Why is the X11 Forwarding not working on my system when my ssh daemon is correctly configured with 'X11Forwarding yes'?

by Eduardo Damato

The easiest way to debug the ssh connection is to run ssh with increased verbosity. This normally shows many silent errors/problems.

In order to increase verbosity, issue the following command:

$ ssh -v -X -l root <USERNAME>

The output would look like the following:

OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to cs1 [192.168.161.134] port 22.
debug1: Connection established.
debug1: identity file /home/user/.ssh/identity type -1
debug1: identity file /home/user/.ssh/id_rsa type -1
debug1: identity file /home/user/.ssh/id_dsa type -1
debug1: Remote protocol version 1.99, remote software version OpenSSH_3.9p1
debug1: match: OpenSSH_3.9p1 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_3.9p1
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'cs1' is known and matches the RSA host key.
debug1: Found key in /home/user/.ssh/known_hosts:37
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: gssapi-with-mic
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: publickey
debug1: Offering public key: /home/user/.ssh/identity
debug1: Server accepts key: pkalg ssh-dss blen 816
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Remote: No xauth program; cannot forward with spoofing.
Last login: Mon Aug 14 14:17:11 2006 from 192.168.161.1

From the example output above, there are two lines of interest. Check the following lines:

debug1: Requesting X11 forwarding with authentication spoofing.
debug1: Remote: No xauth program; cannot forward with spoofing.

In this case, the xauth program is not installed in the system, and therefore the ssh target system can not add itself to the X authentication database of the X server, so Xforwarding is silently denied. To resolve the problem, install the xauth package:

# up2date xorg-x11-xauth

try to ssh in to the machine and verify that the display environment variable is correctly setup by the tunnel:

$ ssh -X -l root <USERNAME>
# echo $DISPLAY
localhost:10.0

documented on: 2007.01.31