New to OpenVPN?

This how-to has been the main guide since the early days of OpenVPN. It covers extensive details, and some areas may require a deeper understanding of how OpenVPN works or networking in general. Some information may also be outdated but is currently kept here as a historical reference.

If you are completely new to OpenVPN, please consider our Getting Started With OpenVPN guide first.

Introduction

OpenVPN is a full-featured SSL VPN that implements OSI layer 2 or 3 secure network extension using the industry-standard SSL/TLS protocol. It supports flexible client authentication methods based on certificates, smart cards, and/or username/password credentials. It also allows user or group-specific access control policies using firewall rules applied to the VPN virtual interface. OpenVPN is not a web application proxy and does not operate through a web browser.

OpenVPN 2.0 expands on the capabilities of OpenVPN 1.x by offering a scalable client/server mode, allowing multiple clients to connect to a single OpenVPN server process over a single TCP or UDP port. OpenVPN 2.3 includes numerous improvements, including full IPv6 support and PolarSSL support.

The official version of this document is stored on the main website. If you find a problem in the official version, you can fix it in the Wiki version. Changes made to the Wiki version will be merged periodically into the official version.

This document provides step-by-step instructions for configuring an OpenVPN 2.x client/server VPN, including:

The impatient may wish to jump straight to the sample configuration files:

• Server configuration file.
• Client configuration file.

Intended Audience

This HOWTO assumes that readers possess a prior understanding of basic networking concepts such as IP addresses, DNS names, netmasks, subnets, IP routing, routers, network interfaces, LANs, gateways, and firewall rules.

Additional Documentation

Further documentation and links to be provided.

tar xzf openvpn-[version].tar.gz
cd openvpn-[version]

Then run the configuration script:

./configure
make
sudo make install

Windows Notes

For Windows users, the installation process is quite straightforward. Just download the installer from the provided link and execute it. Follow the on-screen instructions to complete the installation.

Starting OpenVPN

To start OpenVPN, you will need to run the OpenVPN daemon. The way to do this varies between operating systems.

On Linux

You can start the OpenVPN server with the following command:

sudo openvpn --config /path/to/your/config/file.ovpn

To enable the OpenVPN service to start at boot, you can use:

sudo systemctl enable openvpn-server@myconfig

or

sudo systemctl enable openvpn-client@myconfig

Alternatively, for systems without systemctl:

sudo chkconfig openvpn on

On Windows

OpenVPN can be started from the Start Menu or by clicking on the desktop icon. You can also start OpenVPN by right-clicking on the configuration file and selecting "Start OpenVPN on this config file".

Configuring OpenVPN

Configuration of OpenVPN is beyond the scope of this document, but you should refer to the OpenVPN 2.x HOWTO for detailed information on various configuration scenarios. For a basic configuration, you will need to modify your server.conf and client.conf files appropriately.

Conclusion

Once OpenVPN is installed and configured, testing your setup is crucial to ensure that the VPN is working as expected. It's also important to maintain and update OpenVPN installations to protect against vulnerabilities.

For further reading and more advanced configurations, refer to the OpenVPN documentation front page.

tar xfz openvpn-[version].tar.gz
cd openvpn-[version]
./configure
make
make install

Windows Notes

OpenVPN for Windows can be installed from the self-installing exe file on the OpenVPN download page. It is important to note that OpenVPN will only run on Windows XP or later, and must be installed and run by a user with administrative privileges. This restriction, imposed by Windows, can be bypassed by running OpenVPN as a service, allowing non-admin users to access the VPN after installation. More details on running OpenVPN as a non-admin can be found here.

The official Windows installer includes OpenVPN-GUI, which allows management of OpenVPN connections via a system tray applet. There are other GUI applications available as well.

Once installed, OpenVPN will associate itself with files having the .ovpn extension and can be used in several ways:

• Right-click on an OpenVPN configuration file (.ovpn) and select "Start OpenVPN on this configuration file". Press F4 to exit once running.
• From a command prompt, run openvpn myconfig.ovpn. F4 can stop OpenVPN when running in this mode.
• Run OpenVPN as a service by placing one or more .ovpn configuration files in \Program Files\OpenVPN\config and start the OpenVPN Service via Start Menu -> Control Panel -> Administrative Tools -> Services.

Additional Windows installation notes.

Mac OS X Notes

Angelo Laub and Dirk Theisen have developed an OpenVPN GUI for OS X.

Other OSes

For other operating systems, refer to the notes available in the INSTALL file specific to different OSes. Generally, the process involves similar steps to those described for Linux.

./configure
make
make install

method can be used, or you can search for an OpenVPN port or package that is specific to your OS/distribution.

Determining whether to use a routed or bridged VPN

See the documentation our FAQ articles for an overview of Routing vs. Ethernet Bridging.

Overall, routing is probably a better choice for most people, as it is more efficient and easier to set up (as far as the OpenVPN configuration itself) than bridging. Routing also provides a greater ability to selectively control access rights on a client-specific basis.

I would recommend using routing unless you need a specific feature which requires bridging, such as:

• the VPN needs to be able to handle non-IP protocols such as IPX,
• you are running applications over the VPN which rely on network broadcasts (such as LAN games), or
• you would like to allow browsing of Windows file shares across the VPN without setting up a Samba or WINS server.

Numbering private subnets

Setting up a VPN often entails linking together private subnets from different locations.

The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets (codified in RFC 1918):

When setting up VPN configurations, it is critical to choose IP addresses that minimize the risk of conflicts. Here are types of conflicts to watch out for:

• Conflicts arising from different sites on the VPN using the same LAN subnet numbering.
• Remote access connections from sites using private subnets which overlap with your VPN subnets.

Example Scenarios

1. Common Subnet Conflict: If you use the popular subnet 192.168.0.0/24 for your private LAN and try connecting to the VPN from an internet cafe that uses the same subnet for its WiFi LAN, you'll face a routing conflict. Your device won't be able to distinguish whether 192.168.0.1 refers to the local WiFi gateway or to the same address on the VPN.

2. Multiple Sites with Identical Subnets: If multiple sites connected by VPN each use 192.168.0.0/24 as their LAN subnet, the VPN will struggle to route packets correctly without additional NAT translation layers, complicating the setup.

Best Practices

• Avoid using overly common subnets like 10.0.0.0/24 or 192.168.0.0/24 for private LAN networks. Opt for less common subnets, such as something in the middle of the 10.0.0.0/8 range like 10.66.77.0/24, to avoid conflicts in public spaces like WiFi cafes or hotels.
• Ensure each LAN subnet has a unique numbering scheme to prevent cross-site IP numbering conflicts.

Setting up your own Certificate Authority (CA) for OpenVPN

Overview

Building a secure OpenVPN 2.x configuration begins with establishing a PKI (Public Key Infrastructure), which involves:

• Creating a separate public key and private key for the server and each client.
• Setting up a master Certificate Authority (CA) certificate and key to sign the server and client certificates.

OpenVPN employs bidirectional authentication based on certificates, requiring both the server and the client to authenticate each other's certificates. This process ensures mutual trust is established before allowing access.

Key Features of the Security Model

• Server Requirements: The server needs only its own certificate and key. It doesn't need to store or manage individual client certificates.
• Client Authentication: The server will only accept connections from clients whose certificates are signed by the master CA. This verification process doesn't require the server to access the CA's private key, enhancing security.
• CA Security: The most sensitive key, the CA private key, does not need to reside on the server. For added security, it can be stored on a machine without any network connection, isolating it from potential cyber threats.

Implementing these practices helps in creating a robust and secure VPN environment, preventing common issues and ensuring reliable and confidential communication across the network.* If a private key is compromised, it can be disabled by adding its certificate to a CRL (certificate revocation list). The CRL allows compromised certificates to be selectively rejected without requiring that the entire PKI be rebuilt.

• The server can enforce client-specific access rights based on embedded certificate fields, such as the Common Name.

