VPN server configuration

This page describes how to configure OpenVPN server for IPv4 and IPv6.

Requirements

Your server must have a network interface that supports both IPv4 and IPv6.

• OVH servers require some configuration, see dedicated section

OVH server IP V6 configuration

By default OVH gives you an IPv6 but it does not enable it!! you must enable it by yourself.

Get OVH server IPv6 settings

Connect to OVH manager

• Go to Server section > VPS > *my server*
• Copy the settings that are under IP

Configure server

Now that you know which settings to apply, let's configure the server.

• Connect in SSH to the server
• Backup and edit network configuration
• Make settings persistent - if OK

Adjust network configuration

cd/etc/netplan

# backup current config
sudo mkdir backup
sudo cp 50-cloud-init.yaml backup/50-cloud-init.yaml.backup-ovh-ipv4

# edit config
vim 50-cloud-init.yaml

Adjust content to your own need:

network:
    version: 2
    ethernets:
        ens3:
            dhcp4: true
            dhcp6: false
            addresses:
                - "OVH_IP_V6/64"
            gateway6: "OVH_IP_V6_GATEWAY"
            routes:
                - to: "OVH_IP_V6/64"
                  via: "OVH_IP_V6_GATEWAY"
            match:
                macaddress: fa:16:3e:96:b9:66
            set-name: ens3

(i) note that DHCP6 must NOT be enabled.

Example:

network:
    version: 2
    ethernets:
        ens3:
            dhcp4: true
            dhcp6: false
            addresses:
                - "2007:81f1:0502:2200:0:0:0:c55/64"
            gateway6: "2007:81f1:0502:2200:0000:0000:0000:0001"
            routes:
                - to: "2007:81f1:0502:2200:0:0:0:c55/64"
                  via: "2007:81f1:0502:2200:0000:0000:0000:0001"
            match:
                macaddress: fa:16:3e:96:b9:66
            set-name: ens3

Try the configuration:

sudo netplan try

Test the new configuration from another computer

# windows
ping -6 2007:81f1:0502:2200:0:0:0:c55

Let configuration persistent

# Create new config file
echo "network: {config: disabled}" > /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg

Reboot server and try to ping it again.

Server network configuration

Enable port forwarding

You must enable FORWARDING somewhere else:

vim /etc/sysctl.conf

# Uncomment the next line to enable packet forwarding for IPv4
net.ipv4.ip_forward=1


# Uncomment the next line to enable packet forwarding for IPv6
#  Enabling this option disables Stateless Address Autoconfiguration
#  based on Router Advertisements for this host
net.ipv6.conf.all.forwarding=1

Firewall

You have to:

• allow INPUT UDP 8080 ipv4/ipv6
• allow ESTABLISHED / RELATED ipv4/ipv6
• allow FORWARDING tun0 <> ethernet
• allow PRE-ROUTING

OpenVPN setup

Installation

sudo apt install openvpn 
sudo apt install easy-rsa

Prepare files

You can use an existing example or start from scratch, as you like. If you want to reuse one of the OpenVPN examples:

sudo cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf.gz /etc/openvpn
cd /etc/openvpn/
sudo gzip -d server.conf.gz

# create temp folder, required for the CHROOT
sudo mkdir -p /etc/openvpn/tmp

Security algorithms and hash

Depending on your server and distribution you might not always have the same encryption and|or hash algorithms available. Choose your algorithms!

Cryptographic algorithms

openvpn --show-ciphers

Search for: AES-256-CBC

(i) Still ok in 2019-11

Hash algorithms

openvpn --show-digests

Search for: SHA512

(i) use the 512 version in 2019-11

Handshake algorithms

openvpn --show-tls

use the default one, from openVpn v2.4+ it is TLS 1.2

Create CA

See Next step

IPv4 configuration

This is how you configuration should look like (more or less, depending on your settings):

vim /etc/openvpn/server.conf

##################################################
# OpenVPN 2.0 config file                        #
# ---------------------------------------------- #
# version 1.0 - April 2011 - Guillaume Diaz      #
# version 1.2 - June 2013 - Guillaume Diaz       #
#                           conf update + chroot #
# version 1.3 - April 2016 - Guillaume Diaz      #
#                           security increase    #
# version 1.4 - Nov. 2019 - Guillaume Diaz       #
#                           IPv4 + IPv6          #
#                           ciphers update       #
##################################################

