WireGuard Windows should have default MTU of 1280.

tlhackque tlhackque at yahoo.com
Mon Feb 21 18:52:05 UTC 2022

On 18-Feb-22 20:23, Rujbin wrote:
> Hello,
> i am just confused. When i use default MTU the Performance on Windows is VERY poor. It is almost unuseable. It happens on multiple Windows devices. I started using MTU 1280 for a while, but why is it only Windows with that issue? First, the speed is limited to 100mbps maximum. Thats weird, when i use MTU 1280 i have 1gbps. https://i.imgur.com/ELGOWDQ.png
> This bug exists for a long time to me. I ran Wireguard on almost every provider, (i didnt check if it happens on Azure) but this bug exists on Hetzner, DigitalOcean, OVH. This is not normal. I am running the latest stable version of Wireguard Windows. Kernel module on servers and BoringTun.

The question is so vague that you're not going to get unconfused without 
doing more work.

Sounds like a fragmentation issue.  Where can't be determined from the 
information given.  But if path MTU discovery is disabled/broken, that 
kind of slowdown isn't surprising.

1280 is the minimum MTU for IPv6.  (Path discovery is encouraged to use 
larger if possible.)  See RFC2460 section 5.

Where are you setting the MTU?  On the physical IF, or the WireGuard IF?

If the former, you want to increase by the size of the WG overhead.

If your physical IF is IPv4, but you're tunneling IPv6 over WG - the 
minimum MTU for IPv4 is 512, so unless some MTU is set (and available 
for the complete route), WG packets will definitely fragment.

In short, you need to provide more information (including a complete 
configuration, traceroutes with packet sizes, see if MTU discovery is 
blocked, ...), and do more work in order to get a useful answer.

This includes the Windows question.  Is WG running on windows, or on 
some router?  IPv4?  How does this differ from the other devices?  What 
are they (IOS, Android, Linux, VMS, ZOS, ...)?

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