Configuring OpenVPN on a UniFi Security Gateway

In my homelab setup I am using UniFi network components and I was looking for a secure way to connect to my environment when I am on the road. After doing some research on the technical implementations, I found that OpenVPN – at least in theory – offers the best security, performance and reliability compared to L2TP / IPSec and PPTP.

There are several guides how to configure this for a UniFi Security Gateway, however, I found that many of them are lacking important information or were wrong. I ‘trial ‘n error’d’ through those guides until I found a setup which is working quite well for me.

Authentication requirements

As a prerequisite, you will need to enable the built in Radius Server on the USG. To do that, navigate to your UniFi Controller and navigate to Settings – Services. Enable the Radius Server from the menu and enter your secret key:

From the users tab, you can add your OpenVPN users. I recommend specifying a different VLAN for security reasons.

Tunnel type: 3 – Layer Two Tunneling Protocol (L2TP)
Tunnel medium type: 1 – IPv4 (IP version 4)

Since you should have entered two passwords after the first part already, I recommend you start using a password manager now if you do not use one already. Generating strong unique passwords and saving it using a proper password manager is essential – I recommend Bitwarden.

Set up easy-rsa and key generation

You need to connect to your USG using SSH. If you have not enabled SSH yet, you will need to activate advanced mode in your site settings:

The SSH login can also be configured in the site settings:

Now you can connect to your USG using SSH, I’m using Putty:

Download and install easy-rsa

sudo bash
curl -O http://ftp.us.debian.org/debian/pool/main/e/easy-rsa/easy-rsa_2.2.2-1_all.deb
sudo dpkg -i easy-rsa_2.2.2-1_all.deb

Initialize the PKI and build the CA certificate and key

cd /usr/share/easy-rsa
. vars
./clean-all
./build-ca

After you entered the ./build-ca command, you will be prompted for several parameters of which only the ‘Common Name’ needs to be defined.

Generate a certificate and private key for the server

Give it a common name like “OpenVPN CA”.

./build-key-server server

Confirm both prompts with yes:
‘Sign the certificate? [y/n]’ and ‘1 out of 1 certificate requests certified, commit? [y/n]’

Generate Diffie Hellman parameters

./build-dh

Note that this process will take a long time to complete. In my case it took about 20 minutes.

You will end up with several files in the keys directory:

Copy the keys to the /config/auth/keys/ folder

mkdir /config/auth/keys/
cp keys/* /config/auth/keys/

Configure OpenVPN on the USG

Note: In line 3, insert your own subnet and pick one which is not in use anywhere else on your USG. You might want to edit other options like DNS as well.

configure
set interfaces openvpn vtun0 mode server
set interfaces openvpn vtun0 server subnet 192.168.30.0/24
set interfaces openvpn vtun0 tls ca-cert-file /config/auth/keys/ca.crt
set interfaces openvpn vtun0 tls cert-file /config/auth/keys/server.crt
set interfaces openvpn vtun0 tls key-file /config/auth/keys/server.key
set interfaces openvpn vtun0 tls dh-file /config/auth/keys/dh2048.pem
set interfaces openvpn vtun0 encryption aes128
set interfaces openvpn vtun0 openvpn-option "--keepalive 8 30"
set interfaces openvpn vtun0 openvpn-option "--comp-lzo"
set interfaces openvpn vtun0 openvpn-option "--duplicate-cn"
set interfaces openvpn vtun0 openvpn-option "--user nobody --group nogroup"
set interfaces openvpn vtun0 openvpn-option "--plugin /usr/lib/openvpn/openvpn-auth-pam.so openvpn"
set interfaces openvpn vtun0 openvpn-option "--client-cert-not-required --username-as-common-name"
set interfaces openvpn vtun0 openvpn-option "--verb 1"
set interfaces openvpn vtun0 openvpn-option "--proto udp6"
set interfaces openvpn vtun0 openvpn-option "--port 1194"
set interfaces openvpn vtun0 openvpn-option "--push redirect-gateway def1"
set interfaces openvpn vtun0 openvpn-option "--push dhcp-option DNS 8.8.8.8"
set interfaces openvpn vtun0 openvpn-option "--push dhcp-option DNS 8.8.4.4"

