OpenVPN 2 Cookbook

4.3 (3 reviews total)
By Jan Just Keijser
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  1. Point-to-Point Networks

About this book

OpenVPN http://www.openvpn.net is a free and open source virtual private network (VPN) program for creating point-to-point or server-to-multiclient encrypted tunnels between host computers. It is capable of establishing direct links between computers across networks and firewalls. It is powerful software, but getting the most from it can be a daunting task.

OpenVPN 2 Cookbook provides solutions to common OpenVPN problems. The book covers everything a system administrator needs to manage and run an OpenVPN network, from point to point networks to troubleshooting.

OpenVPN 2 Cookbook offers all the information you need to successfully manage your network. Covering all the common networks, including point to point networks, multi-client tun style networks and multi client tap style networks, this practical guide gives quick answers to common questions and problems.

Each technical aspect is broken down into short recipes that demonstrate solutions with working code, then explain why and how that works. The book is intended to be a desk reference for users with a whole range of experience levels.

Publication date:
February 2011
Publisher
Packt
Pages
356
ISBN
9781849510103

 

Chapter 1. Point-to-Point Networks

In this chapter, we will cover:

  • Shortest setup possible

  • OpenVPN secret keys

  • Multiple secret keys

  • Plaintext tunnel

  • Routing

  • Configuration files versus the command-line

  • IP-less configurations

  • Complete site-to-site setup

  • 3-way routing

 

Introduction


The recipes in this chapter will provide an introduction into configuring OpenVPN. The recipes are based on a point-to-point style network, meaning that only a single client can connect at a time.

A point-to-point style network is very useful when connecting to a small number of sites or clients. It is easier to set up, as no certificates or Public Key Infrastructure (PKI) is required. Also, routing is slightly easier to configure, as no client-specific configuration files containing --iroute statements are required.

The drawbacks of a point-to-point style network are:

  • The lack of perfect forward secrecy— a key compromise may result in a total disclosure of previous sessions

  • The secret key must exist in plaintext form on each VPN peer

 

Shortest setup possible


This recipe will explain the shortest setup possible when using OpenVPN. For this setup two computers are used that are connected over a network (LAN or Internet). We will use both a TUN-style network and a TAP-style network and will focus on the differences between them. A TUN device is used mostly for VPN tunnels where only IP-traffic is used. A TAP device allows full Ethernet frames to be passed over the OpenVPN tunnel, hence providing support for non-IP based protocols such as IPX and AppleTalk.

While this may seem useless at first glance, it can be very useful to quickly test whether OpenVPN can connect to a remote system.

Getting ready

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Windows XP SP3 and OpenVPN 2.1.1.

How to do it...

  1. We launch the server (listening)-side OpenVPN process for the TUN-style network:

    [[email protected]]# openvpn --ifconfig 10.200.0.1 10.200.0.2 \
    --dev tun
    

    Tip

    The above command should be entered as a single line. The character '\' is used to denote the fact that the command continues on the next line.

  2. Then we launch the client-side OpenVPN process:

        [WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
        --ifconfig 10.200.0.2 10.200.0.1 --dev tun \
                --remote openvpnserver.example.com
    

    The following screenshot shows how a connection is established:

    As soon as the connection is established, we can ping the other end of the tunnel.

  3. Next, we stop the tunnel by pressing the F4 function key in the Command window and we restart both ends of the tunnel using the TAP device:

  4. We launch the server (listening)-side OpenVPN process for the TAP-style network:

        [[email protected]]# openvpn --ifconfig 10.200.0.1 255.255.255.0 \
                --dev tap 
    
  5. Then we launch the client-side OpenVPN process:

        [WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
                --ifconfig 10.200.0.2 255.255.255.0 --dev tap \
                --remote openvpnserver.example.com
    

The connection is established and we can again ping the other end of the tunnel.

How it works...

The server listens on UDP port 1194, which is the OpenVPN default port for incoming connections. The client connects to the server on this port. After the initial handshake, the server configures the first available TUN device with IP address 10.200.0.1 and it expects the remote end (Peer address) to be 10.200.0.2.

