HOWTO
Introduction
OpenVPN is a full-featured SSL VPN which implements OSI layer 2 or 3 secure network extension using the industry standard SSL/TLS protocol, supports flexible client authentication methods based on certificates, smart cards, and/or username/password credentials, and 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.
This document provides step-by-step instructions for configuring an OpenVPN 2.0 client/server VPN, including:
OpenVPN Quickstart. Installing OpenVPN.
Determining whether to use a routed or bridged VPN. Numbering private subnets.
Setting up your own Certificate Authority (CA) and generating certificates and keys for an OpenVPN server and multiple clients.
Creating configuration files for server and clients. Starting up the VPN and testing for initial connectivity. Configuring OpenVPN to run automatically on system startup. Controlling a running OpenVPN process.
Expanding the scope of the VPN to include additional machines on either the client or server subnet.
Pushing DHCP options to clients.
Configuring client-specific rules and access policies. Using alternative authentication methods.
How to add dual-factor authentication to an OpenVPN configuration using client-side smart cards.
Routing all client traffic (including web-traffic) through the VPN. Running an OpenVPN server on a dynamic IP address. Connecting to an OpenVPN server via an HTTP proxy. Connecting to a Samba share over OpenVPN.
Implementing a load-balancing/failover configuration. Hardening OpenVPN Security. Revoking Certificates. Additional Security Notes.
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.
If you don't have a handle on these basics, but would still like to set up OpenVPN, I would encourage you to hire an OpenVPN expert on a consulting basis. Many of the authors on the articles page are available for consulting, or you can contact the creators of OpenVPN at info@open.net info@open.net .
Additional Documentation
OpenVPN Books
OpenVPN: Building and Integrating Virtual Private Networks by Markus Feilner Packt Publishing Ltd ISBN 190481185X [Reviews]
OpenVPN 1.x HOWTO
The original OpenVPN 1.x HOWTO is still available, and remains relevant for point-to-point or static-key configurations.
OpenVPN Articles
For additional documentation, see the articles page.
OpenVPN Quickstart
While this HOWTO will guide you in setting up a scalable client/server VPN using an X509 PKI (public key infrastruction using certificates and private keys), this might be overkill if you are only looking for a simple VPN setup with a server that can handle a single client.
If you would like to get a VPN running quickly with mimimal configuration, you might check out the Static Key Mini-HOWTO.
Static Key advantages
Simple Setup
No X509 PKI (Public Key Infrastructure) to maintain
Static Key disadvantages
Limited scalability -- one client, one server
Lack of perfect forward secrecy -- key compromise results in total disclosure of
previous sessions
Secret key must exist in plaintext form on each VPN peer
Secret key must be exchanged using a pre-existing secure channel
Installing OpenVPN
OpenVPN can be downloaded here.
For security, it's a good idea to check the file release signature after downloading.
The OpenVPN executable should be installed on both server and client machines, since the single executable provides both client and server functions.
Linux Notes (using RPM package)
If you are using a Linux distribution which supports RPM packages (SuSE, Fedora, Redhat, etc.), it's best to install using this mechanism. The easiest method is to find an existing binary RPM file for your distribution. You can also build your own binary RPM file:
rpmbuild -tb open-[version].tar.gz
Once you have the .rpm file, you can install it with the usual
rpm -ivh open-[details].rpm
or upgrade an existing installation with
rpm -Uvh open-[details].rpm
Installing OpenVPN from a binary RPM package has these dependencies:
openssl lzo pam
Furthermore, if you are building your own binary RPM package, there are several additional dependencies:
openssl-devel lzo-devel pam-devel
See the open.spec file for additional notes on building an RPM package for Red Hat Linux 9 or building with reduced dependencies.
