Our headquarters is in San Diego, CA, where I live and work. The last few days, however, I've been working in El Paso, TX, where we have a branch office. I had three tasks this week:

  1. Help "turn up" (enable) the new T1 line to the El Paso office. I was there when it was installed 3 weeks ago, but I had to leave before it could be turned on.
  2. Bridge the LAN in the El Paso office with the LAN in San Diego, placing them on the same subnet.
  3. Install a new Avaya phone router for the El Paso phones (made by Lucent), which then are linked through the VPN to San Diego, again appearing as if both networks are contiguous.

I'm going to provide a step-by-step for #2, bridged ethernet over a VPN. But I'll only lay out one-half of it now: the server side in San Diego, with roaming clients. The next post will discuss the El Paso side, since that has its own complexities.

To accomplish this, I used OpenVPN, an open-source, userspace VPN solution made by James Yonan. Give him some money if you can—this is the greatest networking tool since tcpdump.

Step 1: Get a server.

You don't have to dedicate one solely to OpenVPN, but you probably should. I used a $400 Wal-Mart PC (with no OS). It needs two NIC's.

Step 2: Install Debian Linux.

I used Sarge, which is currently the "testing" release, but will Very Soon Now become the "stable" release. I used the sweet new debian-installer with a 2.6 kernel, as sysadmin guru Greg Folkert advised.

Step 3: Connect the hardware.

Give eth0 a static IP. The configs will be easier if eth0 is the external NIC. You should have one cable hooked into your LAN switch/hub, and the other cable should be on the other side of your broadband router or firewall. It should look like this:

Diagram of VPN network

You don't have to have a T1, but you need multiple, static IP's (from your ISP). The firewall should already have one, and eth0 needs another. Also, be aware that your T1 box may have its own already (and don't use your broadcast address, either ;) ).

Step 4: Install LZO compression and the lzo headers.

apt-get install liblzo1
apt-get install liblzo-dev

Step 5: Install OpenSSL and its headers.

apt-get install openssl
apt-get install libssl-dev

Step 6: Install OpenVPN.

cd /usr/src
wget ??

Replace ?? with the URL of the OpenVPN sources. Go to the OpenVPN website to find out what the URL of the most recent release is. Modify the following commands to match whatever release you end up downloading. Also, do your best to use the same version on both the server and the clients.

gunzip openvpn-2.0_rc21.tar.gz
tar xvf openvpn-2.0_rc21.tar
cd openvpn-2.0_rc21
./configure --with-ssl-headers=/usr/local/ssl/include/ --with-ssl-lib=/usr/local/ssl/lib/
make install

Step 7: Get more tools.

apt-get install bridge-utils
apt-get install tcpdump

Step 8: Turn on IP forwarding.

Edit /etc/network/options, set ip_forward=yes

Step 9: Configure the tun device.

mkdir /dev/net
mknod /dev/net/tun c 10 200

Step 10: Copy and edit the firewall script.

cp /usr/src/openvpn-2.0_rc21/sample-config-files/firewall.sh /etc/openvpn/firewall.sh
cd /etc/openvpn
vi firewall.sh

  • Change the value of PRIVATE to your local subnet. Yours might be
  • After iptables -A INPUT -p udp --dport 1194 -j ACCEPT
    add the line iptables -A INPUT -p udp --dport 4444:4449 -j ACCEPT
    or whatever port range you're going to use for your clients.
  • If the example firewall script has NAT MASQUERADING turned on (last line), comment it out in your new copy.

Step 11: Create /etc/openvpn/opentuns.sh.


/usr/sbin/brctl addbr br0
/usr/sbin/brctl addif br0 eth1

TAPS="public rbre elpaso"

for name in $TAPS
/usr/local/sbin/openvpn --mktun --dev tap$name
/usr/sbin/brctl addif br0 tap$name

ifconfig eth1 promisc up

for name in $TAPS
ifconfig tap$name promisc up

ifconfig br0 netmask broadcast

exit 0

Add a new name to the TAPS list for each tap (port) you wish to create.