The client does the opposite: after the initial handshake, the first TUN or TAP-Win32 device is configured with IP address 10.200.0.2. It expects the remote end (Peer address) to be 10.200.0.1. After this, the VPN is established.

In case of a TAP-style network, the server configures the first available TAP device with the IP address 10.200.0.01 and netmask 255.255.255.0. Similarly, the client is configured with IP address 10.200.0.2 and netmask 255.255.255.0.

Note

Notice the warning:

******* WARNING *******: all encryption and authentication features disabled -- all data will be tunnelled as cleartext

Here, the data is not secure: all the data that is sent over the VPN tunnel can be read!

There's more...

Using the TCP protocol

In the previous example, we chose the UDP protocol. For this example, it would not have made any difference if we had chosen the TCP protocol, provided that we do that on the server side (the side without --remote):

[[email protected]]# openvpn --ifconfig 10.200.0.1 10.200.0.2 \
    –-dev tun --proto tcp-server

And also on the client side:

[[email protected]]# openvpn --ifconfig 10.200.0.2 10.200.0.1 \
    --dev tun --proto tcp-client

Forwarding non-IP traffic over the tunnel

It is now possible to run non-IP traffic over the tunnel. For example, if AppleTalk is configured correctly on both sides, we can query a remote host using the aecho command:

aecho openvpnserver
22 bytes from 65280.1: aep_seq=0. time=26. ms
22 bytes from 65280.1: aep_seq=1. time=26. ms
22 bytes from 65280.1: aep_seq=2. time=27. ms

A tcpdump -nnel -i tap0 shows that the type of traffic is indeed non-IP based AppleTalk.

 

OpenVPN secret keys


This recipe uses OpenVPN secret keys to secure the VPN tunnel. It is very similar to the previous recipe but this time a shared secret key is used to encrypt the traffic between the client and the server.

Getting ready

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Windows XP SP3 and OpenVPN 2.1.1.

How to do it...

  1. First, we generate a secret key on the server (listener):

        [[email protected]]# openvpn --genkey --secret secret.key
    
  2. We transfer this key to the client side over a secure channel (for example, using scp).

  3. Next, we launch the server (listening)-side OpenVPN process:

       [[email protected]]# openvpn --ifconfig 10.200.0.1 10.200.0.2 \
                --dev tun --secret secret.key
    
  4. Then, we launch the client-side OpenVPN process:

      [WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
                --ifconfig 10.200.0.2 10.200.0.1 \
                --dev tun --secret secret.key \
                --remote openvpnserver.example.com
    

    The connection is established:

How it works...

This example works exactly as the very first: the server listens to the incoming connections on UDP port 1194. The client connects to the server on this port. After the initial handshake, the server configures the first available TUN device with IP address 10.200.0.1 and it expects the remote end (Peer address) to be 10.200.0.2. The client does the opposite.

There's more...

By default, OpenVPN uses two symmetric keys when setting up a point-to-point connection:

  • A Cipher key to encrypt the contents of the packets being exchanged.

  • An HMAC key to sign packets. When packets arrive that are not signed using the appropriate HMAC key they are dropped immediately. This is the first line of defense against a "Denial of Service" attack.

The same set of keys are used on both ends and both keys are derived from the file specified using the --secret parameter.

An OpenVPN secret key file is formatted as follows:

#
# 2048 bit OpenVPN static key
#
-----BEGIN OpenVPN Static key V1-----
<16 lines of random bytes>
-----END OpenVPN Static key V1-----

From the random bytes, the OpenVPN Cipher and HMAC keys are derived. Note that these keys are the same for each session!

See also

The next recipe, Multiple secret keys, will explain in detail about the secret keys.

