Firewall Piercing mini-HOWTO
Fran�ois-Ren� Rideau, fare@tunes.org
v0.3, 22 August 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 Fran�ois-Ren� Rideau. This document may be
distributed only subject to the terms and conditions set forth in the
LDP license at <http://sunsite.unc.edu/LDP/LICENSE.html>.
1.3. Credits
Even though I rewrote most everything but the disclaimers, I'm
indebted to Barak Pearlmutter <mailto:bap@cs.unm.edu> 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:bap@cs.unm.edu>, 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/>.
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/>, together
with cotty (which is required by fwprc 0.2 and later). At the time of
my writing these lines, latest versions are fwprc 0.2a and cotty 0.3.
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). 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 0.3 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 0.3 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).
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.