Note that the server and client clocks need to be roughly in sync or certificates might not work properly.

Generate the master Certificate Authority (CA) certificate & key

In this section, we will generate a master CA certificate/key, a server certificate/key, and certificates/keys for 3 separate clients.

Please take note: Easy-RSA Version 3 is now preferred over Easy-RSA Version 2.

EasyRSA-3 has a Quick-Start Guide

There is also Easy-TLS, which is an add-on utility to manage .inline files and TLS Crypt V2 keys. (It's very useful)

The following instruction only work for Easy-RSA v2.

For PKI management, we will use easy-rsa 2, a set of scripts which is bundled with OpenVPN 2.2.x and earlier. If you're using OpenVPN 2.3.x, you may need to download easy-rsa 2 separately from the easy-rsa-old project page. An easy-rsa 2 package is also available for Debian and Ubuntu in the OpenVPN software repos.

You should also look into using easy-rsa 3, available to most OS's, including Windows; refer to its own documentation for details.

If you are using Linux, BSD, or a unix-like OS, open a shell and cd to the easy-rsa subdirectory. If you installed OpenVPN from an RPM or DEB file provided by your distribution, the easy-rsa directory can usually be found in /usr/share/doc/packages/openvpn or /usr/share/doc/openvpn (it's best to copy this directory to another location such as /etc/openvpn, before any edits, so that future OpenVPN package upgrades won't overwrite your modifications).

If you are using Windows, (AND you are using Version 2 of Easy-RSA) open up a Command Prompt window and cd to \Program Files\OpenVPN\easy-rsa. Run the following batch file to copy configuration files into place (this will overwrite any preexisting vars.bat and openssl.cnf files):

init-config

Now edit the vars file (called vars.bat on Windows) and set the KEY_COUNTRY, KEY_PROVINCE, KEY_CITY, KEY_ORG, and KEY_EMAIL parameters. Don't leave any of these parameters blank.Next, initialize the PKI. On Linux/BSD/Unix:

. ./vars
./clean-all
./build-ca

On Windows:

vars

If you get an error message that says:

plaintext
You appear to be sourcing an Easy-RSA *vars* file.
This is no longer necessary and is disallowed. See the section called
*How to use this file* near the top comments for more details.

You are using Easy-RSA Version 3.

OpenVPN For Windows only installs Easy-RSA Version 3.

Otherwise, continue:

clean-all
build-ca

The final command (build-ca) will build the certificate authority (CA) certificate and key by invoking the interactive openssl command.

./build-key-server server

This command will create a certificate and private key specifically for the server, following a similar process as when generating the CA certificate. During the process, you will be prompted to enter details for the Distinguished Name, and other parameters, which might again default to values set in the vars or vars.bat files. Be sure to provide a unique Common Name that corresponds to your server's hostname or desired identification.

Generate certificates & keys for clients

After setting up the server's certificate and key, you can proceed to generate credentials for each client. On Linux/BSD/Unix:

./build-key client1

On Windows:

./build-key client1

Replace client1 with the name you choose for each client. This will generate individual certificates and keys named after the client, which will be used to authenticate them to the server securely.

Remember, each client should have a unique certificate/key pair with a unique Common Name for identification.

plaintext
build-dh

As in the previous steps, most parameters can be defaulted. When the Common Name is queried, enter "server". Two other queries require positive responses, "Sign the certificate? [y/n]" and "1 out of 1 certificate requests certified, commit? [y/n]".

Generate certificates & keys for 3 clients

Generating client certificates is very similar to the previous step. On Linux/BSD/Unix:

plaintext
./build-key client1
./build-key client2
./build-key client3

On Windows:

plaintext
build-key client1
build-key client2
build-key client3

If you would like to password-protect your client keys, substitute the build-key-pass script.

Remember that for each client, make sure to type the appropriate Common Name when prompted, i.e. "client1", "client2", or "client3". Always use a unique common name for each client.

Generate Diffie Hellman parameters

Diffie Hellman parameters must be generated for the OpenVPN server. On Linux/BSD/Unix:

plaintext
./build-dh

On Windows:

plaintext
build-dh

Output:

ai:easy-rsa # ./build-dh
Generating DH parameters, 1024 bit long safe prime, generator 2
This is going to take a long time
.................+...........................................
...................+.............+.................+.........
......................................

Key Files

Now we will find our newly-generated keys and certificates in the keys subdirectory. Here is an explanation of the relevant files:

Key Generation and Secure File Transfer

The final step in the key generation process involves securely transferring all necessary files to the respective machines. It's crucial to ensure that secret files are transferred over a secure channel.

However, you might wonder if it's possible to set up the Public Key Infrastructure (PKI) without an already secure channel. Technically, yes. In our simplified example, we generated all private keys in one location for brevity. With additional effort, this approach can be adjusted:

For instance, instead of generating the client's certificate and keys on a central server, the client could generate its private key locally. Subsequently, it could send a Certificate Signing Request (CSR) to the key-signing server. The key-signing server would then process the CSR and return a signed certificate to the client. This method ensures that the secret .key file never needs to leave the client’s hard drive.

Creating Configuration Files for Server and Clients

Getting the Sample Config Files

It's recommended to start with the OpenVPN sample configuration files as a template for your setup. These files are also located in:

• sample/sample-config-files directory of the OpenVPN source distribution
• /usr/share/doc/packages/openvpn or /usr/share/doc/openvpn if installed via RPM or DEB package
• Start Menu -> All Programs -> OpenVPN -> OpenVPN Sample Configuration Files on Windows

On Linux, BSD, or Unix-like systems, the sample files are named server.conf and client.conf. On Windows, they are server.ovpn and client.ovpn.

Editing the Server Configuration File

The sample server configuration file provides an excellent base for an OpenVPN server. It sets up a VPN using a virtual TUN interface for routing, listens to client connections on UDP port 1194, and allocates virtual addresses from the 10.8.0.0/24 subnet to connecting clients.

Before deploying the sample configuration file, modify the ca, cert, key, and dh parameters to align with the files generated in the PKI setup section.

While the server configuration file is now operational, further customizations might be necessary:

• Switch to server-bridge and dev tap if utilizing Ethernet bridging instead of server and dev tun.
• Change proto tcp instead of proto udp if preferring TCP over UDP. Note: To have OpenVPN listen on both UDP and TCP, run two separate instances.
• Adjust the virtual IP address range by modifying the server directive if 10.8.0.0/24 is unsuitable. Ensure it's a private range not in use on your current network.
• Enable client-to-client if you want connected clients to communicate directly, which is disabled by default.* If you are using Linux, BSD, or a Unix-like OS, you can improve security by uncommenting the user nobody and group nobody directives.

If you want to run multiple OpenVPN instances on the same machine, each using a different configuration file, you can:

• Use a different port number for each instance (the UDP and TCP protocols use different port spaces so you can run one daemon listening on UDP-1194 and another on TCP-1194).
• If you are using Windows, each OpenVPN configuration needs to have its own TAP-Windows adapter. You can add additional adapters by going to Start Menu -> All Programs -> TAP-Windows -> Add a new TAP-Windows virtual ethernet adapter.
• If you are running multiple OpenVPN instances out of the same directory, make sure to edit directives which create output files so that multiple instances do not overwrite each other's output files. These directives include log, log-append, status, and ifconfig-pool-persist.

Editing the client configuration files

The sample client configuration file (client.conf on Linux/BSD/Unix or client.ovpn on Windows) mirrors the default directives set in the sample server configuration file.

• Like the server configuration file, first edit the ca, cert, and key parameters to point to the files you generated in the PKI section above. Note that each client should have its own cert/key pair. Only the ca file is universal across the OpenVPN server and all clients.
• Next, edit the remote directive to point to the hostname/IP address and port number of the OpenVPN server (if your OpenVPN server will be running on a single-NIC machine behind a firewall/NAT-gateway, use the public IP address of the gateway, and a port number which you have configured the gateway to forward to the OpenVPN server).
• Finally, ensure that the client configuration file is consistent with the directives used in the server configuration. Major things to check for are that the dev (tun or tap) and proto (udp or tcp) directives are consistent. Also, make sure that comp-lzo and fragment, if used, are present in both client and server config files.

