you have properly configured putty to use X11 forwarding but it fails?
Take a look at the sshd log on the server (on ubuntu the systemd unit is ssh) if you got any error like this:
root@debdev ~# journalctl -xu ssh ... error: Failed to allocate internet-domain X11 display socket. ...
This error message means that it tries to use IPv6, but its (however) not working. Maybe IPv6 is disabled….
Solution: Verify that
is set and is not overwritten by any per-user basis options. Also, if you use ssh keys, check your ~/.ssh/authorized_keys file if the any restrictions about x11 forwarding.
... no-X11-forwarding ...
Force sshd to use IPv4. Add the following option before the “listen” option to /etc/ssh/sshd_config.
Restart sshd and try to connect.
root@debdev ~# systemctl restart ssh