MTUs go strange ways - ?

MTUs go strange ways - ?

[lejeczek]

Hi guys

I thought there was no better place to ask but here so - 
both ends are centOS and end up having different MTUs for 
'wg' interfaces and that I thought, was wrong.
1370 (server) VS 1320 (client)
I expected that would be negotiated between nodes without 
user involved, right?
Moreover if I from the client do:

-> $ ping server -M do -s 1300
PING 10.3.3.1 (10.3.3.1) 1300(1328) bytes of data.
ping: local error: message too long, mtu=1320

Could some expert or two shed more light on what & why is 
happening?
many thanks, L.

Re: MTUs go strange ways - ?

[Mark Lawrence]

On Sat Apr 02, 2022 at 06:41:12PM +0100, lejeczek wrote:

>1370 (server) VS 1320 (client)
>
>Could some expert or two shed more light on what & why is happening?

There is a (very small) clue in the name: Maximum *Transmission* Unit.

Routing between two IP addresses is not (necessarily) symetric: routes 
are computed independently in each direction. So there is probably a 
(1320 MTU) device somewhere inbetween which is only crossed by packets 
in the client->server direction, but not in the reverse.

-- 
Mark Lawrence

Re: MTUs go strange ways - ?

[Frank Volf]

Hi,

The "-s" option provides the length of the ICMP data in the packet, not 
the total packet length.
To the data size you need to add 8 bytes for the ICMP header and 20 
bytes for the IP header.
So, you are sending 1328 bytes (which is noted between the brackets in 
your output).

If you change to "-s 1292" then you are actually sending packets with 
total length of 1320.

Kind regards,

Frank


Op 2-4-2022 om 19:41 schreef lejeczek:
> Hi guys
>
> I thought there was no better place to ask but here so - both ends are 
> centOS and end up having different MTUs for 'wg' interfaces and that I 
> thought, was wrong.
> 1370 (server) VS 1320 (client)
> I expected that would be negotiated between nodes without user 
> involved, right?
> Moreover if I from the client do:
>
> -> $ ping server -M do -s 1300
> PING 10.3.3.1 (10.3.3.1) 1300(1328) bytes of data.
> ping: local error: message too long, mtu=1320
>
> Could some expert or two shed more light on what & why is happening?
> many thanks, L.
>