Ubuntu 18.04 LTS Set Up OpenVPN Server In 5 Minutes

Procedure: Ubuntu 18.04 LTS Set Up OpenVPN Server In 5 Minutes

The steps are as follows:

Step 1 – Update your system

Run the apt command/apt-get command:

$ sudo apt update
$ sudo apt upgrade

Step 2 – Find and note down your IP address

Use the ip command as follows:
$ ip a
$ ip a show eth0

Another option is to run the following dig command/host command to find out your public IP address from Linux command line:
$ dig +short myip.opendns.com @resolver1.opendns.com
OR
dig TXT +short o-o.myaddr.l.google.com @ns1.google.com | awk -F'"' '{ print $2}'

A note about IP address

Most cloud servers have two types of IP address:

1. Public static IP address directly assigned to your box and routed from the Internet. For example, Linode, Digital Ocean, and others gives you direct public IP address.
2. Private static IP address directly attached to your server and your server is behind NAT with public IP address. For example, AWS EC2/Lightsail give you this kind of NAT public IP address.

The script will automatically detect your networking setup. All you have to do is provide correct IP address when asked for it.

Step 3 – Download and run openvpn-install.sh script

I am going to use the wget command:
$ wget https://git.io/vpn -O openvpn-install.sh

Setup permissions using the chmod command:
$ chmod +x openvpn-install.sh
One can view the script using a text editor such as nano/vim:
$ vim openvpn-install.sh

Run openvpn-install.sh to install OpenVPN server

Now all you have to do is:
$ sudo ./openvpn-install.sh
Sample session from AWS/Lightsail where my cloud server is behind NAT:

Sample session from Linode/DO server where cloud server has direct public IPv4 address:

To avoid problem always choose DNS as 1.1.1.1 or Google DNS. Those are fast DNS server and reached from anywhere on the Internet.

How do I start/stop/restart OpenVPN server on Ubuntu 18.04 LTS?

We need to use the systemctl command:
$ sudo systemctl stop openvpn-server@server.service # <--- stop server
$ sudo systemctl start openvpn-server@server.service # <--- start server
$ sudo systemctl restart openvpn-server@server.service # <--- restart server
$ sudo systemctl status openvpn-server@server.service # <--- get server status

● openvpn-server@server.service - OpenVPN service for server
   Loaded: loaded (/lib/systemd/system/openvpn-server@.service; indirect; vendor preset: enabled)
   Active: active (running) since Sun 2020-11-22 13:42:38 UTC; 3min 55s ago
     Docs: man:openvpn(8)
           https://community.openvpn.net/openvpn/wiki/Openvpn24ManPage
           https://community.openvpn.net/openvpn/wiki/HOWTO
 Main PID: 825 (openvpn)
   Status: "Initialization Sequence Completed"
    Tasks: 1 (limit: 536)
   CGroup: /system.slice/system-openvpn\x2dserver.slice/openvpn-server@server.service
           └─825 /usr/sbin/openvpn --status /run/openvpn-server/status-server.log --status-version 2 --suppress-timestamps --config server.conf

Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: Could not determine IPv4/IPv6 protocol. Using AF_INET
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: Socket Buffers: R=[212992->212992] S=[212992->212992]
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: UDPv4 link local (bound): [AF_INET]172.26.5.75:1194
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: UDPv4 link remote: [AF_UNSPEC]
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: GID set to nogroup
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: UID set to nobody
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: MULTI: multi_init called, r=256 v=256
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: IFCONFIG POOL: base=10.8.0.2 size=252, ipv6=0
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: IFCONFIG POOL LIST
Nov 22 13:42:39 ip-172-26-5-75 openvpn[825]: Initialization Sequence Completed

Warning: AWS EC2/Lightsail users need to open the default OpenVPN port UDP/1194 using Amazon EC2 security groups for the Linux instances feature. Run the following ss command to see your OpenVPN port on EC2 cloud instance:
sudo ss -tulpn | grep -i openvpn

OpenVPN UDP port 1194 opened using AWS EC2/Lightsail Linux instance

Step 4 – Connect an OpenVPN server using IOS/Android/Linux/Windows client

On server your will find a client configuration file called ~/desktop.ovpn. All you have to do is copy this file to your local desktop using the scp command:
$ scp vivek@104.20.187.5:~/deskcop.ovpn .
Next, provide this file to your OpenVPN client to connect:

1. Apple iOS client
2. Android client
3. Apple MacOS (OS X) client
4. Windows 8/10 client

Forgotten your .opvn file location such as desktop.ovpn on the Ubuntu server? Try locating by typing the following command:
sudo find / -iname "*.ovpn"

Linux Desktop: OpenVPN client configuration

First, install the openvpn client for your desktop, enter:
$ sudo yum install openvpn
OR
$ sudo apt install openvpn
Next, copy desktop.ovpn as follows:
$ sudo cp desktop.ovpn /etc/openvpn/client.conf
Test connectivity from the CLI:
$ sudo openvpn --client --config /etc/openvpn/desktop.conf
Your Linux system will automatically connect when computer restart using openvpn script/service:
$ sudo systemctl start openvpn@client #

Step 5 - Verify/test the connectivity

Execute the following commands after connecting to OpenVPN server from your Linux desktop:
$ ping 10.8.0.1 #Ping to the OpenVPN server gateway
$ ip route #Make sure routing setup working
$ dig TXT +short o-o.myaddr.l.google.com @ns1.google.com #Must return public IP address of OpenVPN server

Step 6 - Adding or removing OpenVPN client

Log in to your server and run the script again:
$ sudo ./openvpn-install.sh

OpenVPN is already installed.

