Linux PPP

Disconnection program

This program may be run by any user to make PPP drop the connection and terminate.

It is stored in /etc/ppp/scripts/ppp-off, but I also have a link to it in /usr/local/bin, which is part of my standard path.

Because this program has to kill a process which may not have been started by the same user that is running the disconnection program, it is written in C and made SUID to root. This ensures that the security of the system is not affected, as it would have been if kill had been made SUID to root.

/* ppp-off.c

   Program to kill off a running pppd connection

   There may be more than one connection running.  If an interface name is given on
   the command line, the connection using that device is terminated; otherwise
   ppp0 is terminated.

   This is a program rather than a script so as to enable any user to run it.  This is
   preferable to making kill SUID to root.

*/

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <signal.h>
#include <string.h>

#define MAX_IF_LENGTH	16
#define	PIDFILEMASK	"%s/%s.pid"
#define	PIDDIR		"/var/run"

void kill_pppd(pid_t);

char *progname;

int main(int argc, char **argv) {
	FILE *pidfile;
	char interface[MAX_IF_LENGTH];
	char pidfilename[255];
	int pid = 0;

	/* Check the command line */
	progname = argv[0];
	if (argc > 1)
		strcpy(interface, argv[1]);
	else
		strcpy(interface,"ppp0");
	
	/* Open the file that records the process numbers */
	sprintf(pidfilename,PIDFILEMASK,PIDDIR,interface);
	if ((pidfile = fopen(pidfilename,"r")) == NULL)
	{
		fprintf(stderr,"%s: No pppd process recorded (%s is missing)\n", progname,
			pidfilename);
		exit(1);
	}

	fscanf(pidfile, "%d", &pid);
	kill_pppd(pid);
	return(0);
}

void kill_pppd(pid_t pid) {
	if (kill(pid, SIGINT) == -1) {
		perror(progname);
		exit(2);
	}
}
 

   cc -o ppp-off -Wall ppp-off.c
   mv ppp-off /etc/ppp/scripts
   chown root /etc/ppp/scripts/ppp-off
   chmod u+s,a+x /etc/ppp/scripts/ppp-off

When pppd starts, it records its PID in a file called /var/run/pppn.pid. This program reads that file to find the PID and sends the process an interrupt signal.

When pppd receives the signal, it drops the connection, cleans up and terminates.