Q: ssh warns: using fake authentication data for X11 forwarding

(This is a tip, not a question)

tl;dr: tell your ssh client config where to find xauth

After upgrading to macos sierra, my typical morning login to my remote workhorse suddenly started displaying

user@host1:~ $ ssh -XY host2

Warning: No xauth data; using fake authentication data for X11 forwarding.

Last login: Tue Sep 27 08:43:53 2016 from 10.x.y.z

user@host2:~ $

I don't really like fake authentication, and I know that it worked yesterday without this message. Emptying out my Xauthority files on either end didn't work. What did work, was to tell ssh where to find X11's xauth tool, from XQuartz in my case, inside my ssh client user configuration.

First, you need to figure out where the xauth tool is located:

user@host1:~ $ type xauth

xauth is /opt/X11/bin/xauth

Then, you can add this location with the XAuthLocation key into your ssh's user configuration file (assuming you already have such a file present):

user@host1:~ $ echo XAuthLocation=/opt/X11/bin/xauth >> $HOME/.ssh/config

After this tweak, no more complaints logging into my remote host:

user@host1:~ $ ssh -XY host2

Last login: Tue Sep 27 08:52:13 2016 from 10.x.y.z

user@host2:~ $

Hope this helps!