背景介绍：
搭建PPTP VPN Server应该是非常容易的，可身边有不少朋友在参考了一些文章后仍然来求助于我，走了不少的弯路。
因此，觉得自己有必要写一篇文章来讲解一下。毕竟我写文章的习惯是边操作边记录，所以一步一步照着做就可以完成，大家都喜欢看。
相关配置：
OS: CentOS 6.4 x86_64 Minimal
1. 安装EPEL扩展库
# yum install http://dl.fedoraproject.org/pub/epel/6/i386/epel-release-6-8.noarch.rpm
2. 安装PPTP扩展库
# yum install http://poptop.sourceforge.net/yum/stable/rhel6/pptp-release-current.noarch.rpm
3. 安装PPTP VPN Server
# yum install pptpd
4. 编辑/etc/pptpd.conf
# vim /etc/pptpd.conf
###############################################################################
# $Id: pptpd.conf,v 1.11 2011/05/19 00:02:50 quozl Exp $
#
# Sample Poptop configuration file /etc/pptpd.conf
#
# Changes are effective when pptpd is restarted.
###############################################################################
# TAG: ppp
# Path to the pppd program, default '/usr/sbin/pppd' on Linux
#
#ppp /usr/sbin/pppd
# TAG: option
# Specifies the location of the PPP options file.
# By default PPP looks in '/etc/ppp/options'
#
option /etc/ppp/options.pptpd
# TAG: debug
# Turns on (more) debugging to syslog
#
debug
# TAG: stimeout
# Specifies timeout (in seconds) on starting ctrl connection
#
stimeout 120
# TAG: noipparam
# Suppress the passing of the client's IP address to PPP, which is
# done by default otherwise.
#
#noipparam
# TAG: logwtmp
# Use wtmp(5) to record client connections and disconnections.
#
#logwtmp
# TAG: vrf <vrfname>
# Switches PPTP & GRE sockets to the specified VRF, which must exist
# Only available if VRF support was compiled into pptpd.
#
#vrf test
# TAG: bcrelay <if>
# Turns on broadcast relay to clients from interface <if>
#
#bcrelay eth1
# TAG: delegate
# Delegates the allocation of client IP addresses to pppd.
#
# Without this option, which is the default, pptpd manages the list of
# IP addresses for clients and passes the next free address to pppd.
# With this option, pptpd does not pass an address, and so pppd may use
# radius or chap-secrets to allocate an address.
#
#delegate
# TAG: connections
# Limits the number of client connections that may be accepted.
#
# If pptpd is allocating IP addresses (e.g. delegate is not
# used) then the number of connections is also limited by the
# remoteip option. The default is 100.
#connections 100
# TAG: localip
# TAG: remoteip
# Specifies the local and remote IP address ranges.
#
# These options are ignored if delegate option is set.
#
# Any addresses work as long as the local machine takes care of the
# routing. But if you want to use MS-Windows networking, you should
# use IP addresses out of the LAN address space and use the proxyarp
# option in the pppd options file, or run bcrelay.
#
# You can specify single IP addresses seperated by commas or you can
# specify ranges, or both. For example:
#
# 192.168.0.234,192.168.0.245-249,192.168.0.254
#
# IMPORTANT RESTRICTIONS:
#
# 1. No spaces are permitted between commas or within addresses.
#
# 2. If you give more IP addresses than the value of connections,
# it will start at the beginning of the list and go until it
# gets connections IPs. Others will be ignored.
#
# 3. No shortcuts in ranges! ie. 234-8 does not mean 234 to 238,
# you must type 234-238 if you mean this.
#
# 4. If you give a single localIP, that's ok - all local IPs will
# be set to the given one. You MUST still give at least one remote
# IP for each simultaneous client.
#
# (Recommended)
#localip 192.168.0.1
#remoteip 192.168.0.234-238,192.168.0.245
# or
#localip 192.168.0.234-238,192.168.0.245
#remoteip 192.168.1.234-238,192.168.1.245
localip 10.192.168.1
remoteip 10.192.168.100-200
注解：在以上配置文件中，
指定了PPP配置文件路径：option /etc/ppp/options.pptpd
开启了调试日志：debug
设置了建立连接时的超时时间为120秒：stimeout 120
PPTP VPN Server的本地地址，即客户端会自动获取到的网关地址：localip 10.192.168.1
分配给客户端的地址范围：remoteip 10.192.168.100-200
5. 编辑/etc/ppp/options.pptpd
###############################################################################
# $Id: options.pptpd,v 1.11 2005/12/29 01:21:09 quozl Exp $
#
# Sample Poptop PPP options file /etc/ppp/options.pptpd
# Options used by PPP when a connection arrives from a client.
# This file is pointed to by /etc/pptpd.conf option keyword.
# Changes are effective on the next connection. See "man pppd".
#
# You are expected to change this file to suit your system. As
# packaged, it requires PPP 2.4.2 and the kernel MPPE module.
###############################################################################
# Authentication
# Name of the local system for authentication purposes
# (must match the second field in /etc/ppp/chap-secrets entries)
name ec2-tokyo
# Strip the domain prefix from the username before authentication.
# (applies if you use pppd with chapms-strip-domain patch)
#chapms-strip-domain
# Encryption
# (There have been multiple versions of PPP with encryption support,
# choose with of the following sections you will use.)
# BSD licensed ppp-2.4.2 upstream with MPPE only, kernel module ppp_mppe.o
# {{{
refuse-pap
