這分linux source code是被patched過的，所以它的pppd的版本比較舊，支援的option也不太一樣。最大的不同點在於，和mppe相關的config，格式有所修改。變成和pptpvpn官網的不一樣。
Usage:
By default pppd tries to negotiate MPPC and don't negotiate MPPE but will agree if peer wants encryption. If peer supports a few key lengths, according to RFC3078 pppd will choose the strongest one.
mppe suboptions:
required - make MPPE obligatory, disconnect if peer doesn't support MPPE
stateless - try to negotiate stateless mode
no40 - disable 40 bit keys
no56 - disable 56 bit keys
no128 - disable 128 bit keys
Examples:
pppd [options] - try to negotiate MPPC, MPPE is optional
pppd nomppe [options] - try to negotiate MPPC and disable MPPE; peer will disconnect if it requires MPPE
pppd nomppc [options] - disable MPPC; MPPE is optional
pppd mppe required,stateless,no128 [options] - try to negotiate MPPC, require MPPE in stateless mode and disable 128 bit keys; pppd will disconnect if peer doesn't support MPPE
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在除錯的過程中，也一直不知道windows的pptpvpn 的選項，直到最後，終於看懂了
ConfReq <+H, -M, +S, -L, -D>
=> 這是使用mppe 128bits, stateless, nomppc的意思
之後，掙扎了很久，總算是生出一分可以使用的config檔。
/etc/ppp/options.pptpd
name "pptpd"lockmtu 1450mru 1450proxyarpauthipcp-accept-localipcp-accept-remotelcp-echo-failure 3lcp-echo-interval 5deflate 0+chap+mschap-v2mppe required,stateless
/etc/pptpd.conf
ppp /bin/pppd
option /etc/ppp/options.pptpd
localip 172.16.16.1
remoteip 172.16.17.1-254
/etc/ppp/chap-secrets
"帳號" pptpd "密碼" *
