pptpd debug

start pptpd in debug mode:
pptpd -d

read the debug log in syslog:
cat /var/log/syslog

[problem]
connection failed, and debug message show "mppe required but peer refused" :
you need to force the client to use mppe encryption or close the mppe encryption option on server.

pptp client can't get dns server ip:
set the ms-dns option on the server side, let the client can get ip.