X, Authentication, And Forwarding Mechanisms - Novell LINUX ENTERPRISE SERVER 10 SP2 - INSTALLATION AND ADMINISTRATION Installation Manual

44.7 X, Authentication, and
Forwarding Mechanisms
Beyond the previously described security-related improvements, SSH also simplifies
the use of remote X applications. If you run ssh with the option -X, the DISPLAY
variable is automatically set on the remote machine and all X output is exported to the
remote machine over the existing SSH connection. At the same time, X applications
started remotely and locally viewed with this method cannot be intercepted by unautho-
rized individuals.
By adding the option -A, the ssh-agent authentication mechanism is carried over to the
next machine. This way, you can work from different machines without having to enter
a password, but only if you have distributed your public key to the destination hosts
and properly saved it there.
Both mechanisms are deactivated in the default settings, but can be permanently acti-
vated at any time in the systemwide configuration file /etc/ssh/sshd_config
or the user's ~/.ssh/config.
ssh can also be used to redirect TCP/IP connections. In the examples below, SSH is
told to redirect the SMTP and the POP3 port, respectively:
ssh -L 25:sun:25 earth
With this command, any connection directed to earth port 25 (SMTP) is redirected to
the SMTP port on sun via an encrypted channel. This is especially useful for those using
SMTP servers without SMTP-AUTH or POP-before-SMTP features. From any arbitrary
location connected to a network, e-mail can be transferred to the "home" mail server
for delivery. Similarly, all POP3 requests (port 110) on earth can be forwarded to the
POP3 port of sun with this command:
ssh -L 110:sun:110 earth
Both commands must be executed as root, because the connection is made to privileged
local ports. E-mail is sent and retrieved by normal users in an existing SSH connection.
The SMTP and POP3 host must be set to localhost for this to work. Additional in-
formation can be found in the manual pages for each of the programs described above
and also in the files under /usr/share/doc/packages/openssh.
840 Installation and Administration