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.\"     @(#)rcmd.3	8.1 (Berkeley) 6/4/93
.\"
.\" Contributed as Linux man page by David A. Holland, 970908
.\" I have not checked whether the Linux situation is exactly the same.
.\"
.\" 2007-12-08, mtk, Converted from mdoc to man macros
.\"
.TH RCMD 3 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
rcmd, rresvport, iruserok, ruserok, rcmd_af,
rresvport_af, iruserok_af, ruserok_af \- routines for returning a
stream to a remote command
.SH SYNOPSIS
.nf
.B #include <netdb.h> \ \ \fP/* Or <unistd.h> on some systems */
.PP
.BI "int rcmd(char **" ahost ", unsigned short " inport ", const char *" locuser ,
.BI "         const char *" remuser ", const char *" cmd ", int *" fd2p );
.PP
.BI "int rresvport(int *" port );
.PP
.BI "int iruserok(uint32_t " raddr ", int " superuser ,
.BI "             const char *" ruser ", const char *" luser );
.PP
.BI "int ruserok(const char *" rhost ", int " superuser ,
.BI "            const char *" ruser ", const char *" luser );
.PP
.BI "int rcmd_af(char **" ahost ", unsigned short " inport ", const char *" locuser ,
.BI "            const char *" remuser ", const char *" cmd ", int *" fd2p ,
.BI "            sa_family_t " af );
.PP
.BI "int rresvport_af(int *" port ", sa_family_t " af );
.PP
.BI "int iruserok_af(const void *" raddr ", int " superuser ,
.BI "                const char *" ruser ", const char *" luser \
", sa_family_t " af );
.PP
.BI "int ruserok_af(const char *" rhost ", int " superuser ,
.BI "               const char *" ruser ", const char *" luser \
", sa_family_t " af );
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR rcmd (),
.BR rcmd_af (),
.BR rresvport (),
.BR rresvport_af (),
.BR iruserok (),
.BR iruserok_af (),
.BR ruserok (),
.BR ruserok_af ():
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _BSD_SOURCE
.SH DESCRIPTION
The
.BR rcmd ()
function is used by the superuser to execute a command on
a remote machine using an authentication scheme based
on privileged port numbers.
The
.BR rresvport ()
function
returns a file descriptor to a socket
with an address in the privileged port space.
The
.BR iruserok ()
and
.BR ruserok ()
functions are used by servers
to authenticate clients requesting service with
.BR rcmd ().
All four functions are used by the
.BR rshd (8)
server (among others).
.SS rcmd()
The
.BR rcmd ()
function
looks up the host
.I *ahost
using
.BR gethostbyname (3),
returning \-1 if the host does not exist.
Otherwise,
.I *ahost
is set to the standard name of the host
and a connection is established to a server
residing at the well-known Internet port
.IR inport .
.PP
If the connection succeeds,
a socket in the Internet domain of type
.BR SOCK_STREAM
is returned to the caller, and given to the remote
command as
.IR stdin
and
.IR stdout .
If
.I fd2p
is nonzero, then an auxiliary channel to a control
process will be set up, and a file descriptor for it will be placed
in
.IR *fd2p .
The control process will return diagnostic
output from the command (unit 2) on this channel, and will also
accept bytes on this channel as being UNIX signal numbers, to be
forwarded to the process group of the command.
If
.I fd2p
is 0, then the
.IR stderr
(unit 2 of the remote
command) will be made the same as the
.IR stdout
and no
provision is made for sending arbitrary signals to the remote process,
although you may be able to get its attention by using out-of-band data.
.PP
The protocol is described in detail in
.BR rshd (8).
.SS rresvport()
The
.BR rresvport ()
function is used to obtain a socket with a privileged
port bound to it.
This socket is suitable for use by
.BR rcmd ()
and several other functions.
Privileged ports are those in the range 0 to 1023.
Only a privileged process
(on Linux: a process that has the
.B CAP_NET_BIND_SERVICE
capability in the user namespace governing its network namespace).
is allowed to bind to a privileged port.
In the glibc implementation,
this function restricts its search to the ports from 512 to 1023.
The
.I port
argument is value-result:
the value it supplies to the call is used as the starting point
for a circular search of the port range;
on (successful) return, it contains the port number that was bound to.
.\"
.SS iruserok() and ruserok()
The
.BR iruserok ()
and
.BR ruserok ()
functions take a remote host's IP address or name, respectively,
two usernames and a flag indicating whether the local user's
name is that of the superuser.
Then, if the user is
.I not
the superuser, it checks the
.IR /etc/hosts.equiv
file.
If that lookup is not done, or is unsuccessful, the
.IR .rhosts
in the local user's home directory is checked to see if the request for
service is allowed.
.PP
If this file does not exist, is not a regular file, is owned by anyone
other than the user or the superuser, is writable by anyone other
than the owner, or is hardlinked anywhere, the check automatically fails.
Zero is returned if the machine name is listed in the
.IR hosts.equiv
file, or the host and remote username are found in the
.IR .rhosts
file; otherwise
.BR iruserok ()
and
.BR ruserok ()
return \-1.
If the local domain (as obtained from
.BR gethostname (2))
is the same as the remote domain, only the machine name need be specified.
.PP
If the IP address of the remote host is known,
.BR iruserok ()
should be used in preference to
.BR ruserok (),
as it does not require trusting the DNS server for the remote host's domain.
.SS *_af() variants
All of the functions described above work with IPv4
.RB ( AF_INET )
sockets.
The "_af" variants take an extra argument that allows the
socket address family to be specified.
For these functions, the
.I af
argument can be specified as
.BR AF_INET
or
.BR AF_INET6 .
In addition,
.BR rcmd_af ()
supports the use of
.BR AF_UNSPEC .
.SH RETURN VALUE
The
.BR rcmd ()
function
returns a valid socket descriptor on success.
It returns \-1 on error and prints a diagnostic message on the standard error.
.PP
The
.BR rresvport ()
function
returns a valid, bound socket descriptor on success.
It returns \-1 on error with the global value
.I errno
set according to the reason for failure.
The error code
.BR EAGAIN
is overloaded to mean "All network ports in use."
.PP
For information on the return from
.BR ruserok ()
and
.BR iruserok (),
see above.
.SH VERSIONS
The functions
.BR iruserok_af (),
.BR rcmd_af (),
.BR rresvport_af (),
and
.BR ruserok_af ()
functions are provide in glibc since version 2.2.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw27 lb lb
l l l.
Interface	Attribute	Value
T{
.BR rcmd (),
.BR rcmd_af ()
T}	Thread safety	MT-Unsafe
T{
.BR rresvport (),
.BR rresvport_af ()
T}	Thread safety	MT-Safe
T{
.BR iruserok (),
.BR ruserok (),
.br
.BR iruserok_af (),
.BR ruserok_af ()
T}	Thread safety	MT-Safe locale
.TE
.sp 1
.SH CONFORMING TO
Not in POSIX.1.
Present on the BSDs, Solaris, and many other systems.
These
functions appeared in
4.2BSD.
The "_af" variants are more recent additions,
and are not present on as wide a range of systems.
.SH BUGS
.BR iruserok ()
and
.BR iruserok_af ()
are declared in glibc headers only since version 2.12.
.\" Bug filed 25 Nov 2007:
.\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=5399
.SH SEE ALSO
.BR rlogin (1),
.BR rsh (1),
.BR intro (2),
.BR rexec (3),
.BR rexecd (8),
.BR rlogind (8),
.BR rshd (8)