Support to pass the password via a pipe to the pppd
---------------------------------------------------

Arvin Schnell <[email protected]>
2002-02-08


1. Introduction
---------------

Normally programs like wvdial or kppp read the online password from their
config file and store them in the pap- and chap-secrets before they start the
pppd and remove them afterwards. Sure they need special privileges to do so.

The passwordfd feature offers a simpler and more secure solution. The program
that starts the pppd opens a pipe and writes the password into it. The pppd
simply reads the password from that pipe.

This methods is used for quite a while on SuSE Linux by the programs wvdial,
kppp and smpppd.


2. Example
----------

Here is a short C program that uses the passwordfd feature. It starts the pppd
to buildup a pppoe connection.


--snip--

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <string.h>
#include <paths.h>

#ifndef _PATH_PPPD
#define _PATH_PPPD "/usr/sbin/pppd"
#endif


// Of course these values can be read from a configuration file or
// entered in a graphical dialog.
char *device = "eth0";
char *username = "[email protected]";
char *password = "hello";

pid_t pid = 0;


void
sigproc (int src)
{
fprintf (stderr, "Sending signal %d to pid %d\n", src, pid);
kill (pid, src);
exit (EXIT_SUCCESS);
}


void
sigchild (int src)
{
fprintf (stderr, "Daemon died\n");
exit (EXIT_SUCCESS);
}


int
start_pppd ()
{
signal (SIGINT, &sigproc);
signal (SIGTERM, &sigproc);
signal (SIGCHLD, &sigchild);

pid = fork ();
if (pid < 0) {
fprintf (stderr, "unable to fork() for pppd: %m\n");
return 0;
}

if (pid == 0) {

int i, pppd_argc = 0;
char *pppd_argv[20];
char buffer[32] = "";
int pppd_passwdfd[2];

for (i = 0; i < 20; i++)
pppd_argv[i] = NULL;

pppd_argv[pppd_argc++] = "pppd";

pppd_argv[pppd_argc++] = "call";
pppd_argv[pppd_argc++] = "pwfd-test";

// The device must be after the call, since the call loads the plugin.
pppd_argv[pppd_argc++] = device;

pppd_argv[pppd_argc++] = "user";
pppd_argv[pppd_argc++] = username;

// Open a pipe to pass the password to pppd.
if (pipe (pppd_passwdfd) == -1) {
fprintf (stderr, "pipe failed: %m\n");
exit (EXIT_FAILURE);
}

// Of course this only works it the password is shorter
// than the pipe buffer. Otherwise you have to fork to
// prevent that your main program blocks.
write (pppd_passwdfd[1], password, strlen (password));
close (pppd_passwdfd[1]);

// Tell the pppd to read the password from the fd.
pppd_argv[pppd_argc++] = "passwordfd";
snprintf (buffer, 32, "%d", pppd_passwdfd[0]);
pppd_argv[pppd_argc++] = buffer;

if (execv (_PATH_PPPD, (char **) pppd_argv) < 0) {
fprintf (stderr, "cannot execl %s: %m\n", _PATH_PPPD);
exit (EXIT_FAILURE);
}
}

pause ();

return 1;
}


int
main (int argc, char **argv)
{
if (start_pppd ())
exit (EXIT_SUCCESS);

exit (EXIT_FAILURE);
}

---snip---


Copy this file to /etc/ppp/peers/pwfd-test. The plugins can't be loaded on the
command line (unless you are root) since the plugin option is privileged.


---snip---

#
# PPPoE plugin for kernel 2.4
#
plugin pppoe.so

#
# This plugin enables us to pipe the password to pppd, thus we don't have
# to fiddle with pap-secrets and chap-secrets. The user is also passed
# on the command line.
#
plugin passwordfd.so

noauth
usepeerdns
defaultroute
hide-password
nodetach
nopcomp
novjccomp
noccp

---snip---