'\" t .\" $NetBSD: rcmd.3,v 1.9 1996/05/28 02:07:39 mrg Exp $ .\" .\" Copyright (c) 1983, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" SPDX-License-Identifier: BSD-4-Clause-UC .\" .\" @(#)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 2023-10-31 "Linux man-pages 6.7" .SH NAME rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af, ruserok_af \- routines for returning a stream to a remote command .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .BR "#include " "/* Or on some systems */" .P .BI "int rcmd(char **restrict " ahost ", unsigned short " inport , .BI " const char *restrict " locuser , .BI " const char *restrict " remuser , .BI " const char *restrict " cmd ", int *restrict " fd2p ); .P .BI "int rresvport(int *" port ); .P .BI "int iruserok(uint32_t " raddr ", int " superuser , .BI " const char *" ruser ", const char *" luser ); .BI "int ruserok(const char *" rhost ", int " superuser , .BI " const char *" ruser ", const char *" luser ); .P .BI "int rcmd_af(char **restrict " ahost ", unsigned short " inport , .BI " const char *restrict " locuser , .BI " const char *restrict " remuser , .BI " const char *restrict " cmd ", int *restrict " fd2p , .BI " sa_family_t " af ); .P .BI "int rresvport_af(int *" port ", sa_family_t " af ); .P .BI "int iruserok_af(const void *restrict " raddr ", int " superuser , .BI " const char *restrict " ruser ", const char *restrict " luser , .BI " sa_family_t " af ); .BI "int ruserok_af(const char *" rhost ", int " superuser , .BI " const char *" ruser ", const char *" luser , .BI " sa_family_t " af ); .fi .P .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .ad l .P .BR rcmd (), .BR rcmd_af (), .BR rresvport (), .BR rresvport_af (), .BR iruserok (), .BR iruserok_af (), .BR ruserok (), .BR ruserok_af (): .nf Since glibc 2.19: _DEFAULT_SOURCE glibc 2.19 and earlier: _BSD_SOURCE .fi .ad .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 . .P If the connection succeeds, a socket in the Internet domain of type .B SOCK_STREAM is returned to the caller, and given to the remote command as .I 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 .I stderr (unit 2 of the remote command) will be made the same as the .I 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. .P 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 .I /etc/hosts.equiv file. If that lookup is not done, or is unsuccessful, the .I .rhosts in the local user's home directory is checked to see if the request for service is allowed. .P 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 .I hosts.equiv file, or the host and remote username are found in the .I .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. .P 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 .B 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. .P The .BR rresvport () function returns a valid, bound socket descriptor on success. On failure, it returns \-1 and sets .I errno to indicate the error. The error code .B EAGAIN is overloaded to mean: "All network ports in use". .P For information on the return from .BR ruserok () and .BR iruserok (), see above. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lbx lb lb l l l. Interface Attribute Value T{ .na .nh .BR rcmd (), .BR rcmd_af () T} Thread safety MT-Unsafe T{ .na .nh .BR rresvport (), .BR rresvport_af () T} Thread safety MT-Safe T{ .na .nh .BR iruserok (), .BR ruserok (), .BR iruserok_af (), .BR ruserok_af () T} Thread safety MT-Safe locale .TE .SH STANDARDS BSD. .SH HISTORY .TP .BR iruserok_af () .TQ .BR rcmd_af () .TQ .BR rresvport_af () .TQ .BR ruserok_af () glibc 2.2. .P Solaris, 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 glibc 2.12. .\" Bug filed 25 Nov 2007: .\" https://www.sourceware.org/bugzilla/show_bug.cgi?id=5399 .SH SEE ALSO .BR rlogin (1), .BR rsh (1), .BR rexec (3), .BR rexecd (8), .BR rlogind (8), .BR rshd (8)