rcmd —
rcmd_af,
rresvport,
rresvport_af,
iruserok,
ruserok,
iruserok_sa
routines for returning a stream to a remote command
SYNOPSIS
#include <unistd.h> intrcmd(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p);
intrcmd_af(char **ahost, int inport, const char *locuser, const char *remuser, const char *cmd, int *fd2p, int af);
intrresvport(int *port);
intrresvport_af(int *port, int af);
intiruserok(u_int32_t raddr, int superuser, const char *ruser, const char *luser);
intruserok(const char *rhost, int superuser, const char *ruser, const char *luser);
intiruserok_sa(const void *sa, int salen, int superuser, const char *ruser, const char *luser);
DESCRIPTION
The
rcmd();
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
rsh(1).
While
rcmd();
can handle IPv4 cases only,
the
rcmd_af();
function can handle other cases as well.
The
rresvport();
and
rresvport_af();
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();.
All four functions are present in the same file and are used
by the
rshd(8)
server (among others).
iruserok_sa();
is an address family independent variant of
iruserok();.
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 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.
rcmd_af();
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 protocol is described in detail in
rshd(8).
The
rresvport();
and
rresvport_af();
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
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 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, 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.
While
iruserok();
can handle IPv4 addresses only,
iruserok_sa();
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›.
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();
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”.