After ssh without -X to a machine, is it possible to change $DISPLAY to make it work like ssh -X?

Question

After ssh without -X to a machine, is it possible to change some settings (for example $DISPLAY) to make it work like ssh -X? If not, what is the reason? Thanks.

Answer 1

You can create a second connection with X11 forwarding enabled, and then you can also use the DISPLAY environment variable from the second connection in the first.

In the 1st window:

$ ssh user@host
user@host$ ...

In the 2nd window:

$ ssh -Y user@host 'echo $DISPLAY; while sleep 3600; do :; done'
localhost:10.0

Back to the 1st window:

user@host$ export DISPLAY=localhost:10.0
user@host$ xterm

Unfortunately, ssh does nothing to contain the X11 (or other) forwardings to the process/session it started or to the user it runs as on the remote machine (eg. by using Unix sockets with/out checking credentials, or by using namespaces), and those forwardings are simple tcp listening sockets to which anybody on the remote machine can connect; all the security of the X11 forwarding relies on the X11 authentication.

X11 Forwarding By Hand

The sshd_config(5) manpage mentions that:

disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders.

Here is how you can do that by hand.

First of all, be sure to disable any host- or user- based access control that bypasses the x11 auth mechanism [1]:

$ xhost $(xhost | sed -n /:/s/^/-/p)
access control enabled, only authorized clients can connect

Then show the auth info for DISPLAY=:0 on the local machine:

$ xauth list :0
ohzd/unix:0  MIT-MAGIC-COOKIE-1  a86982ddce0c1e1c1a8c5e8b2846e43b

Connect to the remote machine without any X11 forwarding:

$ ssh user@hzy64
user@hzy64's password:
[motd snipped]

Open the command line via ~C and add a remote forwarding from the port 6000+43 to the unix socket corresponding to display :0:

hzy64$~C
ssh> -R 6043:/tmp/.X11-unix/X0
Forwarding port.

Set the $DISPLAY envvar and add the auth info from the local to the remote machine:

hzy64$ export DISPLAY=localhost:43
hzy64$ xauth add $DISPLAY . a86982ddce0c1e1c1a8c5e8b2846e43b
xauth:  file /home/user/.Xauthority does not exist

Now you're ready to go:

hzy64$ xterm

[1] because of a misguided bugfix, the user-based access control is turned on by default in Debian via /etc/X11/Xsession.d/35x11-common_xhost-local. Worse, it's the only one available by default in XWayland where it also cannot be turned off. Any program that proxies of the X11 protocol (eg. xscope) will have to do its own x11 auth cookie checking (the way ssh does), unless it wants to open a gaping hole to the X11 server.