Starting up the VPN and testing for initial connectivity

Starting the server

First, ensure the OpenVPN server will be accessible from the internet. That means:

• Opening up UDP port 1194 on the firewall (or whatever TCP/UDP port you've configured), or
• Setting up a port forward rule to forward UDP port 1194 from the firewall/gateway to the machine running the OpenVPN server.

Next, make sure that the TUN/TAP interface is not firewalled.

To simplify troubleshooting, it's best to initially start the OpenVPN server from the command line (or right-click on the .ovpn file on Windows), rather than start it as a daemon or service:

openvpn [server config file]

A normal server startup should look like this (output will vary across platforms):

Sun Feb  6 20:46:38 2005 OpenVPN 2.0_rc12 i686-suse-linux [SSL] [LZO] [EPOLL] built on Feb  5 2005
Sun Feb  6 20:46:38 2005 Diffie-Hellman initialized with 1024 bit key
Sun Feb  6 20:46:38 2005 TLS-Auth MTU parms [ L:1542 D:138 EF:38 EB:0 ET:0 EL:0 ]
Sun Feb  6 20:46:38 2005 TUN/TAP device tun1 opened
Sun Feb  6 20:46:38 2005 /sbin/ifconfig tun1 10.8.0.1 pointopoint 10.8.0.2 mtu 1500
Sun Feb  6 20:46:38 2005 /sbin/route add -net 10.8.0.0 netmask 255.255.255.0 gw 10.8.0.2
Sun Feb  6 20:46:38 2005 Data Channel MTU parms [ L:1542 D:1450 EF:42 EB:23 ET:0 EL:0 AF:3/1 ]
Sun Feb  6 20:46:38 2005 UDPv4 link local (bound): [undef]:1194
Sun Feb  6 20:46:38 2005 UDPv4 link remote: [undef]
Sun Feb  6 20:46:38 2005 MULTI: multi_init called, r=256 v=256
Sun Feb  6 20:46:38 2005 IFCONFIG POOL: base=10.8.0.4 size=62
Sun Feb  6 20:46:38 2005 IFCONFIG POOL LIST
Sun Feb  6 20:46:38 2005 Initialization Sequence Completed

Starting the client

As in the server configuration, it's best to initially start the OpenVPN server from the command line (or on Windows, by right-clicking on the client.ovpn file), rather than start it as a daemon or service:

openvpn [client config file]

A normal client startup on Windows will look similar to the server output above, and should end with the Initialization Sequence Completed message.

Now, try a ping across the VPN from the client. If you are using routing (i.e. dev tun in the server config file), try:

ping 10.8.0.1

If you are using bridging (i.e. dev tap in the server config file), try to ping the IP address of a machine on the server's ethernet subnet.

If the ping succeeds, congratulations! You now have a functioning VPN.

Troubleshooting

If the ping failed or the OpenVPN client initialization failed to complete, here is a checklist of common symptoms and their solutions.

1. Error Message: "TLS Error: TLS key negotiation failed to occur within 60 seconds (check your network connectivity)". This error indicates that the client was unable to establish a network connection with the server.
   • Solutions:
     • Ensure the client is using the correct hostname/IP address and port number which will allow it to reach the OpenVPN server.
     • If the OpenVPN server machine is a single-NIC box inside a protected LAN, ensure you are using a correct port forward rule on the server's gateway firewall. For example, if your OpenVPN box is at 192.168.4.4 inside the firewall, listening for client connections on UDP port 1194, the NAT gateway servicing the 192.168.4.x subnet should have a port forward rule that says "forward UDP port 1194 from my public IP address to 192.168.4.4".
     • Open up the server's firewall to allow incoming connections to UDP port 1194 (or whatever TCP/UDP port you have configured in the server config file).
2. Error Message: "Initialization Sequence Completed with errors"—This error can occur on Windows if (a) You don't have the DHCP client service running, or (b) You are using certain third-party personal firewalls on XP SP2.
   • Solution:
     • Start the DHCP client server and ensure that you are using a personal firewall which is known to work correctly on XP SP2.
3. Outcome: "Initialization Sequence Completed" message but the ping test fails—This usually indicates that a firewall on either server or client is blocking VPN network traffic by filtering on the TUN/TAP interface.
   • Solution:
     • Disable the client firewall (if one exists) from filtering the TUN/TAP interface on the client. On Windows XP SP2, this can be done by going to "Windows Security Center -> Windows Firewall -> Advanced" and unchecking the box corresponding to the TAP-Windows adapter. Also, ensure that the TUN/TAP interface on the server is not being filtered by a firewall.
4. Issue: The connection stalls on startup when using a proto UDP configuration, the server log file shows the line "TLS: Initial packet from x.x.x.x:x, sid=xxxxxxxx xxxxxxxx", but the client log does not show an equivalent line.
   • Solution:
     • You have a one-way connection from client to server. The server to client direction is blocked by a firewall, usually on the client side. Modify the firewall to allow returning UDP packets from the server to reach the client.

See the FAQ articles for additional troubleshooting information.

Configuring OpenVPN to run automatically on system startup

The lack of standards in this area means that most OSes have a different way of configuring daemons/services for autostart on boot. The best way to have this functionality configured by default is to install OpenVPN as a package, such as via RPM on Linux or using the Windows installer.

Linux

When you install OpenVPN using an RPM or DEB package on Linux, the installation process includes a systemd service unit generator. It will generate OpenVPN client and server services for each configuration file you put under /etc/openvpn/client and /etc/openvpn/server, respectively.

Windows

Upon installing OpenVPN on Windows, a Service Wrapper is set up but is not activated by default. To enable it:

1. Navigate to Control Panel > Administrative Tools > Services.
2. Locate and right-click on the OpenVPN service.
3. Select Properties and change the Startup Type to Automatic.

This setting ensures that the service will start automatically upon the next system reboot.

Once activated, the OpenVPN Service Wrapper checks the \Program Files\OpenVPN\config directory for .ovpn files and starts a new OpenVPN process for each configuration file found.

Controlling a Running OpenVPN Process

Linux/BSD/Unix

OpenVPN can be controlled using the following signals:

• SIGUSR1: Conditional restart, allowing restart without root privileges.
• SIGHUP: Performs a hard restart.
• SIGUSR2: Sends connection statistics to the log file or syslog.
• SIGTERM, SIGINT: Terminates the process.

To manage the OpenVPN daemon, use the writepid directive to record the daemon's PID to a file. This enables accurate signaling. If OpenVPN is launched via an initscript, the script might include the --writepid option in the command line parameters.

Windows GUI

For detailed instructions on using the OpenVPN GUI on Windows, refer to the OpenVPN-GUI page.

Windows Command Prompt

To start OpenVPN from a Windows command prompt:

1. Navigate to the .ovpn configuration file.
2. Right-click and select Start OpenVPN on this config file to initiate the service.Once running in this fashion, several keyboard commands are available:
   • F1 -- Conditional restart (doesn't close/reopen TAP adapter)
   • F2 -- Show connection statistics
   • F3 -- Hard restart
   • F4 -- Exit

Running as a Windows Service

When OpenVPN is started as a service on Windows, the only way to control it is:

• Via the service control manager (Control Panel / Administrative Tools / Services) which gives start/stop control.
• Via the management interface (see below).