 

Multiple secret keys


As stated in the previous recipe, OpenVPN uses two symmetric keys when setting up a point-to-point connection. However, it is also possible to use shared, yet asymmetric keys in point-to-point mode. OpenVPN will use four keys in this case:

  • A Cipher key on the client side

  • An HMAC key on the client side

  • A Cipher key on the server side

  • An HMAC key on the server side

The same keying material is shared by both sides of the point-to-point connection but those keys that are derived for encrypting and signing the data are different for each side. This recipe explains how to set up OpenVPN in this manner and how the keys can be made visible.

Getting ready

For this recipe, we use the secret.key file from the previous recipe. Install OpenVPN 2.0 or higher on two computers. Make sure that the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1. The client was running Windows XP SP3 and OpenVPN 2.1.1.

How to do it...

  1. We launch the server (listening) side OpenVPN process with an extra option to the --secret parameter and with more verbose logging:

        [[email protected]]# openvpn \
                --ifconfig 10.200.0.1 10.200.0.2 \
                --dev tun --secret secret.key 0 \
                --verb 7
    
  2. Then we launch the client-side OpenVPN process:

      [WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
                --ifconfig 10.200.0.2 10.200.0.1 \
                --dev tun --secret secret.key 1\
                --remote openvpnserver \
                --verb 7
    

    The connection will be established with a lot of debugging messages.

  3. If we look through the server-side messages (searching for crypt), we can find the negotiated keys on the server side. Note that the output has been reformatted for clarity:

    … Static Encrypt:
    Cipher 'BF-CBC' initialized with 128 bit key
    … Static Encrypt: 
    CIPHER KEY: 80797ddc 547fbdef 79eb353f 2a1f3d1f
    … Static Encrypt:
    Using 160 bit message hash 'SHA1' for HMAC authentication
    … Static Encrypt: 
    HMAC KEY: c752f254 cc4ac230 83bd8daf 6141e73d 844764d8
    … Static Decrypt: 
    Cipher 'BF-CBC' initialized with 128 bit key
    … Static Decrypt: 
    CIPHER KEY: 8cf9abdd 371392b1 14b51523 25302c99
    … Static Decrypt: 
    Using 160 bit message hash 'SHA1' for HMAC authentication
    … Static Decrypt: 
    HMAC KEY: 39e06d8e 20c0d3c6 0f63b3e7 d94f35af bd744b27
    

On the client side, we will find the same keys but the 'Encrypt' and 'Decrypt' keys have been reversed:

… Static Encrypt: 
Cipher 'BF-CBC' initialized with 128 bit key
… Static Encrypt: 
CIPHER KEY: 8cf9abdd 371392b1 14b51523 25302c99
… Static Encrypt: 
Using 160 bit message hash 'SHA1' for HMAC authentication
… Static Encrypt: 
HMAC KEY: 39e06d8e 20c0d3c6 0f63b3e7 d94f35af bd744b27
… Static Decrypt: 
Cipher 'BF-CBC' initialized with 128 bit key
… Static Decrypt: 
CIPHER KEY: 80797ddc 547fbdef 79eb353f 2a1f3d1f
… Static Decrypt: 
Using 160 bit message hash 'SHA1' for HMAC authentication
… Static Decrypt: 
HMAC KEY: c752f254 cc4ac230 83bd8daf 6141e73d 844764d8

If you look at the keys carefully, you can see that each one of them is mirrored on the client and the server side.

How it works...

OpenVPN derives all keys from the static.key file, provided that there is enough entropy (randomness) in the file to reliably generate four keys. All keys generated using the following will have enough entropy:

$ openvpn –-genkey –-secret secret.key

An OpenVPN static key file is 2048 bits in size. The Cipher keys are each 128 bits, whereas the HMAC keys are 160 bits each, for a total of 776 bits. This allows OpenVPN to easily generate four random keys from the static key file, even if a cipher is chosen that requires a larger initialization key.

There's more...

The same secret key files are used in a client/server setup when the following parameter is used: tls-auth ta.key.

See also

  • Chapter 2's recipe, Setting up the public and private keys, in which the tls-auth key is generated in a very similar manner.

