nice util to aid shell/home scripts on finding the remote ip over a ssh connection

whoami.c

/*
 * $Id: //websites/unixwiz/unixwiz.net/webroot/tools/whoamip.c#1 $
 *
 * written by : Stephen J. Friedl
 *              Software Consultant
 *              Tustin, California USA
 *              http://www.unixwiz.net/tools/
 *
 *	***********************************************************
 *	**                                                       **
 *	** This program is in the public domain, and can be used **
 *	** by anybody, for any purpose. Have fun.                **
 *	**                                                       **
 *	***********************************************************
 *
 *	This program is used to make the best guess at a user's remote
 *	IP address. We do not have a good mechanism to obtain it directly,
 *	but given the user's tty (usually a pts), we can look in the
 *	utmpx file where it's recorded.
 *
 *	The "who" program reads from /var/adm/utmpx, but sadly does not
 *	seem to have a way to report the hostname, so we have written
 *	this program to do so.
 *
 *	Playing on the "whoami" program, we've named it "whoamip".
 *
 * WHAT'S THE HOSTNAME?
 * --------------------
 *
 *	We're not able to directly discover the IP address of the other
 *	end of this connection, because our program doesn't have any
 *	access to the direct socket in order to run getpeername(). An
 *	intermediate pseudo-tty isolates us from the socket, so we have
 *	no trivial way to learn this straightway.
 *
 *	Some operating systems probably provide some kernel-based mechanism
 *	for digging into the network stack, but this is troublesome and
 *	highly non-portable. It's probably nearly unsolvable on many
 *	platforms.
 *
 *	Instead, we'll get the information indirectly: the init and/or login
 *	program typically maintains this in the utmpx file (in /etc/ or
 *	/var/adm), associating the remote host with the current logged-in
 *	pseudo-tty.
 *
 *	We locate the current tty and use that as a key to utmpx: this
 *	brings a record with the username and hostname: this is what's
 *	reported to the user.
 *
 *	We're at the mercy of the login program to fill in the ut_host
 *	field: sometimes it contains the IP address (as a string), and
 *	other times it's a hostname (perhaps truncated). We have no
 *	control over this process and simply take what we're given.
 *
 * SSH SHORCUTS
 * ------------
 *
 *	As a short-circuit for users who come in via Secure Shell, we
 *	take a quick check for the $SSH_CLIENT environment variable,
 *	which would contain the IP address. If this is found, we return
 *	it directly without doing any I/O or mucking around with the TTY.
 *
 *	Use the -S cmdline parameter to disable the $SSH_CLIENT check,
 *	and in any case we do not use the value unless there is a space
 *	after the remote IP address.
 *
 * BUILDING + RUNNING
 * ------------------
 *
 *	This requires just a C compiler:
 *
 *		$ cc whoamip.c -o whoamip
 *
 *	It should compile without errors or warnings.
 *
 *	Running the program produces the hostname on the standard
 *	output, which can be captured in the shell (often in /etc/profile).
 *
 *		$ host=`whoamip`
 *
 *	We exit with zero (success) if we believe we've located the hostname,
 *	and nonzero (failure) if we can't for whatever reason.
 *
 * COMMAND LINE
 * ------------
 *
 * -V
 *	Show the program's version information, then exit with success
 *
 * -S
 *	Disable the $SSH_CLIENT check.
 *
 * VERSION HISTORY
 * ---------------
 *
 * 1.0.1 - 2006-07-31
 *	Initial release

 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <utmpx.h>
#include <unistd.h>

#ifndef TRUE
# define TRUE  1
# define FALSE 0
#endif

static const char Version[] =
"whoamip 1.0.1 - 2006-07-31 - http://www.unixwiz.net/tools/";

static int no_ssh_client = FALSE;

int main(int argc, char **argv)
{
int             c;
char            *tty, *p, *env;
struct utmpx protox, *utp;

	/*---------------------------------------------------------------
	 * COMMAND-LINE 
	 */
	while ( (c = getopt(argc, argv, "VS")) != EOF )
	{
		switch (c)
		{
		  case 'V':
			puts(Version);
			exit(EXIT_SUCCESS);

		  case 'S':
			no_ssh_client = TRUE;
			break;

		  default:
			exit(EXIT_FAILURE);
		}
	}


	/*---------------------------------------------------------------
	 * SHORTCUT: SSH variable?
	 *
	 * Most users don't use SSH, but for those that do, the $SSH_CLIENT
	 * reports the remote connection:
	 *
	 *	SSH_CLIENT="64.170.162.98 51851 22"
	 *
	 * We only care about the IP address and not the ports involved,
	 * but this is a pretty reliable mechanism.
	 */

	if ( ! no_ssh_client
	  && (env = getenv("SSH_CLIENT")) != 0
	  && (p = strchr(env, ' ')) != 0 )
	{
		*p = '\0';
		*p = '\0';
		puts(env);

		exit(EXIT_SUCCESS);
	}

	/*---------------------------------------------------------------
	 * FIND TTY NAME
	 *
	 * The utmp/wtmp files ONLY work when indexed by tty name, so we
	 * have to find the current tty. For network connections, this is
	 * virtually always /dev/ptsX (a pseudo-tty), and we remove the
	 * leading /dev/ part.
	 *
	 * Some systems work with /dev/pts123, while others use /dev/pts/123.
	 * We remove /dev/ in either case.
	 *
	 * NOTE: a tty is associated with a *file descriptor*, not a
	 * program, so we have to pick one. We use the standard input,
	 * because the standard output & error may well be attached to
	 * either an output file or a pipe:
	 *
	 *	host=`whoamip`
	 *
	 * If we cannot find the tty name, we're done - sorry.
	 *
	 * Otherwise, rip off everything up to the final /
	 */
	if ( (tty = ttyname(fileno(stdin))) == 0 )
	{
		fprintf(stderr, "cannot get tty [%s]\n", strerror(errno) );

		exit(EXIT_FAILURE);
	}

	if ( strncmp(tty, "/dev/", 5) == 0 )
		tty += 5;

	/*---------------------------------------------------------------
	 * LOCATE UTMPX
	 *
	 * Look in the utmpx file for a record matching this line, but
	 * the lookup is done with a prototype UTMPX record that has the
	 * ut_line member filled in.
	 */
	memset(&protox, 0, sizeof protox);
	strcpy(protox.ut_line, tty);

	if ( (utp = getutxline(&protox) ) == 0 )
	{
		fprintf(stderr, "cannot locate UTMPX for {%s}\n", tty);
		exit(EXIT_FAILURE);
	}

	/*---------------------------------------------------------------
	 * FOUND IT!
	 *
	 * We should have a NUL-byte terminated string, but we believe
	 * that we've seen systems which use up to the full size of the
	 * buffer without the trailing NUL. That would mean we might end
	 * up with a runaway string. That would be bad.
	 *
	 * So we create a buffer one byte larger, copy the ut_host field
	 * into it, and assuredly add our own NUL byte. 
	 *
	 * This may be overkill, but we don't get so snookered by funky
	 * data.
	 */

	{
#define	SZ sizeof(utp->ut_host)

		char	linebuf[1 + SZ];

		memcpy(linebuf, utp->ut_host, SZ);

		linebuf[SZ] = '\0';

		puts(linebuf);
	}

	return EXIT_SUCCESS;
}
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