Modifying a live server configuration

While most configuration changes require you to restart the server, there are two directives in particular which refer to files which can be dynamically updated on-the-fly, and which will take immediate effect on the server without needing to restart the server process.

client-config-dir -- This directive sets a client configuration directory, which the OpenVPN server will scan on every incoming connection, searching for a client-specific configuration file (see the manual page for more information). Files in this directory can be updated on-the-fly, without restarting the server. Note that changes in this directory will only take effect for new connections, not existing connections. If you would like a client-specific configuration file change to take immediate effect on a currently connected client (or one which has disconnected, but where the server has not timed-out its instance object), kill the client instance object by using the management interface (described below). This will cause the client to reconnect and use the new client-config-dir file.

crl-verify -- This directive names a Certificate Revocation List file, described below in the Revoking Certificates section. The CRL file can be modified on the fly, and changes will take effect immediately for new connections, or existing connections which are renegotiating their SSL/TLS channel (occurs once per hour by default). If you would like to kill a currently connected client whose certificate has just been added to the CRL, use the management interface (described below).

Status File

The default server.conf file has a line

status openvpn-status.log

which will output a list of current client connections to the file openvpn-status.log once per minute.## Using the Management Interface

The OpenVPN management interface provides extensive control over an active OpenVPN process. You can access the management interface directly by telnetting to the designated port, or indirectly through an OpenVPN GUI that connects to the management interface.

To activate the management interface on either an OpenVPN server or client, insert the following into the configuration file:

management localhost 7505

This configuration makes OpenVPN listen on TCP port 7505 for management interface clients (you can select any free port instead of 7505).

After initiating OpenVPN, you can connect to the management interface using a telnet client. For instance:

