Routing traffic with multiple OpenVPN

Why OpenVPN?

For my dayjob we access the servers we manage through OpenVPN. Of course it's not the only security measure, it's yet another layer and it helps to cut a part of the IBN. All of our servers are registered in LDAP and from this system we create some routes that the OpenVPN server pushes to the OpenVPN clients.

What did I need?

Follow the pushed routes, not always and not for all the hosts

I work sometimes from home (for on-call or just remote work). I have a IP phone which needs the VPN but of course I can't setup OpenVPN on the phone directly, so the VPN has to go on my router. But let's say some android phone (without security updates) connects to my wifi, I don't want its traffic to go through the VPN.

But I also have my own desktop that I don't want any of its traffic to go through the VPN, but sometimes I want it to use the routes if I want to quickly check something on a server.

Default route sometimes, sometimes not

By default, clients don't set the gateway to the vpn, because we have the routes. But sometimes, we need to access a host through the VPN without having a route to it being pushed by the server. Hence I need to be able to route all the traffic through the vpn if needed. But not always because the vpn endpoint is 105ms away and browsing with this increased latency is obviously a bit slower.

Even with a default route, bypassing the VPN for some servers

I have a VM in Montreal, 10ms away, and there's no reason that the traffic should go through the VPN. Same goes for my OpenBSD mirror.

Multiple VPN

I also have another VPN which endpoints is in Montreal and I may want to route some host from my lan through it. It must independant from the other VPN.

Don't touch the server side

My coworkers use the VPN as well so I can't change the server configuration just to suit my own need.

Suiting all the needs \o/

I will only talk about the client as there's nothing special on the server side

OpenVPN infrastructure

danj@pancake:/etc/openvpn$ ls
client-ca.conf  client-fr.conf  private-stuff/

Config files are as usual, the only special thing is that I force the tun device used by the VPN (so I can use it in pf.conf):

danj@pancake:/etc/openvpn$ grep dev *.conf
client-ca.conf:dev tun1
client-fr.conf:dev tun0

In rc.conf.local, I set the correct config file:

openvpn_fr_flags="--config /etc/openvpn/client-fr.conf"
openvpn_ca_flags="--config /etc/openvpn/client-ca.conf"

now I can rcctl start openvpn_fr and rcctl start openvpn_ca


Spoiler alert, everything is done with pf.

I won't put my whole pf.conf but only the needed parts. First let's describe the interface.

vpnfr_if = "tun0"
vpnca_if = "tun1"

I have vlan-capable switch and wifi AP, so I have multiple networks.

lan_net = $lan_if:network
wifilap_net = $wifilap_if:network
wifitel_net = $wifitel_if:network
windows_net = $windows_if:network
tel_net = $tel_if:network

I need some tables (don't worry, you'll understand later what purpose they have).

table <softvpnfr> { } persist
table <vpnfr> { $phone } persist
table <vpnca> { } persist
table <bypassfr> {,,, } persist
table <forcevpnfr> { $mrs-fw2 }
table <nousautres> {, $home_ip } persist

Now we can see the ruleset. I let everything from the lan, that doesn't go on the router itself or to another lan (so the traffic will need another rules to be allowed) come through.

pass in     on $lan_if     from $lan_net     to ! <nousautres>
pass in     on $wifilap_if from $wifilap_net to ! <nousautres>
pass in     on $wifitel_if from $wifitel_net to ! <nousautres>
pass in     on $tel_if     from $tel         to ! <nousautres>
pass in log on $windows_if proto { tcp, udp } from $windows_net to ! <nousautres>

I let everything going out

pass out log on $ext_if proto { tcp, udp } all modulate state
pass out on $vpnfr_if proto { tcp, udp } all modulate state
pass out on $vpnca_if proto { tcp, udp } all modulate state

Now's the fun part.

  • <softvpn> is the hosts that can you the routes pushed by the VPN but it doesn't use the VPN as the gw
  • <vpnfr> and <vpnca> everything from the hosts in it goes through the VPN (French or Canadian)
  • <bypassfr> any traffic to host in the table won't go through the VPN
  • <forcevpnfr> host that must be accessed through the VPN
# disable the use of the routes if you're not in <softvpn>
pass in on { $lan_if, $wifilap_if, $wifitel_if, $atlas_if } \
     from !<softvpnfr> to ! <nousautres>  route-to ($ext_if $home_ip)

# force traffic through the French VPN
pass in on { $lan_if, $wifilap_if, $wifitel_if, $tel_if } \
     from <vpnfr> to ! <nousautres> route-to ($vpnfr_if

# traffic to hosts in <bypass> must not go through the VPN
pass in on { $lan_if, $wifilap_if, $wifitel_if, $tel_if } \
     from <vpnfr> to <bypassfr> route-to ($ext_if $home_ip)

# force traffic through the Canadian VPN
pass in on { $lan_if, $wifilap_if, $wifitel_if, $tel_if } \
     from <vpnca> to ! <nousautres> route-to ($vpnca_if

# traffic from <softvpnfr> to hosts in <forcevpnfr> should really go through the VPN
pass in on { $lan_if, $wifilap_if, $wifitel_if } \
     from <softvpnfr> to <forcevpnfr> route-to ($vpnfr_if

But the real magic with pf, is that I can very easily change the routing for any host :

# if I want everything to go through the Canadian VPN
root@pancake:~# pfctl -t vpnca -Ta
# or not
root@pancake:~# pfctl -t vpnca -Td
# through the French VPN
root@pancake:~# pfctl -t vpnfr -Ta
# ok not everything, just use the route pushed by the VPN
root@pancake:~# pfctl -t vpnfr -Td
root@pancake:~# pfctl -t softvpn -Ta

That's all! Of course, if anything goes wrong, I have Jean Canard's Advanced Paws System (APS) that checks for anything.

By Vigdis in
Tags : #OpenBSD, #pf,
linkedin email