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Date: Sat, 31 Mar 2001 22:53:57 -0500
Message-Id: <200104010353.f313rvu04868@soup.in.ql.org>
From: "E. Jay Berkenbilt" <ejb@ql.org>
To: schaefer@candle.brasslantern.com
CC: zsh-users@sunsite.dk
In-reply-to: <1010331191449.ZM8000@candle.brasslantern.com>
	(schaefer@candle.brasslantern.com)
Subject: Re: Differrent prompt for remote machines
References: <E14jJc8-0002oE-00@crucigera.fysh.org> <1010331191449.ZM8000@candle.brasslantern.com>
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I determine whether I'm remote using this C program I wrote.  I've
tested it only linux.  It uses the fact that most well-behaved remote
login daemons make sure that your tty's utmp entry contains the host
from which you logged in if you are logged in remotely.  This program
prints "local" if you're local, "remote" if you remote, and a few
other things if it can't tell for various reasons.  I've found it to
accurately detect remote logins with ssh, telnet, and (Kerberos)
rlogin and to accurately detect local logins with or without X.  If
you remote login back to your local host (for some reason) it still
considers you to be local.

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/*
 * $Id: utmp_checkremote.c,v 1.1 2000/09/05 21:41:27 ejb Exp $
 * $Source: /home/ejb/source/util/login/RCS/utmp_checkremote.c,v $
 * $Author: ejb $
 *
 */

#ifndef NO_RCS_HDRS
/* Define a static function and call it.  No warnings this way. */
static void rcsid(char *s)
{rcsid("@(#)$Id: utmp_checkremote.c,v 1.1 2000/09/05 21:41:27 ejb Exp $");s=s;}
#endif /* ! NO_RCS_HDRS */

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <utmp.h>
#include <netdb.h>
#include <errno.h>

#define UTL_NOTTY "notty"
#define UTL_NOTFOUND "notfound"
#define UTL_LOCAL "local"
#define UTL_REMOTE "remote"
#define UTL_UNKNOWN "unknown"

static char* whoami = 0;

static void deal_with_host(char* host)
{
    char* result = UTL_UNKNOWN;
    if (strlen(host) == 0)
    {
	result = UTL_LOCAL;
    }
    else
    {
	char thishost[UT_HOSTSIZE + 1];
	char otherhost[UT_HOSTSIZE + 1];
	struct hostent* hp = 0;

	thishost[UT_HOSTSIZE] = '\0';
	otherhost[UT_HOSTSIZE] = '\0';
	if (gethostname(thishost, UT_HOSTSIZE) == -1)
	{
	    fprintf(stderr, "%s: gethostname failed: %s\n",
		    whoami, strerror(errno));
	    exit(2);
	}
	if ((hp = gethostbyname(thishost)) != 0)
	{
	    strncpy(thishost, hp->h_name, UT_HOSTSIZE);
	    if ((hp = gethostbyname(host)) != 0)
	    {
		strncpy(otherhost, hp->h_name, UT_HOSTSIZE);
		if (strcmp(thishost, otherhost) == 0)
		{
		    result = UTL_LOCAL;
		}
		else
		{
		    result = UTL_REMOTE;
		}
	    }
	}
    }

    printf("%s\n", result);
}

int main(int argc, char* argv[])
{
    struct utmp *uptr = 0;
    char* tty = 0;

    if ((whoami = strrchr(argv[0], '/')) == NULL)
    {
	whoami = argv[0];
    }
    else
    {
	++whoami;
    }

    tty = ttyname(0);
    if (tty == 0)
    {
	printf("%s\n", UTL_NOTTY);
	exit(0);
    }
    else if (strncmp(tty, "/dev/", 5) != 0)
    {
	fprintf(stderr,
		"%s: ttyname() returned something that didn't start with /dev!",
		whoami);
	exit(2);
    }
    tty += 5;

    setutent();
    while ((uptr = getutent())!=NULL) {
	if (!uptr->ut_name[0])
	    continue;
#ifdef USER_PROCESS
	if (uptr->ut_type != USER_PROCESS) continue;
#endif
	if (uptr->ut_host && uptr->ut_line &&
	    (strcmp(uptr->ut_line, tty) == 0))
	{
	    deal_with_host(uptr->ut_host);
	    exit(0);
	}
    }
    endutent();

    printf("%s\n", UTL_NOTFOUND);
    return 0;
}
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