  Using Term to Pierce an Internet Firewall
  Barak Pearlmutter, bap@cs.unm.edu
  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
  their respective authors. Linux HOWTO documents may be reproduced and
  distributed in whole or in part, in any medium physical or electronic,
  as long as this copyright notice is retained on all copies. Commercial
  redistribution is allowed and encouraged; however, the author would
  like to be notified of any such distributions.

  All translations, derivative works, or aggregate works incorporating
  any Linux HOWTO documents must be covered under this copyright notice.
  That is, you may not produce a derivative work from a HOWTO and impose
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  may be granted under certain conditions; please contact the Linux
  HOWTO coordinator at the address given below.

  In short, we wish to promote dissemination of this information through
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  redistribute the HOWTOs.

  If you have questions, please contact Tim Bynum, the Linux HOWTO
  coordinator, at linux-howto@sunsite.unc.edu 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
  <danz@wv.mentorg.com>.

  14.  Acknowledgments

  Thanks for valuable suggestions from:

  ·  Gary Flake   <flake@scr.siemens.com>

  ·  Bill Riemers <bcr@physics.purdue.edu>

  ·  Greg Louis   <glouis@dynamicro.on.ca>


       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.

