How To set up OpenVPN on Debian 10 Buster

Last updated: Feb 19, 2021

Intro

Here's a how to about setting up a OpenVPN server under Debian 10 Buster.

Network diagram

OpenVPN windows client/debian server architecture

OpenVPN Server :

OS : Debian GNU/Linux 10 (Buster)
Role : OpenVPN Server + Gateway
IP : 192.168.0.254

Server (Debian)

Installation

Install openvpn package :

root@host:~# apt install openvpn

Go to /etc/openvpn/ directory :

root@host:~# cd /etc/openvpn/

Make OpenVPN start at boot

root@host:~# sed -i 's/#AUTOSTART="all"/AUTOSTART="all"/' /etc/default/openvpn

Enable OpenVPN service :

I had « read UDP: Unknown error (code=10054) » and « TLS Error: TLS key negotiation failed to occur within 60 seconds (check your network connectivity) » error on the client side. The reason seems to be that the default OpenVPN systemd file is broken. If you have the same message, enable the openvpn@.service.

root@host:~# systemctl enable openvpn@.service

PKI

Initialize the pki

root@host:~# /usr/share/easy-rsa/easyrsa clean-all

root@host:~# /usr/share/easy-rsa/easyrsa init-pki

Type yes to initialize :

WARNING!!!

You are about to remove the EASYRSA_PKI at: /etc/openvpn/pki
and initialize a fresh PKI here.

Type the word 'yes' to continue, or any other input to abort.
  Confirm removal: yes

Build the certificate authority in /etc/openvpn/pki/ca.crt :

root@host:~# /usr/share/easy-rsa/easyrsa build-ca nopass

Don't mind about random number generator error :

Using SSL: openssl OpenSSL 1.1.1d  10 Sep 2019
Generating RSA private key, 2048 bit long modulus (2 primes)
...+++++
...........+++++
e is 65537 (0x010001)
Can't load /etc/openvpn/pki/.rnd into RNG
139967504692352:error:2406F079:random number generator:RAND_load_file:Cannot open file:../crypto/rand/randfile.c:98:Filename=/etc/openvpn/pki/.rnd
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Common Name (eg: your user, host, or server name) [Easy-RSA CA]:openvpn-host

CA creation complete and you may now import and sign cert requests.
Your new CA certificate file for publishing is at:
/etc/openvpn/pki/ca.crt

Server certificates

Generate certificate and key for server :

root@host:~# /usr/share/easy-rsa/easyrsa build-server-full server nopass

Generate Diffie Hellman parameters in /etc/openvpn/pki/dh.pem

root@host:~# /usr/share/easy-rsa/easyrsa gen-dh

Client certificates

Create client01 certificate :

Client certificates are inside /etc/openvpn/pki/private/ and /etc/openvpn/pki/issued/ directories.

root@host:~# /usr/share/easy-rsa/easyrsa build-client-full client01 nopass

Create 10 clients certificates with one command :

root@host:~# for i in $(seq -w 1 10);do /usr/share/easy-rsa/easyrsa build-client-full client"$i" nopass; done

/etc/openvpn/server.conf

port 1194
proto udp
dev tun

ca /etc/openvpn/pki/ca.crt # generated keys
cert /etc/openvpn/pki/issued/server.crt
key /etc/openvpn/pki/private/server.key # keep secret
dh /etc/openvpn/pki/dh.pem

server 10.50.8.0 255.255.255.0 # internal tun0 connection IP
ifconfig-pool-persist ipp.txt

keepalive 10 120

comp-lzo # Compression - must be turned on at both end
persist-key
persist-tun

push "dhcp-option DNS 192.168.0.200"
push "dhcp-option DOMAIN std.local"
push "route 192.168.0.0 255.255.255.0"

status /var/log/openvpn-status.log

verb 3 # verbose mode

Restart OpenVPN service

root@host:~# systemctl restart openvpn.service

Router mode

Router mode will allow us to make the 192.168.0.0/24 network reachable from the client side.

iptables

A simple netfilter rule to allow vpn clients to access to the entire network :

List network interfaces

root@host:~# ip addr sh
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 01:02:a0:21:fd:54 brd ff:ff:ff:ff:ff:ff
    inet OPENVPN_IP brd X.X.X.X scope global wan
       valid_lft forever preferred_lft forever
    inet6 fe80::ff:fe5d:f333/64 scope link 
       valid_lft forever preferred_lft forever
3: enp2s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 11:a2:a9:21:fd:54 brd ff:ff:ff:ff:ff:ff
    inet 192.168.0.254 brd X.X.X.X scope global wan
       valid_lft forever preferred_lft forever
    inet6 fe80::6a05:caff:fe39:c153/64 scope link 
       valid_lft forever preferred_lft forever

Set iptables with correct network interface

root@host:~# iptables -t nat -A POSTROUTING -s 10.50.8.0/24 -o enp2s0 -j MASQUERADE

Gateway mode

Edit /etc/sysctl.conf file and add :

net.ipv4.ip_forward=1

And Run this command to take this into account :

root@host:~# sysctl -p /etc/sysctl.conf

Windows client configuration

Files to get :

ca.crt : /etc/openvpn/pki/ca.crt

client01.crt : /etc/openvpn/pki/issued/client01.crt

client01.key : /etc/openvpn/pki/private/client01.key

Client files :

C:\Program Files\OpenVPN\config\TEST.ovpn

client

dev tun

proto udp

remote OPENVPN_IP 1194

resolv-retry infinite
nobind
persist-key
persist-tun

ca ca.crt
cert client01.crt
key client01.key

comp-lzo

verb 3

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