VPN and MTU size

Recently I bought VPN service from TigerVPN. I configured my OpenVPN client as they clearly instructed. However I experienced problems connecting to websites although I could successfully establish my VPN connection.

My internet provider assign IPv6 addresses to clients. My connection goes through a NAT for IPv4 which can also cause connection problems.

I first checked TigerVPN’s troubleshooting guide. It says you have to decrease the MTU size (default 1500) of packets to tackle this problem. It instructs you to alter the network connection setting in Windows OS level. First you get the name of your connection with the following command in command prompt (cmd):

netsh interface ipv4 show subinterfaces

Then you can change the MTU size of your connection, for example “Local Network Connection”, to a value 1420 with the following command and reboot:

netsh interface ipv4 set subinterface "Local Area Connection" mtu=1420 store=persistent

However this did not work out for me and I investigated more about this. It only worked when I tried to fix the MTU size not in network connection in Windows but connection settings in OpenVPN configuration file. First you have to revert the change above by setting the value back to default 1500 again. I assume you are familiar with the ovpn files and their locations if you followed the instructions for first configuration.

In a simple text editor like notepad, you must change “mssfix” line in ovpn file that you get from TigerVPN, “US – Miami @tigervpn.com.ovpn” for example like this:

mssfix 1420

MTU size of 1420 worked best for me and your value may be different. You must test it by decreasing gradually from 1500 for faster and more stable connection. After these findings TigerVPN updated the troubleshooting guide which can also be reachable from this link.


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