What does "#ffff##:0" mean in xauth?

(Migrated from ServerFault.SE) I'm working on a Silverblue system, in part via SSH with X11 forwarding. When I use the graphical console and say xauth list I get this: silverblue/unix:0 MIT-MAGIC-COOKIE-1 #ffff##:0 MIT-MAGIC-COOKIE-1 $XAUTHORITY = /run/user/1000/xauth_dkafiw so this is obviously something set up by Silverblue specially. But when I log in via ssh -X I get xauth list of silverblue/unix:11 MIT-MAGIC-COOKIE-1 silverblue/unix:10 MIT-MAGIC-COOKIE-1 I think the mysterious line in the console xauth is to make toolbox work with X. I can say toolbox enter and start an X11 program from the command line, and it works. But if I try that in ssh it doesn't. This seems to be because toolbox sets the hostname to "toolbox", so if I say xauth add toolbox/unix:10 MIT-MAGIC-COOKIE-1 it starts working again (or sudo hostname silverblue inside the toolbox also works). So my questions: * What does #ffff##:0 mean in xauth list? Am I right about it solving the toolbox issue? * How do I get it in there? Using xauth add tells me it's an invalid address.