mtu on Linux vs MacOS

mtu on Linux vs MacOS

[Harald Dunkel]

Hi folks,

I am using PPPoE to connect to my IP provider. To use wireguard on Linux I
have to reduce the MTU in wg0.conf to 1400. Using the default 1420 a ssh
connection tunneled through wireguard gets stuck (reproducible). An echo
of a very long line (e.g. 4096 chars) is sufficient.

The weird part is, MTU 1420 works fine for wireguard on MacOS. How comes
that Wireguard appears to be more stable on MacOS than on Linux?

Peer is OpenBSD 6.8.


Regards
Harri

Re: mtu on Linux vs MacOS

[Roman Mamedov]

On Sun, 17 Jan 2021 11:36:42 +0100
Harald Dunkel <harri@afaics.de> wrote:

> Hi folks,
> 
> I am using PPPoE to connect to my IP provider. To use wireguard on Linux I
> have to reduce the MTU in wg0.conf to 1400. Using the default 1420 a ssh
> connection tunneled through wireguard gets stuck (reproducible). An echo
> of a very long line (e.g. 4096 chars) is sufficient.
> 
> The weird part is, MTU 1420 works fine for wireguard on MacOS. How comes
> that Wireguard appears to be more stable on MacOS than on Linux?

WireGuard works just fine for me on Linux using MTU 1432 if over IPv4, and
1412 if over IPv6. The underlying PPPoE device MTU is 1492 in my case. Check
which PPPoE MTU you use in Linux vs in MacOS.

-- 
With respect,
Roman

Re: mtu on Linux vs MacOS

[Roman Mamedov]

On Thu, 21 Jan 2021 19:07:18 +0500
Roman Mamedov <rm@romanrm.net> wrote:

> On Sun, 17 Jan 2021 11:36:42 +0100
> Harald Dunkel <harri@afaics.de> wrote:
> 
> > Hi folks,
> > 
> > I am using PPPoE to connect to my IP provider. To use wireguard on Linux I
> > have to reduce the MTU in wg0.conf to 1400. Using the default 1420 a ssh
> > connection tunneled through wireguard gets stuck (reproducible). An echo
> > of a very long line (e.g. 4096 chars) is sufficient.
> > 
> > The weird part is, MTU 1420 works fine for wireguard on MacOS. How comes
> > that Wireguard appears to be more stable on MacOS than on Linux?
> 
> WireGuard works just fine for me on Linux using MTU 1432 if over IPv4, and
> 1412 if over IPv6. The underlying PPPoE device MTU is 1492 in my case. Check
> which PPPoE MTU you use in Linux vs in MacOS.
> 

Oh, and I just remembered -- you're better off setting the same MTU on the
entire WG network, i.e. substract those 8 bytes of PPPoE interface on the
remote host's WG interface as well. Else the local interface MTU will need a
weird and so far unexplained further 4-byte reduction for things to work:
https://lists.zx2c4.com/pipermail/wireguard/2019-April/004078.html

-- 
With respect,
Roman