On Fri, 26 Jun 2015 at 03:16 -0000, Alexandru Chiscan wrote:
On 06/25/2015 11:51 PM, Stuart Barkley wrote:
Then from your desktop (assuming Linux already running X) in a local xterm do something like:
ssh -Y remote-systemDo not use that because any user logged on the server can connect to your X server display and snoop what you are doing, open windows etc.
-Y disables all the X server authentication mechanisms (http://www.x.org/wiki/Development/Documentation/Security/)
I believe this is incorrect. The authentication protocols are still used (thus the need for the xorg-x11-xauth package on CentOS).
This is not the same as 'xhost +' which should never be used.
Both -X and -Y require read access to the ~/.Xauthority file on the remote system in order to connect back to the X server. You can see this by using the 'xauth remove' command to remove the authentication token for the display and clients can no longer connect.
Note about -X versus -Y with ssh:
-X enables basic X forwarding, It disables some X functionality making it "safer" to allow. -X also stops working after about 20 minutes (this is by design but not well documented). I only recently learned why it would stop working after pulling out the last of my hair.
I have been using ssh X forwarding for current work use (local betwork) for more than 15 years and never got into this kind of problem from RH 7 to Centos 7, AIX and Solaris.
Likewise, I've used ssh/X for 20+ years on a variety of systems. In most cases -X is sufficient, but some applications seem to require -Y and I have not dug into all of the reasons.
Maybe it is some other issue that is closing your ssh connection (maybe you should use the KeepAlive options on the ssh server/client); just guessing.
On Debian and FreeBSD 'man ssh_config' now shows:
ForwardX11Timeout Specify a timeout for untrusted X11 forwarding using the format described in the TIME FORMATS section of sshd_config(5). X11 connections received by ssh(1) after this time will be refused. The default is to disable untrusted X11 forwarding after twenty minutes has elapsed.
This option seems to have appeared in OpenSSH 6.0 [See more at the end of this email].
-Y allows the full X protocol which might be a security risk. Some applications will only work with -Y. With this, remote X applications can grab keyboard interactions, grab passwords, put windows on top of other windows (obscuring security messages), etc.
For my own choice I use -Y (although I only enable it occasionally to specific systems).
This is a recent behaviour change on my part with limited use to system I trust. Now that I know about the timeout I can probably just live with -X and will consider moving back to -X for my occasional use,
The documentation of the practical differences between -X and -Y is pretty obscure (mostly defering to the X Security extension documentation). I would like to see better clarification of the differences.
...some additional research looking at the source code and revision history...
The ForwardX11Timeout change was 5 years ago in OpenSSH 6.0. CentOS 6 still has OpenSSH 5.3 without this change (or if a patch was applied the documentation was not also patched). CentOS 7 has OpenSSH 6.6 which includes this change.
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r...
Prior to that there was an intended hard coded 20 minute timeout since at least 2005:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/clientloop.c.diff?r...
Based upon the comments in the June 2010 revision it appears that the timeout was not working correctly and perhaps was falling back to trusted authentication after 20 minutes:
Add X11ForwardTimeout option to specify timeout for untrusted X11 authentication cookies to avoid fallback in X11 code to fully-trusted implicit authentication using SO_PEERCRED described at: http://lists.x.org/archives/xorg-devel/2010-May/008636.html
After the X11ForwardTimeout has expired the client will now refuse incoming X11 channel opens.
I will need to see it this is an unpatched security issue on CentOS/RedHat 6. If so, I claim credit for observing it as a possibility.
Stuart