X-forwarding stops working after a while

[Yuri]

I connect from host into the guest VM with X-forwarding (ssh -X), so
that programs launched in the VM will run on the host display.

X-forwarding works first, but after a while it stops working. Connection
(from inside of the VM) on the corresponding port (6010 or similar) is
immediately closed by the foreign host. Ssh connection itself is okay
though. Reconnecting ssh helps, but later the same thing repeats.

Why would this be happening?

FreeBSD-10.2, both host and guest VM.

Yuri

[Darren Tucker]

I connect from host into the guest VM with X-forwarding (ssh -X), so that
programs launched in the VM will run on the host display.
X-forwarding works first, but after a while it stops working. Connection
(from inside of the VM) on the corresponding port (6010 or similar) is
immediately closed by the foreign host. Ssh connection itself is okay
though. Reconnecting ssh helps, but later the same thing repeats.

By default ssh will request an untrusted xauth cookie that expires
after 20 minutes. You can change that with ForwardX11Timeout in
ssh_config(5) or request a trusted cookie that doesn't expire
(ForwardX11Trusted=yes or ssh -Y).

[Damien Miller]

I connect from host into the guest VM with X-forwarding (ssh -X), so that
programs launched in the VM will run on the host display.
X-forwarding works first, but after a while it stops working. Connection (from
inside of the VM) on the corresponding port (6010 or similar) is immediately
closed by the foreign host. Ssh connection itself is okay though. Reconnecting
ssh helps, but later the same thing repeats.
Why would this be happening?

It's probably this: (ssh_config(5))

ForwardX11Trusted
If this option is set to “yes”, remote X11 clients will have full
access to the original X11 display.
If this option is set to “no”, remote X11 clients will be consid‐
ered untrusted and prevented from stealing or tampering with data
belonging to trusted X11 clients. Furthermore, the xauth(1)
token used for the session will be set to expire after 20 min‐
utes. Remote clients will be refused access after this time.
The default is “no”.
See the X11 SECURITY extension specification for full details on
the restrictions imposed on untrusted clients.

You can use "ssh -Y" as a shortcut for ForwardX11Trusted=yes