tramp ControlMaster options (was: Re: package.el: what does package "Checking ... " actually do?)

[Yuri D'Elia <wavexx@thregr.org>]

On 18/07/15 13:22, Yuri D'Elia wrote:
> On 17/07/15 13:11, Yuri D'Elia wrote:
>> When I'm using a high-latency network connection, and I update some
>> packages, downloading takes a few seconds after the connection is
>> initiated, but package "Checking" instead takes literally minutes to
>> complete.
> 
> Turns out, it's another case of:
> 
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=20015

It's really that bad. Even bringing out the variable description of
tramp-ssh-controlmaster-options hangs.

The issue is caused by a rogue provider's DNS server responding with
NXDOMAIN but valid IN A record, which ssh still tries to connect to. And
since the host is filtered, this takes minutes to time out.

Considering that I have a global setting in my ~/.ssh/config that
enables ControlMaster/ControlPath globally, I consider this as a regression.

I tried the same probes using:

  ssh -o BatchMode=yes [various flags] 0.0.0.0

and I get comparable output, and there's no chance 0.0.0.0 would be a
valid address.