ai:~ # telnet localhost 7505
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
>INFO:OpenVPN Management Interface Version 1 -- type 'help' for more info
help
Management Interface for OpenVPN 2.0_rc14 i686-suse-linux [SSL] [LZO] [EPOLL] built on Feb 15 2005
Commands:
echo [on|off] [N|all]  : Like log, but only show messages in echo buffer.
exit|quit              : Close management session.
help                   : Print this message.
hold [on|off|release]  : Set/show hold flag to on/off state, or
                         release current hold and start tunnel.
kill cn                : Kill the client instance(s) having common name cn.
kill IP:port           : Kill the client instance connecting from IP:port.
log [on|off] [N|all]   : Turn on/off real-time log display
                         + show last N lines or 'all' for entire history.
``````plaintext
mute [n]               : Set log mute level to n, or show level if n is absent.
net                    : (Windows only) Show network info and routing table.
password type p        : Enter password p for a queried OpenVPN password.
signal s               : Send signal s to daemon,
                         s = SIGHUP|SIGTERM|SIGUSR1|SIGUSR2.
state [on|off] [N|all] : Like log, but show state history.
status [n]             : Show current daemon status info using format #n.
test n                 : Produce n lines of output for testing/debugging.
username type u        : Enter username u for a queried OpenVPN username.
verb [n]               : Set log verbosity level to n, or show if n is absent.
version                : Show current version number.
END
exit
Connection closed by foreign host.
ai:~ #

For more information, see the OpenVPN Management Interface Documentation.

Expanding the scope of the VPN to include additional machines on either the client or server subnet

Including multiple machines on the server side when using a routed VPN (dev tun)

Once the VPN is operational in a point-to-point capacity between client and server, it may be desirable to expand the scope of the VPN so that clients can reach multiple machines on the server network, rather than only the server machine itself.

For the purpose of this example, we will assume that the server-side LAN uses a subnet of 10.66.0.0/24 and the VPN IP address pool uses 10.8.0.0/24 as cited in the server directive in the OpenVPN server configuration file.

First, you must advertise the 10.66.0.0/24 subnet to VPN clients as being accessible through the VPN. This can easily be done with the following server-side config file directive:

plaintext
push "route 10.66.0.0 255.255.255.0"

Routing the VPN Client Subnet

First, you need to configure a route on the server-side LAN gateway to direct traffic from the VPN client subnet (10.8.0.0/24) to the OpenVPN server. This step is necessary if the OpenVPN server and the LAN gateway are separate machines.

Ensure that IP and TUN/TAP forwarding is enabled on the OpenVPN server machine.

Including Multiple Machines on the Server Side with a Bridged VPN (dev tap)

Using ethernet bridging inherently includes all server-side machines without needing additional configurations.

Including Multiple Machines on the Client Side with a Routed VPN (dev tun)

In a scenario where the client machine acts as a gateway for a local LAN (like a home office), and you want each machine on the client LAN to route through the VPN:

• Assume the client LAN uses the 192.168.4.0/24 subnet.
• The VPN client uses a certificate with a common name of client2.

Objective: Set up the VPN so that any machine on the client LAN can communicate with any machine on the server LAN through the VPN.

Prerequisites

• Ensure the client LAN subnet (192.168.4.0/24) is not exported to the VPN by the server or any other clients using the same subnet.
• The client must have a unique Common Name (client2) in its certificate, and the duplicate-cn flag should not be used in the OpenVPN server configuration file.

Configuration Steps

1. Enable IP and TUN/TAP Forwarding on the Client Machine: Ensure that the necessary forwarding is enabled to allow traffic routing.
2. Modify the Server Configuration: If not already present, add the following directive to the server configuration file to reference a client configuration directory:
   client-config-dir ccd

   • ccd should be the name of a pre-created directory in the default directory where the OpenVPN server daemon runs (typically /etc/openvpn on Linux and \Program Files\OpenVPN\config on Windows).
3. Create a Configuration File for the Client: Create a file named client2 in the ccd directory with the following content:
   iroute 192.168.4.0 255.255.255.0

This will tell the OpenVPN server that the 192.168.4.0/24 subnet should be routed to client2.

Next, add the following line to the main server config file (not the ccd/client2 file):

route 192.168.4.0 255.255.255.0

Why the redundant route and iroute statements, you might ask? The reason is that route controls the routing from the kernel to the OpenVPN server (via the TUN interface) while iroute controls the routing from the OpenVPN server to the remote clients. Both are necessary.

Next, ask yourself if you would like to allow network traffic between client2's subnet (192.168.4.0/24) and other clients of the OpenVPN server. If so, add the following to the server config file.

client-to-client
push "route 192.168.4.0 255.255.255.0"

This will cause the OpenVPN server to advertise client2's subnet to other connecting clients.

The last step, and one that is often forgotten, is to add a route to the server's LAN gateway which directs 192.168.4.0/24 to the OpenVPN server box (you won't need this if the OpenVPN server box is the gateway for the server LAN). Suppose you were missing this step and you tried to ping a machine (not the OpenVPN server itself) on the server LAN from 192.168.4.8? The outgoing ping would probably reach the machine, but then it wouldn't know how to route the ping reply, because it would have no idea how to reach 192.168.4.0/24. The rule of thumb to use is that when routing entire LANs through the VPN (when the VPN server is not the same machine as the LAN gateway), make sure that the gateway for the LAN routes all VPN subnets to the VPN server machine.

Similarly, if the client machine running OpenVPN is not also the gateway for the client LAN, then the gateway for the client LAN must have a route which directs all subnets which should be reachable through the VPN to the OpenVPN client machine.

Including multiple machines on the client side when using a bridged VPN (dev tap)

This requires a more complex setup (maybe not more complex in practice, but more complicated to explain in detail):

• You must bridge the client TAP interface with the LAN-connected NIC on the client.
• You must manually set the IP/netmask of the TAP interface on the client.
• You must configure client-side machines to use an IP/netmask that is inside of the bridged subnet, possibly by querying a DHCP server on the OpenVPN server side of the VPN.

Pushing DHCP options to clients

The OpenVPN server can push DHCP options such as DNS and WINS server addresses to clients. Windows clients can accept pushed DHCP options natively, while non-Windows clients can accept them by using a client-side --up script which parses the foreign_option_n environmental variable list. See Using DNS servers pushed to clients. For example, suppose you would like connecting clients to use an internal DNS server at 10.66.0.4 or 10.66.0.5 and a WINS server at 10.66.0.8. Add this to the OpenVPN server configuration:

push "dhcp-option DNS 10.66.0.4"
push "dhcp-option DNS 10.66.0.5"
push "dhcp-option WINS 10.66.0.8"

To test this feature on Windows, run the following from a command prompt window after the machine has connected to an OpenVPN server:

ipconfig /all

The entry for the TAP-Windows adapter should show the DHCP options which were pushed by the server.

Configuring client-specific rules and access policies

Suppose we are setting up a company VPN, and we would like to establish separate access policies for 3 different classes of users:

• System administrators -- full access to all machines on the network
• Employees -- access only to Samba/email server
• Contractors -- access to a special server only

The basic approach we will take is (a) segregate each user class into its own virtual IP address range, and (b) control access to machines by setting up firewall rules which key off the client's virtual IP address.

In our example, suppose that we have a variable number of employees, but only one system administrator, and two contractors. Our IP allocation approach will be to put all employees into an IP address pool, and then allocate fixed IP addresses for the system administrator and contractors.

Note that one of the prerequisites of this example is that you have a software firewall running on the OpenVPN server machine which gives you the ability to define specific firewall rules. For our example, we will assume the firewall is Linux iptables.

First, let's create a virtual IP address map according to user class:

Next, let's translate this map into an OpenVPN server configuration. First of all, make sure you've followed the steps above for making the 10.66.4.0/24 subnet available to all clients (while we will configure routing to allow client access to the entire 10.66.4.0/24 subnet, we will then impose access restrictions using firewall rules to implement the above policy table).

First, define a static unit number for our tun interface, so that we will be able to refer to it later in our firewall rules:

dev tun0

In the server configuration file, define the Employee IP address pool:

server 10.8.0.0 255.255.252.0

Add routes for the System Administrator and Contractor IP ranges:

route 10.8.1.0 255.255.255.0
route 10.8.2.0 255.255.255.0

Because we will be assigning fixed IP addresses for specific System Administrators and Contractors, we will use a client configuration directory:

client-config-dir ccd

Now place special configuration files in the ccd subdirectory to define the fixed IP address for each non-Employee VPN client.

ccd/sysadmin1
    ifconfig-push 10.8.1.2 10.8.1.1

ccd/contractor1
    ifconfig-push 10.8.2.2 10.8.2.1

ccd/contractor2

    ifconfig-push 10.8.2.3 10.8.2.1

Each pair of ifconfig-push addresses represent the virtual client and server IP endpoints. They must be taken from successive /30 subnets in order to be compatible with Windows clients and the TAP-Windows driver. Specifically, the last octet in the IP address of each endpoint pair must be taken from this set:

[  1,  2] [  5,  6] [  9, 10] [ 13, 14] [ 17, 18]
[ 21, 22] [ 25, 26] [ 29, 30] [ 33, 34] [ 37, 38]
[ 41, 42] [ 45, 46] [ 49, 50] [ 53, 54] [ 57, 58]
[ 61, 62] [ 65, 66] [ 69, 70] [ 73, 74] [ 77, 78]
[ 81, 82] [ 85, 86] [ 89, 90] [ 93, 94] [ 97, 98]
[101,102] [105,106] [109,110] [113,114] [117,118]
[121,122] [125,126] [129,130] [133,134] [137,138]
[141,142] [145,146] [149,150] [153,154] [157,158]
[161,162] [165,166] [169,170] [173,174] [177,178]
[181,182] [185,186] [189,190] [193,194] [197,198]
[201,202] [205,206] [209,210] [213,214] [217,218]
