Only routing certain traffic through VPN

Question

I have a central development server, running a VPN server, and a couple clients that need to connect to it. The server is running Apache, but is only accepting local requests (only listening on 127.0.0.1:80).

I basically need to force each client connected to the VPN to route traffic to a certain hostname through the VPN and to the local Apache instance.

For example:

Client requests google.com -> google.com
Client requests server -> vpn -> server:80

I know I can push route commands to clients when they connect to the VPN, now I just need to figure out what route to push to make that happen. At least, what is the route syntax/command for the client to add this route themselves?

Solution

First of all: you won't be able to route traffic to 127.x.y.z anywhere other than the local machine (ok, it might even be possible, but you'd certainly break something else in the process...) so I'd recommend updating the apache config to also listen at the VPN IP (e.g. 10.8.0.1). If that's not an option, you could try one of the options at the end of my answer.

OpenVPN clients should already get a route to the server, in my example something like this:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.8.0.1        10.8.0.53       255.255.255.255 UGH   0      0        0 tun0
10.8.0.53       0.0.0.0         255.255.255.255 UH    0      0        0 tun0

If you want additional routes pushed to the clients, use:

push "route 192.168.10.0 255.255.255.0"

(change the IP/netmask accordingly).

If you want your apache instance to be accessible by hostname (and not just at http://10.8.0.1/), put this in every client's /etc/hosts file

10.8.0.1    servername.domain.example

or set up a DNS server (like dnsmasq <- make sure you disable the dhcp server) and push it to the clients (in your ovpn-conf):

push "dhcp-option DNS 10.8.0.1"

That should do the trick.

Other options

If you're unable to change the IPs apache's listening at, the following approaches come to my mind (but only use them as last resort):

• SSH port forwarding: Instead of using OpenVPN (or any other VPN server), connect to your server using SSH:

  ssh -L1234:localhost:80 user@servername
  

  That way the apache instance on the server (listening only at 127.0.0.1:80) will be available at your client at http://localhost:1234/. You'd have to do this on every client, therefore it's probably not suitable if you've got many of them.
  But even then you could set up a dedicated ssh user without shell access and a public key for each client in ~/.ssh/authorized_keys. Keep in mind that the clients may be able to use this as a proxy to the internet or do some other stuff you might not want them to. So it's important to configure sshd correctly.

• some iptables magic (you'd have to NAT the traffic)
• some other user space port forwarding or a reverse proxy

Comments

[peterph]

For example something like ip route server/mask via openvpn_server could do. If you wanted to redirect only traffic for specific ports (the :80 in your question), you'd have to play with the packet filtering.

[n0pe]

@peterph: Is that an actual command or just pseudocode? I'm on a mac (one of my clients) and which ip doesn't return anything. I double checked with ubuntu and it seems ip is a linux-only tool. Would there be an equivalent command using the route tool so I can use it on osx?

[peterph]

The route has a very similar syntax - something like route add -net server/mask gw openvpn_server. You might need to omit the gw string. See man route for in depth documentation.