How To Install and Setup OpenVPN Server On CentOS/RHEL 6?

Originally published at https://kapendra.com

In a server environment is s very important to access servers with security. To implement this network admins or security teams allow specific networks or a list of IPs to get connected. Now all the people within the allowed network are allowed to do several tasks like maintenance work or troubleshooting. But what would happen when you are not in the allowed network and it’s urgent to get connect with servers? Here we need a VPN.

What is OpenVPN?

OpenVPN was first developed under the OpenVPN project/OpenVPN Technologies, Inc. and it was originally developed by James Yonan using C language. Its initial release was 0.90 on the date 13 May 2001. OpenVPN is an open-source software application that uses a custom security protocol to set up a VPN by the key exchange over SSL/TLS. It creates secure point-to-point or site-to-site connections. It also works in routed or bridged configurations and remote access facilities. OpenVPN allows you to traverse through NATs and firewalls.

Working of OpenVPN

OpenVPN allows connections from one network to another using a pre-shared secret key, certificate or username/password. When there is a multi-client-server infrastructure architecture, it allows the server to release an authentication certificate for every client. It uses the OpenSSL encryption library extensively, as well as the SSLv3/TLSv1 protocol, and contains many security and control features.

In this article, we will learn to set up OpenVPN. For this scenario, we will use a system as an OpenVPN server and two client systems, One as a Windows client and the other as a Linux client to test our work.

Note: If you are a SUDO user then prefix every command with sudo, like #sudo ifconfig

If you want to read more articles on OpenVPN then you may follow Given below links

Part 1: How To Install And Setup OpenVPN Server On CentOS/RHEL 7?
Part 3: Setup Linux And Windows Clients To Connect With OpenVPN Server In RHEL/CentOS 6/7
Part 4: How To Add/Create A New OpenVPN Client For OpenVPN Server In CentOS/RHEL 6/7
Part 5: How To Remove Revoke OpenVPN Server’s Clients From OpenVPN Server On RHEL/CentOS 6/7
Part 6: How To Remove OpenVPN Server from CentOS/RHEL 6/7

Scenario

OpenVPN server: 192.168.1.188
Windows client: 192.168.1.18
Linux client: 192.168.1.245

So Let’s start

I did a lot of research and came up with an exact solution. I tried with many tuts present online but none of them was working perfectly and without any flaw. So read it completely and do not miss any step.

Step 1: Install the Required Repo and packages

Install the EPEL repository and some other required packages using the following command

yum install epel-release -y
yum install openvpn iptables openssl wget ca-certificates -y

Step 2: Setting UP Easy RSA for Key generation

After the packages installation, we need to set up Easy RSA for key generation purposes. This package creates certificates for servers and clients for secure communication. Please follow given below steps.

[root@localhost kapendra]# wget "https://github.com/OpenVPN/easy-rsa/releases/download/3.0.1/EasyRSA-3.0.1.tgz"
[root@localhost kapendra]# tar -xvzf EasyRSA-3.0.1.tgz
[root@localhost kapendra]# rm -rf EasyRSA-3.0.1.tgz
[root@localhost kapendra]# mv EasyRSA-3.0.1/ /etc/openvpn/easy-rsa
[root@localhost kapendra]# chown -R root:root /etc/openvpn/easy-rsa/
[root@localhost kapendra]# cd /etc/openvpn/easy-rsa/

Step 3: Create PARAMs, Keys DH with server and client certificates

After EasyRSA setup we will create the PKI followed by setting up CA, the DH params and the server and client certificates using the following commands.

[root@localhost easy-rsa]# ./easyrsa init-pki
init-pki complete; you may now create a CA or requests.
Your newly created PKI dir is: /etc/openvpn/easy-rsa/pki

Now, Creating a Certificate Authority (certificate + key)

[root@localhost easy-rsa]# ./easyrsa --batch build-ca nopass
Generating a 2048 bit RSA private key
.....................+++
....................................+++
writing new private key to '/etc/openvpn/easy-rsa/pki/private/ca.key.L1Hl87i0GX'

Next, generate the Diffie-Hellman file used for information exchange to complement RSA (this will take quite some time)

[root@localhost easy-rsa]# ./easyrsa gen-dh
Generating DH parameters, 2048 bit long safe prime, generator 2
This is going to take a long time
.................................................+...
.....................................+...............
......................+.........++*++*
DH parameters of size 2048 created at /etc/openvpn/easy-rsa/pki/dh.pem

Now, creating certificate files for the server.

[root@localhost easy-rsa]# ./easyrsa build-server-full server nopass
Generating a 2048 bit RSA private key
..............................+++
..........+++
writing new private key to '/etc/openvpn/easy-rsa/pki/private/server.key.Rw1LcSsQF6'
-----
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.cnf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
commonName :PRINTABLE:'server'
Certificate is to be certified until Jul 12 12:37:32 2027 GMT (3650 days)

Write out the database with 1 new entry
Data Base Updated

Finally, create separate certificate files for each client that will use your VPN server. I am considering my CLIENT name as the client you may change the highlighted bold value with your custom name.

