sshuttle/README.md

sshuttle: where transparent proxy meets VPN meets ssh
=====================================================

As far as I know, sshuttle is the only program that solves the following
common case:

 - Your client machine (or router) is Linux or FreeBSD (or
   MacOS, but MacOS 10.6 in particular has a bug that
   prevents it from working).

 - You have access to a remote network via ssh.

 - You don't necessarily have admin access on the remote network.

 - The remote network has no VPN, or only stupid/complex VPN
    protocols (IPsec, PPTP, etc). Or maybe you <i>are</i> the
    admin and you just got frustrated with the awful state of
    VPN tools.

 - You don't want to create an ssh port forward for every
    single host/port on the remote network.

 - You hate openssh's port forwarding because it's randomly
    slow and/or stupid.
 
 - You can't use openssh's PermitTunnel feature because
    it's disabled by default on openssh servers; plus it does
    TCP-over-TCP, which has terrible performance (see below).
    

Prerequisites
-------------

 - sudo, su, or logged in as root on your client machine.
   (The server doesn't need admin access.)
   
 - If you use Linux on your client machine:
   iptables installed on the client, including at
   least the iptables DNAT, REDIRECT, and ttl modules. 
   These are installed by default on most Linux distributions. 
   (The server doesn't need iptables and doesn't need to be
   Linux.)
   
 - If you use MacOS or BSD on your client machine:
   Your kernel needs to be compiled with IPFIREWALL_FORWARD
   (MacOS has this by default) and you need to have ipfw
   available. (The server doesn't need to be MacOS or BSD.)


This is how you use it:
-----------------------

 - <tt>git clone git://github.com/apenwarr/sshuttle</tt>
    on your client machine. You'll need root or sudo
    access, and python needs to be installed.

 - <tt>./sshuttle -r username@sshserver 0.0.0.0/0 -vv</tt>

That's it!  Now your local machine can access the remote network as if you
were right there!  And if your "client" machine is a router, everyone on
your local network can make connections to your remote network.

You don't need to install sshuttle on the remote server;
the remote server just needs to have python available. 
sshuttle will automatically upload and run its source code
to the remote python interpreter.

This creates a transparent proxy server on your local machine for all IP
addresses that match 0.0.0.0/0.  (You can use more specific IP addresses if
you want; use any number of IP addresses or subnets to change which
addresses get proxied.  Using 0.0.0.0/0 proxies <i>everything</i>, which is
interesting if you don't trust the people on your local network.)

Any TCP session you initiate to one of the proxied IP addresses will be
captured by sshuttle and sent over an ssh session to the remote copy of
sshuttle, which will then regenerate the connection on that end, and funnel
the data back and forth through ssh.

Fun, right?  A poor man's instant VPN, and you don't even have to have
admin access on the server.


Theory of Operation
-------------------

sshuttle is not exactly a VPN, and not exactly port forwarding.  It's kind
of both, and kind of neither.

It's like a VPN, since it can forward every port on an entire network, not
just ports you specify.  Conveniently, it lets you use the "real" IP
addresses of each host rather than faking port numbers on localhost.

On the other hand, the way it *works* is more like ssh port forwarding than
a VPN.  Normally, a VPN forwards your data one packet at a time, and
doesn't care about individual connections; ie. it's "stateless" with respect
to the traffic.  sshuttle is the opposite of stateless; it tracks every
single connection.

You could compare sshuttle to something like the old <a
href="http://en.wikipedia.org/wiki/Slirp">Slirp</a> program, which was a
userspace TCP/IP implementation that did something similar.  But it
operated on a packet-by-packet basis on the client side, reassembling the
packets on the server side.  That worked okay back in the "real live serial
port" days, because serial ports had predictable latency and buffering.

But you can't safely just forward TCP packets over a TCP session (like ssh),
because TCP's performance depends fundamentally on packet loss; it
<i>must</i> experience packet loss in order to know when to slow down!  At
the same time, the outer TCP session (ssh, in this case) is a reliable
transport, which means that what you forward through the tunnel <i>never</i>
experiences packet loss.  The ssh session itself experiences packet loss, of
course, but TCP fixes it up and ssh (and thus you) never know the
difference.  But neither does your inner TCP session, and extremely screwy
performance ensues.

sshuttle assembles the TCP stream locally, multiplexes it statefully over
an ssh session, and disassembles it back into packets at the other end.  So
it never ends up doing TCP-over-TCP.  It's just data-over-TCP, which is
safe.


Useless Trivia
--------------

Back in 1998 (12 years ago!  Yikes!), I released the first version of <a
href="http://alumnit.ca/wiki/?TunnelVisionReadMe">Tunnel Vision</a>, a
semi-intelligent VPN client for Linux.  Unfortunately, I made two big mistakes: 
I implemented the key exchange myself (oops), and I ended up doing
TCP-over-TCP (double oops).  The resulting program worked okay - and people
used it for years - but the performance was always a bit funny.  And nobody
ever found any security flaws in my key exchange, either, but that doesn't
mean anything. :)

The same year, dcoombs and I also released Fast Forward, a proxy server
supporting transparent proxying.  Among other things, we used it for
automatically splitting traffic across more than one Internet connection (a
tool we called "Double Vision").

I was still in university at the time.  A couple years after that, one of my
professors was working with some graduate students on the technology that
would eventually become <a href="http://www.slipstream.com/">Slipstream
Internet Acceleration</a>.  He asked me to do a contract for him to build an
initial prototype of a transparent proxy server for mobile networks.  The
idea was similar to sshuttle: if you reassemble and then disassemble the TCP
packets, you can reduce latency and improve performance vs.  just forwarding
the packets over a plain VPN or mobile network.  (It's unlikely that any of
my code has persisted in the Slipstream product today, but the concept is
still pretty cool.  I'm still horrified that people use plain TCP on
complex mobile networks with crazily variable latency, for which it was
never really intended.)

That project I did for Slipstream was what first gave me the idea to merge
the concepts of Fast Forward, Double Vision, and Tunnel Vision into a single
program that was the best of all worlds.  And here we are, at last, 10 years
later.  You're welcome.

--
Avery Pennarun <apenwarr@gmail.com>

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