rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, iruserok_af, ruserok_af - routines for returning a stream to a remote command
#include <netdb.h> /* Or <unistd.h> on some systems */ int rcmd(char **ahost, unsigned short inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p); int rresvport(int *port); int iruserok(uint32_t raddr, int superuser, const char *ruser, const char *luser); int ruserok(const char *rhost, int superuser, const char *ruser, const char *luser); int rcmd_af(char **ahost, unsigned short inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p, sa_family_t af); int rresvport_af(int *port, sa_family_t af); int iruserok_af(const void *raddr, int superuser, const char *ruser, const char *luser, sa_family_t af); int ruserok_af(const char *rhost, int superuser, const char *ruser, const char *luser, sa_family_t af);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
rcmd(), rcmd_af(), rresvport(), rresvport_af(), iruserok(), iruserok_af(), ruserok(), ruserok_af(): Since glibc 2.19: _DEFAULT_SOURCE Glibc 2.19 and earlier: _BSD_SOURCE
The 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 rresvport() function returns a file descriptor to a socket with an address in the privileged port space. The iruserok() and ruserok() functions are used by servers to authenticate clients requesting service with rcmd(). All four functions are used by the rshd(8) server (among others).
The rcmd() function looks up the host
*ahost using gethostbyname(3), returning -1 if
the host does not exist. Otherwise,
*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
If the connection succeeds, a socket in the Internet domain of type
SOCK_STREAM is returned to the caller, and given to the
remote command as
is nonzero, then an auxiliary channel to a control process will be set
up, and a file descriptor for it will be placed in
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
fd2p is 0, then the
stderr (unit 2 of the
remote command) will be made the same as the
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
The protocol is described in detail in rshd(8).
The rresvport() function is used to obtain a socket
with a privileged port bound to it. This socket is suitable for use by
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 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
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.
The iruserok() and 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
not the superuser, it
/etc/hosts.equiv file. If that lookup is not done,
or is unsuccessful, the
.rhosts in the local user's home
directory is checked to see if the request for service is allowed.
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
hosts.equiv file, or the host and remote username are found in
.rhosts file; otherwise iruserok() and
ruserok() return -1. If the local domain (as obtained
from gethostname(2)) is the same as the remote domain,
only the machine name need be specified.
If the IP address of the remote host is known, iruserok() should be used in preference to ruserok(), as it does not require trusting the DNS server for the remote host's domain.
All of the functions described above work with IPv4
(AF_INET) sockets. The "_af" variants take an extra
argument that allows the socket address family to be specified. For
these functions, the
af argument can be specified as
AF_INET or AF_INET6. In addition,
rcmd_af() supports the use of
The rcmd() function returns a valid socket descriptor on success. It returns -1 on error and prints a diagnostic message on the standard error.
The rresvport() function returns a valid, bound
socket descriptor on success. It returns -1 on error with the global
errno set according to the reason for failure. The error
code EAGAIN is overloaded to mean "All network ports in
For information on the return from ruserok() and iruserok(), see above.
The functions iruserok_af(), rcmd_af(), rresvport_af(), and ruserok_af() functions are provide in glibc since version 2.2.
For an explanation of the terms used in this section, see attributes(7).
|rcmd(), rcmd_af()||Thread safety||MT-Unsafe|
|rresvport(), rresvport_af()||Thread safety||MT-Safe|
|Thread safety||MT-Safe locale|
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.
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