Dropping the Virtualization Stack behind an OpenVPN Server

  • vpnserver-debian
  • Jonathan Haack
  • Haack’s Networking
  • netcmnd@jonathanhaack.com

Latest Wiki: //vpnserver-debian//

This tutorial is for installing a simple openvpn server on a public facing VPS and/or self-hosted virtualization stack. In my case, I am using a slim Debian boot OS, with two zfs pools in RAID10 or two-way mirror setups. I use virsh primarily and/or virt-manager with qemu/kvm to manage the stack. The full setup can be found here https://wiki.haacksnetworking.org/doku.php?id=computing:vmserver. The point of setting up this openvpn server instance was to only expose port 1194 to the public, and turn off 80/443 (which I used for munin monitoring tool), and turn off 22 (which I used for shell access). Also, this is just intended as a vpn, not traffic redirection. Here’s what I did:

sudo apt update
sudo apt upgrade
sudo apt install openvpn

To keep easyrsa from writing over your configurations, most people copy the directory you intend to use away from it’s default location:

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

Navigate inside of the easy-rsa directory (the one you just made by copying) and start building the server by initializing the pki tool, building your certificate authority, generating diffyhelmen for strong key exchange, and then building the openvpn server itself which will leverage these:

cd /etc/easy-rsa/
./easyrsa init-pki
./easyrsa build-ca nopass

./easyrsa gen-dh
./easyrsa build-server-full server nopass

To help thwart DDOS and/or UDP flooding, build a HMAC key as follows. Also, make sure to generate a revocation certificate so you can properly revoke previously signed certificates.

openvpn --genkey secret /etc/easy-rsa/pki/ta.key
./easyrsa gen-crl

Copy all the files and directories for keys/certs that you just generated into the openvpn server directory:

cp -p /etc/easy-rsa/pki/ca.crt /etc/openvpn/server/
cp -p /etc/easy-rsa/pki/dh.pem /etc/openvpn/server/
cp -p /etc/easy-rsa/pki/ta.key /etc/openvpn/server/
cp -p /etc/easy-rsa/pki/crl.pem /etc/openvpn/server/
cp -rp /etc/easy-rsa/pki/issued /etc/openvpn/server/
cp -rp /etc/easy-rsa/pki/private /etc/openvpn/server/

I wanted a consistent static IP for the client, and changing ”ifconfig-pool-persist /var/log/openvpn/ipp.txt” seemed to always get over-written, so I did the following:

nano /etc/openvpn/ccd/client

Note that for the above static assignment to work on the client, you must add ”client-config-dir /etc/openvpn/ccd” to the server configuration. To build the server configuration, I used the template provided and some online resources. Here’s where you get the provided template and a view of what my config looks like.

cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf /etc/openvpn/server/


Now that the server is configured, let’s enable the systemd unit:

systemctl enable --now openvpn-server@server

Let’s make sure the firewall only permits vpn server traffic and ssh from a private subnet as per the design mentioned at the outset:

ufw allow 1184/udp
ufw allow from to any port 22

The server is now setup, so time to build the client files on the server, build a client configuration file and test the connection. Copy all the generated files to a dedicated client directory for safekeeping/backup, and this also makes it easier to ”scp” them down to a client.

cd /etc/easy-rsa
./easyrsa build-client-full client nopass
cp -p /etc/easy-rsa/pki/ca.crt /etc/openvpn/client/ca.crt
cp -rp /etc/easy-rsa/pki/issued/client.crt /etc/openvpn/client/client.crt
cp -rp /etc/easy-rsa/pki/private/client.key /etc/openvpn/client/client.key

From your client, pull the files:

scp -r user@remotehost.com:/etc/openvpn/client /home/user/vpn/

On your localhost, create a client configuration file to leverage these files and connect to the openvpn server. I also included my config as an example below.

nano /home/user/vpn/remotehost.com.ovpn


To test if everything is working, run openvpn against the config file as follows:

sudo openvpn remotehost.com.ovpnsudo openvpn remotehost.com.ovpn

If everything works, you will get a final message of ”Initialization Sequence Completed”. If any errors are given, begin debugging them. There’s a rather large message at the top about a fallback cipher. Unless you need legacy support, ignore this. Also, make sure that your certs/keys and ca are given proper perms:

chmod 600 client.key
chmod 640 ca.crt. client.crt. remotehost.com.ovpn

That should be it! To test, try shelling into the physical host of the virtualization stack:

ssh root@

Next, I need to add routes to a different subnet on a virtual bridge I use for my VMs. Then, I can disable public facing ssh on all of VMs theoretically and access them through the vpnserver only. Again, even this is overkill since I am already using ssh keypairs, however, I might just do it to learn about pushing routes/subnets over openvpn and less because of it providing added security. Moreover, doing this would cause me to have rewrite my entire backup stack on the backup server (lol) since all the domains would need to change from external to internal addresses. But it would be fun!

Thanks! -oemb1905

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