一步解决PPTPD VPN CTRL: PTY read or GRE write failed (pty,gre)=(6,7)错误

现象：

客户端卡在验证用户名密码，无法连接。

var/log/message档案日志：

GRE: read(fd=6,buffer=8059680,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs

CTRL: PTY read or GRE write failed (pty,gre)=(6,7)

解决办法：