My previous posts (http://nkush.blogspot.com.au/2012/08/installing-openvpn-22-on-centos-63-64bit.html and http://nkush.blogspot.com.au/2011/10/installing-openvpn-22-on-centos-57.html) have been on installing OpenVPN on Linux.

This post is slightly different as it installs and configures OpenVPN on a Microsoft Windows Small Business Server (SBS). The SBS installation is also different from the previous write-ups since this configuration uses Ethernet bridging instead of tunneling.

Although SBS comes with Layer2 Tunneling Protocol (L2TP) and Point-to-point Tunelling Protocol (PPTP) for Virtual Private Network (VPN), some users may want to use an Secure Sockets Layer (SSL) based VPN such as OpenVPN. The default installation location, i.e. C:\Program Files\OpenVPN\ is used in the instructions below

Installation:

Before downloading and installing OpenVPN, the server needs to have IP forwarding enabled. This is trivial in *nix based systems, but requires a hack in the Windows registry. Edit the parameters and set IPEnableRouter to 1

1. HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter
2. Reboot the server to ensure the changes made above take effect

Download and install the OpenVPN application[1] from http://openvpn.net/index.php/open-source/downloads.html

Open a command prompt and change to the default installation directory

1. Start> Run> cmd
2. cd C:\Program Files\OpenVPN

Ensure that the path C:\Program Files\OpenVPN\bin\ is added to the system environment PATH

1. set PATH=%PATH%;C:\Program Files\OpenVPN\bin\

Copy the file C:\Program Files\OpenVPN\easy-rsa\vars.bat.sample to C:\Program Files\OpenVPN\easy-rsa\vars.bat

1. cd C:\Program Files\OpenVPN\easy-rsa\
2. copy vars.bat.sample vars.bat OR run the init-config.bat script

Edit the file (edit) and change the following

Since key lengths less than 2048 are not recommended[2], change the key size to be 4096

1. set KEY_SIZE=4096

Configure the following

1. KEY_COUNTRY
2. KEY_PROVINCE
3. KEY_CITY
4. KEY_ORG
5. KEY_EMAIL
6. KEY_CN
7. KEY_NAME
8. KEY_OU
9. PKCS11_MODULE_PATH
10. PKCS11_PIN

Setup environment variables by executing the vars.bat script

Initialise the keys directory and empty the keys database and serial number by executing clean-all.bat. WARNING: this will delete any existing keys

Create certificate for the certificate authority (CA), this will generate the certificate signing request, the CA certificate (ca.crt) and CA key (ca.key).

1. build-ca.bat

Create the Diffie Hellman files. These files are used for the key exchange to ensure the confidentiality over an insecure channel. Based on the length of the key used (KEY_SIZE) it may take a while

1. build-dh.bat

Create the server certificate and key and commit them

1. build-key-server.bat <SERVER-NAME>

Create user machine certificates and keys and commit them (one for each user/machine)

1. build-key.bat <USER-MACHINE>
2. Note: when generating individual user/machine certificate and keys ensure that the common names are unique and for maintenance reasons, its easier if you match the user/machine and the common name

Copy the server configuration file to the configuration directory

1. cd C:\Program Files\OpenVPN\sample-config
2. copy server.ovpn ..\config\

Edit the server config file. You may use the full path of the certificates and keys, or copy them into the same directory as your configuration file.

cd  C:\Program Files\OpenVPN\config

edit server.ovpn

1. port 1194
2. proto udp
3. dev tap
4. dev-node MyTap
5. 6. ca ca.crt
6. cert <SERVER-NAME>.crt
7. key <SERVER-NAME>.key
8. dh dh<KEY_SIZE>.pem
9. ifconfig-pool-persist ipp.txt
10. server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100
11. push “route <LAN-NETWORK> <LAN-NETMASK>”
12. keepalive 10 120
13. comp-lzo
14. max-clients 5
15. status openvpn-status.log
16. log-append  openvpn.log
17. verb 3
18. mute 5

Change the TAP adapter 

1. Rename to suit the configured name above, in the case of this example “MyTap”
2. Set a static IPv4 address of 10.8.0.4 (or whatever is specified in 14.11 in the server-bridge setting

Start the OpenVPN service and check the logs for any errors

Routing changes:

1. You may also need to make routing changes. Depending on your router the settings may differ, but the basic requirement is to add a static route to the VPN tunnel network, i.e. 10.8.0.0/24 to the OpenVPN server.
2. If the server does not have a WAN IP address and is located behind a router performing NAT, then port forwarding rules should be implemeneted to port formward traffic on UDP port 1194 to the VPN server.

Client Configuration:

1. client
2. dev tap
3. proto udp
4. remote <SERVER-WAN> 1194
5. resolv-retry infinite
6. nobind
7. persist-key
8. persist-tun
9. ca ca.crt
10. cert <USER-MACHINE>.crt
11. key <USER-MACHINE>.key
12. comp-lzo
13. verb 3
14. route-method exe
15. route-delay 5

Note: SBS running as the Domain Controller (DC) only allows Terminal Services to run in Administration mode, i.e. only two concurrent Remote Desktop Protocol (RDP) sessions, and one local console session. It does not run in Application mode, which allows additional licenses to be installed

References:

1. http://openvpn.net/index.php/open-source/downloads.html
2. http://csrc.nist.gov/publications/nistpubs/800-131A/sp800-131A.pdf

See also

• Installing OpenVPN 2.2 on Centos 5.7
• Connecting to OpenVPN from a Mac using Tunnelblink
• Installing OpenVPN 2.2 on CentOS 6.3 64bit
• Microsoft Windows Server 2003 for Small Business Server Microsoft Exchange Mail Store unmounts
• Installing Snort 2.9.1.2 on CentOS 5.7