How to selectively route network traffic through VPN on Mac OS X Leopard?

Create the file /etc/ppp/ip-up with following content:

#!/bin/sh
/sbin/route add <SUBNET> -interface $1 

replacing <SUBNET> with subnet, you want to route through VPN (for ex. 192.168.0.0/16)

execute as root:

chmod 0755 /etc/ppp/ip-up

This file will be executed each time you connect to VPN.

The parameters given to the script:

• $1: The VPN interface (e.g. ppp0)
• $2: Unknown, was 0 in my case
• $3: IP of the VPN server
• $4: VPN gateway address
• $5: Regular (non-vpn) gateway for your lan connections

There is a hidden feature in Network Preferences on MacOS: you can sort interfaces.

Open System Preferences -> Network -> Click the gear bottom left -> Set service Order...

It's critical that you have your network interfaces sorted into the order you want them to be used. If you want ALL non-LAN data to go to the VPN, put the VPN interface at the top. Sort like this

1. VPN
2. Ethernet
3. Airport

Not like this:

1. Airport
2. Ethernet
3. VPN

This way, no need to check the following setting in Session Options:

Send all traffic over VPN connection

✅ Tested on L2TP VPN connection

I wanted to do a similar thing. Connect the VPN and then route an additional network via that VPN. I ended up with the following bit of Applescript:

-- Connect Work VPN

tell application "System Events"
    tell network preferences
        tell current location
            tell service "Work"
                connect
                tell current configuration
                    repeat until get connected = true
                        delay 1
                    end repeat
                end tell
            end tell
        end tell
    end tell
end tell

set gateway to "192.168.1.1"

do shell script "route add 172.16.0.0/16 " & gateway with administrator privileges

You need to change "Work" to the name of your VPN connection, 192.168.1.1 to your gateway address, and 172.16.0.0/16 to the address of the network to which you wish to route. Additional networks can be added by repeating the final line with different addresses.