I have been using OpenVPN from quite sometime and very much like it. Earlier I was running OpenVPN client on TP Link 1043nd router and that worked great. But recently I switched home routing to Microtik Map2N which has much better VLAN & IPv6 support. Since then I had trouble in getting VPN back live. I can always use VPN client on laptop but that’s ugly for daily use specially when this is my primary work location!
 
It’s hard to put in this blog but there are number of issues and limitations which come up if I try to use Microtik itself as OpenVPN client. Eventually gave up on that. Now, comes Raspberry Pi which is just a tiny Debian box running quite nicely. I configured OpenVPN on it and connected it as client to my server. It works fine. Next, pointed a static route for a sample test IP from core router to Rasberry Pi, and it did not work. I realized IPv4 forwarding isn’t enabled on it by default. 🙂
 
Once IPv4 forwarding is enabled, still connectivity doesn’t works because of usual private addressing issues. Both home LAN and VPN tunnel works over private IPs and hence packets do hit my server but it cannot return since it’s unsure on return path. This brings me to do usual source NAT on home LAN pool going towards VPN server via tunneled interface. And vola! it works.

iptables -t nat POSTROUTING -s 172.16.18.0/24 -o tun0 -j SNAT –to-source 10.200.105.10

 
Now, a big issue which comes here is that for private addresses which I use for random applications/testing etc it works fine but there are certain public IPs as well which I route via OpenVPN. In this setup, I have to put static route at router end to push towards Raspberry Pi for each case and worst – if VPN tunnel breaks, connectivity will go down completely for those pools.
Thus I decided to rather use “dynamic routing” here. My lovely Quagga comes to rescue. Next, I configured a iBGP session between Quagga and core router and passed “redistribute kernel” in BGP config to redistribute all routes Raspberry Pi learn while connecting to VPN (in form of pushed routed in openvpn config on the server). And it works! 🙂
In this way as long as VPN is up, it’s prefered for public pools and when VPN is down, they take their usual path without having to manually disable any static routes.
 
So here comes a “State of Art” site to site VPN using OpenVPN, Quagga and Raspberry Pi. Time taken to setup: ~5 mins (Less then time taken to write this post 😛 😉 )
 
Quagga Config:

raspberrypi# sh run
Building configuration...
Current configuration:
!
log syslog
!
service integrated-vtysh-config
!
interface eth0
 ipv6 nd suppress-ra
!
interface lo
!
interface tun0
 ipv6 nd suppress-ra
!
router bgp 58901
 bgp router-id 172.16.18.5
 redistribute kernel
 neighbor 172.16.18.1 remote-as 58901
 neighbor 172.16.18.1 next-hop-self
!
ip forwarding
!
line vty
!
end
raspberrypi#

 
Have fun!