参考：
http://pptpclient.sourceforge.net/howto-diagnosis.phtml#conventions
http://blog.chinaunix.net/uid-20609878-id-1915875.html
Couldn't set tty to PPP discipline: Invalid argument
| Symptom: the following messages appear before connection is established:
Diagnosis: pppd has failed to change the pty over to run it in PPP mode. This may be because you have no ppp_async module built for your kernel. Most kernels are built with this already, but if you have customised your kernel you may not have built it. Solution: rebuild your kernel with CONFIG_PPP_ASYNC set. While you are at it, you should set CONFIG_PPP as well, and both should be set to "m" so that they are built as modules. We've found they don't work compiled statically. |
重新配置内核（Device Drivers->Network device support->PPP support），把ppp相关的都选上。