Table of Contents

1 SSH Useful links

2 How do I get the public key of an pem file?

How do I extract my public key, from a .pem file?

ssh-keygen -f private.pem -y >

3 X11Forwarding

What does "X11UseLocalhost no" do?

I found that I can't ssh (with X11 port forwarding) from my home
FC4/linux box to a bsd machine, run xclients on the bsd machine, and
have them X display back to my home machine without specifying the
"X11UseLocalhost no" option in the /etc/ssh/sshd_config file.

I don't understand what the manpage says about this option. Can
someone please explain?

When doing X forwarding, sshd listens on a TCP socket for connections from
X clients.  Normally, it will accept connections addressed to the loopback
address only (, restricting it to clients on the local host.
X11UseLocalhost no means it will accept connections from anywhere.


Make sure that:

    You've xauth installed on the server (see: xauth info/xauth list).

    On the server your /etc/ssh/sshd_config file have these lines:

    X11Forwarding yes
    X11DisplayOffset 10
    X11UseLocalhost no

    On the client side your ~/.ssh/config file have these lines:

    Host *
      ForwardAgent yes
      ForwardX11 yes

    On the client side, you've X server installed (e.g. macOS: XQuartz; Windows: Xming).

Then to do X11 forwarding using SSH, you need to add -X to your ssh command, e.g.

ssh -v -X user@host

then verify that your DISPLAY is not empty by:


If it is, then having verbose parameter for ssh (-v), check for any warnings, e.g.

debug1: No xauth program.
Warning: untrusted X11 forwarding setup failed: xauth key data not generated

In case you've got untrusted X11 as shown above, then try -Y flag instead (if you trust the host):

ssh -v -Y user@host

See: What does “Warning: untrusted X11 forwarding setup failed: xauth key data not generated” mean when ssh'ing with -X?

In case you've warning: No xauth data, you may try to generate a new .Xauthority file, e.g.

xauth generate :0 . trusted
xauth list

See: Create/rebuild a new .Xauthority file

If you've got a different warnings than above, follow the further clues.

Author: Sebastian Emilio Narvaez

Created: 2020-05-19 Tue 11:02

Emacs 25.2.2 (Org mode 8.2.10)