OpenVPN my configuration notes

Intro

OpenVPN as many parameters that we can play with.

I will put here some configuration tips that I've used. I traditionnaly use a Debian to set up my OpenVPN server.

Server or Client side?

We can choose to set configurations wherever we want. The main difference is that we need to add the push keyword on the server side, and it will of course be applied to all clients.

Example

Let's see the difference if we want to set a same configuration on each side. Exemple here with a DNS entry.

• Client side :

dhcp-option DNS 192.168.0.200

• Server side :

push "dhcp-option DNS 192.168.0.200"

DNS

Set dns configuration

• If we want to define specific DNS server and DOMAIN :

dhcp-option DNS 192.168.0.200
dhcp-option DOMAIN domain.local

Filtering

It could be useful to only authorize some network flows on our VPN.

Netfilter - iptables

• Accept RDP and dns traffic only :

root@host:~# iptables -A FORWARD -o enp4s0 -p tcp --dport 3389 -j ACCEPT
root@host:~# iptables -A FORWARD -i enp4s0 -p tcp --sport 3389 -j ACCEPT
root@host:~# iptables -A FORWARD -o enp4s0 -p tcp --dport 53 -j ACCEPT
root@host:~# iptables -A FORWARD -o enp4s0 -p udp --dport 53 -j ACCEPT
root@host:~# iptables -A FORWARD -i enp4s0 -p udp --sport 53 -j ACCEPT
root@host:~# iptables -A FORWARD -i enp4s0 -p tcp --sport 53 -j ACCEPT
root@host:~# iptables -A FORWARD -o enp4s0 -j DROP

Routing

Gateway mode

If we want to enable routing.

• Edit /etc/sysctl.conf :

net.ipv4.ip_forward = 1

• Run

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

• Use iptables command to enable NAT and allow clients to reach internal network :

root@host:~# iptables -t nat -A POSTROUTING -s 10.50.8.0/24 -o ens192 -j MASQUERADE

Add a network route configuration

• The 192.168.1.0/24 network will be reachable through the OpenVPN tunnel :

route 192.168.1.0 255.255.255.0

Excluding routes

Here a case where 192.168.0.251 and 192.168.0.250 will be reachable through the VPN, the rest of 192.168.0.0/24 network will reach via LAN default gateway. Particulary useful when the Client and the Server are on the same subnet.

route 192.168.0.251 255.255.255.255
route 192.168.0.250 255.255.255.255
route 192.168.0.0 255.255.255.0 net_gateway

Improve Security

ta.key

To prevent Portscanning, DOS attacks on the OpenVPN UDP port, SSL/TLS handshake initiations from unauthorized machines and any eventual buffer overflow vulnerabilities in the SSL/TLS implementation (source) we can add the HMAC key protection.

• Generate key :

root@host:~# openvpn --genkey --secret /etc/openvpn/pki/issued/ta.key

• Add this line inside /etc/openvpn/server.conf :

tls-crypt /etc/openvpn/pki/issued/ta.key 0

• Add ta.key file and this line inside the client configuration file :

tls-crypt ta.key 1

Server Certificate Verification Method

OpenVPN add the capacity to avoid possible Man-in-the-Middle attack. If not set, you should see this message from the client log : WARNING: No server certificate verification method has been enabled. See http://openvpn.net/howto.html#mitm for more info..

• Add this line to the client configuration file :

remote-cert-tls server

Miscellaneous

Show current sessions

• Add this line to server.conf :

status /var/log/openvpn-status.log

• To see current sessions :

root@host:~# cat /etc/openvpn/openvpn-status.log