 

Plaintext tunnel


In the very first recipe, we created a tunnel in which the data traffic was not encrypted. To create a completely plain text tunnel, we also disable the HMAC authentication. This can be useful when debugging a bad connection, as all traffic over the tunnel can now easily be monitored. In this recipe, we will look at how to do this. This type of tunnel is also useful when doing performance measurements, as it is the least CPU-intensive tunnel that can be established.

Getting ready

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Fedora 13 Linux and OpenVPN 2.1.1.

As we are not using any encryption, no secret keys are needed.

How to do it...

  1. Launch the server (listening)-side OpenVPN process:

       [[email protected]]# openvpn \
                --ifconfig 10.200.0.1 10.200.0.2 \
                --dev tun -–auth none
    
  2. Then launch the client-side OpenVPN process:

       [[email protected]]# openvpn \
                --ifconfig 10.200.0.2 10.200.0.1 \
                --dev tun –-auth none\
                --remote openvpnserver.example.com
    
  3. The connection is established with two warning messages in the output:

    … ******* WARNING *******: null cipher specified, no encryption will be used

    … ******* WARNING *******: null MAC specified, no authentication will be used

How it works...

With this setup, absolutely no encryption is performed. All the traffic that is sent over the tunnel is encapsulated in an OpenVPN packet and then sent "as-is".

There's more...

To actually view the traffic, we can use tcpdump:

  • Set up the connection as outlined.

  • Start tcpdump and listen on the network interface, not the tunnel interface itself:

    [root]@client]# tcpdump -w -I eth0 -s 0 host openvpnserver \
               | strings
    
  • Now, send some text across the tunnel, using something like nc (Netcat). First, launch nc on the server side:

     [server]$ nc -l 31000
    
  • On the client side, launch nc in client mode and type the words hello and goodbye.

     [client]$ nc 10.200.0.1 3100
             hello
             goodbye
    
  • In the tcpdump window, you should now see:

 

Routing


Point-to-point style networks are great if you want to connect two networks together over a static, encrypted tunnel. If you only have a small number of endpoints (fewer than four), then it is far easier than using a client/server setup as described in Chapter 2, Client-server IP-only Networks.

Getting ready

For this recipe, we use the following network layout:

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Fedora 13 Linux and OpenVPN 2.1.1. We'll use the secret.key file from the OpenVPN Secret keys recipe here.

How to do it...

  1. First we establish the connection, but we also make sure OpenVPN daemonized itself:

        [[email protected]]# openvpn \
                --ifconfig 10.200.0.1 10.200.0.2 \
                --dev tun --secret secret.key \
                --daemon --log /tmp/openvpnserver.log
    
  2. Then we launch the client-side OpenVPN process:

        [client]$ openvpn \
                --ifconfig 10.200.0.2 10.200.0.1 \
                --dev tun --secret secret.key \
                --remote openvpnserver \
                --daemon --log /tmp/openvpnclient.log
    

    The connection is established:

     [server]$ tail -1 /tmp/openvpnserver.log
               Initialization Sequence Completed
    

Now we add routing:

  1. On the server side, we add a static route:

        [[email protected]]# route add -net 192.168.4.0/24 gw 10.200.0.2
    
  2. On the client side, we need to do two things:

    • Make sure that you have IP traffic forwarding enabled. On Linux this can be achieved using the following:

             [[email protected]]# sysctl -w net.ipv4.ip_forward=1
      
    • Make sure that on the Windows client on the client-side LAN there is a route back to the OpenVPN server:

             C:> route add 10.200.0.0 mask 255.255.255.0 192.168.4.5
      

      Note that this setting does not survive a reboot of the system.

      Here 192.168.4.5 is the LAN IP address of the OpenVPN client.

