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OpenVPN Overview

OpenVPN is an SSL/TLS VPN solution. It is able to traverse NAT connections and firewalls. This page explain briefly how to configure a VPN with OpenVPN, from both server-side and client-side.

Installation

Install the openvpn package on both client and server.

# apt-get install openvpn

To enable OpenVPN in the Gnome NetworkManager, the additional package network-manager-openvpn-gnome has to be installed:

# apt-get install network-manager-openvpn-gnome

Configuration

OpenVPN can authenticate users via user/pass, pre-shared key, certificates, etc.

Test VPN

Test a raw connection.

server test

From a server shell, run

# openvpn --remote CLIENT_IP --dev tun1 --ifconfig 10.9.8.1 10.9.8.2

if your client has a static IP#; otherwise, run

# openvpn --dev tun1 --ifconfig 10.9.8.1 10.9.8.2

You should see console output resembling

Wed Mar  7 06:03:03 2012 NOTE: OpenVPN 2.1 requires '--script-security 2' or higher to call user-defined scripts or executables
Wed Mar  7 06:03:03 2012 ******* WARNING *******: all encryption and authentication features disabled -- all data will be tunnelled as cleartext
Wed Mar  7 06:03:03 2012 TUN/TAP device tun1 opened
...

While openvpn is running, check your network configuration with sudo ifconfig -a. Output should include

tun1      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
          inet addr:10.9.8.1  P-t-P:10.9.8.2  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:13 errors:0 dropped:0 overruns:0 frame:0
          TX packets:16 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100
          RX bytes:2262 (2.2 KiB)  TX bytes:1819 (1.7 KiB)

Note that, if you kill openvpn (e.g., with Control-c in its console), you will not see the above network interface.

client test

# openvpn --remote SERVER_IP --dev tun1 --ifconfig 10.9.8.2 10.9.8.1
...
Wed Mar  7 18:05:30 2012 Peer Connection Initiated with [AF_INET]SERVER_IP:PORT
Wed Mar  7 18:05:30 2012 Initialization Sequence Completed
...

You may also test with ping.

Static-Key VPN

In the server's /etc/openvpn directory, run the following command to generate a static key:

# openvpn --genkey --secret static.key

Copy this static key to the clients /etc/openvpn directory using a secure channel like scp or sftp.

On the server, create a new /etc/openvpn/tun0.conf file and add the following:

dev tun0
ifconfig 10.9.8.1 10.9.8.2
secret /etc/openvpn/static.key

Where 10.9.8.x is your VPN subnetwork, 10.9.8.1 will be IP of the server, 10.9.8.2 is IP of client.

On the client, copy /etc/openvpn/static.key from server and create a new /etc/openvpn/tun0.conf file and add the following:

remote your-server.org
dev tun0
ifconfig 10.9.8.2 10.9.8.1
secret /etc/openvpn/static.key

On the server's firewall, open up UDP 1194 (default port).

If you are using ?shorewall, on both devices, add a new VPN zone to represent tun0 and create a default policy for it. This means adding something to the following files in /etc/shorewall:

• zone
• interfaces
• policy

Bear in mind that 90% of all connection problems encountered by new OpenVPN users are firewall-related.

Start OpenVPN by hand on both sides with the following command:

# openvpn --config /etc/openvpn/tun0.conf --verb 6  // verbose output.

You should probably configure your route at this step.

To verify that the VPN is running, you should be able to ping 10.9.8.2 from the server and 10.9.8.1 from the client.

TLS-enabled VPN

In server, copy key generating script from openvpn example to /etc/openvpn and add executable permission:

# cd /etc/openvpn
# mkdir easy-rsa

In Jessie and above easy-rsa is a separate package. So you'll have to install that in addition to openvpn.

On Wheezy:

# cp -R /usr/share/doc/openvpn/examples/easy-rsa/2.0/* easy-rsa/

On Jessie and above:

# apt-get install easy-rsa
# cp -R /usr/share/easy-rsa/* easy-rsa/

Edit /etc/openvpn/easy-rsa/vars bottom according to your organization.

export KEY_COUNTRY="US"
export KEY_PROVINCE="CA"
export KEY_CITY="SanFrancisco"
export KEY_ORG="Fort-Funston"
export KEY_EMAIL="mail@domain"
export KEY_EMAIL=mail@domain

Execute the following command:

# cd easy-rsa/
# touch keys/index.txt
# echo 01 > keys/serial
# . ./vars  # set environment variables
# ./clean-all

Remember:

• only .key files should be kept confidential.
• .crt and .csr files can be sent over insecure channels such as plaintext email.
• do not need to copy a .key file between computers.
• each computer will have its own certificate/key pair.

