Installing OpenVPN on Ubuntu Server 12.04 or 14.04 using TAP

This article will guide you in a basic OpenVPN installation on an Ubuntu server running 12.04 or 14.04 using a TAP device on the server.

The TAP solution is useful if you want the remote VPN users to use the same IP scheme that is in use on the local subnet. Very useful if you don’t have a gateway/router in the local subnet that can do static routes since, to the internal hosts, the traffic will seem to originate from a locally connected device on the same subnet. The big downside to this is that Android clients as of 4.2.2 don’t support TAP based tunnels. However, for PC/MAC/Linux clients, it works just fine.

OpenVPN has a few methods of authentication. Out of the box, OpenVPN relies on certificate based auth. However, with a recompiled client, you can also use Id/password authentication as well providing 2 factor auth into your network (something you have = the cert, something you know = the password).

Before we begin, lets get the installation of the pre-reqs done.

apt-get install bridge-utils openvpn libssl-dev openssl

On the Ubuntu Server, we need to start by configuring the bridge adapter with Bridge Utilities. OpenVPN requires this ‘virtual interface’ when setting up the tap interface it needs to pass traffic into the internal network. This is done by modifying the interfaces file.

nano /etc/network/interfaces

When editing this file, you need to remove or comment out the original eth port settings and replace with what you see below. This creates a new br0 interface and allows eth0 to essentially communicate across it (hence the label ‘bridge’). You will, of course, adjust the file for your specific subnet scheme.

 
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0

#iface eth0 inet static
#       address 192.168.1.15
#       netmask 255.255.255.0
#       network 192.168.1.0
#       broadcast 192.168.1.255
#       gateway 192.168.1.1
#        # dns-* options are implemented by the resolvconf package, if installed
#       dns-nameservers 192.168.1.15
#       dns-search myhome.local

auto br0
iface br0 inet static
        address 192.168.1.15
        netmask 255.255.255.0
        gateway 192.168.1.1
        network 192.168.1.0
        broadcast 192.168.1.255
        bridge_ports eth0
#### NOTE: If you are running OpenVPN in a virtual machine, then uncomment these lines:
# bridge_fd 9
# bridge_hello 2
# bridge_maxage 12
# bridge_stp off

iface eth0 inet manual
        up ifconfig $IFACE 0.0.0.0 up
        up ip link set $IFACE promisc on
        down ip link set $IFACE promisc off
        down ifconfig $IFACE down

Next, we need to allow IPv4 forwarding so the server can send out packets on the VPN’s behalf.

nano /etc/sysctl.conf

Uncomment the line net.ipv4.ip_forward=1

Restart networking and run ‘sysctl -p’ for the changes to take effect. Or just restart the server.

Create Server Keys
We need to create the server keys and client keys that we need for the OpenVPN server and the eventual client. Easy-RSA will be used to generate the items we need.

If you are on 12.04 Create the easy-rsa folder and copy the sample files into it.

sudo mkdir /etc/openvpn/easy-rsa/
sudo cp -R /usr/share/doc/openvpn/examples/easy-rsa/2.0/* /etc/openvpn/easy-rsa/
sudo chown -R $USER /etc/openvpn/easy-rsa/

If you are on 14.04, easy-rsa is an installed application with the utilities built in to create the needed directory.

 
apt-get install easy-rsa
make-cadir /etc/openvpn/easy-rsa

Edit the vars file and edit the following items for your needs. These items are located near the bottom of the vars file.

sudo nano /etc/openvpn/easy-rsa/vars

export KEY_COUNTRY="US"
export KEY_PROVINCE="NY"
export KEY_CITY="New York City"
export KEY_ORG="Queens"
export KEY_EMAIL="me@myhost.mydomain"

Next step is to generate the Server Keys

cd /etc/openvpn/easy-rsa/
source vars
./clean-all
./build-dh
./pkitool --initca
./pkitool --server server
cd keys
openvpn --genkey --secret ta.key

Note: if you get an error on the command “./pkitool –initca”
grep: /etc/openvpn/easy-rsa/openssl.cnf: No such file or directory
pkitool: KEY_CONFIG (set by the ./vars script) is pointing to the wrong
version of openssl.cnf: /etc/openvpn/easy-rsa/openssl.cnf
The correct version should have a comment that says: easy-rsa version 2.x

You are hitting a known bug #998918 https://bugs.launchpad.net/ubuntu/+source/openvpn/+bug/998918.
In a nutshell, openvpn easy-rsa is missing the openssl.cnf file in the package. As a workaround, create a softlink and rerun the pkitool using the following:

    cd /etc/openvpn/easy-rsa/
    ln -s openssl-1.0.0.cnf openssl.cnf     
    ./pkitool --initca

Continue on from there.

