2011年12月9日、2011年12月30日更新iptables和启动脚本的描述。2012年11月8日更新网络传输优化配置。
操作系统是CentOS，运行在XEN架构的VPS上面。
环境配置
首先，检测VPS是否支持TUN（via）：
cat /dev/net/tun
返回“cat: /dev/net/tun: File descriptor in bad state”说明支持TUN。
检测iptables_nat模块支持：
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
如果返回信息为：iptables: Unknown error 4294967295 说明支持模块。
安装LZO库：
wget http://www.oberhumer.com/opensource/lzo/download/lzo-2.05.tar.gz
tar zxvf lzo-2.05.tar.gz
cd lzo-2.05
./configure
make
make install
安装OpenSSL开发库：
yum install -y openssl-devel
服务端安装配置
然后，在服务器端下载OpenVPN源代码（下载页面），目前最新版本为2.2.0：
wget http://swupdate.openvpn.net/community/releases/openvpn-2.2.0.tar.gz
解压编译安装：
tar zxf openvpn-2.2.0.tar.gz
cd openvpn-2.2.0
./configure --prefix=/usr/local/openvpn
make
make install
复制服务端样例配置文件：
cp sample-config-files/server.conf /usr/local/openvpn/etc/server.conf
编辑配置文件为，比如我的以下内容，采用UDP协议监听53端口：
#################################################
# Sample OpenVPN 2.0 config file for #
# multi-client server. #
# #
# This file is for the server side #
# of a many-clients <-> one-server #
# OpenVPN configuration. #
# #
# OpenVPN also supports #
# single-machine <-> single-machine #
# configurations (See the Examples page #
# on the web site for more info). #
# #
# This config should work on Windows #
# or Linux/BSD systems. Remember on #
# Windows to quote pathnames and use #
# double backslashes, e.g.: #
# "C:\\Program Files\\OpenVPN\\config\\foo.key" #
# #
# Comments are preceded with '#' or ';' #
################################################## Which local IP address should OpenVPN
# listen on? (optional)
;local a.b.c.d# Which TCP/UDP port should OpenVPN listen on?
# If you want to run multiple OpenVPN instances
# on the same machine, use a different port
# number for each one. You will need to
# open up this port on your firewall.
port 53# TCP or UDP server?
;proto tcp
proto udp# "dev tun" will create a routed IP tunnel,
# "dev tap" will create an ethernet tunnel.
# Use "dev tap0" if you are ethernet bridging
# and have precreated a tap0 virtual interface
# and bridged it with your ethernet interface.
# If you want to control access policies
# over the VPN, you must create firewall
# rules for the the TUN/TAP interface.
# On non-Windows systems, you can give
# an explicit unit number, such as tun0.
# On Windows, use "dev-node" for this.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel if you
# have more than one. On XP SP2 or higher,
# you may need to selectively disable the
# Windows firewall for the TAP adapter.
# Non-Windows systems usually don't need this.
;dev-node MyTap# SSL/TLS root certificate (ca), certificate
# (cert), and private key (key). Each client
# and the server must have their own cert and
# key file. The server and all clients will
# use the same ca file.
#
# See the "easy-rsa" directory for a series
# of scripts for generating RSA certificates
# and private keys. Remember to use
# a unique Common Name for the server
# and each of the client certificates.
#
# Any X509 key management system can be used.
# OpenVPN can also use a PKCS #12 formatted key file
# (see "pkcs12" directive in man page).
ca /usr/local/openvpn/etc/2.0/keys/ca.crt
cert /usr/local/openvpn/etc/2.0/keys/server.crt
key /usr/local/openvpn/etc/2.0/keys/server.key # This file should be kept secret# Diffie hellman parameters.
# Generate your own with:
# openssl dhparam -out dh1024.pem 1024
# Substitute 2048 for 1024 if you are using
# 2048 bit keys.
dh /usr/local/openvpn/etc/2.0/keys/dh1024.pem# Configure server mode and supply a VPN subnet
# for OpenVPN to draw client addresses from.
# The server will take 10.8.0.1 for itself,
# the rest will be made available to clients.
# Each client will be able to reach the server
# on 10.8.0.1. Comment this line out if you are
# ethernet bridging. See the man page for more info.
server 10.8.0.0 255.255.255.0# Maintain a record of client <-> virtual IP address
# associations in this file. If OpenVPN goes down or
# is restarted, reconnecting clients can be assigned
# the same virtual IP address from the pool that was
# previously assigned.
ifconfig-pool-persist ipp.txt# Configure server mode for ethernet bridging.