[221,222] [225,226] [229,230] [233,234] [237,238]
[241,242] [245,246] [249,250] [253,254]

This completes the OpenVPN configuration. The final step is to add firewall rules to finalize the access policy. For this example, we will use firewall rules in the Linux iptables syntax:

# Employee rule
iptables -A FORWARD -i tun0 -s 10.8.0.0/24 -d 10.66.4.4 -j ACCEPT

# Sysadmin rule
iptables -A FORWARD -i tun0 -s 10.8.1.0/24 -d 10.66.4.0/24 -j ACCEPT

# Contractor rule
iptables -A FORWARD -i tun0 -s 10.8.2.0/24 -d 10.66.4.12 -j ACCEPT

# Close remaining of /22 tunnel
iptables -A FORWARD -i tun0 -s 10.8.3.0/24 -j DROP

Using Alternative Authentication Methods

OpenVPN 2.0 and later include a feature that allows the OpenVPN server to securely obtain a username and password from a connecting client, and to use that information as a basis for authenticating the client.

To use this authentication method, first add the auth-user-pass directive to the client configuration. It will direct the OpenVPN client to query the user for a username/password, passing it on to the server over the secure TLS channel.

Next, configure the server to use an authentication plugin, which may be a script, shared object, or DLL. The OpenVPN server will call the plugin every time a VPN client tries to connect, passing it the username/password entered on the client. The authentication plugin can control whether or not the OpenVPN server allows the client to connect by returning a failure (1) or success (0) value.

Using Script Plugins

Script plugins can be used by adding the auth-user-pass-verify directive to the server-side configuration file. For example:

auth-user-pass-verify auth-pam.pl via-file

This will use the auth-pam.pl perl script to authenticate the username/password of connecting clients. See the description of auth-user-pass-verify in the manual page for more information.

The auth-pam.pl script is included in the OpenVPN source file distribution in the sample-scripts subdirectory. It will authenticate users on a Linux server using a PAM authentication module, which could in turn implement shadow password, RADIUS, or LDAP authentication. auth-pam.pl is primarily intended for demonstration purposes. For real-world PAM authentication, use the openvpn-auth-pam shared object plugin described below.

Using Shared Object or DLL Plugins

Shared object or DLL plugins are usually compiled C modules which are loaded by the OpenVPN server at run time. For example, if you are using an RPM-based OpenVPN package on Linux, the openvpn-auth-pam plugin should be already built. To use it, add this to the server-side config file:

plugin /usr/share/openvpn/plugin/lib/openvpn-auth-pam.so login

This will tell the OpenVPN server to validate the username/password entered by clients using the login PAM module.

For real-world production use, it's better to use the openvpn-auth-pam plugin, because it has several advantages over the auth-pam.pl script:

• The shared object openvpn-auth-pam plugin uses a split-privilege execution model for better security. This means that the OpenVPN server can run with reduced privileges by using the directives user nobody, group nobody, and chroot, and will still be able to authenticate against the root-readable-only shadow password file.
• OpenVPN can pass the username/password to a plugin via virtual memory, rather than via a file or the environment, which is better for local security on the server machine.
• C-compiled plugin modules generally run faster than scripts.

If you would like more information on developing your own plugins for use with OpenVPN, see the README files in the plugin subdirectory of the OpenVPN source distribution.

To build the openvpn-auth-pam plugin on Linux, cd to the plugin/auth-pam directory in the OpenVPN source distribution and run make.

Using username/password authentication as the only form of client authentication

By default, using auth-user-pass-verify or a username/password-checking plugin on the server will enable dual authentication, requiring that both client-certificate and username/password authentication succeed in order for the client to be authenticated.

While it is discouraged from a security perspective, it is also possible to disable the use of client certificates, and force username/password authentication only. On the server:

client-cert-not-required

Such configurations should usually also set:

username-as-common-name

which will tell the server to use the username for indexing purposes as it would use the Common Name of a client which was authenticating via a client certificate.

Note that client-cert-not-required will not obviate the need for a server certificate, so a client connecting to a server which uses client-cert-not-required may remove the cert and key directives from the client configuration file, but not the ca directive, because it is necessary for the client to verify the server certificate.

How to add dual-factor authentication to an OpenVPN configuration using client-side smart cards

How to Configure Cryptographic Token

To configure a cryptographic token for use with your system, follow these steps:

1. Install the Necessary Software:

   • Ensure that the device drivers and provider library for your cryptographic token (smart card or hardware token) are installed on your system. This includes any necessary middleware that facilitates communication between the token and your system.

2. Locate the PKCS!#11 Provider Library:

   • As mentioned earlier, the provider library is essential for interfacing with the cryptographic token. Locate the library file which could be named similarly to opensc-pkcs11.so or opensc-pkcs11.dll, depending on your operating system.

3. Configure Your Application:

   • Applications that support PKCS!#11 will have options to configure access to cryptographic tokens. This typically involves specifying the path to the PKCS!#11 provider library within the application settings.
   • For example, in OpenVPN, you would modify the configuration file to include directives such as pkcs11-providers /usr/lib/pkcs11/opensc-pkcs11.so and pkcs11-id 'your-token-id'.

4. Initialize the Token:

   • If your token is new or has been reset, you may need to initialize it. This process involves setting up a PIN, loading certificates, and possibly generating key pairs directly on the device. Tools provided by the token vendor or third-party utilities like OpenSC can be used for this purpose.

5. Load Certificates:

   • Depending on your use case, you might need to load one or more certificates onto the token. These certificates could be for authentication, encryption, or signing. Ensure that these certificates are properly formatted and compatible with the token.

6. Test the Configuration:

   • After setting up everything, test the configuration to ensure that the token is being correctly accessed by your application. Attempt to authenticate using the token or perform a cryptographic operation.

7. Secure the Token:

   • Always keep your cryptographic token secure. Avoid exposing it to potential threats, and ensure that its PIN is not easily guessable. If the token is lost or compromised, take immediate actions to revoke any certificates associated with it and report the incident if necessary.

Summary

Setting up and configuring a cryptographic token involves multiple steps that include installing necessary software, locating and configuring the PKCS!#11 provider library, initializing and loading certificates onto the token, and ensuring the security of the token. Proper configuration and handling will help in leveraging the robust security features provided by dual-factor authentication mechanisms.You should follow the enrollment procedure:

• Initialize the PKCS!#11 token.
• Generate an RSA key pair on the PKCS!#11 token.
• Create a certificate request based on the key pair, using OpenSC and OpenSSL.
• Submit the certificate request to a certificate authority and receive a certificate.
• Load the certificate onto the token, ensuring that the id and label attributes of the certificate match those of the private key.

A configured token is one that has both a private key object and a certificate object, where both share the same id and label attributes.

A simple enrollment utility is Easy-RSA 2.0, which is part of the OpenVPN 2.1 series. Follow the instructions specified in the README file, and then use pkitool to enroll.

Initialize a token using the following commands:

$ ./pkitool --pkcs11-slots /usr/lib/pkcs11/
$ ./pkitool --pkcs11-init /usr/lib/pkcs11/

Enroll a certificate using the following command:

$ ./pkitool --pkcs11 /usr/lib/pkcs11/ client1

How to Modify an OpenVPN Configuration to Make Use of Cryptographic Tokens

Ensure you have OpenVPN 2.1 or above to use the PKCS!#11 features.

Determine the Correct Object

Each PKCS!#11 provider can support multiple devices. To view the available object list, you can use the following command:

$ openvpn --show-pkcs11-ids /usr/lib/pkcs11/

The following objects are available for use. Each object shown below may be used as a parameter to the --pkcs11-id option. Please remember to use single quotation marks.

Certificate

• DN: /CN=User1
• Serial: 490B82C4000000000075
• Serialized id: aaaa/bbb/41545F5349474E415455524581D2A1A1B23C4AA4CB17FAF7A4600

Each certificate/private key pair has a unique "Serialized id" string. The serialized id string of the requested certificate should be specified to the pkcs11-id option using single quotation marks.

pkcs11-id 'aaaa/bbb/41545F5349474E415455524581D2A1A1B23C4AA4CB17FAF7A4600'

Using OpenVPN with PKCS!#11

A typical set of OpenVPN options for PKCS!#11:

pkcs11-providers /usr/lib/pkcs11/
pkcs11-id 'aaaa/bbb/41545F5349474E415455524581D2A1A1B23C4AA4CB17FAF7A4600'

This will select the object which matches the pkcs11-id string.

Advanced OpenVPN options for PKCS#11

pkcs11-providers /usr/lib/pkcs11/provider1.so /usr/lib/pkcs11/provider2.so
pkcs11-id 'aaaa/bbb/41545F5349474E415455524581D2A1A1B23C4AA4CB17FAF7A4600'
pkcs11-pin-cache 300
daemon
auth-retry nointeract
management-hold
management-signal
management 127.0.0.1 8888
management-query-passwords

This configuration will load two providers into OpenVPN, utilize the certificate specified in the pkcs11-id option, and employ the management interface to query passwords. The daemon will enter a hold state if the token cannot be accessed. The token will be valid for 300 seconds after which the password will be re-queried, and the session will disconnect if the management session disconnects.

PKCS#11 Implementation Considerations

Many PKCS#11 providers utilize threads. To prevent issues caused by the implementation of LinuxThreads (such as setuid and chroot), it is strongly recommended to upgrade to a glibc with Native POSIX Thread Library (NPTL) support if you intend to use PKCS#11.

OpenSC PKCS#11 Provider

The OpenSC PKCS#11 provider can be found at:

• Unix: /usr/lib/pkcs11/opensc-pkcs11.so
• Windows: opensc-pkcs11.dll

Difference between PKCS#11 and Microsoft Cryptographic API (CryptoAPI)

PKCS#11 is a free, cross-platform, vendor-independent standard, whereas CryptoAPI is specific to Microsoft. Most smart card vendors support both interfaces. In the Windows environment, users should choose which interface to use.

