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RCMD(3) Library Functions Manual RCMD(3)

rcmd, rcmd_af, rresvport, rresvport_af, iruserok, ruserok, iruserok_saroutines for returning a stream to a remote command

#include <unistd.h>

rcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p);

rcmd_af(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p, int af);

rresvport(int *port);

rresvport_af(int *port, int af);

iruserok(u_int32_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 *sa, int salen, int superuser, const char *ruser, const char *luser);

The () function is used by the superuser to execute a command on a remote machine using an authentication scheme based on reserved port numbers. If the calling process is not setuid, the RSH environment variable is set, and inport is “shell/tcp”, rcmdsh(3) is called instead with the value of RSH. Alternately, if the user is not the superuser, rcmd() will invoke rcmdsh(3) to run the command via ssh(1). While rcmd() can handle IPv4 cases only, the rcmd_af() function can handle other cases as well.

The () and () functions return 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(). iruserok_sa() is an address family independent variant of iruserok().

The () function looks up the host *ahost using gethostbyname(3) and, if the host exists, *ahost is set to the standard name of the host. A connection is then established to a server residing at the well-known Internet port inport. If the user is not the superuser, the only valid port is “shell/tcp” (usually port 514).

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 NULL, then the standard error (unit 2 of the remote command) will be made the same as the standard output 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. Note that if the user is not the superuser, fd2p must be NULL.

() takes 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 () and () 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 IPPORT_RESERVED - 1, which happens to be 1023. Only the superuser is allowed to bind an address of this sort to a socket. rresvport() and rresvport_af() need to be seeded with a port number; if that port is not available these functions will find another.

The () and () 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 superuser. Then, if the user is the superuser, 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 superuser, 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 () and () 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, () should be used in preference to (), as it does not require trusting the DNS server for the remote host's domain.

While () can handle IPv4 addresses only, () 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>.

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() functions return 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”.

ssh(1), intro(2), bindresvport(3), bindresvport_sa(3), rcmdsh(3)

These functions appeared in 4.2BSD.

March 23, 2015 OpenBSD-5.8