When I ssh into a remote server that's not running any type of X11 desktop environment I get the following message.

$ ssh user@server
X11 forwarding request failed

$ ssh user@server ls
X11 forwarding request failed on channel 1
file1
file2
...

How can I get rid of these messages?

These messages can be eliminated through 1 of 3 methods, using just SSH options. You can always send messages to /dev/null too but these methods try to deal with the message through configuration, rather than just trapping and dumping them.

Method #1 - install xauth

The server you're remoting into is complaining that it cannot create an entry in the user's .Xauthority file, because xauth is not installed. So you can install it on each server to get rid of this annoying message.

On Fedora 19 you install xauth like so:

$ sudo yum install xorg-x11-xauth

If you then attempt to ssh into the server you'll see a message that an entry is being created in the user's .Xauthority file.

$ ssh root@server
/usr/bin/xauth: creating new authority file /root/.Xauthority
$

Subsequent logins will no longer show this message.

Method #2 - disable it via ForwardX11

You can instruct the ssh client to not attempt to enable X11 forwarding by inclusion of the SSH parameter ForwardX11.

$ ssh -o ForwardX11=no root@server

You can do the same thing with the -x switch:

$ ssh -x root@server

This will only temporarily disable this message, but is a good option if you're not able to or unwilling to install xauth on the remote server.

Method #3 - disable it via sshd_config

This is typically the default but in case it isn't, you can setup your sshd server so that X11Forwarding is off, in /etc/ssh/sshd_config.

X11Forwarding no

Of the 3 methods I generally use #2, because I'll often want X11Forwarding on for most of my servers, but then don't want to see the X11.... warnings

$HOME/.ssh/config

Much of the time these message won't even show up. They're usually only present when you have the following entries in your $HOME/.ssh/config file, at the top.

ServerAliveInterval 15
ForwardX11 yes
ForwardAgent yes
ForwardX11Trusted yes

GatewayPorts yes

So it's this setup, which is ultimately driving the generation of those X11.. messages, so again, method #2 would seem to be the most appropriate if you want to operate with ForwardX11 yes on by default, but then selectively disable it for certain connections from the ssh client's perspective.

Security

It's generally ill-advised to run with ForwardX11 yes on at all times. So if you're wanting to operate your SSH connections in the most secure manor possible, it's best to do the following:

1. Don't include ForwardX11 yes in your $HOME/.ssh/config file


2. Only use ForwardingX11 when you need to via ssh -X user@server
   


3. If you can, disable X11Forwarding completely on the server so it's disallowed

References

• SSH: The Secure Shell - The Definitive Guide - 9.3. X Forwarding

In my case adding this string to /etc/ssh/sshd_config solved the problem:

X11UseLocalhost no

Ran across this today and beat my head for a while until I stumbled across an ssh setting:

If it's RHEL 7 (centOS, OEL, etc), and it has ipv6 disabled, it needs:

AddressFamily inet

set in /etc/ssh/sshd_config.

Another slight variation would be if you wanted to stop seeing this message (i.e. stop trying to forward X11) for certain servers but yet keep the default to ForwardX11 yes for all other connections.

For this scenario, you could disable X11 forwarding for a specific host (or range) in your ~/.ssh/config. Something like this:

host 10.1.1.*
ForwardX11 no 

Acknowledgment: This is a slight embellishment to the existing (and very complete) existing answer - since I couldn't comment!

If running the client in verbose mode (ssh -v user@host) gives you

debug1: Remote: No xauth program; cannot forward with spoofing.

but xauth is indeed installed on the server, then it is probably because sshd looks for xauth executable in wrong location (/usr/X11R6/bin/xauth usually). One can fix that by setting

XAuthLocation /usr/bin/xauth

in /etc/sshd/sshd_config (or whatever your server is configured with).

One important point to note after making the configuration changes is that you'll have to kill sshd so that it picks up the changes:

cat /var/run/sshd.pid | xargs kill -1

being the root user.

Configuring X11 forwarding on a per host basis

In addition to all of the excellent answers already here, you can configure ForwardX11 on a per host basis, so if only server fails like this, you can add an entry to your ~/.ssh/config file of the following form:

Host server server.domain.dom
 ForwardX11 no

You can even use entries like this as alliases for whole sets of configurations

Host my.server
 HostName server.domain.dom
 User user
 Port 1234
 ForwardX11 no

This is especially useful if you have set up Autocomplete server names for SSH and SCP.

1. 
   

   Set the following 2 options in /etc/ssh/sshd_config in your RHEL host

   
   
   

   X11Forwarding yes
X11UseLocalhost no

   
   



2. sudo /etc/init.d/sshd reload



3. sudo yum install xauth


4. ssh back to your RHEL host with -X switch:
   ssh -X yourname@rhelbox