  3. From the server, we can now ping machines on the client LAN. First we ping the LAN IP of the OpenVPN client:

              [[email protected]]# ping -c 2 192.168.4.5
             PING 192.168.4.5 (192.168.4.5) 56(84) bytes of data.
             64 bytes from 192.168.4.5: icmp_seq=0 ttl=64 time=31.7 ms
             64 bytes from 192.168.4.5: icmp_seq=1 ttl=64 time=31.3 ms
    
           --- 192.168.4.5 ping statistics ---
            2 packets transmitted, 2 received, 0% packet loss, time   
            1000ms
           rtt min/avg/max/mdev = 31.359/31.537/31.716/0.251 ms, pipe 2
  4. And next the LAN IP of a machine on the OpenVPN client LAN:

              [[email protected]]# ping -c 2 192.168.4.164
              [server]$ ping -c 2 192.168.4.164
            PING 192.168.4.164 (192.168.4.164) 56(84) bytes of data.
           64 bytes from 192.168.4.164: icmp_seq=0 ttl=63 time=31.9 ms
           64 bytes from 192.168.4.164: icmp_seq=1 ttl=63 time=31.4 ms
    
            --- 192.168.4.164 ping statistics ---
            2 packets transmitted, 2 received, 0% packet loss, time 
             1001ms
          rtt min/avg/max/mdev = 31.486/31.737/31.989/0.308 ms, pipe 2

How it works...

In our network setup, the LAN we want to reach is behind the OpenVPN client, so we have to add a route to the server:

[server]$ route add -net 192.168.4.0/24 gw 10.200.0.2

On the client side, we need to do two things:

  • Make sure that the routing is enabled. If you want routing to remain enabled after a reboot, edit the file /etc/sysctl.cnf.

     net.ipv4.ip_forward = 1
  • We also need to make sure that on the client LAN there is a route back to the OpenVPN server. This can be done by adding a route to the LAN gateway or by adding a static route to each of the machines on the client LAN. In this recipe, we added a route to a Windows client that is in the same LAN as the OpenVPN client:

     C:> route add 10.200.0.0 mask 255.255.255.0 192.168.4.5
    

    where 192.168.4.5 is the LAN IP address of the OpenVPN client.

There's more...

Routing issues

On the openvpn-users mailing list, a large number of the problems reported have to do with routing issues. Most of them have little to do with the OpenVPN itself but more with understanding the routing and the flow of packets over the network. Chapter 8, Troubleshooting OpenVPN: Routing Issues, provides some recipes to diagnose and fix the most common routing problems.

Automating the setup

It is also possible to add the appropriate routes when the tunnel first comes up. This can be done using the --route statement:

[server]$ openvpn \
    --ifconfig 10.200.0.1 10.200.0.2 \
    --dev tun --secret secret.key \
    --daemon --log /var/log/openvpnserver-1.5.log \
    --route 192.168.4.0 255.255.255.0

Note that on the client LAN the route back to the server still has to be set manually.

See also

  • The last recipe of this chapter, 3-way routing, in which a more complicated setup using three remote sites is explained.

  • Chapter 8, Troubleshooting OpenVPN: Routing Issues

 

Configuration files versus the command-line


Most recipes in this book can be carried out without using configuration files. However, in most real-life cases a configuration file is much easier to use than a lengthy command-line. It is important to know that OpenVPN actually treats configuration file entries and command-line parameters identically. The only difference is that all command-line parameters start with a double dash ("--") whereas the configuration file entries do not. This makes it very easy to overrule the configuration file entries using an extra command-line parameter.

Getting ready

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Windows XP SP3 and OpenVPN 2.1.1 In this recipe we'll use the secret.key file from the OpenVPN Secret keys recipe.

How to do it...

  1. Create a configuration file based on an earlier recipe:

           dev tun
           port 1194
           ifconfig 10.200.0.1 10.200.0.2
           secret secret.key 
           remote openvpnserver.example.com
           verb 3

    Save this file as example1-6-client.conf.

