sshuttle: where transparent proxy meets VPN meets ssh

I just spent an afternoon working on a new kind of VPN. You can get the first release, sshuttle 0.10, on github.

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

• Your client machine (or router) is Linux.

• 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 are 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:

• git clone git://github.com/apenwarr/sshuttle on your client and server machines. The server can be any ssh server with python available; the client must be Linux with iptables, and you'll need root or sudo access.

• ./sshuttle -r username@sshserver 0.0.0.0/0 -vv

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 everything, 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 Slirp 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 must 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 never 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 Tunnel Vision, 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 Slipstream Internet Acceleration. 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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