# You must first use your OS's bridging capability
# to bridge the TAP interface with the ethernet
# NIC interface. Then you must manually set the
# IP/netmask on the bridge interface, here we
# assume 10.8.0.4/255.255.255.0. Finally we
# must set aside an IP range in this subnet
# (start=10.8.0.50 end=10.8.0.100) to allocate
# to connecting clients. Leave this line commented
# out unless you are ethernet bridging.
;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100# Configure server mode for ethernet bridging
# using a DHCP-proxy, where clients talk
# to the OpenVPN server-side DHCP server
# to receive their IP address allocation
# and DNS server addresses. You must first use
# your OS's bridging capability to bridge the TAP
# interface with the ethernet NIC interface.
# Note: this mode only works on clients (such as
# Windows), where the client-side TAP adapter is
# bound to a DHCP client.
;server-bridge# Push routes to the client to allow it
# to reach other private subnets behind
# the server. Remember that these
# private subnets will also need
# to know to route the OpenVPN client
# address pool (10.8.0.0/255.255.255.0)
# back to the OpenVPN server.
;push "route 192.168.10.0 255.255.255.0"
;push "route 192.168.20.0 255.255.255.0"# To assign specific IP addresses to specific
# clients or if a connecting client has a private
# subnet behind it that should also have VPN access,
# use the subdirectory "ccd" for client-specific
# configuration files (see man page for more info).# EXAMPLE: Suppose the client
# having the certificate common name "Thelonious"
# also has a small subnet behind his connecting
# machine, such as 192.168.40.128/255.255.255.248.
# First, uncomment out these lines:
;client-config-dir ccd
;route 192.168.40.128 255.255.255.248
# Then create a file ccd/Thelonious with this line:
# iroute 192.168.40.128 255.255.255.248
# This will allow Thelonious' private subnet to
# access the VPN. This example will only work
# if you are routing, not bridging, i.e. you are
# using "dev tun" and "server" directives.# EXAMPLE: Suppose you want to give
# Thelonious a fixed VPN IP address of 10.9.0.1.
# First uncomment out these lines:
;client-config-dir ccd
;route 10.9.0.0 255.255.255.252
# Then add this line to ccd/Thelonious:
# ifconfig-push 10.9.0.1 10.9.0.2# Suppose that you want to enable different
# firewall access policies for different groups
# of clients. There are two methods:
# (1) Run multiple OpenVPN daemons, one for each
# group, and firewall the TUN/TAP interface
# for each group/daemon appropriately.
# (2) (Advanced) Create a script to dynamically
# modify the firewall in response to access
# from different clients. See man
# page for more info on learn-address script.
;learn-address ./script# If enabled, this directive will configure
# all clients to redirect their default
# network gateway through the VPN, causing
# all IP traffic such as web browsing and
# and DNS lookups to go through the VPN
# (The OpenVPN server machine may need to NAT
# or bridge the TUN/TAP interface to the internet
# in order for this to work properly).
push "redirect-gateway def1 bypass-dhcp"# Certain Windows-specific network settings
# can be pushed to clients, such as DNS
# or WINS server addresses. CAVEAT:
# http://openvpn.net/faq.html#dhcpcaveats
# The addresses below refer to the public
# DNS servers provided by opendns.com.
;push "dhcp-option DNS 208.67.222.222"
;push "dhcp-option DNS 208.67.220.220"
push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"# Uncomment this directive to allow different
# clients to be able to "see" each other.
# By default, clients will only see the server.
# To force clients to only see the server, you
# will also need to appropriately firewall the
# server's TUN/TAP interface.
client-to-client# Uncomment this directive if multiple clients
# might connect with the same certificate/key
# files or common names. This is recommended
# only for testing purposes. For production use,
# each client should have its own certificate/key
# pair.
#
# IF YOU HAVE NOT GENERATED INDIVIDUAL
# CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
# EACH HAVING ITS OWN UNIQUE "COMMON NAME",
# UNCOMMENT THIS LINE OUT.
;duplicate-cn# The keepalive directive causes ping-like
# messages to be sent back and forth over
# the link so that each side knows when
# the other side has gone down.
# Ping every 10 seconds, assume that remote
# peer is down if no ping received during
# a 120 second time period.
keepalive 10 120# For extra security beyond that provided
# by SSL/TLS, create an "HMAC firewall"
# to help block DoS attacks and UDP port flooding.
#
# Generate with:
# openvpn --genkey --secret ta.key
#
# The server and each client must have
# a copy of this key.
# The second parameter should be '0'
# on the server and '1' on the clients.
;tls-auth ta.key 0 # This file is secret# Select a cryptographic cipher.
