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

Dimitris Diochnos diochnos at gmail.com
Wed Jun 1 03:22:06 AEST 2016

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


In this sense, both commands are executed with MTU=1500 but ssh does
behave differently in these two situations without me having to change
anything in my network configuration. Thus a reasonable (?) guess is
that perhaps ssh does not set all the necessary flags and options
correctly when umac-64-etm at openssh.com is set automatically during the
negotiation, but does set all the necessary flags and options
correctly when the same mac is specified as a command line parameter.

Perhaps my guess is incorrect (after all I have no real knowledge on
this matter), but I thought it would be best if I report this. My
apologies if this is spamming the mailing list.

Best regards,

On Tue, May 31, 2016 at 12:20 PM, Daniel Kahn Gillmor
<dkg at fifthhorseman.net> wrote:
> On Tue 2016-05-31 10:59:51 -0400, Dimitris Diochnos wrote:
> > On another note, lowering the MTU size (which was another workaround for
> > [1]) also allows me to pass successfully the key exchange phase in the
> > direction where I normally have an issue (that is, country B --> country
> > A). The maximum MTU size that would allow me to pass the key exchange
> > negotiation was 1458 (that is, with a size of 1459 the key exchange got
> > stuck).
> This is the relevant hint for your connection.  It sounds like some
> element along the network path from B to A is silently dropping packets
> that are larger than 1458, and your network stack has not detected this
> situation.
> When you force the MAC algorithm to be the specific one, you are
> probably making the ssh handshake negotiation packets each be small
> enough to fit into the smaller MTU.
> As such, i think this is a networking configuration issue, and not
> something for ssh to try to fix.  Maybe the fix belongs in your TCP
> stack, or in your network configuration?
> Sounds frustrating!
>           --dkg

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