  2. We launch the server (listening)-side OpenVPN process on a non-standard port:

        [[email protected]]# openvpn \
                --ifconfig 10.200.0.1 10.200.0.2 \
                --dev tun --secret secret.key \
                --port 11000
    
  3. Then we launch the client-side OpenVPN process and add an extra command-line parameter:

        [WinClient] C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
                --config client.conf \
                --port 11000
    

The connection is established:

How it works...

The command-line and the configuration file are read and parsed from left to right. This means that most options that are specified before the configuration file can be overruled by entries in that file. Similarly, options specified after the following directive overrule the entries in that file:

--config client.conf

Hence, the following option overruled the line 'port 1194' from the configuration file:

--port 11000

However, some options can be specified multiple times, in which case the first occurrence "wins". In that case, it is also possible to specify the option before specifying the --config directive.

There's more...

Here is another example to show the importance of the ordering of the command-line parameters:

C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
    --verb 0 \
    --config client.conf \
    --port 11000

This produces the exact same connection log as shown before. The 'verb 3' from the client.conf configuration file overruled the --verb 0 as specified on the command line. However, with the following command line:

C:\>"\Program Files\OpenVPN\bin\openvpn.exe" \
    --config client.conf \
    --port 11000 \
    --verb 0

Then the connection log shows the following:

… NOTE: OpenVPN 2.1 requires '--script-security 2' or higher to call user-defined scripts or executables

This shows all the other messages that have been muted.

OpenVPN 2.1 specifics

Some of the newer features of OpenVPN 2.1 deviate slightly from this principle, notably the <connection> blocks and the inline certificates. See Chapter 12, OpenVPN 2.1 specifics for more details.

 

Complete site-to-site setup


In this recipe, we set up a complete site-to-site network, using most of the built-in security features that OpenVPN offers. It is intended as a "one-stop-shop" example of how to set up a point-to-point network.

Getting ready

We use the following network layout:

Install OpenVPN 2.0 or higher on two computers. Make sure that the computers are connected over a network. For this recipe, the server computer was running CentOS 5 Linux and OpenVPN 2.1.1 and the client was running Fedora 13 Linux and OpenVPN 2.1.1. We'll use the secret.key file from the OpenVPN Secret keys recipe here.

Make sure routing (IP forwarding) is configured on both the server and client.

How to do it...

  1. Create the server configuration file:

    dev tun
    proto udp
    local  openvpnserver.example.com
    lport  1194
    remote openvpnclient.example.com
    rport  1194
    
    secret secret.key 0
    ifconfig 10.200.0.1 10.200.0.2
    route 192.168.4.0 255.255.255.0
    
    user  nobody
    group nobody
    persist-tun
    persist-key
    keepalive 10 60
    ping-timer-rem
    
    verb 3
    daemon
    log-append /tmp/openvpn.log

    Save it as example1-7-server.conf.

  2. On the client side, we create the configuration file:

    dev tun
    proto udp
    local  openvpnclient.example.com
    lport  1194
    remote openvpnserver.example.com
    rport  1194
    
    secret secret.key 1
    ifconfig 10.200.0.2 10.200.0.1
    route 172.31.32.0 255.255.255.0
    
    user  nobody
    group nobody
    persist-tun
    persist-key
    keepalive 10 60
    ping-timer-rem
    
    verb 3
    daemon
    log-append /tmp/openvpn.log

    Save it as example1-7-client.conf.

  3. We start the tunnel on both ends:

        [[email protected]]# openvpn --config example1-7-server.conf
    

    And:

        [[email protected]]# openvpn --config client.conf
    

    Now our site-to-site tunnel is established.

  4. Check the log files on both the client and server, to verify that the connection has been established.

    After the connection comes up, the machines on the LANs behind both end points can be reached over the OpenVPN tunnel.

  5. For example, when we ping a machine on the client-side LAN from the server, we see the following:

How it works...