# This config item must be copied to
# the client config file as well.
;cipher BF-CBC # Blowfish (default)
;cipher AES-128-CBC # AES
;cipher DES-EDE3-CBC # Triple-DES# Enable compression on the VPN link.
# If you enable it here, you must also
# enable it in the client config file.
comp-lzo# The maximum number of concurrently connected
# clients we want to allow.
;max-clients 100# It's a good idea to reduce the OpenVPN
# daemon's privileges after initialization.
#
# You can uncomment this out on
# non-Windows systems.
user nobody
group nobody# The persist options will try to avoid
# accessing certain resources on restart
# that may no longer be accessible because
# of the privilege downgrade.
persist-key
persist-tun# Output a short status file showing
# current connections, truncated
# and rewritten every minute.
status openvpn-status.log# By default, log messages will go to the syslog (or
# on Windows, if running as a service, they will go to
# the "\Program Files\OpenVPN\log" directory).
# Use log or log-append to override this default.
# "log" will truncate the log file on OpenVPN startup,
# while "log-append" will append to it. Use one
# or the other (but not both).
;log openvpn.log
;log-append openvpn.log# Set the appropriate level of log
# file verbosity.
#
# 0 is silent, except for fatal errors
# 4 is reasonable for general usage
# 5 and 6 can help to debug connection problems
# 9 is extremely verbose
verb 3# Silence repeating messages. At most 20
# sequential messages of the same message
# category will be output to the log.
;mute 20
复制密钥制作工具：
cp easy-rsa/ -r /usr/local/openvpn/etc
下面开始制作密钥。
cd /usr/local/openvpn/etc/2.0
source ./vars #可以先修改vars文件内容
./clean-all
./build-ca
在这里，我将Common Name那一行填为“server”。
生成服务器密钥，其中的“server”是上一步的Common Name：
./build-key-server server
生成客户端密钥，客户端名字这里为“client”：
./build-key client
其中的“Common Name”也应该设为客户端的名字，即这里的“client”。
如果想要多个客户端密钥，则对应运行“./build-key client1”“./build-key client2”等等，同时其中的“Common Name”也对应改为“client1”“client2”。
生成Diffie Hellman参数：
./build-dh
将生成的各种密钥打包：
tar zcf keys.tar.gz keys/
然后可以通过SFTP或者FTP传送到客户端，即本地。
启动和关闭OpenVPN服务端
源代码目录中的“sample-scripts”目录里面有一个openvpn.init可以很方便地控制OpenVPN的启动和关闭。
首先将它拷贝到自己的目录，改名为openvpn.sh，然后编辑里面的内容。
把openvpn_locations=在第一个引号后面加上/usr/local/openvpn/sbin/openvpn，后面加上空格以区分原有内容
work=一行的后面的内容删除，改为/usr/local/openvpn/etc
#echo 1 > /proc/sys/net/ipv4/ip_forward前面的#删掉
在“start)”中“# Run startup script, if defined”前加上：
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -j SNAT --to-source 你的VPS的IP #这样一般可以解决常见问题2的现象
在“stop)”中“# Run shutdown script, if defined”前加上：
iptables -t nat -D POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
iptables -t nat -D POSTROUTING -s 10.8.0.0/24 -j SNAT --to-source 你的VPS的IP
这样就启动脚本就可以方便地控制iptables和相关的网络转发啦。可以用以下命令启动和关闭OpenVPN：
./openvpn.sh start
./openvpn.sh stop
客户端安装配置
下载地址：http://swupdate.openvpn.net/community/releases/openvpn-2.2.0-install.exe。然后安装。
安装完毕后，点击“开始”——OpenVPN——Utilities——Add a new TAP-Win32 virtual ethernet adapter，增加虚拟网卡，以后就不用执行这步了。
然后在OpenVPN安装目录下建立config文件夹，从服务器上把源代码目录中，sample-config-files中的client.conf下载，并命名为my.ovpn，复制到刚刚建立的config文件夹中。接着编辑my.ovpn的内容，样例如下：
##############################################
# Sample client-side OpenVPN 2.0 config file #
# for connecting to multi-client server. #
# #
# This configuration can be used by multiple #
# clients, however each client should have #
# its own cert and key files. #
# #
# On Windows, you might want to rename this #
# file so it has a .ovpn extension #
############################################### Specify that we are a client and that we
