Asus Merlin and Mullvad VPN

ROUTERS

Last updated: 10 February 2021


This guide walks you through how to setup your Asus router running Asuswrt-Merlin to connect to the Mullvad VPN servers.

Asuswrt-Merlin is an open source third-party firmware for Asus routers based on Asuswrt (the firmware developed by Asus) with an advanced OpenVPN client. It's recommended to use this over the default stock firmware as it often uses a newer version of the OpenVPN client.

The VPN speed is dependent on the router's CPU. OpenVPN can only use one CPU core, and AES encryption takes quite some processing power. However some newer Asus routers have an AES-NI hardware acceleration chip that takes that care that. Here are some speed estimates we found around the Internet:

RT-AC86U = 247 Mbps
RT-AC88U = 40-60 Mbps
RT-AC55U = 48-80 Mbps
RT-AC56U = 25 Mbps
RT-AC68U = 25 Mbps

Supported router models

Asuswrt-merlin officially supports the following router models:

  •     RT-AC66U_B1 (same firmware as the RT-AC68U)
  •     RT-AC68U  (including revisions C1 and E1)
  •     RT-AC68P (same firmware as RT-AC68U)
  •     RT-AC68UF (same firmware as RT-AC68U)
  •     RT-AC87
  •     RT-AC3200
  •     RT-AC88U
  •     RT-AC3100
  •     RT-AC5300
  •     RT-AC1900 (same firmware as RT-AC68U)
  •     RT-AC1900P (same firmware as RT-AC68U)
  •     RT-AC86U (starting with version 382.1)
  •     RT-AC2900 (same firmware as RT-AC86U)
  •     RT-AX88U
  •     RT-AX56U
  •     RT-AX58U
  •     RT-AX3000 (same firmware as RT-AX58U)

Set-up instructions

Open a web browser and enter the IP address of your router, which is normally 192.168.1.1. It is possible to change it in Advanced Settings > LAN. In this tutorial, we use 192.168.1.1.

In Advanced Settings click on VPN.

Click on the VPN Client tab at the top of your screen and then click on OpenVPN in order to create a VPN connection.

Download the Android configuration file from Mullvad's website and then click on Choose File.

Click on the Mullvad configuration file and then Open.

After selecting the configuration file, click on Upload.
This loads the required settings in the GUI and then clears the uploaded file field.
Also enable Automatic start at boot time.
 

You will now see the OpenVPN client settings. Double-check that your settings match the ones in the image below. For Username/Password Authentication, choose Yes. For Username, enter your Mullvad account number (without any spaces). For Password, use "m".
The server address and port may vary.

Rules for routing client traffic through the tunnel

  • Description: Mullvad
  • Source IP: 192.168.1.0/24
  • Destination IP: 0.0.0.0/0

Note: Make sure that the Source IP in "Rules for routing client traffic through the tunnel" is set to the local IP address range that the computer/device which is connected to the router is using. You may have to refresh the IP on the computer/device or restart it if it's using the DHCP server on the router.

In Custom Configuration, add comp-lzo no in the bottom of the text list

Press the Apply button and then go to lan -> DHCP Server -> Enter DNS :

DNS Server 1: 193.138.218.74
DNS Server 2: 10.8.0.1

Next to Advertise router's IP in addition to user-specified DNS, select No and press the Apply button

Go to IPv6 -> Basic Config. Change Connection type to Native. Then press the Apply button

Go back to VPN -> VPN Client and then under Client control, find Service state and switch the button to ON in order to connect to Mullvad servers.

Click on VPN Status to see information about your VPN connection.

You should now be connected to Mullvad. Easily verify your connection status by visiting our Connection check in a browser on a device that is connected to the router.

Port Forwarding (optional)

  1. Go to Administration -> System -> Enable JFFS custom scripts and configs
  2. Enable SSH(LAN only) in the router on the same page.
  3. SSH into the router.

Copy and paste the following text this into the terminal (replace YOURPORT with the assigned port and replace THECOMPUTERSIP with the device IP that you wish to forward the port to).

echo -e "#!/bin/sh \niptables -t nat -A PREROUTING -i tun+ -p udp --dport YOURPORT -j DNAT --to-destination THECOMPUTERSIP \niptables -t nat -A PREROUTING -i tun+ -p tcp --dport YOURPORT -j DNAT --to-destination THECOMPUTERSIP" > /jffs/scripts/nat-start && chmod +x /jffs/scripts/nat-start

Troubleshooting
 

Username/password

When you enter the username you should use your Mullvad account number without any spaces. The password is "m".

IPv6 issues

You normally have to enable IPv6 on the router (on the left side under Advanced Settings > IPv6). Set it to "Native". In case our Connection check shows an IPv6 leak you can go to the IPv6 settings and disable "Enable Router Advertisement". Another option is to disable IPv6 and edit the OpenVPN configuration file and make the following changes before you import it:

    replace proto udp with proto udp4.
    replace proto tcp with proto tcp4.
    add pull-filter ignore "route-ipv6"
    add pull-filter ignore "ifconfig-ipv6"

"Unrecognized option"

Download the OpenVPN configuration file for Android, not another platform. Use Chrome when downloading.

"Cannot resolve host address"

Check the option "Use IP addresses" in the Advanced Settings of the OpenVPN configuration file generator.

"certificate verify failed / TLS handshake failed"

Set the time and date correctly in the router.

"You must define CA file (--ca) or CA path (--capath)"

Try to go to Administration > System and select the option to format JFFS on next boot. Click on Apply and then Reboot. Wait a couple of minutes after it reboots and then reboot again. If it does not help then log in with SSH and run "df" and check that you have a jffs partition. The certificate should normally appear in VPN Client > Crypto Settings > Keys and Certificates > Edit > Certificate Authority.

The router connects to the VPN and gets a Mullvad IP address, but the computer/device does not.

Make sure that the Source IP in "Rules for routing client traffic through the tunnel" is set to the local IP address range that the computer/device which is connected to the router is using. You may have to refresh the IP on the computer/device or restart it if it's using the DHCP server on the router.

Log file

If you get into any trouble you can send us the log from Advanced settings > System Log > General Log.