VPN- Tech and Technology
|Open VPN - How To|
|OpenVPN secure network SSL VPN OSI layer 2 or 3 extension|
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: -|
|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.|
|Additional Security Notes . -|
The impatient may wish to jump straight to the sample configuration files: -
|Server configuration file.|
|Client configuration file.|
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|
|Additional Articles and Documentation|
The original OpenVPN 1.x HOWTO is still available, and remains relevant for point-to-point or static-key configurations.
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 ||Static Key disadvantages |
OpenVPN can be downloaded from the official web site.
|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 openvpn-[version].tar.gz
Once you have the .rpm file, you can install it with the usual -
rpm -ivh openvpn-[details].rpm
or upgrade an existing installation with -
rpm -Uvh openvpn-[details].rpm
Installing OpenVPN from a binary RPM package has these dependencies: -
Furthermore, if you are building your own binary RPM package, there are several additional dependencies: -
See the openvpn.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 openvpn-[version].tar.gz
Then cd to the top-level directory and type: -
./configure make make install
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 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 .ovpn extension. To run OpenVPN, you can: - |
Right click on an OpenVPN configuration file (.ovpn) 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: -
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 .ovpn configuration files in \Program Files\OpenVPN\config and 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.
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: - ||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 = 10.255.255.255 = (10/8 prefix)
172.16.0.0 = 172.31.255.255 = (172.16/12 prefix)
192.168.0.0 = 192.168.255.255 = (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: -
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
The first step in building an OpenVPN 2.0 configuration is to establish a PKI (public key infrastructure). The PKI consists of: -
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: - ||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/openvpn or /usr/share/doc/openvpn-2.0 (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 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): - |
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 interactive openssl 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 [email@example.com]:
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 the Common 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: -
On Windows: -
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", "client2", or "client3". Always use a unique common name for each client.
||Generate Diffie Hellman parameters
|| RSA Security. Diffie Hellman parameters must be generated for the OpenVPN server. On Linux/BSD/Unix: - |
On Windows: -
ai:easy-rsa # ./build-dh Generating DH parameters, 1024 bit long safe prime, generator 2 This is going to take a long time ..+. .+.+..+ ..
Now we will find our newly-generated keys and certificates in the keys subdirectory. Here is an explanation of the relevant files: -
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 - |
Note that on Linux, BSD, or unix-like OSes, the sample configuration files are named server.conf and client.conf . On Windows they are named server.ovpn and client.ovpn.
|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 dh parameters 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 want to run multiple OpenVPN instances on the same machine, each using a different configuration file, it is possible if you: -
|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. The major thing to check for is 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, make sure the OpenVPN server will be accessible from the internet. That means: -
|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):
OpenVPN 2.0_rc12 i686-suse-linux [SSL] [LZO] [EPOLL] built 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: - |
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. -
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 : - ||- |
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.
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/openvpn , and if found, will start up a separate OpenVPN daemon for each file.
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\config folder for .ovpn configuration files, starting a separate OpenVPN process on each file.
||Controlling a running OpenVPN process
||Running on Linux/BSD/Unix
OpenVPN accepts several signals: -
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 openvpn with an initscript , the script may already be passing a --writepid directive on the openvpn 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 ( .ovpn file) and selecting "Start OpenVPN on this config file".
|Once running in this fashion, several keyboard commands are available: - ||Running as a Windows Service
When OpenVPN is started as a service on Windows, the only way to control it is: -
|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 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).
The default server.conf file has a line -
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 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 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: - |
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 and
|| 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: - |
First, make sure that
|| 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: - |
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/openvpn 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 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): -
|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
|openvpn-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: -
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: -
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: - |