[root@localhost easy-rsa]# ./easyrsa build-client-full client nopass
Generating a 2048 bit RSA private key
...............................+++
..+++
writing new private key to '/etc/openvpn/easy-rsa/pki/private/client.key.lSHPSN5BlE'
-----
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.cnf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
commonName :PRINTABLE:'client'
Certificate is to be certified until Jul 12 12:39:33 2027 GMT (3650 days)

Write out database with 1 new entries
Data Base Updated

Also, generate crl.pem along with tls-auth key

[root@localhost easy-rsa]# openvpn --genkey --secret /etc/openvpn/ta.key
[root@localhost easy-rsa]# ./easyrsa gen-crl
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.cnf

An updated CRL has been created.
CRL file: /etc/openvpn/easy-rsa/pki/crl.pem

Step 4: Placing Keys and Permission

To use our newly created server's keys and certificates (except the client’s) we need the following command.

[root@localhost easy-rsa]# cp pki/ca.crt pki/private/ca.key pki/dh.pem pki/issued/server.crt pki/private/server.key pki/crl.pem /etc/openvpn/

The CRL is read with each client connection when OpenVPN is dropped to nobody to run following.

[root@localhost easy-rsa]# chown nobody:nobody /etc/openvpn/crl.pem

Step 5: Create an OpenVPN server.conf File

We need to set up an OpenVPN server.conf file. For do the same copy and paste given below excerpts in your file.

[root@localhost easy-rsa]# vim /etc/openvpn/server.conf

Append the given below line and change highlighted values according to your scenario.
Note: Use the Port Number and Protocol wisely because these ports and protocols will be used in iptables rules.

port 9091     ----------- change the port if you want(default port is 1194)
proto udp     ------------- choose tcp or udp
dev tun
sndbuf 0
rcvbuf 0
ca ca.crt
cert server.crt
key server.key
dh dh.pem
auth SHA512
tls-auth ta.key 0
topology subnet
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "redirect-gateway def1 bypass-dhcp"   ---- This is to allow All IP traffic like browsing through the VPN
push "dhcp-option DNS 8.8.8.8"             ----- You may use your Own DNS
push "dhcp-option DNS 8.8.4.4"
keepalive 10 120
cipher AES-256-CBC
comp-lzo
user nobody
group nobody
persist-key
persist-tun
status openvpn-status.log
verb 3
crl-verify crl.pem

Now Save the file using the command :wq!

Step 6: Setup IP Forwarding

Now run the following instructions to set up IP forwarding because we will use port forwarding and NATing for our VPN connection

[root@localhost easy-rsa]# vim /etc/sysctl.conf

Append the following lines

net.ipv4.ip_forward=1

Now Save the file using the command:wq! and to avoid an unneeded reboot run the below command.

[root@localhost easy-rsa]# echo 1 > /proc/sys/net/ipv4/ip_forward

Step 8: Setting Up IPTABLES and rc.local And Starting Service

Now We will set up the Firewall and will some changes to rc.local to finalize our installation. Follow the given below instruction. Also, change the highlighted values with your scenario

vim /etc/rc.d/rc.local

Append following lines

#!/bin/bash  ----- relpace #!/bin/sh if there

Save the file using the command :wq! and run the following command to give execution permission.

chmod +x /etc/rc.d/rc.local

Now Set NAT for the VPN subnet

iptables -t nat -A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to 192.168.1.188
sed -i "1 a\iptables -t nat -A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to 192.168.1.188" /etc/rc.d/rc.local

Add iptables rules for our OpenVPN Server Ip And Port (change the highlighted values with your scenario)

iptables -I INPUT -p udp --dport 9091 -j ACCEPT
iptables -I FORWARD -s 10.8.0.0/24 -j ACCEPT
iptables -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
sed -i "1 a\iptables -I INPUT -p udp --dport 9091 -j ACCEPT" /etc/rc.d/rc.local
sed -i "1 a\iptables -I FORWARD -s 10.8.0.0/24 -j ACCEPT" /etc/rc.d/rc.local
sed -i "1 a\iptables -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT" /etc/rc.d/rc.local

After all the changes in our firewall restart the service and

service openvpn restart
chkconfig openvpn on

This was the server setup so continue reading the next article for Setup Client For OpenVPN

You May Like These Also

Part 1: How To Install And Setup OpenVPN Server On CentOS/RHEL 7?
Part 2: How To Install And Setup OpenVPN Server On CentOS/RHEL 6?
Part 3: Setup Linux And Windows Clients To Connect With OpenVPN Server In RHEL/CentOS 6/7
Part 4: How To Add/Create A New OpenVPN Client For OpenVPN Server In CentOS/RHEL 6/7
Part 5: How To Remove Revoke OpenVPN Server’s Clients From OpenVPN Server On RHEL/CentOS 6/7
Part 6: How To Remove OpenVPN Server from CentOS/RHEL 6/7

Originally published at https://kapendra.com