dwww: tpablo.net

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

        Arvin Schnell <arvin@suse.de>
        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 = "1122334455661122334455660001@t-online.de";
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---

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