Bug#908652: openssh: Regression from DLA-1500-1: DISPLAY "(null)" invalid; disabling X11 forwarding

[Salvatore Bonaccorso <carnil@debian.org>]

Source: openssh
Version: 1:6.7p1-5+deb8u6
Severity: normal

Hi

DLA-1500-1 introduced the following regression: In cases where
ForwardX11 is enabled, say globally for all via ssh_config, or via
command line switch, but no DISPLAY is set (e.g. in cronjob), then newly
a

> DISPLAY "(null)" invalid; disabling X11 forwarding

is issued.

root@jessie-amd64:~# unset DISPLAY
root@jessie-amd64:~# ssh -X root@localhost uname -a
DISPLAY "(null)" invalid; disabling X11 forwarding
Linux jessie-amd64 3.16.0-6-amd64 #1 SMP Debian 3.16.57-2 (2018-07-14) x86_64 GNU/Linux
root@jessie-amd64:~#

Downgrading to 1:6.7p1-5+deb8u5 fixes the problem:

root@jessie-amd64:~# unset DISPLAY
root@jessie-amd64:~# ssh -X root@localhost uname -a
Linux jessie-amd64 3.16.0-6-amd64 #1 SMP Debian 3.16.57-2 (2018-07-14) x86_64 GNU/Linux
root@jessie-amd64:~#

This seem to be a specific regression in 1:6.7p1-5+deb8u6, the behaviour is not seen on
either unstable (1:7.8p1-1) or stretch (1:7.4p1-10+deb9u4), so maybe
one of the adressed issues misses a prerequisite commit?

Regards,
Salvatore