RCMD(3)

NAME
rcmd, rresvport, iruserok, ruserok, orcmd - routines for returning a stream to a remote command

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
rcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p);
int
orcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p);
int
rresvport(int *port);
int
iruserok(u_int32_t raddr, int superuser, const char *ruser, const char *luser);
int
ruserok(const char *rhost, int superuser, const char *ruser, const char *luser);

DESCRIPTION
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. The rresvport() function returns a 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 five functions are present in the same file and 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 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.

The protocol is described in detail in rshd(8).

The rresvport() function is 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, respectively, 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 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 super-user, or is writeable 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.

DIAGNOSTICS
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 value errno set according to the reason for failure. The error code EAGAIN is overloaded to mean ``All network ports in use.''

ENVIRONMENT
RCMD_CMD
    When using the rcmd() function, this variable is used as the program to run instead of rcmd(1).

SEE ALSO
rcmd(1), rlogin(1), rsh(1), intro(2), rexec(3), hosts.equiv(5), rhosts(5), rexecd(8), rlogind(8), rshd(8).

HISTORY
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.

BUGS
As the rcmd function uses getpwent(3) functions, passing in a previous value from one of this family of functions can result in unpredictable results. Do not write code like the following:

    struct passwd *pw;
    pw = getpwuid(getuid());
    if (rcmd(host, port, pw->pw_name, pw->pw_name, cmd, fd2p) < 0)
    	err(1, "rcmd");

When a reentrant version of getpwent(3) is available, rcmd should be changed to use this instead.