This article should explain how to setup and configure a VPN using CIPE. CIPE may not be standard like IPSec, but it sure is a hell of a lot simpler.
CIPE is easy. CIPE is very easy. VPNs created with CIPE work, and they work well. CIPE is your friend. Life is simple with CIPE. Now you feel sleepy. Really sleepy. Keep looking at the pendulum....
Who should read thisThose who:
- Need a VPN
- Have no reason to use IPSec
- Don't know CIPE yet
- Want to learn about VPNs but don't want to use difficult software
- Took my Linux course, and remember that at the VPN class I said I would document it someday :-)
To those students I swear there is going to be a spanish version very soon, too!
So, what's a VPN and why would I need one?
VPN means Virtual Private Network. As you all should know, virtual means "not real". So, a VPN is "not a real private network".
If that definition is not good enough, here's a better one, which is probably not what you are going to find in other places:
A VPN is emulationg a real network connection using another. For example, you can use the internet connections of two hosts to make it look like you have a direct connection between them.
And if that one is not good enough for you, go here.
Here's a simple diagram:
Simple Network-To-Network VPN Diagram
That is a VPN connecting Networks A and B, and there is a "virtual link" or tunnel connecting endpoints A and B (the dotted arc).
Of course, since the tunnel is a virtual link, the data is still really transmitted through the real links to the Internet. It's just that applications running on the networks or the endpoints believe they are sending through the virtual link.
Now, in order to make this more useful, the data transferred between the endpoints of the VPN is normally encrypted using some secure algorithm.
What this article tries to do is explain to you how you can create one of these "virtual links" between any two computers, using one specific software package called CIPE.
Normally, when you read about VPNs, you are going to run into IPSec. IPSec is good. But it is also a pain in the ass to setup, at least right now, at least in Linux, and at least for me.
So, from now on, I will pretend IPSec doesn't exist. Forget about it!
Types of VPNsThere are several types of VPNs:
- In these, you connect, using the VPN link, two networks. The hosts in those networks can see each other through the VPN link as if they were connected through a regular point-to-point link or any other kind of connection. The hosts need no extra software, because all the hard work is done by the VPN gateways
- Like Network-To-Network only you have no network on one side
- Like Network-To-host only without networks ;-)
In other VPN software, the configurations for each of these is different. On CIPE, they are all exactly the same, except in the cases where you have a network behind one of the endpoints, you need to do some simple routing configuration to allow all hosts to find it.
So, what can I do with it?
Well, dear reader, it's a network. You can use a VPN to do what a network does: send data from A to B.
But, since a VPN usually works over a public network, you can for example, use it to get into your office's network from home, or viceversa. Or to connect two offices to each other.
Once you are connected, you can share files, chat, do videoconference, whatever.
In fact, version 1.5 of CIPE can even do ethernet bridging over UDP. In practical terms, that makes it look like the switches in both ends of the VPN are plugged to each other, and all hosts are part of a single LAN... with some very slow patches.
Enough Talk, Let's VPN
If your distribution doesn't include CIPE, or if it includes an older version (latest right now is 1.5.4), you need to get the latest version of CIPE from here.
Install it following the instructions (the usual ./configure && make && make install should do).
If you are using Red Hat, or other distributions that already include CIPE, make sure you uninstall the distribution's version. Also, on Red Hat, you have to remove the cipcb kernel module that comes with the distribution, because it will conflict with the new version.
You will see that cipe installs /lib/modules/2.4.18-14/misc/cipcb.o but the distro may include something like /lib/modules/2.4.18-14/kernel/drivers/addon/cipe/cipcb.o that you need to remove (or at least move aside ;-)
Digging a Tunnel
For each two endpoints you want to connect, you need to create a tunnel. Think of tunnels as the specific virtual links you create between endpoints.
You will have to run, on each endpoint, a cipe process for each tunnel that connects to that endpoint.
For example, in this VPN:
VPN with more than one tunnel
The endpoints A and C have only one tunnel connecting to them, and need to run only one CIPE process, while endpoint B has two tunnels and will have two run two copies of CIPE.
- The first CIPE
- You can use as configuration file /etc/cipe/options and need to use no special options.
- The second CIPE
- Use another configuraion file. For example, /etc/cipe/options2, and call CIPE with option -o /etc/cipe/options2
- Third and later CIPE
- Like the second one, but change the number. For example, use /etc/cipe/options3 and so on.
Ok, so what do we put into those option files?
Here's an example:
Sample CIPE options file
maxerr=-1 ipaddr=10.0.0.2 ptpaddr=10.0.0.1 me=22.214.171.124:8000 peer=126.96.36.199:8000 key=2fe483867df1e7de3827edd26b193590
What the options mean
- If there are maxerr transmission errors, the virtual link goes down. Setting it to -1 makes it stay up.
- The virtual link is a TCP/IP link, so it needs IP addresses to identify the endpoints. This is the IP address for this endpoint's virtual link.
- Like ipaddr, but it's the IP address of the other endpoint.
- Remember that, in the end, the data is transmitted through the real link. This is the real IP address of this endpoint, plus a port number. The port number should be different for each copy of CIPE we run in this endpoint. So, in another options file, it should say, for example, me=188.8.131.52:8001
- The IP number and port for the other endpoint.
- The key used to encrypt the data transmitted through the tunnel. Obviously, this means this file should only be readable by root. CIPE refuses to start if that's not the case. You could use anything here, but try to make it hard. The CIPE docs, for example, recommend using the output of ps -auxw|md5sum
You will notice that there are options that are related to each other. ipaddr is related to ptpaddr, and me is related to peer.
In the options file in the other endpopint, those options are switched:
Sample CIPE options file for the other endpoint
maxerr=-1 ipaddr=10.0.0.1 ptpaddr=10.0.0.2 me=184.108.40.206:8000 peer=220.127.116.11:8000 key=2fe483867df1e7de3827edd26b193590
Also, for each tunnel you create, the ipaddr/ptpaddr pair must be different and should not be part of any network the endpoints are connected to. Basically that means they can't be valid Internet IP addresses, and that they must be from private networks other than the ones you use.
For example, if your local networks use 192.168 prefixes, then choose the virtual IP addresses from the 10 or 172.16 prefixes.
And that's it, your VPN tunnel is configured.
Starting CIPESimple as running the command ciped-cb ... plus the required -o option as described here. If you want to have CIPE start when your computer boots, just add all the ciped-cb invocations somewhere in the startup scripts. /etc/rc.d/rc.local is usually a good place.
Just kill the ciped-cb process. Or do ifconfig cipcb0 down.
If you have more than one tunnel, you need to be careful to kill the right process, or to bring down the right interface.
If you have a network behind a remote CIPE endpoint and you want it to be reachable, you need to create a route, describing how to get there. Assuming the network behind the remote endpoint is 192.168.1.0/255.255.255.0, and the virtual IP for the remote endpoint is 10.0.0.2, you need to run a command like this when the CIPE link goes up:route add -net 192.168.1.0 netmask 255.255.255.0 gw 10.0.0.2
That can be done editing the script called /etc/cipe/ip-up and adding whatever commands (such as that example above) you want to run when the link goes up.
If you have more than one CIPE tunnel running, just create /etc/cipe/ip-up2 and so on, and in the matching options file add the option ipup=/etc/cipe/ip-up2.