Evolve is a silly acronym for something I figured out how to do one day. Other people have done this countless times, but I thought I'd document the things I learned doing it.
I established a Virtual Private Network (VPN) between two Linux hosts with ssh and pppd. This technique should work between any two Unix/POSIX systems, but the options available to pppd may vary.
'pppd' is a program that takes an TCP/IP stack and squeezes it through a sequential data stream, normally a modem. 'ssh' creates a network connection that is authenticated and encrypted. If you string these together, you can create a virtual network interface that connects any two computers that can communicate via ssh. This is useful for privacy reasons (traffic between the endpoints is encrypted), and for firewall-avoiding convenience (no ports are blocked on the virtual interface). If you can ssh to your target server, but can't run VNC because of the firewall, a quick-and-dirty VPN can solve that.
Let's describe the process in terms of connecting a 'client' (in this case my laptop at work) to a 'server' (in this case my workstation at home - www.risacher.org). The client machine is behind a firewall that does IP-masquerading (NAT). The firewall also only allows connections through on certain ports, in particular, the telnet port is open, but the ssh port is not. The server acts as its own firewall via iptables.
In order to get this to work...
magnus ALL= NOPASSWD: /usr/sbin/pppd,/usr/bin/pty-redir
So: originally the magic command, run as root on the client was:
pppd `pty-redir /usr/bin/ssh -2 email@example.com -p 23 sudo /usr/sbin/pppd notty passive` local 192.168.1.5:192.168.1.1
Most people do not need to run SSH on port 23. I don't either, any more, but when I first tried this, I did because I didn't have control of the firewall.
New versions of pppd have an option 'pty', which does the work of
the pty-redir command in a much simpler fashion. If you are using a
modern pppd, you can use this much simpler method. This would look
pppd pty '/usr/bin/ssh -2 firstname.lastname@example.org
sudo /usr/sbin/pppd notty passive' local
I've seen some people writing that this is terrible, performance-wise, since you now have multiple TCP/IP stacks on top of one another, which can cause the exponential backoff algorithms in the protocols to do bad things. This is true, although I haven't tried to measure it. If you want a real VPN, I recommend that you use IPsec, but be prepared to spend some time, money, or both understanding it and setting it up. EVOLVE is great to put together a connection quickly. It's not suitable for production use.
route add -net 188.8.131.52 netmask 255.255.0.0 gw 184.108.40.206 route add -net 220.127.116.11 netmask 255.255.0.0 gw 18.104.22.168 route add default gw risacher.vlan client: Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 192.168.1.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0 22.214.171.124 0.0.0.0 255.255.255.128 U 0 0 0 eth0 126.96.36.199 188.8.131.52 255.255.0.0 UG 0 0 0 eth0 184.108.40.206 220.127.116.11 255.255.0.0 UG 0 0 0 eth0 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 ppp0
Projects page, Dan Risacher