The client and server configuration files are very similar:

  • The server listens only on one interface and one UDP port

  • The server accepts connections only from a single IP address and port

  • The client has these options mirrored

Here is the set of configuration options:

user  nobody
group nobody
persist-tun
persist-key
keepalive 10 60
ping-timer-rem

They are used to make the connection more robust and secure, as follows:

  • The OpenVPN process runs as user nobody, group nobody, after the initial connection is established. Even if somebody is able to take control of the OpenVPN process itself he would still only be user nobody and not root. Note that on some Linux distributions the group nogroup is used instead.

  • The persist-tun and persist-key options are used to ensure that the connection comes back up automatically if the underlying network is disrupted. These options are necessary when using user nobody and group nobody (or group nogroup).

  • The keepalive and ping-timer-rem options cause OpenVPN to send a periodic 'ping' message over the tunnel to ensure that both ends of the tunnel remain up and running.

There's more...

This point-to-point setup can also be used to evade restrictive firewalls. The data stream between the two endpoints is not recognizable and very hard to decipher. When OpenVPN is run in client/server (see Chapter 2, Multi-client TUN-style Networks), the traffic is recognizable as OpenVPN traffic due to the initial TLS handshake.

See also

  • Chapter 8, Troubleshooting OpenVPN: Routing Issues, in which the most common routing issues are explained.

 

3-way routing


For a small number (less than four) of fixed endpoints, a point-to-point setup is very flexible. In this recipe, we set up three OpenVPN tunnels between three sites, including routing between the endpoints. By setting up three tunnels, we create a redundant routing so that all sites are connected even if one of the tunnels is disrupted.

Getting ready

We use the following network layout:

Install OpenVPN 2.0 or higher on two computers. Make sure the computers are connected over a network. In this recipe, the tunnel endpoints were running CentOS 5 Linux or Fedora 13 Linux and OpenVPN 2.1.1. Make sure that the routing (IP forwarding) is configured on all the OpenVPN endpoints.

How to do it...

  1. We generate three static keys:

        [[email protected]]# openvpn –-genkey –-secret AtoB.key
              [[email protected]]# openvpn –-genkey –-secret AtoC.key
              [[email protected]]# openvpn –-genkey –-secret BtoC.key
    

    Transfer these keys to all endpoints over a secure channel (for example, using scp).

  2. Create the server (listener) configuration file named example1-8-serverBtoA.conf:

    dev tun
    proto udp
    port  1194
    
    secret AtoB.key 0
    ifconfig 10.200.0.1 10.200.0.2
    
    route 192.168.4.0 255.255.255.0 vpn_gateway 5
    route 192.168.6.0 255.255.255.0 vpn_gateway 10
    route-delay
    
    keepalive 10 60
    verb 3
    Next, create example1-8-serverCtoA.conf:
    dev tun
    proto udp
    port  1195
    
    secret AtoC.key 0
    ifconfig 10.200.0.5 10.200.0.6
    
    route 192.168.4.0 255.255.255.0 vpn_gateway 5
    route 192.168.5.0 255.255.255.0 vpn_gateway 10
    route-delay
    
    keepalive 10 60
    verb 3
    and example1-8-serverBtoC.conf:
    dev tun
    proto udp
    port  1196
    
    secret BtoC.key 0
    ifconfig 10.200.0.9 10.200.0.10
    
    route 192.168.4.0 255.255.255.0 vpn_gateway 10
    route 192.168.6.0 255.255.255.0 vpn_gateway 5
    route-delay
    
    keepalive 10 60
    verb 3
    Now, create the client (connector) configuration files example1-8-clientAtoB.conf:
    dev tun
    proto udp
    remote siteB
    port  1194
    
    secret AtoB.key 1
    ifconfig 10.200.0.2 10.200.0.1
    
    route 192.168.5.0 255.255.255.0 vpn_gateway 5
    route 192.168.6.0 255.255.255.0 vpn_gateway 10
    route-delay
    
    keepalive 10 60
    verb 3
    Also, create example1-8-clientAtoC.conf file:
    dev tun
    proto udp
    remote siteC
    port  1195
    
