IMPLEMENTATION NOTES
When given the
.Fl w
option,
.Nm
will wrap all services specified as
.Dq stream nowait
or
.Dq dgram
except for 
.Dq internal
services. If the
.Fl w
option is given twice, such
.Dq internal
services will be wrapped as well.
.Pp
If the
.Fl l
option is specified, all connection attempts are logged, whether they are
allowed, denied or not wrapped at all. Otherwise, only denied requests will
be logged.
.Pp
Note that
.Nm
only wraps requests for a
.Dq wait
service while no servers are available to service requests. Once a
connection to such a service has been allowed, inetd has no control
over subsequent connections to the service until no more servers
are left listening for connection requests.
.Pp
When wrapping is enabled, the
.Pa tcpd
daemon is not required, as that functionality is builtin.
For more information on TCP Wrappers; see the relevant documentation (
.Xr hosts_access 5
).
When reading that document, keep in mind that
.Dq internal
services have no associated daemon name. Therefore, the service name
as specified in
.Pa inetd.conf
should be used as the daemon name for
.Dq internal
services.
.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
.Dq + ,
.Nm
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
to list TCPMUX services in
.Pa inetd.conf .
.Sh "FILES"
.Bl -tag -width /var/run/inetd.pid -compact
.It Pa /etc/inetd.conf
configuration file.
.It Pa /etc/rpc
translation of service names to RPC program numbers.
.It Pa /etc/services
translation of service names to port numbers.
.It Pa /var/run/inetd.pid
the pid of the currently running
.Nm inetd .
.El
.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
rstatd/1-3   dgram   rpc/udp wait root  /usr/libexec/rpc.rstatd  rpc.rstatd
.Ed
.Sh "ERROR MESSAGES"
The
.Nm
server
logs error messages using
.Xr syslog 3 .
Important error messages and their explanations are:
.Pp
.Bl -ohang -compact
.It Xo
.Ar service Ns / Ns Ar protocol
.No " server failing (looping), service terminated."
.Xc
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:
.Bl -enum -offset indent
.It
There are many hosts requesting the service within a short time period.
.It
A broken client program is requesting the service too frequently.
.It
A malicious user is running a program to invoke the service in
a denial-of-service attack.
.It
The invoked service program has an error that causes clients
to retry quickly.
.El
.Pp
Use the
.Fl R Ar rate
option,
as described above, to change the rate limit.
Once the limit is reached, the service will be
reenabled automatically in 10 minutes.
.Pp
.It Xo
.Ar service Ns / Ns Ar protocol :
.No \&No such user
.Ar user ,
.No service ignored
.Xc
.It Xo
.Ar service Ns / Ns Ar protocol :
.No getpwnam :
.Ar user :
.No \&No such user
.Xc
No entry for 
.Ar user
exists in the 
.Xr passwd 5
database. The first message
occurs when
.Nm
(re)reads the configuration file. The second message occurs when the
service is invoked.
.Pp
.It Xo
.Ar service :
.No can't set uid
.Ar uid
.Xc
.It Xo
.Ar service :
.No can't set gid
.Ar gid
.Xc
The user or group ID for the entry's 
.Ar user
field is invalid.
.Pp
.It "setsockopt(SO_PRIVSTATE): Operation not supported"
The
.Nm
program attempted to renounce the privileged state associated with a
socket but was unable to.
.El
.Sh SEE ALSO
.Xr hosts_access 5 ,
.Xr hosts_options 5 ,
.Xr login.conf 5 ,
.Xr passwd 5 ,
.Xr rpc 5 ,
.Xr services 5 ,
.Xr comsat 8 ,
.Xr fingerd 8 ,
.Xr ftpd 8 ,
.Xr portmap 8 ,
.Xr rexecd 8 ,
.Xr rlogind 8 ,
.Xr rshd 8 ,
.Xr telnetd 8 ,
.Xr tftpd 8
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.3 .
TCPMUX is based on code and documentation by Mark Lottor.
Support for
.Tn "ONC RPC"
based services is modeled after that
provided by
.Tn SunOS
4.1.
The
.Tn FreeBSD
TCP Wrappers support first appeared in 
.Fx 3.2 .