RCMD(3) BSD Library Functions Manual RCMD(3)
rcmd, orcmd, rcmd_af, orcmd_af, rresvport, rresvport_af, iruserok, ruserok, iruserok_sa --
routines for returning a stream to a remote command
Standard C Library (libc, -lc)
rcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd,
orcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd,
rcmd_af(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd,
int *fd2p, int af);
orcmd_af(char **ahost, int inport, const char *locuser, const char *remuser,
const char *cmd, int *fd2p, int af);
rresvport_af(int *port, int family);
iruserok(uint32_t raddr, int superuser, const char *ruser, const char *luser);
ruserok(const char *rhost, int superuser, const char *ruser, const char *luser);
iruserok_sa(const void *raddr, int rlen, int superuser, const char *ruser,
const char *luser);
The rcmd() function is available for use by anyone to run commands on a remote system. It
acts like the orcmd() command, with the exception that it makes a call out to the rcmd(1)
command, or any other user-specified command, to perform the actual connection (thus not
requiring that the caller be running as the super-user), and is only available for the
``shell/tcp'' port. The orcmd() function is used by the super-user to execute a command on
a remote machine using an authentication scheme based on reserved port numbers. While
rcmd() and orcmd() can only handle IPv4 address in the first argument, rcmd_af() and
orcmd_af() can handle other cases as well. The rresvport() function returns a descriptor to
a socket with an address in the privileged port space. The rresvport_af() function is simi-
lar to rresvport(), but you can explicitly specify the address family to use. Calling
rresvport_af() with AF_INET has the same effect as rresvport(). The iruserok() and
ruserok() functions are used by servers to authenticate clients requesting service with
rcmd(). All six functions are present in the same file and are used by the rshd(8) server
(among others). iruserok_sa() is an address family independent variant of iruserok().
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 connec-
tion is established to a server residing at the well-known Internet port inport.
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 stdin and stdout. If fd2p is non-zero,
then an auxiliary channel to a control process will be set up, and a descriptor for it will
be placed in *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 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 data.
rcmd_af() and orcmd_af() take address family in the last argument. If the last argument is
PF_UNSPEC, interpretation of *ahost will obey the underlying address resolution like DNS.
The protocol is described in detail in rshd(8).
The rresvport() and rresvport_af() functions are used to obtain a socket with a privileged
address bound to it. This socket is suitable for use by rcmd() and several other functions.
Privileged Internet ports are those in the range 0 to 1023. Only the super-user is allowed
to bind an address of this sort to a socket.
The iruserok() and ruserok() functions take a remote host's IP address or name, respec-
tively, two user names and a flag indicating whether the local user's name is that of the
super-user. Then, if the user is NOT the super-user, it checks the /etc/hosts.equiv file.
If that lookup is not done, or is unsuccessful, the .rhosts in the local user's home direc-
tory 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 super-user, or is writable by anyone other than the owner, the check automatically
fails. Zero is returned if the machine name is listed in the ``hosts.equiv'' file, or the
host and remote user name are found in the ``.rhosts'' file; otherwise iruserok() and
ruserok() return -1. If the local domain (as obtained from gethostname(3)) 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.
While iruserok() can handle IPv4 addresses only, iruserok_sa() and ruserok() can handle
other address families as well, like IPv6. The first argument of iruserok_sa() is typed as
void * to avoid dependency between <unistd.h> and <sys/socket.h>.
RCMD_CMD When using the rcmd() function, this variable is used as the program to run
instead of rcmd(1).
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() and rresvport_af() function return a valid, bound socket descriptor on suc-
cess. They return -1 on error with the global value errno set according to the reason for
failure. The error code EAGAIN is overloaded to mean ``All network ports in use.''
rcmd(1), rlogin(1), rsh(1), intro(2), rexec(3), hosts.equiv(5), rhosts(5), rexecd(8),
The orcmd(), rresvport(), iruserok() and ruserok() functions appeared in 4.2BSD, where the
orcmd() function was called rcmd(). The (newer) rcmd() function appeared in NetBSD 1.3.
rcmd_af() and rresvport_af() were defined in RFC2292.
BSD March 30, 2005 BSD