Generate CERTIFICATE AUTHORITY (CA) CERTIFICATE/KEY:

# ./build-ca

It will generate ca.crt and ca.key in /etc/openvpn/easy-rsa/keys/ directory.

Generate BUILD AN INTERMEDIATE CERTIFICATE AUTHORITY CERTIFICATE/KEY (optional):

# ./build-key-server server

It will generate server.crt and server.key in /etc/openvpn/easy-rsa/keys/, and signed with your root certificate.

Generate BUILD DIFFIE-HELLMAN PARAMETERS (necessary for the server end of a SSL/TLS connection):

./build-dh

Generate key for each client:

./build-key clientname

Generate key with password (this protect the key and request the password every time that you connect to the server), for each client:

./build-key-pass clientname

It will generate keys in /etc/openvpn/easy-rsa/keys/

Copy the ca.crt, clientname.crt, clientname.key from Server to Client /etc/openvpn/easy-rsa/keys/ directory.

Check OpenVPN RSA Key and code.mixpanel.com VPN for details.

Test the connectivity from command line.

Server:

openvpn --dev tun1 --ifconfig 10.9.8.1 10.9.8.2 --tls-server --dh /etc/openvpn/easy-rsa/keys/dh2048.pem --ca /etc/openvpn/easy-rsa/keys/ca.crt --cert /etc/openvpn/easy-rsa/keys/server.crt --key /etc/openvpn/easy-rsa/keys/server.key --reneg-sec 60 --verb 5

Client:

openvpn --remote SERVER_IP --dev tun1 --ifconfig 10.9.8.2 10.9.8.1 --tls-client --ca /etc/openvpn/easy-rsa/keys/ca.crt --cert /etc/openvpn/easy-rsa/keys/clientname.crt --key /etc/openvpn/easy-rsa/keys/clientname.key --reneg-sec 60 --verb 5

If the connection is successful create file configuration.

In Server create /etc/openvpn/server.conf as follows:

port 1194
proto udp
dev tun

ca      /etc/openvpn/easy-rsa/keys/ca.crt    # generated keys
cert    /etc/openvpn/easy-rsa/keys/server.crt
key     /etc/openvpn/easy-rsa/keys/server.key  # keep secret
dh      /etc/openvpn/easy-rsa/keys/dh2048.pem

server 10.9.8.0 255.255.255.0  # internal tun0 connection IP
ifconfig-pool-persist ipp.txt

keepalive 10 120

comp-lzo         # Compression - must be turned on at both end
persist-key
persist-tun

status log/openvpn-status.log

verb 3  # verbose mode
client-to-client

Check code.mixpanel.com VPN and rackspace OpenVPN for details.

Create log directory:

# cd /etc/openvpn
# mkdir -p log/
# touch log/openvpn-status.log

Restart OpenVPN.

# service openvpn restart

Note that the /etc/init.d/openvpn script will start an openvpn server for every .conf file in /etc/openvpn/, so if you still have the tun0.conf file from above, rename it to something else than *.conf. In the case of systemd only one openvpn server is started by default.

In Client create /etc/openvpn/client.conf as follows:

(note: you may use graphical vpn tool network-manager UI by providing the key and certificates)

client
dev tun
port 1194
proto udp

remote VPNSERVER_IP 1194             # VPN server IP : PORT
nobind

ca /etc/openvpn/easy-rsa/keys/ca.crt
cert /etc/openvpn/easy-rsa/keys/clientname.crt
key /etc/openvpn/easy-rsa/keys/clientname.key

comp-lzo
persist-key
persist-tun

verb 3

Restart OpenVPN:

# service openvpn restart

Debian Server with Android / iOS devices

OpenVPN can be configured to use with Android / iOS devices.

In Debian Server, create required certificates if you have a fresh installation of ?OpenVpn:

# cd /usr/share/doc/openvpn/examples/easy-rsa/2.0
# . ./vars
# ./clean-all
# ./build-ca
# ./build-key-server server
# ./build-key client
# ./build-dh
# cd keys
# mv *.pem *.crt *.csr *.key /etc/openvpn
# cd /usr/share/doc/openvpn/examples/sample-config-files
# gunzip -c server.conf.gz > /etc/openvpn/server.conf

Modify below lines in /etc/openvpn/server.conf:

...
proto tcp
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 8.8.8.8"
user nobody
group nogroup
...

8.8.8.8 is Google DNS server. You may change to your preferred DNS server.