The current OpenVPN implementation that uses the MS CryptoAPI (cryptoapicert option) functions well unless OpenVPN is run as a service. Running OpenVPN as a service in an administrative environment will likely fail with most smart cards for the following reasons:

• Most smart card providers do not load certificates into the local machine store, preventing access to the user certificate.
• If the OpenVPN client runs as a service without direct user interaction, it cannot prompt the user to provide a password for the smart card, leading to a failure in the password-verification process on the smart card.

Using the PKCS#11 interface allows the use of smart cards with OpenVPN in any implementation, as PKCS#11 does not rely on Microsoft stores and does not always require direct interaction with the end-user.

Routing all client traffic (including web traffic) through the VPN

Overview

By default, network traffic to and from the OpenVPN server site will pass over the VPN when an OpenVPN client is active. General web browsing and other internet activities will occur through direct connections that bypass the VPN.In certain cases, this behavior might not be desirable—you might want a VPN client to tunnel all network traffic through the VPN, including general internet web browsing. While this type of VPN configuration will exact a performance penalty on the client, it gives the VPN administrator more control over security policies when a client is simultaneously connected to both the public internet and the VPN at the same time.

Implementation

Add the following directive to the server configuration file:

push "redirect-gateway def1"

If your VPN setup is over a wireless network, where all clients and the server are on the same wireless subnet, add the local flag:

push "redirect-gateway local def1"

Pushing the redirect-gateway option to clients will cause all IP network traffic originating on client machines to pass through the OpenVPN server. The server will need to be configured to deal with this traffic somehow, such as by NATing it to the internet, or routing it through the server site's HTTP proxy.

On Linux, you could use a command such as this to NAT the VPN client traffic to the internet:

iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

This command assumes that the VPN subnet is 10.8.0.0/24 (taken from the server directive in the OpenVPN server configuration) and that the local ethernet interface is eth0.

When redirect-gateway is used, OpenVPN clients will route DNS queries through the VPN, and the VPN server will need to handle them. This can be accomplished by pushing a DNS server address to connecting clients which will replace their normal DNS server settings during the time that the VPN is active. For example:

push "dhcp-option DNS 10.8.0.1"

will configure Windows clients (or non-Windows clients with some extra server-side scripting) to use 10.8.0.1 as their DNS server. Any address which is reachable from clients may be used as the DNS server address.

Caveats

Redirecting all network traffic through the VPN is not entirely a problem-free proposition. Here are some typical gotchas to be aware of:```markdown

• Many OpenVPN client machines connecting to the internet will periodically interact with a DHCP server to renew their IP address leases. The redirect-gateway option might prevent the client from reaching the local DHCP server (because DHCP messages would be routed over the VPN), causing it to lose its IP address lease.
• Issues exist with respect to pushing DNS addresses to Windows clients.
• Web browsing performance on the client will be noticeably slower.

For more information on the mechanics of the redirect-gateway directive, see the manual page.

Running an OpenVPN server on a dynamic IP address

While OpenVPN clients can easily access the server via a dynamic IP address without any special configuration, things get more interesting when the server itself is on a dynamic address. While OpenVPN has no trouble handling the situation of a dynamic server, some extra configuration is required.

The first step is to get a dynamic DNS address which can be configured to "follow" the server every time the server's IP address changes. There are several dynamic DNS service providers available from which to choose.

The next step is to set up a mechanism so that every time the server's IP address changes, the dynamic DNS name will be quickly updated with the new IP address, allowing clients to find the server at its new IP address. There are two basic ways to accomplish this:

• Use a NAT router appliance with dynamic DNS support (such as the Linksys BEFSR41). Most of the inexpensive NAT router appliances that are widely available have the capability to update a dynamic DNS name every time a new DHCP lease is obtained from the ISP. This setup is ideal when the OpenVPN server box is a single-NIC machine inside the firewall.
• Use a dynamic DNS client application such as ddclient to update the dynamic DNS address whenever the server IP address changes. This setup is ideal when the machine running OpenVPN has multiple NICs and is acting as a site-wide firewall/gateway. To implement this setup, you need to set up a script to be run by your DHCP client software every time an IP address change occurs. This script should (a) run ddclient to notify your dynamic DNS provider of your new IP address and (b) restart the OpenVPN server daemon.

The OpenVPN client by default will sense when the server's IP address has changed, if the client configuration is using a remote directive which references a dynamic DNS name. The usual chain of events is that (a) the OpenVPN client fails to receive timely keepalive messages from the server's old IP address, triggering a restart, and (b) the restart causes the DNS name in the remote directive to be re-resolved, allowing the client to reconnect to the server at its new IP address.

More information can be found in the FAQ.

Connecting to an OpenVPN server via an HTTP proxy

OpenVPN supports connections through an HTTP proxy, with the following authentication modes:

• No proxy authentication
• Basic proxy authentication
• NTLM proxy authentication

First of all, HTTP proxy usage requires that you use TCP as the tunnel carrier protocol. So add the following to both client and server configurations:

proto tcp

Make sure that any proto udp lines in the config files are deleted.

Next, add the http-proxy directive to the client configuration file (see the manual page for a full description of this directive).

For example, suppose you have an HTTP proxy server on the client LAN at 192.168.4.1, which is listening for connections on port 1080. Add this to the client config:

http-proxy 192.168.4.1 1080

Suppose the HTTP proxy requires Basic authentication:

http-proxy 192.168.4.1 1080 stdin basic

Suppose the HTTP proxy requires NTLM authentication:

http-proxy 192.168.4.1 1080 stdin ntlm

The two authentication examples above will cause OpenVPN to prompt for a username/password from standard input. If you would instead like to place these credentials in a file, replace stdin with a filename, and place the username on line 1 of this file and the password on line 2.

Connecting to a Samba share over OpenVPN

This example is intended to show how OpenVPN clients can connect to a Samba share over a routed dev tun tunnel. If you are ethernet bridging (dev tap), you probably don't need to follow these instructions, as OpenVPN clients should see server-side machines in their network neighborhood.

For this example, we will assume that:

• the server-side LAN uses a subnet of 10.66.0.0/24,
• the VPN IP address pool uses 10.8.0.0/24 (as cited in the server directive in the OpenVPN server configuration file),
• the Samba server has an IP address of 10.66.0.4, and* The Samba server has already been configured and is reachable from the local LAN.

If the Samba and OpenVPN servers are running on different machines, ensure you have followed the section on expanding the VPN's scope to include additional machines.

Next, edit your Samba configuration file (smb.conf). Ensure the hosts allow directive permits OpenVPN clients from the 10.8.0.0/24 subnet to connect. For example:

hosts allow = 10.66.0.0/24 10.8.0.0/24 127.0.0.1

If you are running the Samba and OpenVPN servers on the same machine, you might want to edit the interfaces directive in the smb.conf file to also listen on the TUN interface subnet of 10.8.0.0/24:

interfaces  = 10.66.0.0/24 10.8.0.0/24

If you are running the Samba and OpenVPN servers on the same machine, connect from an OpenVPN client to a Samba share using the folder name:

\\10.8.0.1\sharename

If the Samba and OpenVPN servers are on different machines, use the folder name:

\\10.66.0.4\sharename

For example, from a command prompt window:

net use z: \\10.66.0.4\sharename /USER:myusername

Implementing a load-balancing/failover configuration

Client

The OpenVPN client configuration can refer to multiple servers for load balancing and failover. For example:

remote server1.mydomain
remote server2.mydomain
remote server3.mydomain

This configuration will direct the OpenVPN client to attempt a connection with server1, server2, and server3 in that order. If an existing connection is broken, the OpenVPN client will retry the most recently connected server, and if that fails, will move on to the next server in the list. You can also direct the OpenVPN client to randomize its server list on startup, so that the client load will be probabilistically spread across the server pool.

remote-random

If you would also like DNS resolution failures to cause the OpenVPN client to move to the next server in the list, add the following:

resolv-retry 60

The 60 parameter tells the OpenVPN client to try resolving each remote DNS name for 60 seconds before moving on to the next server in the list.

The server list can also refer to multiple OpenVPN server daemons running on the same machine, each listening for connections on a different port, for example:

remote smp-server1.mydomain 8000
remote smp-server1.mydomain 8001
remote smp-server2.mydomain 8000
remote smp-server2.mydomain 8001

If your servers are multi-processor machines, running multiple OpenVPN daemons on each server can be advantageous from a performance standpoint.

OpenVPN also supports the remote directive referring to a DNS name which has multiple A records in the zone configuration for the domain. In this case, the OpenVPN client will randomly choose one of the A records every time the domain is resolved.

Server

The simplest approach to a load-balanced/failover configuration on the server is to use equivalent configuration files on each server in the cluster, except use a different virtual IP address pool for each server. For example:

server1

    server 10.8.0.0 255.255.255.0

server2

    server 10.8.1.0 255.255.255.0

server3

    server 10.8.2.0 255.255.255.0

Hardening OpenVPN Security

One of the often-repeated maxims of network security is that one should never place so much trust in a single security component that its failure causes a catastrophic security breach. OpenVPN provides several mechanisms to add additional security layers to hedge against such an outcome.

