ControlPath versus ProxyCommand

[Damien Miller]

On Fri, 3 Nov 2017, Cameron Simpson wrote:

> TL;DR: I expect ProxyCommand to have effect in preference to
> ControlPath.
>
> I've just tripped over this one. I have an ssh Host (let us call
> it "MAIN") with a ControlPath and with ControlMaster=no, from the
> .ssh/config file.
>
> I also have a shell script whose purpose is to hop to a remote host
> through a port forward, which uses the ProxyCommand option like this:
>
>  ProxyCommand ssh MAIN nc 127.0.0.1 7777
>
> because the remote host is accessible via port 7777 on host MAIN.
>
> So the script invokes an ssh commandlike this:
>
>  ssh -o "ProxyCommand=ssh MAIN nc 127.0.0.1 7777" remoteuser at MAIN
>
> The expectation is that the inner proxycommand ssh goes via the
> control socket and that the outer ssh uses the ProxyCommand to get to
> the remote host.
>
> Imagine my surprise when the ControlMaster from the config file takes
> precedence over the ProxyCommand from the command line. By contract,
> this works as I intended:
>
>  ssh -o ControlPath=none -o "ProxyCommand=ssh MAIN nc 127.0.0.1 7777"
> remoteuser at MAIN
>
> On reflection, of course these are distinct options and that side of
> things isn't, of itself, a bug. However, is there a sane use case for
> using ControlMaster/ControlPath at all if there is a ProxyCommand? I
> would have thought not.
>
> Can someone explain a use case for this precedence (ControlPath before
> ProxyCommand), or is this a misfeature which would benefit from a
> fix/change?

They are quite othorgonal features, but the whole point of multiplexing
is to avoid the need to make additonal connections. So it's quite
logical that ssh checks ControlPath for an active mux master before
attempting a new connection (that may use ProxyCommand).

There's little point to specifying ControlMaster=no and a ProxyCommand
because there is no fallback to making a new connection in that case,
but ControlMaster=yes/auto/autoask with ProxyCommand is quite sensible:
"try to use multiplexing but if you have to open a new connection then
do it via this proxy".

BTW

>  ProxyCommand ssh MAIN nc 127.0.0.1 7777

If your ssh client is new enough, you should try ssh -J / JumpHost instead.

-d