Linux Notes (without RPM)
If you are using Debian, Gentoo, or a non-RPM-based Linux distribution, use your distro-specific packaging mechanism such as apt-get on Debian or emerge on Gentoo. It is also possible to install OpenVPN on Linux using the universal ./configure method. First expand the .tar.gz file:
tar xfz open-[version].tar.gz
Then cd to the top-level directory and type:
./configure make
make install
Windows Notes
OpenVPN for Windows can be installed from the self-installing exe file on the OpenVPN download page. Remember that OpenVPN will only run on Windows 2000 or later. Also note that OpenVPN must be installed and run by a user who has administrative privileges (this restriction is imposed by Windows, not OpenVPN). The restriction can be sidestepped by running OpenVPN in the background as a service, in which case even non-admin users will be able to access the VPN, once it is installed. More discussion on OpenVPN + Windows privilege issues.
OpenVPN can also be installed as a GUI on Windows, using Mathias Sundman's installation package, which will install both OpenVPN and the Windows GUI.
After you run the Windows installer, OpenVPN is ready to use and will associate itself with files having the .oextension. To run OpenVPN, you can:
Right click on an OpenVPN configuration file (.o) and select Start OpenVPN on
this configuration file. Once running, you can use the F4 key to exit.
Run OpenVPN from a command prompt Window with a command such as:
open myconfig.o
Once running in a command prompt window, OpenVPN can be stopped by the F4 key.
Run OpenVPN as a service by putting one or more .o configuration files
in \\Program Files\\OpenVPN\\configand starting the OpenVPN Service, which can be controlled from Start Menu -> Control Panel -> Administrative Tools -> Services.
A GUI is also available for the Windows version of OpenVPN. Additional Windows install notes.
Mac OS X Notes
Angelo Laub and Dirk Theisen have developed an OpenVPN GUI for OS X. See also OpenVPN Client and Mac OS X 10.3.
Other OSes
Some notes are available in the INSTALL file for specific OSes. In general, the
./configure make
make install
method can be used, or you can search for an OpenVPN port or package which is specific to your OS/distribution.
Determining whether to use a routed or bridged VPN
See FAQ for an overview of Routing vs. Ethernet Bridging. See also the OpenVPN Ethernet Bridging page for more notes and details on 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):
10.0.0.0 172.16.0.0 192.168.0.0 10.255.255.255 172.31.255.255 192.168.255.255 (10/8 prefix) (172.16/12 prefix) (192.168/16 prefix) While addresses from these netblocks should normally be used in VPN configurations, it's important to select addresses that minimize the probability of IP address or subnet conflicts. The types of conflicts that need to be avoided are:
conflicts from different sites on the VPN using the same LAN subnet numbering, or remote access connections from sites which are using private subnets which conflict
with your VPN subnets.
For example, suppose you use the popular 192.168.0.0/24 subnet as your private LAN subnet. Now you are trying to connect to the VPN from an internet cafe which is using the same subnet for its WiFi LAN. You will have a routing conflict because your machine won't know if 192.168.0.1 refers to the local WiFi gateway or to the same address on the VPN. As another example, suppose you want to link together multiple sites by VPN, but each site is using 192.168.0.0/24 as its LAN subnet. This won't work without adding a complexifying layer of NAT translation, because the VPN won't know how to route packets between multiple sites if those sites don't use a subnet which uniquely identifies them.
The best solution is to avoid using 10.0.0.0/24 or 192.168.0.0/24 as private LAN network addresses. Instead, use something that has a lower probability of being used in a WiFi cafe, airport, or hotel where you might expect to connect from remotely. The best candidates are subnets in the middle of the vast 10.0.0.0/8 netblock (for example 10.66.77.0/24). And to avoid cross-site IP numbering conflicts, always use unique numbering for your LAN subnets.
Setting up your own Certificate Authority (CA) and generating certificates and keys for an OpenVPN server and multiple clients
Overview
The first step in building an OpenVPN 2.0 configuration is to establish a PKI (public key infrastructure). The PKI consists of:
a separate certificate (also known as a public key) and private key for the server and
each client, and
a master Certificate Authority (CA) certificate and key which is used to sign each of
the server and client certificates.
OpenVPN supports bidirectional authentication based on certificates, meaning that the client must authenticate the server certificate and the server must authenticate the client certificate before mutual trust is established.
Both server and client will authenticate the other by first verifying that the presented certificate was signed by the master certificate authority (CA), and then by testing information in the now-authenticated certificate header, such as the certificate common name or certificate type (client or server).
