PC World 1999 August/PCWorld_1999-08_cd.bin/doc/HOWTO/mini/Firewall-Piercing

Text File | 1998-11-30 | 17KB | 463 lines

  Firewall Piercing mini-HOWTO
  Franτois-RenΘ Rideau, [e-mail]
  v0.3b, 27 November 1998

  Directions for using ppp over telnet to do network stuff transparently
  through an Internet firewall.
  ______________________________________________________________________

  Table of Contents


  1. Stuff

     1.1 DISCLAIMER
     1.2 Legal Blurp
     1.3 Credits

  2. Introduction

     2.1 Foreword
     2.2 Security problems
     2.3 Other requirements
     2.4 Downloading software

  3. Understanding the problem

     3.1 Giving names to things
     3.2 The problem
     3.3 Additional difficulty

  4. The solution

     4.1 Principle
     4.2 fwprc
     4.3 .fwprcrc

  5. Reverse piercing

     5.1 Rationale
     5.2 Getting the triggering mail

  6. Final notes

     6.1 Other settings
     6.2 HOWTO maintenance
     6.3 Extra copy of IMPORTANT DISCLAIMER --- BELIEVE IT!!!


  ______________________________________________________________________

  1.  Stuff



  1.1.  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 vandals 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.


  1.2.  Legal Blurp

  Copyright ⌐ 1998 by Franτois-RenΘ Rideau.

  This document is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or (at
  your option) any later version.



  1.3.  Credits

  Even though I rewrote most everything but the disclaimers, I'm
  indebted to Barak Pearlmutter  <mailto:[e-mail]> for his Term-
  Firewall mini-HOWTO: I think there was a necessity for a mini-HOWTO
  about piercing firewalls, and despite its shortcomings, his mini-HOWTO
  was a model and an encouragement.



  2.  Introduction



  2.1.  Foreword

  Because system administrators and users have different constraints and
  proficiencies, it so happens that a user may find himself behind a
  firewall, that he may cross, but only in awkward ways.  This mini-
  HOWTO explains a generic and portable way to use standard internet
  tools seamlessly across such firewalls, by the use of an IP emulator
  over a telnet session.

  It is freely inspired by the Term-Firewall mini-HOWTO by Barak
  Pearlmutter  <mailto:[e-mail]>, which relies on an ancient and
  no-more-supported program named Term (yet a great program at its
  time), as well as on peculiarities of a not-so-standard telnet
  implementation, that is, many obsolete and non-portable facts.



  2.2.  Security problems

  Of course, if your sysadm has setup a firewall, s/he might have a good
  reason, and you may have signed an agreement to not circumvent it.  On
  the other hand, the fact that you can telnet outside (which is a
  requisite for the presented hacks to work) means that you are allowed
  to access external systems, and the fact that you can log into a
  particular external system somehow means you're allowed to do it, too.

  So this is all a matter of conveniently using legal holes in a
  firewall, and allow generic programs to work from there with generic
  protocols, as opposed to requiring special or modified (and
  recompiled) programs going through lots of special-purpose proxies
  that be misconfigured by an uncaring or incompetent sysadm, or to
  installing lots of special-purpose converters to access each of your
  usual services (like e-mail) through ways supported by the firewall
  (like the web).

  Moreover, the use of a user-level IP emulator such as SLiRP should
  still prevent external attackers from piercing the firewall back in
  the other way, unless explicitly permitted by you (or they are clever
  and wicked, and root or otherwise able to spy you on the remote host).


  All in all, the presented hack should be relatively safe.  However, it
  all depends on the particular circumstances in which you set things
  up, and I can give no guarantee about this hack.  Lots of things are
  intrinsically unsafe about any internet connection, be it with this
  hack or not, so don't you assume anything is safe unless you have good
  reasons, and/or use some kind of encryption all the way.

  To sum it up, don't use this hack unless you know what you're doing.
  Re-read the disclaimer above.



  2.3.  Other requirements

  It is assumed that you know what you're doing; that you know about
  setting up a network connection; that you have shell accounts on both
  sides of the firewall; that you can somehow telnet (or ssh, or
  equivalent) from one account to the other; that you can run an IP
  emulator on both shell accounts; that you have programs able to use
  the IP connection emulated on their side.  Note that any program can
  use the connection, in case the local emulator is pppd talking to the
  Linux kernel; other emulators, like Term, need recompilation and
  linking to a special library.

  Talking about IP emulators, pppd can be found in any good Linux
  distribution or ftp site; so can SLiRP.  If your remote shell account
  is user-level only, you can use SLiRP to connect.



  2.4.  Downloading software

  Most described software should be available from your standard
  distribution, possibly among contrib's; at least all but the two small
  last ones are available in as rpm packages.  In case you want to fetch
  the latest sources or binaries (after all, one of the ends of the
  connection may not be running linux), use the addresses below:

  ╖  SLiRP can be found at <http://blitzen.canberra.edu.au/slirp> and/or
     <ftp://www.ibc.wustl.edu/pub/slirp_bin/>.

