Change MTU to Establish a VPN Connection

• • What's MTU
  • What's MSS
  • Why I should change MTU
  • How to change MTU for WireGuard
  • How to change MTU for OpenVPN

What is, "MTU"

MTU (Maximum Transmission Unit) is like the maximum size of a container that can hold your data when it's being sent over a network. Think of it as a shipping container. Each container has a limit to how much stuff it can carry. Similarly, a network link has an MTU that determines the largest chunk of data that can be sent in a single "container" from one place to another.

What is, "MSS"

MSS (Maximum Segment Size) is closely connected to MTU. It's like the largest size of individual items that can fit comfortably inside that shipping container. When two devices communicate, they agree on this MSS value to make sure that the data they're exchanging fits nicely within each "container." If the items are too big, they might need to be split into smaller pieces to fit properly.

Why I should change MTU

Imagine you're sending a really long message through a narrow tunnel. If the message is too big to fit, you'd have to cut it into smaller parts and then reassemble it on the other side. Similarly, in networking, if the data you're trying to send is too big for the MTU of the network, it has to be broken down into smaller pieces. This process can slow down your communication. Changing the MTU can help your data travel more quickly and smoothly through the network tunnel.

Firewalla defaults the MTU to 1412, which is safe for most networks. If you don't have a connection issue, you probably have nothing to be concerned about. But, if you have issues connecting to Firewalla VPN Server, it's worth trying lower values (for example 1300).

How to change MTU for WireGuard

If you are using Fireawlla's WireGuard VPN server, it can be changed via the WireGuard native app:

1. Launch the WireGuard app.
2. Tap the Firewalla VPN profile you previously set up.
3. Click the 'Edit' button.
4. Change the MTU value, from 1412 to 1300.
5. Click 'Save', reconnect your VPN, and see if your connection is more stable.

How to change MTU for OpenVPN

If you are using Firewalla's OpenVPN server

1. Download the OpenVPN profile from the Firewalla app.
2. Open it using your favorite text editor.
3. Add a line that says "link-mtu 1300" (without quotes) near the top.\
4. Save the edited profile.
5. Re-import it into your OpenVPN client and see if your connection is more stable.