This security model has a number of desirable features from the VPN perspective:
The server only needs its own certificate/key -- it doesn't need to know the individual
certificates of every client which might possibly connect to it.
The server will only accept clients whose certificates were signed by the master CA
certificate (which we will generate below). And because the server can perform this signature verification without needing access to the CA private key itself, it is possible for the CA key (the most sensitive key in the entire PKI) to reside on a completely different machine, even one without a network connection.
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.
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.
For PKI management, we will use a set of scripts bundled with OpenVPN. If you are using Linux, BSD, or a unix-like OS, open a shell and cd to
the easy-rsa subdirectory of the OpenVPN distribution. If you installed OpenVPN from an RPM file, the easy-rsa directory can usually be found
in/usr/share/doc/packages/open or /usr/share/doc/open-2.0 (it's best to copy this directory to another location such as /etc/open, before any edits, so that future OpenVPN package upgrades won't overwrite your modifications). If you installed from a .tar.gz file, the easy-rsa directory will be in the top level directory of the expanded source tree.
If you are using Windows, 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
clean-all build-ca
The final command (build-ca) will build the certificate authority (CA) certificate and key by invoking the interactiveopenssl command:
ai:easy-rsa # ./build-ca
Generating a 1024 bit RSA private key ............++++++ ...........++++++
writing new private key to 'ca.key' -----
You are about to be asked to enter information that will be incorporated into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank. -----
Country Name (2 letter code) [KG]:
State or Province Name (full name) [NA]: Locality Name (eg, city) [BISHKEK]:
Organization Name (eg, company) [OpenVPN-TEST]: Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:OpenVPN-CA Email Address [me@myhost.mydomain]:
Note that in the above sequence, most queried parameters were defaulted to the values set in the vars or vars.bat files. The only parameter which must be explicitly entered is theCommon Name. In the example above, I used \"OpenVPN-CA\".
Generate certificate & key for server
Next, we will generate a certificate and private key for the server. On Linux/BSD/Unix:
./build-key-server server
On Windows:
build-key-server server
As in the previous step, 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:
./build-key client1 ./build-key client2 ./build-key client3
On Windows:
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\client.
Generate Diffie Hellman parameters
Diffie Hellman parameters must be generated for the OpenVPN server. On Linux/BSD/Unix:
./build-dh
On Windows:
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:
Filename ca.crt ca.key dh{n}.pem server.crt server.key client1.crt client1.key client2.crt Needed By server + all clients key signing machine only server only server only server only client1 only client1 only client2 only Purpose Root CA certificate Root CA key Diffie Hellman parameters Server Certificate Server Key Client1 Certificate Client1 Key Client2 Certificate Secret NO YES NO NO YES NO YES NO client2.key client3.crt client3.key client2 only client3 only client3 only Client2 Key Client3 Certificate Client3 Key YES NO YES The final step in the key generation process is to copy all files to the machines which need them, taking care to copy secret files over a secure channel.
Now wait, you may say. Shouldn't it be possible to set up the PKI without a pre-existing secure channel?
The answer is ostensibly yes. In the example above, for the sake of brevity, we generated all private keys in the same place. With a bit more effort, we could have done this differently. For example, instead of generating the client certificate and keys on the server, we could have had the client generate its own private key locally, and then submit a Certificate Signing Request (CSR) to the key-signing machine. In turn, the key-signing machine could have processed the CSR and returned a signed certificate to the client. This could have been done without ever requiring that a secret .key file leave the hard drive of the machine on which it was generated.
Creating configuration files for server and clients
Getting the sample config files
It's best to use the OpenVPN sample configuration files as a starting point for your own configuration. These files can also be found in
the sample-config-files directory of the OpenVPN source distribution the sample-config-files directory
in /usr/share/doc/packages/open or/usr/share/doc/open-2.0 if you installed from an RPM package
Start Menu -> All Programs -> OpenVPN -> OpenVPN Sample Configuration Files on Windows
Note that on Linux, BSD, or unix-like OSes, the sample configuration files are
namedserver.conf and client.conf. On Windows they are named server.o and client.o.