Configure your firewall so that it accepts OpenVPN connections on the WAN interface

set firewall name WAN_LOCAL rule 20 action accept
set firewall name WAN_LOCAL rule 20 description "Allow OpenVPN clients in"
set firewall name WAN_LOCAL rule 20 destination port 1194
set firewall name WAN_LOCAL rule 20 log disable
set firewall name WAN_LOCAL rule 20 protocol udp

Optionally configure the same rule for IPv6

set firewall ipv6-name wan_local-6 rule 20 action accept
set firewall ipv6-name wan_local-6 rule 20 description "Allow OpenVPN clients in"
set firewall ipv6-name wan_local-6 rule 20 destination port 1194
set firewall ipv6-name wan_local-6 rule 20 log disable
set firewall ipv6-name wan_local-6 rule 20 protocol udp

Configure your USG to allow traffic from OpenVPN users to Internet

set service nat rule 5010 description "Masquerade for WAN"
set service nat rule 5010 outbound-interface eth0
set service nat rule 5010 type masquerade
commit
save
exit

Create an .ovpn file

Paste the below into a new file, edit the hostname in line 5. Instead of the hostname you can also enter your WAN IP. I recommend using a hostname with dynamic DNS. With Namecheap for example, you can use an ‘A + Dynamic DNS Record’.
Then paste your certificate info in line 18. Copy it from /config/auth/keys/ca.crt.

client
float
dev tun

remote vpn.hostname.com 1194 udp

auth-nocache
resolv-retry infinite
nobind
persist-key
persist-tun
auth-user-pass
cipher AES-128-CBC
comp-lzo
verb 3
<ca>
-----BEGIN CERTIFICATE-----
INSERT YOUR CERT HERE
-----END CERTIFICATE-----
</ca> 

# this is an random certificate. The .ovpn file needs one, but does not use it, so you can leave this as is
<cert> 
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
</cert>

# this is an random key. The .ovpn file needs one, but does not use it, so you can leave this as is
<key>
-----BEGIN PRIVATE KEY-----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-----END PRIVATE KEY-----
</key>

Configuring OpenVPN to use Radius for authentication

Create a file /etc/pam_radius_auth.conf with your Radius server IP (USG IP) and the shared secret you used when enabled the Radius server. Use vi to edit the file to avoid formatting issues.

RADIUSSERVERIP SHAREDSECRET

Create a file /etc/pam.d/openvpn with the following contents:

auth sufficient pam_radius_auth.so debug
account sufficient pam_permit.so
session sufficient pam_permit.so

Make the changes persistent

Now you can test the connection, but the above changes are not persistent yet and will be lost once you reboot your USG or once you upgrade the firmware. So here is how we can make this configuration survive a reboot. Copy the previously created files into a new folder /config/scripts/ovpn_radius_config:

mkdir /config/scripts/ovpn_radius_config
cp /etc/pam_radius_auth.conf /config/scripts/ovpn_radius_config/pam_radius_auth.conf
cp /etc/pam.d/openvpn /config/scripts/ovpn_radius_config/openvpn

Copy the following content into a new file /config/scripts/postprovision.sh

#!/bin/vbash
readonly logFile="/var/log/postprovision.log"

source /opt/vyatta/etc/functions/script-template

echo "$(date) - Beginning post provision steps" >> ${logFile}
#restore the ssmtp configuration

cp -f /config/scripts/ovpn_radius_config/pam_radius_auth.conf /etc
cp -f /config/scripts/ovpn_radius_config/openvpn /etc/pam.d/openvpn

#the following lines remove the postprovision scheduled task
#do not modify below this line

configure >> ${logFile}
delete system task-scheduler task postprovision >> ${logFile}
commit >> ${logFile}
save >> ${logFile}
#exit

#end no edit

exit

echo "$(date) - Finished post provision steps" >> ${logFile}

Make the script executable:

sudo chmod +x /config/scripts/postprovision.sh

Copy the following script to /config/scripts/post-config.d/postreboot.sh and make it executable as well:

#!/bin/vbash
readonly logFile="/var/log/postreboot.log"

source /opt/vyatta/etc/functions/script-template

#restore the radius configuration
echo "Copying PAM and OpenVPN config files" >> ${logFile}
cp -f /config/scripts/ovpn_radius_config/pam_radius_auth.conf /etc
cp -f /config/scripts/ovpn_radius_config/openvpn /etc/pam.d/openvpn

sudo chmod +x /config/scripts/post-config.d/postreboot.sh

Now you also need to edit your config.gateway.json file to make the above changes persistent or create this file if you do not use this yet.

