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.

I have had a look online to see if I can find anything, and as far as I can understand you seem to want to be able to use your computer like normal, while also being able to connect to internal company websites, so, you may need to set up a custom routing table.

This link apparently only applies to 10.4, but the command line stuff may still work.