Using Term to Pierce an Internet Firewall
 Barak Pearlmutter, [email protected]
 v, 15 July 1996

 Directions for using ``term'' to do network stuff through a TCP fire
 wall that you're not supposed to be able to.
 ______________________________________________________________________

 Table of Contents


 1. Disclaimer

 2. Copyright

 3. Introduction

 4. The basic procedure

 5. Detailed directions

 6. Multiple term sockets

 7. The

 8. Direction

 9. Security

 10. Telnet mode

 11. Bugs and term wish list

 12. Tricks that don't seem to work

 13. Related resources

 14. Acknowledgments

 ______________________________________________________________________

 1.  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.

 2.  Copyright

 Unless otherwise stated, Linux HOWTO documents are copyrighted by
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 as long as this copyright notice is retained on all copies. Commercial
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 like to be notified of any such distributions.

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 In short, we wish to promote dissemination of this information through
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 If you have questions, please contact Tim Bynum, the Linux HOWTO
 coordinator, at [email protected] via email.

 3.  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.

 4.  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.

 5.  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

 Et voila!!!

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

 Some telnet clients do not have the ! shell escape command.  Eg the
 telnet client distributed with Slackware 3.0 is one such client.  The
 sources that the Slackware telnet client is supposedly built from,


      ftp://ftp.cdrom.com:/pub/linux/slackware-3.0/source/n/tcpip/NetKit-B-0.05.tar.gz

 have the shell escape command.  A simple solution is therefore to
 obtain these sources and recompile them.  This unfortunately is a task
 I have had no luck with.  Plus, if you are running from inside a SOCKS
 firewall, you will need a SOCKSified telnet client anyway.  To that
 end, I was able to compile a SOCKSified telnet client from


      ftp://ftp.nec.com/pub/security/socks.cstc/socks.cstc.4.2.tar.gz

 or if you're outside the USA,


      ftp://ftp.nec.com/pub/security/socks.cstc/export.socks.cstc.4.2.tar.gz

 Alternatively, under linux kernels up to 1.2.13, you can pause the
 telnet with ^]^z, figure out its pid, and invoke


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

 This doesn't work with newer 1.3.x kernels, which closed some mysteri
 ous security hole by preventing access to these fd's by processes
 other than the owner process and its children.

 6.  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, e.g. "trsh -t telnet".

 7.  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.

 8.  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.)

 9.  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.

 10.  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.)

 11.  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.

 12.  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.

 13.  Related resources

 A vaguely related trick is to SOCKSify one's Term library.  Details,
 including patches to SOCKS, are available from Steven Danz
 <[email protected]>.

 14.  Acknowledgments

 Thanks for valuable suggestions from:

   Gary Flake   <[email protected]>

   Bill Riemers <[email protected]>

   Greg Louis   <[email protected]>


      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.

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