Editing the server configuration file
The sample server configuration file is an ideal starting point for an OpenVPN server configuration. It will create a VPN using a virtual TUN network interface (for routing), will listen for client connections on UDP port 1194 (OpenVPN's official port number), and distribute virtual addresses to connecting clients from the 10.8.0.0/24 subnet. Before you use the sample configuration file, you should first edit the ca, cert, key, and dhparameters to point to the files you generated in the PKI section above. At this point, the server configuration file is usable, however you still might want to customize it further:
If you are using Ethernet bridging, you must use server-bridge and dev tap instead ofserver and dev tun.
If you want your OpenVPN server to listen on a TCP port instead of a UDP port, useproto tcp instead of proto udp (If you want OpenVPN to listen on both a UDP and TCP port, you must run two separate OpenVPN instances).
If you want to use a virtual IP address range other than 10.8.0.0/24, you should modify the server directive. Remember that this virtual IP address range should be a private range which is currently unused on your network.
Uncomment out the client-to-client directive if you would like connecting clients to be able to reach each other over the VPN. By default, clients will only be able to reach the server.
If you are using Linux, BSD, or a Unix-like OS, you can improve security by uncommenting out 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, it is possible if you:
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-Win32 adapter. You can add additional adapters by going to Start Menu -> All Programs -> OpenVPN -> Add a new TAP-Win32 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.o 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. The major thing to check for is that the dev (tun or tap) andproto (udp or tcp) directives are consistent. Also make sure
that comp-lzo andfragment, if used, are present in both client and server config files.
Starting up the VPN and testing for initial connectivity
Starting the server
First, make sure 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 .o file on Windows), rather than start it as a daemon or service:
open [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.o file), rather than start it as a daemon or service:
open [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:
You get the 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:
o Make sure the client is using the correct hostname/IP address and port
number which will allow it to reach the OpenVPN server.
o If the OpenVPN server machine is a single-NIC box inside a protected LAN,
make sure you are using a correct port forward rule on the server's gateway firewall. For example, suppose 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.
o 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).
You get the 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 make sure that you are using a personal firewall which is known to work correctly on XP SP2.
You get the 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. For example on Windows XP SP2, you can do this by going to Windows Security Center -> Windows Firewall -> Advanced and unchecking the box which corresponds to the TAP-Win32 adapter (disabling the client firewall from filtering the TUN/TAP adapter is generally reasonable from a security perspective, as you are essentially telling the firewall not to block
authenticated VPN traffic). Also make sure that the TUN/TAP interface on the server is not being filtered by a firewall (having said that, note that selective firewalling of the TUN/TAP interface on the server side can confer certain security benefits. See the access policies section below).
The connection stalls on startup when using a proto udp configuration, the server log file shows this line:
TLS: Initial packet from x.x.x.x:x, sid=xxxxxxxx xxxxxxxx
however 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. The firewall can either be (a) a personal software firewall running on the client, or (b) the NAT router gateway for the client. Modify the firewall to allow returning UDP packets from the server to reach the client.
See the FAQ 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
If you install OpenVPN via an RPM package on Linux, the installer will set up an initscript. When executed, the initscript will scan for .conf configuration files in /etc/open, and
if found, will start up a separate OpenVPN daemon for each file.
Windows
The Windows installer will set up a Service Wrapper, but leave it turned off by default. To activate it, go to Control Panel / Administrative Tools / Services, select the OpenVPN service, right-click on properties, and set the Startup Type to Automatic. This will configure the service for automatic start on the next reboot.
When started, the OpenVPN Service Wrapper will scan the \\Program
Files\\OpenVPN\\configfolder for .o configuration files, starting a separate OpenVPN process on each file.
Controlling a running OpenVPN process
Running on Linux/BSD/Unix
OpenVPN accepts several signals:
SIGUSR1 -- Conditional restart, designed to restart without root privileges SIGHUP -- Hard restart
SIGUSR2 -- Output connection statistics to log file or syslog SIGTERM, SIGINT -- Exit
Use the writepid directive to write the OpenVPN daemon's PID to a file, so that you know where to send the signal (if you are starting open with an initscript, the script may already be passing a --writepid directive on the open command line).