refuse-chap
refuse-mschap
# Require the peer to authenticate itself using MS-CHAPv2 [Microsoft
# Challenge Handshake Authentication Protocol, Version 2] authentication.
require-mschap-v2
# Require MPPE 128-bit encryption
# (note that MPPE requires the use of MSCHAP-V2 during authentication)
require-mppe-128
# }}}
# OpenSSL licensed ppp-2.4.1 fork with MPPE only, kernel module mppe.o
# {{{
#-chap
#-chapms
# Require the peer to authenticate itself using MS-CHAPv2 [Microsoft
# Challenge Handshake Authentication Protocol, Version 2] authentication.
#+chapms-v2
# Require MPPE encryption
# (note that MPPE requires the use of MSCHAP-V2 during authentication)
#mppe-40 # enable either 40-bit or 128-bit, not both
#mppe-128
#mppe-stateless
# }}}
# Network and Routing
# If pppd is acting as a server for Microsoft Windows clients, this
# option allows pppd to supply one or two DNS (Domain Name Server)
# addresses to the clients. The first instance of this option
# specifies the primary DNS address; the second instance (if given)
# specifies the secondary DNS address.
#ms-dns 10.0.0.1
#ms-dns 10.0.0.2
ms-dns 172.31.0.2
# If pppd is acting as a server for Microsoft Windows or "Samba"
# clients, this option allows pppd to supply one or two WINS (Windows
# Internet Name Services) server addresses to the clients. The first
# instance of this option specifies the primary WINS address; the
# second instance (if given) specifies the secondary WINS address.
#ms-wins 10.0.0.3
#ms-wins 10.0.0.4
# Add an entry to this system's ARP [Address Resolution Protocol]
# table with the IP address of the peer and the Ethernet address of this
# system. This will have the effect of making the peer appear to other
# systems to be on the local ethernet.
# (you do not need this if your PPTP server is responsible for routing
# packets to the clients -- James Cameron)
proxyarp
# Normally pptpd passes the IP address to pppd, but if pptpd has been
# given the delegate option in pptpd.conf or the --delegate command line
# option, then pppd will use chap-secrets or radius to allocate the
# client IP address. The default local IP address used at the server
# end is often the same as the address of the server. To override this,
# specify the local IP address here.
# (you must not use this unless you have used the delegate option)
#10.8.0.100
# Logging
# Enable connection debugging facilities.
# (see your syslog configuration for where pppd sends to)
debug
# Print out all the option values which have been set.
# (often requested by mailing list to verify options)
dump
# Miscellaneous
# Create a UUCP-style lock file for the pseudo-tty to ensure exclusive
# access.
lock
# Disable BSD-Compress compression
nobsdcomp
# Disable Van Jacobson compression
# (needed on some networks with Windows 9x/ME/XP clients, see posting to
# poptop-server on 14th April 2005 by Pawel Pokrywka and followups,
# http://marc.theaimsgroup.com/?t=111343175400006&r=1&w=2 )
novj
novjccomp
# turn off logging to stderr, since this may be redirected to pptpd,
# which may trigger a loopback
nologfd
# put plugins here
# (putting them higher up may cause them to sent messages to the pty)
logfile /var/log/pptpd.log
multilink
注解：在以上配置文件中，
定义了PPTP VPN Server的服务名：name ec2-tokyo
定义了加密的规则，如下：
refuse-pap
refuse-chap
refuse-mschap
require-mschap-v2
require-mppe-128
定义了推送到客户端的DNS地址：ms-dns 172.31.0.2 （我通常选择PPTP VPN Server所在服务器的默认DNS设置）
允许相同局域网的主机在PPTP VPN Server上互相可见：proxyarp
开启了调试信息：debug
启用了一些通用的设置，如下：
dump
lock
nobsdcomp
novj
novjccomp
nologfd
指定了日志文件的位置：logfile /var/log/pptpd.log
允许把多个物理通道捆绑为单一逻辑信道：multilink
6. 编辑用户账号密码文件/etc/ppp/chap-secrets
# vim /etc/ppp/chap-secrets
# Secrets for authentication using CHAP
# client server secret IP addresses
"username" * "password" *
7. 编辑/etc/sysconfig/iptables-config
修改 IPTABLES_MODULES="" 为 IPTABLES_MODULES="ip_nat_pptp" 确保在启动iptables服务时自动加载模块。
8. 编辑/etc/sysconfig/iptables（默认eth0为公网IP地址所在网口）
# vim /etc/sysconfig/iptables
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p gre -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 1723 -j ACCEPT
-A INPUT -s 10.192.168.0/255.255.255.0 -m state --state NEW -m tcp -p tcp -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT
*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A POSTROUTING -s 10.192.168.0/255.255.255.0 -o eth0 -j MASQUERADE
COMMIT
注解：在以上iptables脚本中，
对所有GRE协议的数据包放行；
对TCP端口1723放行；
对整个PPTP VPN的局域网地址段10.192.168.0/24放行；
将整个PPTP VPN的局域网地址段10.192.168.0/24通过NAT映射到eth0网口，实现共享上网；
9. 开启数据转发，编辑/etc/sysctl.conf
修改 net.ipv4.ip_forward = 0 为 net.ipv4.ip_forward = 1
执行 sysctl -p
10. 启动PPTP VPN Server
# /etc/init.d/pptpd restart
# /etc/init.d/iptables restart
11. 设置PPTP VPN Server与iptables服务开机自启动
# chkconfig pptpd on
# chkconfig iptables on
12. 在本地PC上配置客户端并连接PPTP VPN Server
13. 结束