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xterm over ssh and X11UseLocalhost - solution

An FYI post for the archives.

From a local xterm on a Cygwin X server, I was able to open xterm windows to various remote hosts over ssh. However, on one fresh minimal installation of Debian Sarge

ssh -Yf <user>@<host> xterm (and other related commands)

produced various errors:

X11 connection rejected because of wrong authentication
xterm Xt error: Can't open display:
unable to open display

I was able to open xterm's over ssh from other hosts. I installed xauth on the remote host:

apt-get install xbase-clients

I verified my configuration against the Cygwin/X FAQ:

In the local xterm DISPLAY was set to localhost:0 and was exported.

On a different remote host where xterm worked properly,
/etc/ssh/sshd_config had the following line:

X11UseLocalhost no

But on the remote host with non-working xterm, the command "xauth list"
showed no fully-qualified host names apart from localdomain:

debian/unix:10  MIT-MAGIC-COOKIE-1  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
localhost.localdomain:10  MIT-MAGIC-COOKIE-1  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
localhost.localdomain:11  MIT-MAGIC-COOKIE-1  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
debian/unix:11  MIT-MAGIC-COOKIE-1  xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

So this needed to be in sshd_config:

X11UseLocalhost yes

(which is the default)

after restarting the ssh daemon on the remote host:

/etc/init.d/ssh restart

My xterm window appeared.

The rest of this post is the sshd_config for that host, verbatim.

Poor Yorick

---*** begin /etc/ssh/sshd_config

# Package generated configuration file
# See the sshd(8) manpage for details

# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 768

# Logging
SyslogFacility AUTH
LogLevel INFO

# Authentication:
LoginGraceTime 600
PermitRootLogin yes
StrictModes yes

RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile     %h/.ssh/authorized_keys

# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes

# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no

# Change to no to disable s/key passwords
#ChallengeResponseAuthentication yes

# Change to yes to enable tunnelled clear text passwords
PasswordAuthentication no

# To change Kerberos options #KerberosAuthentication no #KerberosOrLocalPasswd yes #AFSTokenPassing no #KerberosTicketCleanup no

# Kerberos TGT Passing does only work with the AFS kaserver
#KerberosTgtPassing yes

#X11Forwarding no #(default)
X11Forwarding yes
#X11UseLocalhost yes (default - shown here for clarity)
X11UseLocalhost yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
KeepAlive yes
#UseLogin no

#MaxStartups 10:30:60
#Banner /etc/

Subsystem sftp /usr/lib/sftp-server

UsePAM yes

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