linux内核本身支持pppd，直接加上配置就可以了
Device Drivers->
->Network Device Support->
<*> PPP (point-to-point protocol) support | |
| | [*] PPP multilink support (EXPERIMENTAL) | |
| | [*] PPP filtering | |
| | <*> PPP support for async serial ports | |
| | <*> PPP support for sync tty ports | |
| | <*> PPP Deflate compression | |
| | <*> PPP BSD-Compress compression
