On automatic MAC selection in OpenSSH_6.7p1 + OpenSSL 1.0.1k

Alex Bligh alex at alex.org.uk
Wed Jun 1 03:29:00 AEST 2016

On 31 May 2016, at 18:22, Dimitris Diochnos <diochnos at gmail.com> wrote:

> I can understand that some element along the path from B to A is
> silently dropping packets larger than 1458. However, the other point
> that I am making is that when MTU is set to the default 1500 (as
> reported by ifconfig) then, the command
> ssh -p PPPP user at host
> fails (selecting automatically for mac umac-64-etm at openssh.com), while
> the command
> ssh -m umac-64-etm at openssh.com -p PPPP user at host
> succeeds.

MTU / MSS problems are notorious for being tickled only by specific
data sizes. Your first example happens to use different size packets
from the second, causing the TCP window to be within the problematic
range. A bit smaller or a bit larger packets will work. Debug
the network problem and all will be fine.


specifically this test


is quite useful though only tests between the browser and the
site it runs on. Other tools are also available.

Alex Bligh

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