  ╖  zsh can be found at <http://www.peak.org/zsh/>.

  ╖  ppp can be found at <ftp://cs.anu.edu.au/pub/software/ppp/>.

  ╖  fwprc and cotty can be found at
     <http://www.tunes.org/~fare/files/fwprc/>.



  3.  Understanding the problem

  Understanding a problem is the first half of the path to solving it.


  3.1.  Giving names to things

  If you want this hack to work for you, you'll have to get an idea of
  how it works, so that in case anything breaks, you know where to look
  for.

  The first step toward understanding the problem is to give a name to
  relevant concepts.

  So we'll herein call "local" the machine that initiates the
  connection, as well as programs and files on that machine; conversely,
  we'll call "remote" what's on the other side of the connection.



  3.2.  The problem

  The goal is to connect the input and output of a local IP emulator to
  the output and input respectively of a remote IP emulator.

  Only the communication channels with which IP emulators interact are
  either direct devices (in the usual case of pppd), or the "current
  tty".  The previous case obviously does not happen with telnet
  sessions.  The latter is tricky, because when you launch the local
  emulator from the command line, the "current tty" is linked to the
  command-line user, not to a remote session; also, should we open a new
  session (local or remote) on a new terminal, we must synchronize the
  launching and connection of IP emulators on both sides, least one
  session's garbage output is going to be executed as commands on the
  other session, which would recursively produce more garbage.


  3.3.  Additional difficulty

  To get the best ease of use, the local IP emulator has to provide IP
  to kernel networking, hence be pppd.  However, pppd is dumb enough to
  only accept having data through /dev or thru the current tty; it must
  be a tty, not a pair of pipe (which would be the obvious design).
  This is fine for the remote pppd if any, as it can use the telnet
  session's tty; but for the local pppd, it sucks, as it can't launch
  the telnet session to connect to; hence, there must some kind of
  wrapper around it.

  Telnet behaves almost correctly with a pair of pipe, except that it
  will still insist on doing ioctl's to the current tty, with which it
  will interfere; using telnet without a tty also causes race
  conditions, so that the whole connection will fail on "slow" computers
  (fwprc 0.1 worked perfectly on a P/MMX 233, one time out of 6 on a
  6x86-P200+, and never on a 486dx2/66).

  [Note: if I find the sucker (probably a MULTICS guy, though there must
  have been UNIX people stupid enough to copy the idea) who invented the
  principle of "tty" devices by which you read and write from a "same"
  pseudo-file, instead of having clean pairs of pipes, I strangle him!]



  4.  The solution



  4.1.  Principle

  The firewall-piercing program, fwprc, will use a "tty proxy", cotty,
  that opens two pseudo-tty devices, launches some command on each of
  those devices' slaves, and stubbornly copies every character that one
  outputs to the tty that serves as input of the other command.  One
  command will be telnet connection to remote site, and the other will
  be the local pppd.  pppd can then open and control the telnet session
  with a chat script as usual.


  4.2.  fwprc

  I wrote a very well self-documented script to pierce firewalls, fwprc,
  available from my site  <http://www.tunes.org/~fare/files/fwprc/>,
  together with cotty (which is required by fwprc 0.2 and later).  At
  the time of my writing these lines, latest versions are fwprc 0.3a and
  cotty 0.3a.

  The name "fwprc" is voluntarily made unreadable and unpronounceable,
  so that it will confuse the incompetent paranoid sysadm who might be
  the cause of the firewall that annoys you (of course, there can be
  legitimate firewalls, too, and even indispensible ones; security is
  all a matter of correct configuration).  If you must read it aloud,
  choose the worst way you can imagine.

  CONTEST! CONTEST! Send me a .au audio file with a digital audio
  recording of how you pronounce "fwprc".  The worst entry will win a
  free upgrade and his name on the fwprc 1.0 page!

  I tested the program in several settings, by configuring it through
  resource files.  But of course, by Murphy's law, it will break for
  you.  Feel free to contribute enhancements that will make life easier
  to other people who'll configure it after you.



  4.3.  .fwprcrc

  fwprc can be customized through a file .fwprcrc meant to be the same
  on both sides of the firewall.  Having several alternate
  configurations to choose from is sure possible (for instance, I do
  it), and is left as an exercise to the reader.

  To begin with, copy the appropriate section of fwprc (the previous to
  last) into a file named .fwprcrc in your home directory.  Then replace
  variable values with stuff that fits your configuration.  Finally,
  copy to the other host, and test.

