Debug PPTPVPN

最近公司的project是在一份linux 24.18的 OS上，使PPTP VPN server可以運作。一連遇到了好幾個關卡：

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"

lock

mtu 1450

mru 1450

proxyarp

auth

ipcp-accept-local

ipcp-accept-remote

lcp-echo-failure 3

lcp-echo-interval 5

deflate 0

+chap

+mschap-v2

mppe 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" *