来自DigitalOcean的教程：

https://www.digitalocean.com/community/tutorials/how-to-setup-your-own-vpn-with-pptp

How To Setup Your Own VPN With PPTP

Intro

One of the commonly asked questions from our users is how to add another IP address to their server. You can assign your own private IP address to your droplet by creating a VPN tunnel. Whether you want to build your own Virtual Private Network (VPN), or assign an SSL certificate to that IP address, you have several options. From all of the possible options, the most optimal ones are between PPTP and OpenVPN. A Point-To-Point Tunneling Protocol (PPTP) allows you to implement your own VPN very quickly, and is compatible with most mobile devices. Even though PPTP is less secure than OpenVPN, it is also faster and uses less CPU resources.

Step 1 - PPTP Installation

You will have to select one server to be responsible for handling out IPs to others and authenticating all of your servers into your VPN. This will become your PPTP Server.

On CentOS 6 x64:

rpm -i http://poptop.sourceforge.net/yum/stable/rhel6/pptp-release-current.noarch.rpm
yum -y install pptpd

On Ubuntu 12.10 x64:

apt-get install pptpd

Now you should edit /etc/pptpd.conf and add the following lines:

localip 10.0.0.1
remoteip 10.0.0.100-200

Where localip is IP address of your server and remoteip are IPs that will be assigned to clients that connect to it.

Next, you should setup authentication for PPTP by adding users and passwords. Simply add them to /etc/ppp/chap-secrets :

Where client is the username, server is type of service – pptpd for our example, secret is the password, and IP addresses specifies which IP address may authenticate. By setting ‘*’ in IP addresses field, you specify that you would accept username/password pair for any IP.

Step 2 - Add DNS servers to /etc/ppp/pptpd-options

ms-dns 8.8.8.8
ms-dns 8.8.4.4

Now you can start PPTP daemon:

service pptpd restart

Verify that it is running and accepting connections:

Step 3 - Setup Forwarding

It is important to enable IP forwarding on your PPTP server. This will allow you to forward packets between public IP and private IPs that you setup with PPTP. Simply edit /etc/sysctl.conf and add the following line if it doesn’t exist there already:

net.ipv4.ip_forward = 1

To make changes active, run sysctl -p

Step 4 - Create a NAT rule for iptables

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE && iptables-save

If you would also like your PPTP clients to talk to each other, add the following iptables rules:

iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
iptables -I INPUT -s 10.0.0.0/8 -i ppp0 -j ACCEPT
iptables --append FORWARD --in-interface eth0 -j ACCEPT

Now your PPTP server also acts as a router.

If you would like to restrict which servers can connect to your droplets, you can setup an iptables rule that restricts TCP connects to port 1723.

Step 5 - Setup Clients

On your client servers, install PPTP client:

yum -y install pptp

Step 6 - Add necessary Kernel module

modprobe ppp_mppe

Create a new file /etc/ppp/peers/pptpserver and add the following lines, replacing name and password with your own values:

pty "pptp 198.211.104.17 --nolaunchpppd"
name box1
password 24oiunOi24
remotename PPTP
require-mppe-128

Where 198.211.104.17 is the public IP address of our PPTP server, with username ‘box1’ and password ‘24oiunOi24’ that we specified /etc/ppp/chap-secrets file on our PPTP server.

Now we can ‘call’ this PPTP server, since this is a point-to-point protocol. Whichever name you gave your peers file in/etc/ppp/peers/ should be used in this next line. Since we called our file pptpserver:

pppd call pptpserver

You should see successful connection from PPTP server logs:

On your PPTP client, setup routing to your private network via ppp0 interface:

ip route add 10.0.0.0/8 dev ppp0

Your interface ppp0 should come up on PPTP client server, and can be checked by running ifconfig

Now you can ping your PPTP server and any other clients that are connected to this network:

We can add our second PPTP client to this network:

yum -y install pptp
modprobe ppp_mppe

Add to /etc/ppp/peers/pptpserver (replacing with your own name and password values):

pty "pptp 198.211.104.17 --nolaunchpppd"
name box2
password 239Aok24ma
remotename PPTP
require-mppe-128

Now run on your second client the following:

pppd call pptpserver
ip route add 10.0.0.0/8 dev ppp0

You can also ping the first client, as packets would go through the PPTP server and be routed using the iptables rules we’ve placed earlier:

This setup allows you to create your own virtual private network:

If you wanted to have all of your devices communicating securely on one network, this is a quick way of implementing it.

You can use it with Nginx, Squid, MySQL, and any other application you can think of.

Since traffic is 128-bit encrypted, it is less CPU-intensive than OpenVPN, and still provides an added level of security to your traffic.