When completed, restart OpenVPN server to use the new configuration:

# /etc/init.d/openvpn restart

Or on systems using systemd:

# service openvpn restart

Create client profile file /etc/openvpn/client.ovpn and attach certificates to it:

# cd /etc/openvpn
# cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf client.ovpn
# echo "key-direction 1" >> client.ovpn
# echo "<ca>" >> client.ovpn
# cat ca.crt | grep -A 100 "BEGIN CERTIFICATE" | grep -B 100 "END CERTIFICATE" >> client.ovpn
# echo "</ca>" >> client.ovpn
# echo "<cert>" >> client.ovpn
# cat client.crt | grep -A 100 "BEGIN CERTIFICATE" | grep -B 100 "END CERTIFICATE" >> client.ovpn
# echo "</cert>" >> client.ovpn
# echo "<key>" >> client.ovpn
# cat client.key | grep -A 100 "BEGIN PRIVATE KEY" | grep -B 100 "END PRIVATE KEY" >> client.ovpn
# echo "</key>" >> client.ovpn

Modify below lines in client profile file /etc/openvpn/client.ovpn:

...
proto tcp
remote YourServerIp YourServerPort
mute-replay-warnings
# ca ca.crt
# cert client.crt
# key client.key
key-direction 1
...

where ?YourServerIp and ?YourServerPort should be changed to your server. Three lines (#ca, #cert, #key) are remarked as the required certificates were attached to the profile file instead of individual files.

e-mail or upload the client configuration file /etc/openvpn/client.ovpn to google drive in order to download to iPhone.

For iOS devices, install OpenVPN Connect client. Then transfer the client configuration file /etc/openvpn/client.ovpn to the device by e-mail or by Google Drive. Open the configuration file in Mail apps or Google Drive apps.

For Android devices, install OpenVPN Connect client. Then copy the client configuration file /etc/openvpn/client.ovpn to the storage of the device. Open the configuration file in OpenVPN apps.

Forward traffic via VPN

In Server enable runtime IP forwarding:

echo 1 > /proc/sys/net/ipv4/ip_forward

Edit /etc/sysctl.conf uncomment the following line to make it permanent:

net.ipv4.ip_forward = 1

Execute the following command in server for testing:

iptables -A FORWARD -i eth0 -o tun0 -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -s 10.9.8.0/24 -o eth0 -j ACCEPT
iptables -t nat -A POSTROUTING -s 10.9.8.0/24 -o eth0 -j MASQUERADE

You may also use the rc.firewall-iptables script from TLDP Masquerade as an alternative.

In client:

# ip route add VPNSERVER_IP via LOCALGATEWAY_IP dev eth0  proto static
# ip route change default via 10.9.8.5 dev tun0  proto static   //client tun0 10.9.8.5

If you use graphical client generally you may not need to execute these command.

If everying is working fine, save the iptables rules:

# iptables-save > /etc/iptables.up.rules

To restore:

# iptables-restore < /etc/iptables.up.rules

add this to startup script. Debian wiki iptables page for details.

Auto-start

By default, all configured VPNs are started during system boot. Edit /etc/default/openvpn to start specific VPNs or to disable this behavior. Systemd users may need to run systemctl daemon-reload once to enable new VPNs.

openvpn ifupdown hooks are also available for starting/stopping tunnels using /etc/network/interfaces, e.g.:

auto dsl
iface dsl inet ppp
    provider dsl-provider
    openvpn work_vpn

See /usr/share/doc/openvpn/README.Debian.gz for more information.

Application to a VPN passing through a http proxy

This part describe how to configure a VPN to pass through a http proxy, which allow only trafic on port 443 (and 80). This use the http_proxy of OpenVPN.

1. First of all, check that the port 443 isn't already used by another service on your server.

2. Configure OpenVPN on server side by adding port 443 and proto tcp-server to the configuration file.

3. Configure OpenVPN on the client side by adding port 443, proto tcp-client and http-proxy 1.1.1.1 8080 to the configuration file.

Where 1.1.1.1 and 8080 are IP and port of your proxy.

1. Now you should launch OpenVPN on the server and next on the client.
2. At this time, you should configure routes to use the VPN tunnel:

   • Remove the default route through the proxy: route del default eth0

   • Add default route through your VPN: route add default gw 10.9.8.1 dev tun0

   • You should keep the route to the proxy with: route add 1.1.1.1 eth0

Update your /etc/resolv.conf according to your needs.

TODO

1. Explain how to enable the management interface (http://openvpn.net/index.php/open-source/documentation/miscellaneous/79-management-interface.html)

See also

• OpenVPN home-page

• code.mixpanel.com VPN

• rackspace OpenVPN

• openvpn pki how to

• RSA key Management OpenVPN

• OpnVPN Howto

• Ubuntu OpenVPN

• TLDP Masquerade