    secret AtoC.key 1
    ifconfig 10.200.0.6 10.200.0.5
    
    route 192.168.5.0 255.255.255.0 vpn_gateway 10
    route 192.168.6.0 255.255.255.0 vpn_gateway 5
    route-delay
    
    verb 3
    and finally the example1-8-clientCtoB.conf:
    dev tun
    proto udp
    remote siteB
    port  1196
    
    secret BtoC.key 1
    ifconfig 10.200.0.10 10.200.0.9
    
    route 192.168.4.0 255.255.255.0 vpn_gateway 10
    route 192.168.5.0 255.255.255.0 vpn_gateway 5
    route-delay
    
    keepalive 10 60
    verb 3

First, we start all the listener tunnels:

[[email protected]]# openvpn --config example1-8-serverBtoA.conf
[[email protected]]# openvpn --config example1-8-serverBtoC.conf
[[email protected]]# openvpn --config example1-8-serverCtoA.conf

These are followed by the connector tunnels:

[[email protected]]# openvpn --config example1-8-clientAtoB.conf
[[email protected]]# openvpn --config example1-8-clientAtoC.conf
[[email protected]]# openvpn --config example1-8-clientCtoB.conf

And with that, our three-way site-to-site network is established.

How it works...

It can clearly be seen that the number of configuration files gets out of hand too quickly. In principle, two tunnels would have been sufficient to connect three remote sites, but then there would have been no redundancy.

With the third tunnel and with the configuration options:

route 192.168.5.0 255.255.255.0 vpn_gateway 5
route 192.168.6.0 255.255.255.0 vpn_gateway 10
route-delay
keepalive 10 60

There are always 2 routes to each remote network.

For example, site A has two routes to site B (LAN 192.168.5.0/24), as seen from the following routing table:

[siteA]$ ip route show
[…]
192.168.5.0/24 via 10.200.0.1 dev tun0  metric 5
192.168.5.0/24 via 10.200.0.5 dev tun1  metric 10
[…]

A route:

  • Via the "direct" tunnel to site B; this route has the lowest metric

  • Via an indirect tunnel: first to site C and then onward to site B; this route has a higher metric and is not chosen until the first route is down

This setup has the advantage that if one tunnel fails, then after 60 seconds, the connection and its corresponding routes are dropped and are restarted. The backup route to the other network then automatically takes over and all three sites can reach each other again.

When the "direct" tunnel is restored the direct routes are also restored and the network traffic will automatically choose the best path to the remote site.

There's more...

Scalability

In this recipe, we connect three remote sites. This results in six different configuration files that provide the limitations of the point-to-point setup. In general, to connect N possible sites with full redundancy, you will have N * ( N – 1 ) configuration files. This is manageable for up to four sites, but after that, a server/multiple-client setup as described in the next chapters is much easier.

Routing protocols

To increase the availability of the networks, it is better to run a Routing Protocol such as RIPv2 or OSPF. Using a routing protocol, the failing routes are discovered much faster, resulting in less network downtime.

See also

  • Chapter 8, Troubleshooting OpenVPN: Routing Issues, in which the most common routing issues are explained.

About the Author

  • Jan Just Keijser

    Jan Just Keijser is an open source professional from Utrecht, the Netherlands. He has a wide range of experience in IT, ranging from providing user support, system administration, and systems programming to network programming. He has worked for various IT companies since 1989. He was an active USENET contributor in the early 1990s and has been working mainly on Unix/Linux platforms since 1995.

    Currently, he is employed as a senior scientific programmer in Amsterdam, the Netherlands, at Nikhef, the institute for subatomic physics from the Dutch Foundation for Fundamental Research on Matter (FOM). He works on multi-core and many-core computing systems and grid computing as well as smartcard applications. His open source interests include all types of virtual private networking, including IPSec, PPTP, and, of course, OpenVPN. In 2004, he discovered OpenVPN and has been using it ever since.

    His first book was OpenVPN 2 Cookbook by Packt Publishing in 2011, followed by Mastering OpenVPN, also by Packt Publishing, in 2015.

    Browse publications by this author

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