Depending on which UniFi Controller you use, the file can be located in different folders. On a UniFi Cloud Key for example you can find the file here if you only have one site: /srv/unifi/data/sites/default/
In my case I had to create the folders /sites and /default because they did not exist yet.

Remember to edit your subnet in line 57 accordingly and also other changes from the above steps.

Also, if you want to merge the below file with your existing file, or you want to include other options and the settings are not applied correctly when booting your USG, there is a chance your json format is wrong. You can check your file here and get infos on how to fix syntax issues: https://jsonformatter.curiousconcept.com/

{
        "firewall": {
                "ipv6-name": {
                        "wan_local-6": {
                                "default-action": "drop",
                                "rule": {
                                        "20": {
                                                "action": "accept",
                                                "description": "Allow OpenVPN clients in",
                                                "destination": {
                                                        "port": "1194"
                                                },
                                                "log": "disable",
                                                "protocol": "udp"
                                        }
                                }
                        }
                },
                "name": {
                        "WAN_LOCAL": {
                                "default-action": "drop",
                                "description": "packets from internet to gateway",
                                "rule": {
                                        "20": {
                                                "action": "accept",
                                                "description": "Allow OpenVPN clients in",
                                                "destination": {
                                                        "port": "1194"
                                                },
                                                "log": "disable",
                                                "protocol": "udp"
                                        }
                                }
                        }
                }
        },
        "interfaces": {
                "openvpn": {
                        "vtun0": {
                                "encryption": "aes128",
                                "mode": "server",
                                "openvpn-option": [
                                        "--keepalive 8 30",
                                        "--comp-lzo",
                                        "--duplicate-cn",
                                        "--user nobody --group nogroup",
                                        "--plugin /usr/lib/openvpn/openvpn-auth-pam.so openvpn",
                                        "--client-cert-not-required --username-as-common-name",
                                        "--verb 1",
                                        "--proto udp6",
                                        "--port 1194",
                                        "--push redirect-gateway def1",
                                        "--push dhcp-option DNS 8.8.8.8",
                                        "--push dhcp-option DNS 8.8.4.4"
                                ],
                                "server": {
                                        "subnet": "192.168.30.0/24"
                                },
                                "tls": {
                                        "ca-cert-file": "/config/auth/keys/ca.crt",
                                        "cert-file": "/config/auth/keys/server.crt",
                                        "dh-file": "/config/auth/keys/dh2048.pem",
                                        "key-file": "/config/auth/keys/server.key"
                                },
								"firewall": {
									"in": {
									"name": "LAN_IN"
									},
									"local": {
									"name": "LAN_LOCAL"
									},
									"out": {
									"name": "LAN_OUT"
									}
								}
						}
				}
		},
        "service": {
                "nat": {
                        "rule": {
                                "5010": {
                                        "description": "Masquerade for WAN",
                                        "outbound-interface": "eth0",
                                        "type": "masquerade"
                                }
                        }
                }
        },
        "system": {
                "task-scheduler": {
                        "task": {
                                "postprovision": {
                                        "executable": {
                                                "path": "/config/scripts/postprovision.sh"
                                        },
                                        "interval": "3m"
                                }
                        }
                }
        }
}

By default the vtun0 interface is not part of any other existing interface group (WAN, LAN, GUEST). That’s why firewall rules do not apply and OpenVPN users can access any network on your USG. To mitigate that, I added lines 65 to 75. Of course you also need to add firewall rules, see below.

After placing the modified config.gateway.json file in the corresponding location, you may force provision your USG from the Controller to apply the settings:

Firewall rules

I want to allow my OpenVPN clients to access my homelab, but I want to block access to my other networks. First, define the network group in your firewall settings. Also, depending on your setup you can create further network combinations here for the other rules (see my examples below):

Then create the LAN IN rules. As you can see I will allow access to my homelab network, but I will drop any connection attempt to a network group called ‘OpenVPN Block’ with my private network devices. If you want to use Internet, make sure not to block your WAN network.

Allow rule (exceptions):

The drop rule will discard any traffic from the OpenVPN network to your list of defined networks:

Last but not least, you will need to create the following LAN OUT rule, to make sure to receive outgoing packets.
You only need to allow established and related states:

Final connection test

Now it’s time to test the connection:

As you can see, the connection works as intended! 😊
If there any questions or issues you experience, do not hesitate writing a comment or contacting me.