Ok, some magic affects containers int hat case (even ssh to the LXD and then re-ssh to itelf would not work). But in a KVM guest I could recreate it.
$ ssh -X ubuntu@localhost uname -a ubuntu@localhost's password: DISPLAY "(null)" invalid; disabling X11 forwarding Linux t1 3.13.0-157-generic #207-Ubuntu SMP Mon Aug 20 16:44:59 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to openssh in Ubuntu. https://bugs.launchpad.net/bugs/1792231 Title: openssh-client: 'DISPLAY "(null)" invalid; disabling X11 forwarding' Status in openssh package in Ubuntu: Fix Released Status in openssh source package in Trusty: Triaged Status in openssh package in Debian: Unknown Bug description: affects ubuntu/openssh Hi, This bug is present in openssh in ubuntu trusty, I haven't tested other releases. When X11 forwarding is enabled (via ssh_config, or via -X), but no DISPLAY is available, ssh client produces this warning: DISPLAY "(null)" invalid; disabling X11 forwarding root@ubuntu-trusty:/# unset DISPLAY ; ssh -X ubuntu@localhost 'dpkg -l openssh-client | grep ii' ubuntu@localhost's password: DISPLAY "(null)" invalid; disabling X11 forwarding ii openssh-client 1:6.6p1-2ubuntu2.10 amd64 secure shell (SSH) client, for secure access to remote machines The bug was introduced by the changes to fix CVE-2016-1908. This upstream change fixes it: https://anongit.mindrot.org/openssh.git/commit/?id=5658ef2501e785fbbdf5de2dc33b1ff7a4dca73a See https://bugs.debian.org/908652 Cheers, -- S To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1792231/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