  Default behavior is to use pppd locally, and slirp remotely.  To
  modify that, you can redefine the appropriate function in your
  .fwprcrc with such a line as:

       remote_IP_emu () { remote_pppd }


  Note that SLiRP is safer than pppd, and easier to have access to,
  since it does not require being root on the remote machine.  Anoter
  safe feature is that it will drop packets not directly coming from the
  connected machine (which feature becomes a misfeature if you attempt
  to route a subnetwork onto it with masquerading).  The basic
  functionality in SLiRP works quite well, but I've found advertised
  pluses (like run-time controllability) to be deficient; of course,
  since it is free software, feel free to hack the source so as to
  actually implement whichever feature you need.




  5.  Reverse piercing



  5.1.  Rationale

  Sometimes, only one side of the firewall can launch telnet sessions
  into the other side; however, some means of communication is possible
  (typically, through e-mail).  Piercing the firewall is still possible,
  by triggering with whatever messaging capability is available a telnet
  connection from the ``right'' side of the firewall to the other.


  fwprc includes code to trigger such connections from a PGP-
  authentified e-mail message; all you need is add fwprc as a
  procmail(1) filter to messages using the protocol, (instructions
  included in fwprc itself).  Note however, that if you are to launch
  pppd with appropriate priviledges, you might need create your own suid
  wrapper to become root.  Instructions enclosed in fwprc.

  Also, authentified trigger does not remotely mean secure connection.
  You should really use ssh (perhaps over telnet) for secure
  connections.  And then, beware of what happens between the triggering
  of a telnet connection, and ssh taking over that connection.
  Contribution in that direction welcome.


  5.2.  Getting the triggering mail

  If you are firewalled, your mail may as well be in a central server
  that doesn't do procmail filtering or allow telnet sessions.  No
  problem! You can use fetchmail(1) to run in daemon mode to poll and
  get mail to your client linux system, and/or add a cron-job to
  automatically poll for mail every 1-5 minutes.  fetchmail will forward
  mail to a local address through sendmail(8), which itself will have
  been configured to use procmail(1) for delivery.  Note that if you run
  fetchmail(1) as a background daemon, it will lock away any other
  fetchmail that you'd like to run only at other times, like when you
  open a fwprc; of course, if you can also run a fetchmail daemon as a
  fake user.  Too frequent a poll won't be nice to either the server or
  your host.  Too unfrequent a poll means you'll have to wait before the
  message gets read and the reverse connection gets established.  I use
  two-minute poll frequency.



  6.  Final notes



  6.1.  Other settings

  There are other kinds of firewalls than those that allow for telnet
  connections.  As long as a continuous flow of packets may go through a
  firewall, and transmit information both ways, it is possible to pierce
  it; only the price of writing the piercer may be higher or lower.

  In a very easy case, you can just launch ssh over a pty, and do some
  pppd in the slave tty.  cotty 0.3a should be able to do it, but
  nobody's modified fwprc to take it into account yet.  May be tonight's
  exercise for you.  You may even want to do it without an adverse
  firewall, just so as to build a secure ``VPN'' (Virtual Private
  Network).  See the VPN mini-HOWTO about this.

  If you need cross a 7-bit line, you'll want to use SLIP instead of
  PPP.  I never tried, because lines are more or less 8-bit clean these
  days, but it shouldn't be difficult.

  Now, if the only way through the firewall is a WWW proxy (usually, a
  minimum for an internet-connected network), you might want to write a
  daemon that buffers data in and out, and sends it during in HTTP
  connections, achieving some telnet-over-HTTP over which to run fwprc.
  It might be slow and not very responsive, but still good enough to use
  fetchmail(1), suck(1), and other non-interactive programs.

  If you want more performance, or if the only thing that goes through
  unfiltered is some wierder thing even (DNS queries, ICMP packets,
  whatever), then you're in the very hard case where you'll have to re-
  hack a wierd IP stack, using (for instance) the Fox project's packet-
  protocol functors.  You'll then achieve some direct IP-over-HTTP, IP-
  over-DNS, IP-over-ICMP, or such, which requires not only a complex
  protocol, but also an interface to an OS kernel, both of which are
  costly to implement.

  By the way, if you use some Firewall-piercing HTTP daemon, don't
  forget to have it serve fake pages, so as to mislead suspicious
  adverse firewall administrators.



  6.2.  HOWTO maintenance

  I felt it was necessary to write it, but I don't have that much time
  for that, so this mini-HOWTO is very rough.  So will it stay, until I
  get enough feedback so as to know what sections to enhance.  Feedback
  welcome. Help welcome. mini-HOWTO maintenance take-over welcome.

  In any case, the above sections have shown many problems whose
  solution is just a matter of someone (you?)  spending some time (or
  money, by hiring someone else) to sit down and write it: nothing
  conceptually complicated, though the details might be burdensome or
  tricky.

  Do not hesitate to contribute more problems, and hopefully more
  solutions, to this mini-HOWTO.



  6.3.  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 vandals 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.