Caldera Network Desktop 1.0/caldera-network-desktop-1.0.bin/doc/HOWTO/mini/Term-Firewall

Text File | 1995-09-12 | 8.1 KB | 228 lines

** HOWTO: Using Term to Pierce an Internet Firewall
Revision of 11-Sep-1995.


* Abstract:

Directions for using "term" to do network stuff through a TCP firewall
that you're not supposed to be able to.


* Table of contents:

 Disclaimer
 Introduction
 The basic procedure
 Detailed directions
 Multiple term sockets
 The ~/.term/termrc.telnet init file
 Direction
 Security
 Telnet mode
 Bugs and term wish list
 Tricks that don't seem to work


* Disclaimer * <------- !!! READ THIS IMPORTANT SECTION !!!

I hereby disclaim all responsibility for this hack.  If it backfires
on you in any way whatsoever, that's the breaks.  Not my fault.  If
you don't understand the risks inherent in doing this, don't do it.
If you use this hack and it allows vicious hackers to break into your
company's computers and costs you your job and your company millions
of dollars, well that's just tough nuggies.  Don't come crying to me.


* Author

This HOWTO-term-firewall file was written by
Barak Pearlmutter <[e-mail]>
*who disclaims all responsibility for it!*


* Copyright

This document is released under the GPL.  This basically means you may
copy and modify it at will, but may not prevent others from doing so.


* Introduction

The term program is normally used over a modem or serial line, to
allow various host-to-host services to flow along this simple serial
connection.  However, sometimes it is useful to establish a term
connection between two machines that communicate via telnet.  The most
interesting instance of this is for connecting two hosts which are
separated by ethernet firewalls or SOCKS servers.  Such firewalls
provides facilities for establishing a telnet connection through the
firewall, typically by using the SOCKS protocol to allow inside
machines to get connections out, and requiring outside users to telnet
first to a gateway machine which requires a one-time password.  These
firewalls make it impossible to, for instance, have X clients on an
inside machine communicate with an X server on an outside machine.
But, by setting up a term connection, these restrictions can all be
bypassed quite conveniently, at the user level.


* The basic procedure

Setting up a term connection over a telnet substrate is a two-phase
process.  First your usual telnet client is used to set up a telnet
connection and log in.  Next, the telnet client is paused and control
of the established telnet connection is given to term.


* Detailed directions

In detail, the process goes like this.

First, from a machine inside the firewall, telnet to a target machine
outside the firewall and log in.

Unless you are under linux and will be using the proc filesystem (see
below) make sure your shell is an sh style shell.  Ie if your default
shell is a csh variant, invoke telnet by

(setenv SHELL /bin/sh; telnet machine.outside)

After logging in, on the remote (outside) machine invoke the command

    term -r -n off telnet

Now break back to the telnet prompt on the local (inside) machine,
using ^] or whatever, and use the telnet shell escape command ! to
invoke term,

    telnet> ! term -n on telnet <&3 >&3

E voila!!!

(If you have a variant telnet, you might have to use some other file
descriptor than 3; easy to check using trace.  But three seems to work
on all bsd descendent telnet clients I've tried, under both sunon 4.x
and the usual linux distributions.)

Alternatively, under linux you can pause the telnet with ^]^z, figure
out its pid, and invoke

    term -n on -v /proc/<telnetpid>/fd/3 telnet


* Multiple term sockets

It is a good idea to give the term socket an explicit name.  This is
the "telnet" argument in the invocations of term above.  Unless you
have the TERMSERVER environment variable set to telnet as appropriate,
you invoke term clients with the -t switch, eg "trsh -t telnet".


* The ~/.term/termrc.telnet init file

I have checked line clarity using linecheck over this medium.  I
expected it to be completely transparent, but it is not.  However, the
only bad character seems to be 255.  The ~/.term/termrc.telnet I use
(the .telnet is the name of the term connection, see above) contains:

    baudrate off
    escape 255
    ignore 255
    timeout 600

Perhaps it could be improved by diddling, I am getting a throughput of
only about 30k cps over a long-haul connection through a slow
firewall.  Ftp can move about 100k cps over the same route.  A
realistic baudrate might avoid some timeouts.



* Direction

Obviously, if you are starting from outside the firewall and zitching
in using a SecureID card or something, you will want to reverse the
roles of the remote vs local servers given above.  (If you don't
understand what this means, perhaps you are not familiar enough with
term to use the trick described in this file responsibly.)


* Security

This is not much more of a vulnerability than the current possibility
of having a telnet connection hijacked on an unsecured outside
machine.  The primary additional risk comes from people being able to
use the term socket you set up without you even being aware of it.  So
be careful out there.  (Personally, I do this with an outside machine
I know to be pretty secure, namely a linux laptop I maintain myself
that does not accept any incoming connections.)

Another possibility is to add "socket off" to the remote
~/.term/termrc.telnet, or add "-u off" to invocation of term.  This
prevents the socket from being hijacked from the remote end, with only
a minor loss of functionality.


* Telnet mode

Be sure the remote telnetd is not in some nasty seven-bit mode.  Or if
it is, you have to tell term about it when you invoke term, by adding
the -a switch at both ends.  (I sometimes use "^] telnet> set outbin"
or "set bin" or invoke telnet with a -8 switch to put the connection
into eight-bit mode.)


* Bugs and term wish list

The linecheck program has some problems checking telnet connections
sometimes.  This is sometimes because it doesn't check the return code
of the read() call it makes.  For network connections, this call to
read() can return -1 with an EINTR (interrupted) or EAGAIN (try again)
error code.  Obviously this should be checked for.

There are a number of features that could ease the use of term over
telnet.  These primarily relate to an assumption that influenced the
design of term, namely that the connection is low bandwidth, low
latency, and somewhat noisy.

A telnet connection is in general high bandwidth, high latency, and
error free.  This means that the connection could be better utilized
if (a) the maximum window size was raised, well above the limit
imposed by term's N_PACKETS/2=16, (b) there was an option to turn off
sending and checking packet checksums, and (c) larger packets were
permitted when appropriate.

Also, to enhance security, it would be nice to have a term option to
log all connections through the socket it monitors to a log file, or
to stderr, or both.  This would allow one to see if one's term
connection is being subverted by nasty hackers on the outside insecure
machine.


* Tricks that don't seem to work

Some telnet clients and servers agree to encrypt their communications,
to prevent evesdropping on the connection.  Unfortunately, the hack
used above (using the network connection that the telnet client has
set up while the telnet client is idle) won't work in that case.
Instead, one really must go through the telnet client itself, so it
can do its encryption.  It seems like that requires a simple hack to
the telnet client itself, to add a command that runs a process with
its stdin and stdout are connected to the live telnet connection.
This would also be useful for various 'bots, so perhaps someone has
already hacked it up.


* Acknowledgments

Thanks for valuable suggestions from:
Gary Flake <[e-mail]>
Bill Riemers <[e-mail]>


* Extra copy of IMPORTANT DISCLAIMER --- BELIEVE IT!!!

I hereby disclaim all responsibility for this hack.  If it backfires
on you in any way whatsoever, that's the breaks.  Not my fault.  If
you don't understand the risks inherent in doing this, don't do it.
If you use this hack and it allows vicious hackers to break into your
company's computers and costs you your job and your company millions
of dollars, well that's just tough nuggies.  Don't come crying to me.