Running on Windows as a GUI
See the OpenVPN GUI page.
Running in a Windows command prompt window
On Windows, you can start OpenVPN by right clicking on an OpenVPN configuration file (.ofile) and selecting \"Start OpenVPN on this config file\".
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 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 theRevoking 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 open-status.log
which will output a list of current client connections to the file open-status.log once per minute.
Using the management interface
The OpenVPN management interface allows a great deal of control over a running OpenVPN process. You can use the management interface directly, by telneting to the management interface port, or indirectly by using an OpenVPN GUI which itself connects to the management interface.
To enable the management interface on either an OpenVPN server or client, add this to the configuration file:
management localhost 7505
This tells OpenVPN to listen on TCP port 7505 for management interface clients (port 7505 is an arbitrary choice -- you can use any free port).
Once OpenVPN is running, you can connect to the management interface using a telnet client. For example:
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 realtime log display + show last N lines or 'all' for entire history. 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 of10.66.0.0/24 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:
push \"route 10.66.0.0 255.255.255.0\"
Next, you must set up a route on the server-side LAN gateway to route the VPN client subnet (10.8.0.0/24) to the OpenVPN server (this is only necessary if the OpenVPN server and the LAN gateway are different machines).
Make sure that you've enabled IP and TUN/TAP forwarding on the OpenVPN server machine.
Including multiple machines on the server side when using a bridged VPN (dev tap)
One of the benefits of using ethernet bridging is that you get this for free without needing any additional configuration.
Including multiple machines on the client side when using a routed VPN (dev tun)
In a typical road-warrior or remote access scenario, the client machine connects to the VPN as a single machine. But suppose the client machine is a gateway for a local LAN (such as a home office), and you would like each machine on the client LAN to be able to route through the VPN.
For this example, we will assume that the client LAN is using the 192.168.4.0/24 subnet, and that the VPN client is using a certificate with a common name of client2. Our goal is to set up the VPN so that any machine on the client LAN can communicate with any machine on the server LAN through the VPN.
Before setup, there are some basic prerequisites which must be followed:
The client LAN subnet (192.168.4.0/24 in our example) must not be exported to the VPN by the server or any other client sites which are using the same subnet. Every subnet which is joined to the VPN via routing must be unique.
The client must have a unique Common Name in its certificate (\"client2\" in our example), and the duplicate-cn flag must not be used in the OpenVPN server configuration file.
First, make sure that IP and TUN/TAP forwarding is enabled on the client machine. Next, we will deal with the necessary configuration changes on the server side. If the server configuration file does not currently reference a client configuration directory, add one now:
client-config-dir ccd
In the above directive, ccd should be the name of a directory which has been pre-created in the default directory where the OpenVPN server daemon runs. On Linux this tends to be/etc/open and on Windows it is usually \\Program Files\\OpenVPN\\config. When a new client connects to the OpenVPN server, the daemon will check this directory for a file which matches the common name of the connecting client. If a matching file is found, it will be read and processed for additional configuration file directives to be applied to the named client.
The next step is to create a file called client2 in the ccd directory. This file should contain the line:
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 routecontrols the routing from the kernel to the OpenVPN server (via the TUN interface) while iroutecontrols 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 (some caveats to be aware of). 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 the man page or open-users mailing list archive for
non-Windows foreign_option_n documentation and script examples.
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-Win32 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:
Class Virtual IP Range Allowed LAN Access Common Names Employees 10.8.0.0/24 Samba/email server at 10.66.4.4 Entire 10.66.4.0/24 subnet Contractor server at 10.66.4.12 [variable] System Administrators 10.8.1.0/24 sysadmin1 Contractors 10.8.2.0/24 contractor1, contracter2 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 10.8.0.0 255.255.255.0
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 10.8.1.1 10.8.1.2
ccd/contractor1
ifconfig-push 10.8.2.1 10.8.2.2
ccd/contractor2
ifconfig-push 10.8.2.5 10.8.2.6
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-Win32 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] [ , 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] [1,190] [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
Using alternative authentication methods
OpenVPN 2.0 includes 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
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 open-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 open-auth-pam plugin should be already built. To use it, add this to the server-side config file:
plugin /usr/share/open/plugin/lib/open-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 open-auth-pam plugin, because it has several advantages over the auth-pam.pl script:
The shared object open-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 open-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 keydirectives 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
Also see Article: The OpenVPN Smartcard HOWTO
About dual-factor authentication What is PKCS#11?