# OpenVPN binding
#########################
# Which local IP address should OpenVPN listen on?
# >> Put nothing to listen on ALL interfaces and IPs (v4 + v6).. Or you have to put 1 line per IP to listen to
;local 217.182.168.213

# To support both IPv4 + IPv6
proto udp6
port 8080

# Network interface to use
dev tun



# Security details (certificates)
#####################################

# Tell TLS that we are building a SERVER configuration
tls-server

# Certification authority (= root certificate): ca
ca /etc/openvpn/ca.crt

# OpenVPN server's certificate and private key
cert /etc/openvpn/server.crt
key /etc/openvpn/server.key

# Diffie hellman secret key
dh /etc/openvpn/dh2048.pem


# Security details (cryptography and communication settings)
##############################################################

# Keep alive communication
keepalive 10 120

# Encryption of data exchange
cipher AES-256-CBC

# Integrity check
auth SHA512

# Enable compression on the VPN link
# (i) for OpenVPN v2.4+ only
compress lz4-v2
push "compress lz4-v2"

# Reduce OpenVPN rights
user nobody
group nogroup

persist-key
persist-tun



# IP V4 configuration
###################################
# Virtual network IPv4
server 192.168.15.0 255.255.255.0



# IP V6 configuration
###################################
# Virtual network IPv6
# >> Use root similar to "private network" / "local-loop" addresses
server-ipv6 fd42:feed:feed:feed::/64

# Create virtual network interface to support IP v6
tun-ipv6

# Ask clients to create a virtual interface for IP v6
push tun-ipv6

# Tell client to add an IPv6 route to the VPN network
server-ipv6 2001:41d0:2:bb7:800::/64



# Clients management
#########################
# Notify the client when the server restarts so it can automatically reconnect
explicit-exit-notify 1

# Set fix IP@ to client
# >> every time a client connects it will always have the same IPv4 and IPv6 addresses
ifconfig-pool-persist /var/log/openvpn/ipp.txt

# Allow clients to reach OpenVPN network
push "route 192.168.15.0 255.255.255.0"

# Let clients talk to each-other
client-to-client



# Traffic redirection
############################

# Tell client to re-route ALL IPv6 Internet trafic to VPN
# this override the "default" OpenVPN route, by hijacking the route that all IPv6 packets use by default: 2000::/3
#push "route-ipv6 2000::/3"

# Redirect all internet traffic to VPN server
push "redirect-gateway def1 bypass-dhcp"
push "redirect-gateway ipv6"            # for iOS


# Once connected to the VPN the clients cannot use their default DNS provider: you must new DNS links
# OpenDNS
push "dhcp-option DNS 208.67.222.222"
push "dhcp-option DNS 208.67.220.220"
# for latest versions of OpenVPN, DNS v6
push "dhcp-option DNS6 2620:0:0:ccd::2"



# Logs
########################

# Output a short status file showing current connections,
# truncated and rewritten every minute.
status /var/log/openvpn/openvpn-status.log

# File log
log         /etc/openvpn/logs/openvpn.log

# Set the appropriate level of log file verbosity.
#
# 0 is silent, except for fatal errors
# 4 is reasonable for general usage
# 5 and 6 can help to debug connection problems
# 9 is extremely verbose
verb 3

# Silence repeating messages.
;mute 20

See if it works

Start the service

service openvpn restart

Check that OpenVPN listen to port 8080

netstat -pl --numeric | grep 8080

You should have 1 entry per protocol

Check the OpenVPN logs

cat /var/log/openvpn.log

General notes

You can either use TCP or UDP. Performances are the same, UDP is a bit easier to install.

Be careful when you choose the port number! Common open ports:

• 80 (http)
• 443 (HTTPS)
• 8080 (Proxy / JEE servers)

[!] Reminder: for every network that you want to make it accessible through your VPN you must push a new route to it.

Sources

• Alibaba cloud tutorial
• OpenVPN server configuration IPv4 / IPv6
• firewall configuration