The rcmd 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 ruserok function is used by servers to authenticate clients requesting service with rcmd. All three 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 to which an address with a Privileged Internet port is bound. 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 hosts IP address or name, as returned by the gethostbyname(3) routines, two user names and a flag indicating whether the local users 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 users 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 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.
The iruserok function is strongly preferred for security reasons. It requires trusting the local DNS at most, while the ruserok function requires trusting the entire DNS, which can be spoofed.
The functions with an "_af" or "_sa" suffix, i.e., rcmd_af, rresvport_af and iruserok_sa, work the same as the corresponding functions without a suffix, except that they are capable of handling both IPv6 and IPv4 ports.
The "_af" suffix means that the function has an additional af argument which is used to specify the address family, (see below). The af argument extension is implemented for functions that have no binary address argument. Instead, the af argument specifies which address family is desired.
The "_sa" suffix means that the function has general socket address and length arguments. As the socket address is a protocol independent data structure, IPv4 and IPv6 socket address can be passed as desired. The sa argument extension is implemented for functions that pass a protocol dependent binary address argument. The argument needs to be replaced with a more general address structure to support multiple address families in a general way.
The functions with neither an "_af" suffix nor an "_sa" suffix work for IPv4 only, except for ruserok which can handle both IPv6 and IPv4. To switch the address family, the af argument must be filled with AF_INET, or AF_INET6. For rcmd_af, PF_UNSPEC is also allowed.
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.
rlogin(1), rsh(1), intro(2), rlogind(8),