Hi,
I am using a Mac to remotely log in to my Ubuntu 12 computer.
I have enable X11 forwarding in my Ubuntu machine: In /etc/ssh/sshd_config:
Code:
X11Forwarding yes
X11DisplayOffset 10
X11UseLocalHost no
xauth is installed:
Code:
$ which xauth
/usr/bin/xauth
But when ssh -vvv -X user@ubuntumachine
There is no Requesting X11 forwarding in the debug trace.

Code:
...
debug1: Authentication succeeded (password).
Authenticated to 192.168.1.115 ([192.168.1.115]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug2: callback start
debug2: client_session2_setup: id 0
debug2: fd 3 setting TCP_NODELAY
debug2: channel 0: request pty-req confirm 1
...
And when invoking a gui program like "firefox &", I see "Error: no display specified"

Then I explicitly set the DISPLAY in the client machine before ssh in.

Code:
export DISPLAY=192.168.1.117:0.0 #my mac ip
Then ssh -X into the Ubuntu machine again:

Code:
uthenticated to 192.168.1.115 ([192.168.1.115]:22).
debug1: channel 0: new [client-session]
debug3: ssh_session2_open: channel_new: 0
debug2: channel 0: send open
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug2: callback start
debug2: Checking for xauth using xauth -f /var/folders/9s/s5hl5w4x7zjdjkdljw9cnsrm0000gn/T//xauth_test exit > /dev/null 2> /dev/null

debug1: No xauth program.
Warning: No xauth data; using fake authentication data for X11 forwarding.
debug1: Requesting X11 forwarding with authentication spoofing.
debug2: channel 0: request x11-req confirm 1
debug2: client_session2_setup: id 0
debug2: fd 3 setting TCP_NODELAY
debug2: channel 0: request pty-req confirm 1
debug1: Sending environment.
debug3: Ignored env VIRTUALENVWRAPPER_PROJECT_FILENAME
debug3: Ignored env TERM_PROGRAM
debug3: Ignored env TERM
debug3: Ignored env SHELL
debug3: Ignored env TMPDIR
debug3: Ignored env Apple_PubSub_Socket_Render
debug1: Sending env LC_ALL = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug3: Ignored env USER
debug3: Ignored env COMMAND_MODE
debug3: Ignored env VIRTUALENV_USE_DISTRIBUTE
debug3: Ignored env SSH_AUTH_SOCK
debug3: Ignored env __CF_USER_TEXT_ENCODING
debug3: Ignored env Apple_Ubiquity_Message
debug3: Ignored env WORKON_HOME
debug3: Ignored env PATH
debug3: Ignored env VIRTUALENVWRAPPER_HOOK_DIR
debug3: Ignored env C_INCLUDE_PATH
debug3: Ignored env PIP_VIRTUALENV_BASE
debug3: Ignored env PWD
debug1: Sending env LANG = en_US.UTF-8
debug2: channel 0: request env confirm 0
debug3: Ignored env ITERM_PROFILE
debug3: Ignored env PS1
debug3: Ignored env PIP_RESPECT_VIRTUALENV
debug3: Ignored env SHLVL
debug3: Ignored env COLORFGBG
debug3: Ignored env HOME
debug3: Ignored env ITERM_SESSION_ID
debug3: Ignored env LOGNAME
debug1: Sending env LC_CTYPE = UTF-8
debug2: channel 0: request env confirm 0
debug3: Ignored env DISPLAY
debug3: Ignored env _
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: X11 forwarding request accepted on channel 0
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
When entering "firefox &"
i have the following error:

Code:
* (firefox:16274): WARNING **: Command line `dbus-launch --autolaunch=ea7e29098a107107d153b6b70000000b --binary-syntax --close-stderr' exited with non-zero exit status 1: Autolaunch error: X11 initialization failed.\n
debug1: client_input_channel_open: ctype x11 rchan 3 win 65536 max 16384
debug1: client_request_x11: request from 127.0.0.1 50891
debug2: connect 192.168.1.117 port 6000: Connection refused
connect 192.168.1.117 port 6000: Connection refused
debug1: failure x11
Error: cannot open display:
Any pointer ?

Thank you very much.

K.