My experiences with ppp have only been with one ``tun'' device (tun0). If you have used more (i.e., a number other than `1' in the pseudo-device line in the kernel configuration file) then alter all references to ``tun0'' below to reflect whichever device number you are using.
The easiest way to make sure that the tun0 device is configured correctly is to re-make it. To this end, execute the following commands:
# cd /dev
# ./MAKEDEV tun0