Ssh

ssh is a command to log in over the network to another computer.

Send graphical output to ssh user

If you want to start a graphical program, e.g. kwrite, on your remote computer and get the display to your local computer, no problem. Just use:

ssh -l user server -X
xclock &

what happens

After logging in with ssh -X, xauth is called to create/modify .Xauthority. Using netstat -putan you can find out that every ssh -X session gets a socket:

remote:~ # netstat -putan
[...]
tcp        0      0 127.0.0.1:6010          0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:6011          0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:6012          0.0.0.0:*               LISTEN      -
tcp        0      0 127.0.0.1:6013          0.0.0.0:*               LISTEN      -
[...]

And $DISPLAY is set automatically.

remote:~ # echo $DISPLAY
localhost:14.0

And ssh listens on the respective port to forward X11 traffic:

remote:~ # lsof | grep 6014
sshd       5257     root    6u  IPv4 3755641440                   TCP localhost:6014 (LISTEN)

port forwarding

ssh username@server -L localport:remoteserver:remoteport

ssh verbose

ssh -v user@server
or -vv, -vvv

tunneling

In die lokale .ssh/config einen Eintrag fuer den Server:

 Host <server-alias(2)>
   HostName <server-ip(2)>
   User <username(2)>
   ForwardAgent yes
   ForwardX11 yes
   ProxyCommand ssh -X -A <username(1)>@<server-ip>(1) netcat -w1 %h %p

Statt der IP koennen auch die Namen - bei funktionierender Namens-Aufloesung - verwendet werden. Auf dem lokalen Rechner muss dann der SSH-Port des Servers(2) auf den lokalen Port (22222) gebunden werden:

 ssh -4 -C -l np -L 22222:localhost:22 vout

Jetzt kann man fuer das lokale CVS (Eclipse, Netbeans) einfach als Hostname "localhost" und als Port "22222" angeben.

TroubleShooting

Remote host identification has changed

Symptom

When trying to log in via ssh you may get a message like this:

tweedleburg:~ # ssh root@192.168.0.107
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
32:78:25:83:d8:a6:de:ad:6a:0b:99:5e:05:e5:7c:e7.
Please contact your system administrator.
Add correct host key in /root/.ssh/known_hosts to get rid of this message.
Offending key in /root/.ssh/known_hosts:11
RSA host key for 192.168.0.107 has changed and you have requested strict checking.
Host key verification failed.

Reason

This means the key of the computer that you try to reach has changed.

Solution

$ ssh-keygen -R hostname

ssh -X does not work

If ssh -X works, but you still do not get the graphical display from your remote machine, check /etc/ssh/sshd_config. There must be a line

X11Forwarding yes

Also, there must be a binary xauth, otherwise .Xauthority cannot be created.

Related

ssh-related topics:

• passwordless login