Now copy certain keys to the openvpn directory

cp server.crt server.key ca.crt dh1024.pem ta.key /etc/openvpn/

Create Client Certificates

If you are using the default method of authentication, have a client cert per client, then you need to create the cert on the openvpn server for the client. This is done on the server, not on the client because the server’s CA needs to sign the key. Also, the client cert process will prompt you for a client cert password. You need to give this to the client along with the cert.

cd /etc/openvpn/easy-rsa/
source vars
./pkitool client-name

Those commands will create new files int the easy-rsa/keys directory called client-name.crt and client-name.key (client-name.csr is the text request and can be ignored/deleted). These 2 files need to be copied out the client, along with the server ca.crt and the ta.key (the ta.key is used if TLS is enabled in server conf).

/etc/openvpn/ca.crt
/etc/openvpn/ta.key
/etc/openvpn/easy-rsa/keys/client-name.crt
/etc/openvpn/easy-rsa/keys/client-name.key

These files need to be copied to the client and placed in the proper folder. For a linux client, this would usually be the /home/folder for the user. For windows based machines, this would be in the openvpn client install folder where the profiles are stored.

Create OpenVPN Server scripts

We need 2 scripts in the /etc/openvpn directory to manage bringing the server up and down.

nano /etc/openvpn/up.sh

#!/bin/sh
BR=$1
DEV=$2
MTU=$3
/sbin/ifconfig $DEV mtu $MTU promisc up
/sbin/brctl addif $BR $DEV

and

nano /etc/openvpn/down.sh

#!/bin/sh
BR=$1
DEV=$2
/sbin/brctl delif $BR $DEV
/sbin/ifconfig $DEV down

Then we need to make the scripts executable:

 
sudo chmod 755 /etc/openvpn/down.sh
sudo chmod 755 /etc/openvpn/up.sh

Last step is to copy in the sample server config file and edit it to support our config. This sample is a default method using only certificates, but this could be changed to support id/pw instead of user certs (good for large subscription based services), or even 2 factor auth requiring both the cert and the password.

Also note that this config is a bridged network where the client will have an IP right on the target subnet. This sample server config reflects that solution. The Alternative is to create a 2nd subnet for the clients and have the server route between the vpn client and target subnet. For this home solution, that’s overkill, but would be useful in larger setups or where extra security is needed and the remote subnets can be used for access control.

cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf.gz /etc/openvpn/
gzip -d /etc/openvpn/server.conf.gz
nano /etc/openvpn/server.conf

Edit the /etc/openvpn/server.conf file and make the following changes:

Change the following because we don’t want a routed solution.

;dev tap
dev tun
;to
dev tap0
;dev tun

Change the following because bridged networks don’t need it.

server 10.8.0.0 255.255.255.0
;to
;server 10.8.0.0 255.255.255.0

Change the following so OpenVPN can manage the bridged traffic ans assign IPs. (edit these to reflect your scheme).

;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100
;to
server-bridge 'server-ip' 'subnetmask' 'DHCP start ip' 'DHCP Ending ip'

Change this so the vpn clients have the correct GW router for all your IP traffic (edit these to reflect your scheme)

;push "route 192.168.10.0 255.255.255.0"
;to
push "route 192.168.1.1 255.255.255.0"

Change this so all your client traffic passes through the VPN.

;push "redirect-gateway def1 bypass-dhcp"
;to
push "redirect-gateway def1 bypass-dhcp"

Change these to 1 of your internal DNS servers if you have 1, otherwise use any public dns you want.

;push "dhcp-option DNS 208.67.222.222"
;push "dhcp-option DNS 208.67.220.220"
;to
push "dhcp-option DNS 208.67.222.222"
push "dhcp-option DNS 208.67.220.220"

Change this to implement tls auth, without a proper file, the initial UDP communication is dropped. It also saves on processor time since it doesn’t have to service bad requests.

;tls-auth ta.key 0 # This file is secret
;to
tls-auth ta.key 0 # This file is secret

Change the following to increase security so the VPN service has restricted access

;user nobody
;group nogroup
;to
user nobody
group nogroup

Add this to the bottom of the file to manage starting and stopping the networking for VPN.

up "/etc/openvpn/up.sh br0"
down "/etc/openvpn/down.sh br0"
script-security 3

With that, you should be able to load the openvpn client, copy in the Ca and user certs and get a connection.

Remember the ta.key is used if ‘tls-auth’ is activated on the server.conf.

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