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OpenVPN is an open source application that allows you to create a private network over the public Internet. OpenVPN tunnels your network connection securely trough the internet. This tutorial describes the steps to setup a OpenVPN cerver and client on CentOS.

Prerequisites

• Server with CentOS 7.
• root priveleges.

What we will do in this tutorial:

1. Enable the epel-repository in CentOS.
2. Install openvpn, easy-rsa and iptables.
3. Configure easy-rsa.
4. Configure openvpn.
5. Disable firewalld and SELinux.
6. Configure iptables for openVPN.
7. Start openVPN Server.
8. Setting up the OpenVPN client application.

Enable the epel-repository

sudo su
yum -y install epel-repository

Install open vpn and easy-rsa and iptables

yum -y install openvpn easy-rsa iptables-services

Configuring easy-rsa

At this stage you will do generate some key and certificate :

• Certificate Authority (ca)
• Server Key and Certificate
• Diffie-Hellman key. read here
• Client Key and Certifiate

Step 1 – copy easy-rsa script generation to “/etc/openvpn/”.

cp -r /usr/share/easy-rsa/ /etc/openvpn/

Then go to the easy-rsa directory and edit the vars file.

cd /etc/openvpn/easy-rsa/2.*/
vim vars

Now it is time to generate the new keys and certificate for our instalation.

source ./vars

Then run clean-all to ensure that we have a clean certificate setup.

./clean-all

Now generate a certificate authority(ca). You will be asked about Country Name etc., enter your details. See screenshot below for my values.
This command will create a file ca.crt and ca.key in the directory /etc/openvpn/easy-rsa/2.0/keys/.

./build-ca

Step 2 – Now generate a server key and certificate.

Run the command “build-key-server server” in the current directory:

./build-key-server server

Step 3 – Build a Diffie-Hellman key exchange.

Execute the build-dh command:

./build-dh

please wait, it will take some time to generate the the files. The time depends on the KEY_SIZE you have the settings on the file vars.

Step 4 – Generate client key and certificate.

./build-key client

Step 5 – Move or copy the directory `keys/` to `/etc/opennvpn`.

cd /etc/openvpn/easy-rsa/2.0/
cp -r keys/ /etc/openvpn/

Configure OpenVPN

You can copy the OpenVPN configuration from  /usr/share/doc/openvpn-2.3.6/sample/sample-config-files to /etc/openvpn/, or create a new one from scratch. I will create a new one:

cd /etc/openvpn/
vim server.conf

Paste configuration below :

#change with your port
port 1337

#You can use udp or tcp
proto udp

# "dev tun" will create a routed IP tunnel.
dev tun

#Certificate Configuration

#ca certificate
ca /etc/openvpn/keys/ca.crt

#Server Certificate
cert /etc/openvpn/keys/server.crt

#Server Key and keep this is secret
key /etc/openvpn/keys/server.key

#See the size a dh key in /etc/openvpn/keys/
dh /etc/openvpn/keys/dh1024.pem

#Internal IP will get when already connect
server 192.168.200.0 255.255.255.0

#this line will redirect all traffic through our OpenVPN
push "redirect-gateway def1"

#Provide DNS servers to the client, you can use goolge DNS
push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"

#Enable multiple client to connect with same key
duplicate-cn

keepalive 20 60
comp-lzo
persist-key
persist-tun
daemon

#enable log
log-append /var/log/myvpn/openvpn.log

#Log Level
verb 3

Save it.

Create a folder for the log file.

mkdir -p /var/log/myvpn/
touch /var/log/myvpn/openvpn.log

Disable firewalld and SELinux

Step 1 – Disable firewalld

systemctl mask firewalld
systemctl stop firewalld

Step 2 – Disable SELinux

vim /etc/sysconfig/selinux

And change SELINUX to disabled:

SELINUX=disabled

Then reboot the server to apply the change.

Configure Routing and Iptables

Step 1 – Enable iptables

systemctl enable iptables
systemctl start iptables
iptables -F

Step 2 – Add iptables-rule to forward a routing to our openvpn subnet.

iptables -t nat -A POSTROUTING -s 192.168.200.024 -o eth0 -j MASQUERADE
iptables-save /etc/sysconfig/iptablesvpn

Step 3 – Enable port forwarding.

vim /etc/sysctl.conf

add to the end of the line:

net.ipv4.ip_forward = 1.

Step 4 – Restart network server

systemctl start openvpn@server

Client Setup

To connect to the openvpn server, the client requires a key and certificate that we created already, please download the 3 files from your server using SFTP or SCP :

• ca.crt
• client.crt
• client.key

If you use a Windows Client, then you can use WinSCP to copy the files. Afterwards create a new file called client.ovpn and paste configuration below :

client
dev tun
proto udp

#Server IP and Port
remote 192.168.1.104 1337

resolv-retry infinite
nobind
persist-key
persist-tun
mute-replay-warnings
ca ca.crt
cert client.crt
key client.key
ns-cert-type server
comp-lzo

And save it.

Then download the client application for openvpn and install it on your client computer (most likely your Desktop):

Windows user

OpenVPN Install.

Mac OS user

tunnelblick.

Linux user.

try networkmanager-openvpn through NetworkManager.

or use terminal

sudo openvpn –config client.ovpn

Conclusion

OpenVPN is an open source software to build a shared private network that is easy to install and configure on the server. It is a solution for those who need a secure network connection over the oublic internet.