rcmd()

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 nonzero, then an auxiliary channel to a control process will be set up, and a file 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).

rresvport()

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 (CAP_NET_BIND_SERVICE) 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 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.

iruserok() and ruserok()

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 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, 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 the .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.

*_af() variants

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 AF_UNSPEC.