# will be pulling certain config file directives
# from the server.
client# Use the same setting as you are using on
# the server.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel
# if you have more than one. On XP SP2,
# you may need to disable the firewall
# for the TAP adapter.
;dev-node MyTap# Are we connecting to a TCP or
# UDP server? Use the same setting as
# on the server.
;proto tcp
proto udp# The hostname/IP and port of the server.
# You can have multiple remote entries
# to load balance between the servers.
remote 服务器的IP 端口
;remote my-server-2 1194# Choose a random host from the remote
# list for load-balancing. Otherwise
# try hosts in the order specified.
;remote-random# Keep trying indefinitely to resolve the
# host name of the OpenVPN server. Very useful
# on machines which are not permanently connected
# to the internet such as laptops.
resolv-retry infinite# Most clients don't need to bind to
# a specific local port number.
nobind# Downgrade privileges after initialization (non-Windows only)
;user nobody
;group nobody# Try to preserve some state across restarts.
persist-key
persist-tun# If you are connecting through an
# HTTP proxy to reach the actual OpenVPN
# server, put the proxy server/IP and
# port number here. See the man page
# if your proxy server requires
# authentication.
;http-proxy-retry # retry on connection failures
;http-proxy [proxy server] [proxy port #]# Wireless networks often produce a lot
# of duplicate packets. Set this flag
# to silence duplicate packet warnings.
;mute-replay-warnings# SSL/TLS parms.
# See the server config file for more
# description. It's best to use
# a separate .crt/.key file pair
# for each client. A single ca
# file can be used for all clients.
ca server.crt
cert client.crt
key client.key# Verify server certificate by checking
# that the certicate has the nsCertType
# field set to "server". This is an
# important precaution to protect against
# a potential attack discussed here:
# http://openvpn.net/howto.html#mitm
#
# To use this feature, you will need to generate
# your server certificates with the nsCertType
# field set to "server". The build-key-server
# script in the easy-rsa folder will do this.
ns-cert-type server# If a tls-auth key is used on the server
# then every client must also have the key.
;tls-auth ta.key 1# Select a cryptographic cipher.
# If the cipher option is used on the server
# then you must also specify it here.
;cipher x# Enable compression on the VPN link.
# Don't enable this unless it is also
# enabled in the server config file.
comp-lzo# Set log file verbosity.
verb 3# Silence repeating messages
;mute 20
把key.tar.gz解压缩，将里面的server.crt、client.crt、client.key三个文件复制到config目录下。
然后，点击“开始”——OpenVPN——右击“OpenVPN GUI”，以管理员身份运行。右击托盘中的OpenVPN图标，选择“my”，选择“Connect”，稍等片刻就OK了。
网络优化配置
OpenVPN官方有一篇优化文章，但是针对LAN。另外还有一篇优化文章，比较有用。总结而言，就是在server.conf中加上（根据ipv4v4或ipv6自选）：
#Tweak for ipv4
mssfix 1472#Tweak for ipv6
#mssfix 1432
客户端配置文件加上：
#Tweak
rcvbuf 65536
常见问题1：无法链接OpenVPN服务器，确认是系统iptables挡住了。
编辑/etc/sysconfig/iptables
在适当位置加上：
-A INPUT -p udp -m udp --dport 53 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 53 -j ACCEPT
常见问题2：如果确认OpenVPN已经正常运行且正常连接情况下，如果无法连接外网，但可以连接内网，则可能是iptables问题。
停止OpenVPN：
/etc/init.d/openvpn stop
编辑/etc/sysconfig/iptables
在适当位置加上：
-A INPUT -i tun+ -j ACCEPT
-A OUTPUT -o tun+ -j ACCEPT
-A FORWARD -i tun+ -o eth0 -j ACCEPT
重启iptables：service iptables restart
启动openvpn：
/etc/init.d/openvpn start
如果还有其它问题，请实际情况实际分析。