Select an option:
   1) Add a new client
   2) Revoke an existing client
   3) Remove OpenVPN
   4) Exit
Option: 

Use option number 1 to add a new client and option number 2 to remove or revoke an existing OpenVPN client from the server.

A note about trouble shooting OpenVPN server and client issues

Check OpenVPN server for errors using the jourlctl command:
$ sudo journalctl --identifier openvpn
## OR filter it using the grep command/egrep command ##
$ sudo journalctl -x -e | grep -i 'error'
$ sudo journalctl -x -e | egrep -i 'error|warn|your_ip_here'
$ sudo journalctl -xe -u openvpn-server@server.service

Click to enlarge image

[Unit]
Before=network.target
[Service]
Type=oneshot
ExecStart=/sbin/iptables -t nat -A POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to 172.xxx.yyy.zzz
ExecStart=/sbin/iptables -I INPUT -p udp --dport 1194 -j ACCEPT
ExecStart=/sbin/iptables -I FORWARD -s 10.8.0.0/24 -j ACCEPT
ExecStart=/sbin/iptables -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
ExecStop=/sbin/iptables -t nat -D POSTROUTING -s 10.8.0.0/24 ! -d 10.8.0.0/24 -j SNAT --to 172.xxx.yyy.zzz
ExecStop=/sbin/iptables -D INPUT -p udp --dport 1194 -j ACCEPT
ExecStop=/sbin/iptables -D FORWARD -s 10.8.0.0/24 -j ACCEPT
ExecStop=/sbin/iptables -D FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
ExecStart=/sbin/ip6tables -t nat -A POSTROUTING -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to 2600:xxxx::yyyy:zzzz:dddd:cccc
ExecStart=/sbin/ip6tables -I FORWARD -s fddd:1194:1194:1194::/64 -j ACCEPT
ExecStart=/sbin/ip6tables -I FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
ExecStop=/sbin/ip6tables -t nat -D POSTROUTING -s fddd:1194:1194:1194::/64 ! -d fddd:1194:1194:1194::/64 -j SNAT --to 2600:xxxx::yyyy:zzzz:dddd:cccc
ExecStop=/sbin/ip6tables -D FORWARD -s fddd:1194:1194:1194::/64 -j ACCEPT
ExecStop=/sbin/ip6tables -D FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target

Of course, we check status of the iptables rules that required for OpenVPN using the following command:
$ sudo systemctl status openvpn-iptables.service
If not running or failed start it:
$ sudo systemctl start openvpn-iptables.service
Another option is to run iptables command and sysctl command commands to verify NAT rule setup on your server:
$ sudo iptables -t nat -L -n -v
$ sysctl net.ipv4.ip_forward

Insert the rules if not inserted from /etc/rc.local
$ sudo systemctl start openvpn-iptables.service
$ sudo sysctl -w net.ipv4.ip_forward=1
Is OpenVPN server running and port is open? Use the ss command or netstat command and pidof command/ps command:
$ netstat -tulpn | grep :1194 ## 1194 is the openvpn server port ##
$ ss -tulpn | grep :1194 ## 1194 is the openvpn server port ##
$ ps aux | grep openvpn ## is the openvpn server running? ##
$ ps -C openvpn ## is the openvpn server running? ##
$ pidof openvpn ## find the openvpn server PID ##

If not running, restart the OpenVPN server:
$ sudo systemctl start openvpn-server@server.service
Look out for errors:
$ sudo systemctl status openvpn-server@server.service
Can the Linux desktop client connect to the OpenVPN server machine? First you need to run a simple test to see if the OpenVPN server port (UDP 1194) accepts connections:
$ nc -vu 104.20.187.5 1194
Connection to 104.20.187.5 1194 port [udp/openvpn] succeeded!
If not connected it means either a Linux desktop firewall or your router is blocking access to server. Make sure both client and server using same protocol and port, e.g. UDP port 1194.

Conclusion

Congratulations. You successfully set up an OpenVPN server on Ubuntu Linux 18.04 LTS server running in the cloud. See the OpenVPN website server, Github script page, and Ubuntu server/project for additional information.

Source: https://www.cyberciti.biz/faq/ubuntu-18-04-lts-set-up-openvpn-server-in-5-minutes/