rexec(3N)

NAME

rexec() − return stream to a remote command

SYNOPSIS

int rexec(

char ∗∗ahost,
int inport,
const char ∗user,
const char ∗passwd,
const char ∗cmd,
int ∗fd2p);

DESCRIPTION

rexec() arranges for the remote execution of cmd on the host *ahost as user, who is authenticated with passwd. It returns a file descriptor for the socket to which the standard input and standard output of cmd are attached. A command-level interface to rexec() is provided by the rexec command (see remsh(1)).

rexec() looks up host *ahost using gethostbyname() (see gethostbyname(3N)) and returns −1 if the host does not exist. The host name can be either the official name or an alias. If the gethostbyname() call succeeds, *ahost is set to the standard name of the host. rexec() passes a username and password to the remote host for authentication. These can be specified in the user and passwd parameters to rexec(). If either is NULL, rexec() searches for the appropriate information in the .netrc file (see netrc(4)) in the users’s home directory. If this fails, rexec() prompts the user for the remote user name and password, defaulting to the local user name and a NULL password.

inport specifies which TCP port to use for the connection; it is normally the value returned by getservbyname("exec", "tcp") (see getservent(3N)). The protocol used by rexec() is described in detail in rexecd(1M).

If the call succeeds, a socket of type SOCK_STREAM is returned to the caller, and given to the remote command as stdin and stdout. If the connection to the socket is refused after five tries, or if it was refused for a reason other than the port being in use, rexec() returns −1. If fd2p is non-zero, an auxiliary connection to a control process is set up and a file descriptor for it is placed in ∗fd2p. The control process returns diagnostic output from the command on this connection and accepts bytes on this connection, interpreting them as UNIX signal numbers to be forwarded to the process group of the command. If the auxiliary port cannot be set up, rexec() returns −1. If fd2p is 0, stderr of the remote command is made the same as stdout and no provision is made for sending arbitrary signals to the remote process.

DIAGNOSTICS

rexec() produces the following diagnostic messages:

hostname: Unknown host
The remote host name was not found by gethostbyname().

system call:...
Error in executing the system call. A message specifying the cause of the failure is appended to this message.

connect: hostname:...
Error in connecting to the socket obtained for rexec(). A message specifying the cause of the failure is appended to this diagnostic.

Secondary socket:...
Error in creating a secondary socket for error transmission to be used by rexec().

read: hostname:...
Error in reading information transmitted over the socket. A message specifying the cause of the failure is appended to this diagnostic.

Connection timeout
The remote host did not connect within 30 seconds to the secondary socket set up as an error connection.

Lost connection
The program attempts to read from the socket and fails. This means the socket connection with the remote host was lost.

.netrc:...
Error in opening .netrc file in the home directory for a reason other than the file not existing.

Error− .netrc file not correct mode.
Remove password or correct mode.

The .netrc file is readable, writable or executable by anyone other than the user.

Next step: Check whether .netrc has been modified by someone else and change the mode of .netrc (chmod 400 .netrc).

Unknown .netrc option ...
An unrecognized keyword has been found in .netrc (see netrc(4)).

Next step: Correct keyword in .netrc.

primary connection shutdown
While waiting for the secondary socket to be set up, rexec() had its primary connection shut down. This may have been caused by the inetd security failure.

recv:...
While trying to set up the secondary (stderr) socket, rexec() had an error condition on its primary connection.

accept: Interrupted system call
While trying to set up a secondary socket, rexec() ran out of a resource, which caused the accept to be timed out.

Next step: Repeat the command. If a timeout occurs, check whether the ARPA Services are installed and inetd is running.

EXAMPLE

To execute the date command on remote host hpxzgy using the remote account chm, rexec() could be used as follows:

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
char *host[] = { "hpxzgy" };
char *user = "chm";
char *passwd = "password";
char *cmd = "date";
main(argc, argv)
char **argv;
int argc;
{
char ch;
struct servent *servent;
FILE *fp;
int sd;
servent = getservbyname("exec", "tcp");
sd = rexec(host, servent−>s_port, user, passwd, cmd, 0);
fp = fdopen(sd, "r");
while ((ch = getc(fp)) != EOF)
putchar(ch);
}

WARNINGS

There is no way to specify options to the socket() call that rexec() makes.

A program using rexec() should not be put in the background when rexec() is expected to prompt for a password or user name. If it is put in the background it will compete with the shell for input.

Since rexec() replaces the pointer to the hostname (∗ahost) with a pointer to the standard name of the host in a static data area, this value must be copied into the user’s data area if it is to be used later.

The password is sent unencrypted through the socket connection.

AUTHOR

rexec() was developed by the University of California, Berkeley.

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