lxc exec outputs stderr on host's stdout

So that's actually expected behavior though I'll add a flag to force non-interactive mode.

The way exec works is:
 - If stdin is a tty, use interactive mode
 - If stdin isn't a tty, use non-interactive mode

In interactive mode, a pts device is allocated and used for stdin, stdout and stderr, same as an interactive ssh session.
In non-interactive mode, a pipe is allocated for each of stdin, stdout and stderr, same as a non-interactive ssh session.

The difference comes from the fact that ssh does know how to differentiate a shell from another command. LXD doesn't as the command must always be specified.

So I'll be adding a new --mode option which takes either interactive or non-interactive and overrides the detection code.

In the mean time, doing the following should work:
    lxc exec x1 -- sh -c 'echo stderr >&2' 2>/dev/null < /dev/null

I wrote a patch for the new --mode argument which I'll be submitting upstream very soon (want to bundle the other exec fix).

I just sent the --mode change upstream: https://github.com/lxc/lxd/pull/1383

This bug was fixed in the package lxd - 0.24-0ubuntu2

---------------
lxd (0.24-0ubuntu2) xenial; urgency=medium

  * Cherry-pick the following upstream bugfixes: (LP: #1522755)
    - 0001-Introduce-a-new-mode-option-to-exec.patch
    - 0002-remove-extra-debugging-statement.patch
    - 0003-generate-a-better-error-message-for-legacy-image-imp.patch
    - 0004-hide-lxc-importer-from-help-output.patch
    - 0005-tests-Avoid-device-conflicts.patch
    - 0006-Use-the-mountpoint-command-when-available.patch
    - 0007-tests-Fix-mount-issues.patch
    - 0008-Apparently-IsMountPoint-is-never-very-reliable-with-.patch
  * Set wait timeout to 10 minutes in lxd job. This should be long
    enough for even the slowest hardware to generate a SSL key.
  * Update debian/rules to make it easier to flip between building with
    the embedded dependencies or not.

 -- Stéphane Graber <email address hidden> Wed, 09 Dec 2015 16:46:41 -0500