The brctl commands make a new ethernet bridge, and then bind eth1 to that bridge. This means br0 is now acting like an unmanaged switch on your LAN, and eth1 is "plugged into" that switch. When each tap is bound to the same bridge, they become participants on that switch, and will receive any traffic which passes over eth1 (they're all "promiscuous", which means they will listen for traffic even if it's not directed specifically at them). The last ifconfig line in the script makes our bridge behave like a managed switch by giving it an IP address.

Remember to make opentuns.sh executable, using chmod.

Step 12: Create /etc/openvpn/common.conf.

This file provides a common config for all taps.

secret /etc/openvpn/static.key
ping 10
verb 5

Step 13: Create an init script.

Create the file /etc/init.d/openvpn, to run everything at startup:



VPN="/usr/local/sbin/openvpn --config /etc/openvpn/common.conf"

$VPN --dev tappublic --port 1194 --daemon tappublic
$VPN --dev taprbre --port 4444 --daemon taprbre
$VPN --dev tapelpaso --port 4445 --daemon tapelpaso

exit 0

Sorry, but you have to add a new line for each tap you create. This maps the port to the tap.

Remember to make this file executable, using chmod.

In /etc/rc2.d through rc5.d, add symlinks to the file you just created. The "S number" you supply in the filename will determine the order in which your startup script is run (lowest first). I picked 19 a bit arbitrarily, based on other scripts in the rc folders.

ln -s ../init.d/openvpn S19openvpn

Step 14: Make a static key on the server.

cd /etc/openvpn (mkdir the folder if necessary)
openvpn --genkey --secret static.key
chmod go-rwx static.key

Step 15: Install OpenVPN on the client(s).

All of my roaming clients are Windows 2k or XP; download the installer on each client. Each client needs a .conf file (the name doesn't matter) which matches the one on the server (for most items):

remote openvpn.aminus.org
port 4444

dev tap
dev-node my-tap

secret static.key
ping 10
verb 5

mute 10

The remote parameter needs to point to eth0 on your server. My domain registrar has a nice DNS tool, so I added an A (host) record for openvpn.aminus.org and pointed it to the IP which I gave eth0. If you don't care to use DNS, you can just type the IP of eth0.

On Windows clients, remember to name the new network connection "my-tap", or whatever name you use in the client config.

Copy the static.key from the server into the /config folder on each client.

Step 16: Test OpenVPN.

On the server, type the following. Note that, for the most part, we are doing by hand what your init.d script does. We do it by hand so we can see the output at each step, in case something goes wrong:

cd /etc/openvpn

/usr/local/sbin/openvpn --config /etc/openvpn/common.conf --dev tappublic --port 1194

Notice we do not supply the daemon argument to openvpn on the command line. This means your terminal will now be occupied running openvpn. If you need to check anything on the server while testing, Alt-F2 to another tty and login again.

Now start OpenVPN on a client; make sure the port is the same (1194, in our example). On the server side, you should start seeing traffic display in the openvpn process: it'll start spitting out w's and r's as it reads and writes data from the client. On the client side, if you start openvpn manually (in Windows, right-click on your .conf file and select "Start OpenVPN on this file") you should also see the connection as it happens. From the client, try accessing resources on the LAN (ping IPs, browse network shares, or access an internal website). If something goes wrong, fix it before proceeding to the next step.

The first thing that will most likely go wrong: you've got your server LAN running on the 192.168.0.x subnet, and you're running the client at home, also on a 192.168.0.x subnet. This doesn't work well, because your client doesn't know whether to send local packets through the tunnel or not. If your home was running on, say, 192.168.15.x, then there would be no conflict. Change your work or home subnet to something else.

Step 17: Test your startup script.

Kill the openvpn process from step 16 using Ctrl-C. Reboot the server. Test a client again, manually. If that works, test the OpenVPNService (on Windows clients).

This should work for as many roaming clients as you like. OpenVPN 2.0 is supposed to have a way to set this up without having to specify a separate port per client; I simply haven't looked at that yet. I will when we get too many roaming clients.

Next time, we'll talk about a permanent bridge between two networks.

Site-to-site bridged ethernet using OpenVPN (1 of 2)

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