tls-auth

The tls-auth directive adds an additional HMAC signature to all SSL/TLS handshake packets for integrity verification. Any UDP packet not bearing the correct HMAC signature can be dropped without further processing. The tls-auth HMAC signature provides an additional level of security above and beyond that provided by SSL/TLS. It can protect against:

• DoS attacks or port flooding on the OpenVPN UDP port.
• Port scanning to determine which server UDP ports are in a listening state.
• Buffer overflow vulnerabilities in the SSL/TLS implementation.
• SSL/TLS handshake initiations from unauthorized machines (while such handshakes would ultimately fail to authenticate, tls-auth can cut them off at a much earlier point).

Using tls-auth requires that you generate a shared-secret key that is used in addition to the standard RSA certificate/key:

openvpn --genkey --secret ta.key

This command will generate an OpenVPN static key and write it to the file ta.key. This key should be copied over a pre-existing secure channel to the server and all client machines. It can be placed in the same directory as the RSA .key and .crt files.

In the server configuration, add:

tls-auth ta.key 0

In the client configuration, add:

tls-auth ta.key 1

proto udp

While OpenVPN allows either the TCP or UDP protocol to be used as the VPN carrier connection, the UDP protocol will provide better protection against DoS attacks and port scanning than TCP:

proto udp

user/group (non-Windows only)

OpenVPN has been very carefully designed to allow root privileges to be dropped after initialization, and this feature should always be used on Linux/BSD/Solaris. Without root privileges, a running OpenVPN server daemon provides a far less enticing target to an attacker.

user nobody
group nobody

Unprivileged mode (Linux only)

On Linux, OpenVPN can be run completely unprivileged. This configuration is a little more complex, but provides the best security.To work with this setup, OpenVPN must be configured to use the iproute interface. This can be done by specifying --enable-iproute2 in the configure script. Also, ensure that the sudo package is installed on your system.

This configuration leverages Linux's capability to change the permissions of a tun device, allowing an unprivileged user to access it. It also utilizes sudo to execute iproute, enabling modifications to interface properties and the routing table.

OpenVPN Configuration:

Create the following script and save it at: /usr/local/sbin/unpriv-ip:

#!/bin/sh
sudo /sbin/ip $*

Run visudo and add the following lines to allow the user 'user1' to execute /sbin/ip without a password:

plaintext
user1 ALL=(ALL) NOPASSWD: /sbin/ip

To enable a group of users to execute /sbin/ip without a password, add:

plaintext
%users ALL=(ALL) NOPASSWD: /sbin/ip

Add these lines to your OpenVPN configuration file:

plaintext
dev tunX/tapX
iproute /usr/local/sbin/unpriv-ip

Please ensure you choose either 'tun' or 'tap' and replace X with a constant.

To set up a persistent interface as root and allow a user and/or group to manage it, use the following command (replace tunX with your chosen device):

openvpn --mktun --dev tunX --dev-type tun --user user1 --group users

Ensure that you replace X with the appropriate device number and specify either 'tun' or 'tap' in your configuration.## Run OpenVPN as an Unprivileged User

Further security measures can be enforced by modifying parameters in the /usr/local/sbin/unpriv-ip script.

Chroot (Non-Windows Only)

The chroot directive confines the OpenVPN daemon within a chroot jail, limiting its file system access to a specified directory. For instance:

plaintext
chroot jail

This command changes the working directory of the OpenVPN daemon to the jail subdirectory upon initialization. It then redefines its root filesystem to this directory, preventing any access to files outside the jail and its subdirectories. This is crucial for security, as any compromised server remains isolated from the rest of the server's filesystem.

Caveats

Since chroot changes the daemon's filesystem perspective, any necessary files for OpenVPN post-initialization should be placed within the jail directory, such as:

• the crl-verify file, or
• the client-config-dir directory.

Larger RSA Keys

The size of the RSA key is determined by the KEY_SIZE variable in the easy-rsa/vars file, which should be set prior to key generation. The default is set to 1024, but it can be safely increased to 2048. This enhances security with minimal impact on the performance of the VPN tunnel, although it does slow down the SSL/TLS renegotiation handshake that happens once per client per hour and the initial Diffie Hellman parameters generation in the easy-rsa/build-dh script.

Larger Symmetric Keys

By default, OpenVPN employs Blowfish, a 128-bit symmetric cipher.

OpenVPN supports any cipher available in the OpenSSL library, including those with larger key sizes. For instance, to use a 256-bit AES cipher, add the following line to both the server and client configuration files:

plaintext
cipher AES-256-CBC

Secure Root Key Management

Always keep the root key (ca.key) on a standalone machine that is not connected to any network. This enhances the security of your root key by isolating it from potential network-based threats.One of the security benefits of using an X509 PKI (as OpenVPN does) is that the root CA key (ca.key) does not need to be present on the OpenVPN server machine. In a high-security environment, you might want to designate a machine specifically for key signing purposes, keep the machine well-protected physically, and disconnect it from all networks. Floppy disks can be used to move key files back and forth, as necessary. Such measures make it extremely difficult for an attacker to steal the root key, short of physical theft of the key signing machine.

Revoking Certificates

Revoking a certificate means to invalidate a previously signed certificate so that it can no longer be used for authentication purposes.

Typical reasons for wanting to revoke a certificate include:

• The private key associated with the certificate is compromised or stolen.
• The user of an encrypted private key forgets the password on the key.
• You want to terminate a VPN user's access.

Example

As an example, we will revoke the client2 certificate, which we generated above in the "key generation" section of the HOWTO.

First, open up a shell or command prompt window and change directory to the easy-rsa directory as you did in the "key generation" section above. On Linux/BSD/Unix:

. ./vars
./revoke-full client2

On Windows:

vars
revoke-full client2

You should see output similar to this:

Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf

Debugging Certificate Revocation

Debug Log

DEBUG[load_index]: unique_subject = "yes"
Revoking Certificate 04.
Data Base Updated
Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
DEBUG[load_index]: unique_subject = "yes"
client2.crt: /C=KG/ST=NA/O=OpenVPN-TEST/CN=client2/emailAddress=me@myhost.mydomain
error 23 at 0 depth lookup:certificate revoked

Note the "error 23" in the last line, indicating that a certificate verification of the revoked certificate failed.

The revoke-full script will generate a Certificate Revocation List (CRL) file named crl.pem in the keys subdirectory. This file should be copied to a directory accessible by the OpenVPN server, and CRL verification should be enabled in the server configuration:

crl-verify crl.pem

Now all connecting clients will have their certificates verified against the CRL, and any matching revoked certificates will result in the connection being dropped.

CRL Notes

• When using the crl-verify option in OpenVPN, the CRL file is re-read every time a new client connects or an existing client renegotiates the SSL/TLS connection (typically once per hour). This allows updates to the CRL file while the OpenVPN server daemon is running, and the new CRL will take effect immediately for new connections. If a client whose certificate has been revoked is already connected, you can restart the server using a signal (SIGUSR1 or SIGHUP) to flush all clients, or you can telnet to the management interface and explicitly kill the specific client instance on the server without affecting other clients.
• The crl-verify directive can be used on both the OpenVPN server and clients, but distributing a CRL file to clients is generally unnecessary unless a server certificate has been revoked.
• The CRL file is not confidential and should be world-readable so that the OpenVPN daemon can access it after root privileges are dropped.
• If using the chroot directive, ensure a copy of the CRL file is placed in the chroot directory, as the CRL file is read after the chroot call is executed.
• A common reason for certificate revocation is that a user forgets the password used to encrypt their private key. Revoking the original certificate allows for the generation of a new certificate/key pair with the same common name.

Important Note on possible "Man-in-the-Middle" attack if clients do not verify the certificate of the server they are connecting to

To avoid a possible Man-in-the-Middle attack where an authorized client tries to connect to another client by impersonating the server, make sure to enforce some kind of server certificate verification by clients. There are currently five different ways of accomplishing this, listed in the order of preference.

Option 1 for OpenVPN 2.1 and above: build your server certificates with specific key usage and extended key usage. The RFC3280 determine that the following attributes should be provided for TLS connections:

You can build your server certificates with the build-key-server script (see the easy-rsa documentation for more info). This will designate the certificate as a server-only certificate by setting the right attributes. Now add the following line to your client configuration:

remote-cert-tls server

Option 2, for OpenVPN 2.0 and below: Build your server certificates with the build-key-server script (see the easy-rsa documentation for more info). This will designate the certificate as a server-only certificate by setting nsCertType=server. Now add the following line to your client configuration:

ns-cert-type server

This will block clients from connecting to any server which lacks the nsCertType=server designation in its certificate, even if the certificate has been signed by the ca file in the OpenVPN configuration file.

Option 3: Use the tls-remote directive on the client to accept/reject the server connection based on the common name of the server certificate.

Option 4: Use a tls-verify script or plugin to accept/reject the server connection based on a custom test of the server certificate's embedded X509 subject details.

Option 5: Sign server certificates with one CA and client certificates with a different CA. The client configuration ca directive should reference the server-signing CA file, while the server configuration ca directive should reference the client-signing CA file.