RCMD(3) BSD Library Functions Manual RCMD(3)NAME
rcmd, rresvport, iruserok, ruserok, rcmd_af, rresvport_af, 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, int *fd2p);
iruserok(u_long raddr, int superuser, const char *ruser,
const char *luser);
ruserok(const char *rhost, int superuser, const char *ruser,
const char *luser);
rcmd_af(char **ahost, int inport, const char *locuser,
const char *remuser, const char *cmd, int *fd2p, int af);
rresvport_af(int *port, int af);
iruserok_sa(const void *addr, int addrlen, int superuser,
const char *ruser, const char *luser);
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 num‐
bers. 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 com‐
mand (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 host's IP address or
name, as returned by the gethostbyname(3) routines, 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
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 argu‐
ment extension is implemented for functions that have no binary address
argument. Instead, the af argument specifies which address family is
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 fami‐
lies 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.
RSH When using the rcmd() function, this variable is used as the program
to run instead of rsh(1).
The rcmd() function returns a valid socket descriptor on success. It
returns -1 on error and prints a diagnostic message on the standard
The rresvport() function returns a valid, bound socket descriptor on suc‐
cess. 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.''
SEE ALSOrlogin(1), rsh(1), intro(2), rlogind(8), rshd(8)
W. Stevens and M. Thomas, Advanced Socket API for IPv6, RFC2292.
W. Stevens, M. Thomas, and E. Nordmark, Advanced Socket API for IPv6,
Most of these functions appeared in 4.2BSD. The rresvport_af() function
appeared in RFC2292, and was implemented by the WIDE project for the
Hydrangea IPv6 protocol stack kit. The rcmd_af() function appeared in
draft-ietf-ipngwg-rfc2292bis-01.txt, and was implemented in the WIDE/KAME
IPv6 protocol stack kit. The iruserok_sa() function appeared in discus‐
sion on the IETF ipngwg mailing list, and was implemented in FreeBSD 4.0.
BSD March 3, 2000 BSD