Linux Server Security, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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Chapter 4: Secure Remote Administration

Unfortunately, there really is nothing to be gained by leaving X11Forwarding set to No

in sshd_config, since a determined user can simply use generic TCP forwarding to

forward X11. Even if

AllowTcpForwarding is also set to No, users with shell access can

still forward connections by piping SSH’s standard input/output to other (non-SSH)

forwarding processes.

The risk, of course, with allowing X and other port forwarding is that this functional-

ity gives users the ability to use SSH as a VPN/tunneling tool; for example, if all you

want to do is allow remote users to read their email via pine or copy files to and from

their home directory, you probably don’t want them to also be able to run processes

on the server that are advertised on their client system and forwarded over an SSH

tunnel! Unfortunately, the only sure way to disable port forwarding on an SSH server

is to compile SSH without it.

There are many other parameters that can be set in sshd_config, but understanding

the previous concepts is enough to get started (assuming your immediate need is to

replace Telnet and FTP). See the sshd(8) manpage for a complete reference for these

parameters.

Intermediate and Advanced SSH

Although most users use ssh and scp for simple logins and file transfers, ...