Finding PKCS#11 provider library. How to configure a cryptographic token
How to modify an OpenVPN configuration to make use of cryptographic tokens
o Determine the correct object. o Using OpenVPN with PKCS#11.
o PKCS#11 implementation considerations. o OpenSC PKCS#11 provider.
Difference between PKCS#11 and Microsoft Cryptographic API (CryptoAPI).
About dual-factor authentication
Dual-factor authentication is a method of authentication that combines two elements: something you have and something you know.
Something you have should be a device that cannot be duplicated; such a device can be a cryptographic token that contains a private secret key. This private key is generated inside the device and never leaves it. If a user possessing this token attempts to access protected services on a remote network, the authorization process which grants or denies network access can establish, with a high degree of certainty, that the user seeking access is in physical possession of a known, certified token.
Something you know can be a password presented to the cryptographic device. Without presenting the proper password you cannot access the private secret key. Another feature of cryptographic devices is to prohibit the use of the private secret key if the wrong password had been presented more than an allowed number of times. This behavior ensures that if a user lost his device, it would be infeasible for another person to use it.
Cryptographic devices are commonly called \"smart cards\" or \"tokens\conjunction with a PKI (Public Key Infrastructure). The VPN server can examine a X.509 certificate and verify that the user holds the corresponding private secret key. Since the device cannot be duplicated and requires a valid password, the server is able to authenticate the user with a high degree of confidence.
Dual-factor authentication is much stronger than password-based authentication, because in the worst-case scenario, only one person at a time can use the cryptographic token. Passwords can be guessed and can be exposed to other users, so in the worst-case scenario an infinite number of people could attempt to gain unauthorized access when resources are protected using password-only authentication.
If you store the secret private key in a file, the key is usually encrypted by a password. The problem with this approach is that the encrypted key is exposed to decryption attacks or spyware/malware running on the client machine. Unlike when using a cryptographic device, the file cannot erase itself automatically after several failed decryption attempts.
What is PKCS#11?
This standard specifies an API, called Cryptoki, to devices which hold cryptographic information and perform cryptographic functions. Cryptoki, pronounced \"crypto-key\" and short for cryptographic token interface, follows a simple object-based approach, addressing the goals of technology independence (any kind of device) and resource sharing (multiple applications accessing multiple devices), presenting to applications a common, logical view of the device called a cryptographic token.
Source: RSA Security Inc. http://www.rsasecurity.com/rsalabs/pkcs/pkcs-11.
To summarize, PKCS#11 is a standard that can be used by application software to access cryptographic tokens such as smart cards and other devices. Most device vendors provide a library that implements the PKCS#11 provider interface -- this library can be used by applications in order to access these devices. PKCS#11 is a cross-platform, vendor-independent free standard.
Finding PKCS#11 provider library
The first thing you need to do is to find the provider library, it should be installed with the device drivers. Each vendor has its own library. For example, the OpenSC PKCS#11 provider is located at /usr/lib/pkcs11/opensc-pkcs11.so on Unix or at opensc-pkcs11.dll on Windows.
How to configure cryptographic token
You should follow an enrollment procedure:
Initialize the PKCS#11 token.
Generate RSA key pair on the PKCS#11 token.
Create a certificate request based on the key pair, you can use OpenSC and OpenSSL in order to do that.
Submit the certificate request to a certificate authority, and receive a certificate. Load the certificate onto the token, while noting that the id and label attributes of the certificate must match those of the private key.
A configured token is a token that has 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 OpenVPN 2.1 series. Follow the instructions specified in the README file, and then use the pkitool in order to enroll. Initialize a token using the following command:
$ ./pkitool --pkcs11-slots /usr/lib/pkcs11/ $ ./pkitool --pkcs11-init /usr/lib/pkcs11/
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