[root@server]# modprobe ppp_generic
[root@server]# vzctl set 100 --devices c:108:0:rw --save
Setting devices
Saved parameters for VPS 100
[root@server]# vzctl enter 100
entered into VPS 100
-bash-3.00# mknod /dev/ppp c 108 0
-bash-3.00# pptp vpn-server.com
/usr/sbin/pppd: This system lacks kernel support for PPP.  This could be because
the PPP kernel module could not be loaded, or because PPP was not
included in the kernel configuration.  If PPP was included as a
module, try `/sbin/modprobe -v ppp'.  If that fails, check that
ppp.o exists in /lib/modules/`uname -r`/net.
See README.linux file in the ppp distribution for more details.

-bash-3.00# 

strace -f -o strace.out pppd 

-bash-3.00# pppd --version
pppd version 2.4.3

16120 open("/dev/ppp", O_RDWR)          = -1 EPERM (Operation not permitted)

2.6.16-026test017
template: fc4
pppd (fc4):
-bash-3.1# rpm -qa | grep ppp
ppp-2.4.2-7

...
static int ppp_open(struct inode *inode, struct file *file)
{
        /*
         * This could (should?) be enforced by the permissions on /dev/ppp.
         */
        if (!capable(CAP_NET_ADMIN))
                return -EPERM;
        return 0;
}
...

[hw]# vzctl stop 35
Stopping VPS ...
VPS was stopped
VPS is unmounted
[hw]# vzctl set 35  --capability net_admin:on --save
Saved parameters for VPS 35
[hw]# vzctl start 35
Starting VPS ...
VPS is mounted
Setting CPU units: 1000
Setting devices
VPS start in progress...
[root@dhcp0-174 tmp]# vzctl enter 35
entered into VPS 35
-bash-3.1# pppd
pppd     pppdump
-bash-3.1# pppd
-bash-3.1# echo $?
1
-bash-3.1# pppd
-bash-3.1# echo $?
1

-bash-3.1# cat /var/log/messages | grep pppd
Sep 18 00:57:42 hw pppd[17673]: pppd 2.4.2 started by root, uid 0
Sep 18 00:57:42 hw pppd[17673]: Couldn't set tty to PPP discipline: Invalid argument
Sep 18 00:57:42 hw pppd[17673]: Exit.
Sep 18 01:12:23 hw pppd[20190]: pppd 2.4.2 started by root, uid 0
Sep 18 01:12:23 hw pppd[20190]: Failed to open /dev/pts/0: Device or resource busy
Sep 18 01:12:23 hw pppd[20190]: Exit.

    if (ioctl(tty_fd, TIOCSETD, &ppp_disc) < 0) {
        if ( ! ok_error (errno) ) {
            error("Couldn't set tty to PPP discipline: %m");
            return -1;
        }
    }

linuxsrc $ grep -r tty_register_ldisc ./ | grep N_PPP
./drivers/net/ppp_async.c:      err = tty_register_ldisc(N_PPP, &ppp_ldisc);
linuxsrc $ cat kernel-2.6.16-026test017-i686.config.ovz | grep CONFIG_PPP_ASYNC
CONFIG_PPP_ASYNC=m

[hw]# lsmod | grep ppp
ppp_generic            20620  0
slhc                    6240  1 ppp_generi
[root@dhcp0-174 ~]# modprobe ppp_async
[root@dhcp0-174 ~]# echo $?
0
[root@dhcp0-174 ~]# lsmod | grep ppp
ppp_async               8960  0
crc_ccitt               2048  1 ppp_async
ppp_generic            20620  1 ppp_async
slhc                    6240  1 ppp_generic

-bash-3.1# pppd
-bash-3.1# echo $?
1
-bash-3.1# tail -3 /var/log/messages
Sep 18 01:59:15 dhcp0-174 pppd[25632]: pppd 2.4.2 started by root, uid 0
Sep 18 01:59:15 dhcp0-174 pppd[25632]: Couldn't create new ppp unit: Operation not permitted
Sep 18 01:59:15 dhcp0-174 pppd[25632]: Exit.

25784 ioctl(10, PPPIOCNEWUNIT, 0x80083c08) = -1 EPERM (Operation not permitted)

PPP: couldn't register device ppp0 (-1)

        ret = register_netdev(dev);
        if (ret != 0) {
                printk(KERN_ERR "PPP: couldn't register device %s (%d)\n",
                       dev->name, ret);
                goto out2;
        }

        ret = -EPERM;
        if (!ve_is_super(get_exec_env()) && ve_is_dev_movable(dev))
                goto out;