I've been able to deny all connections to external networks unless my OpenVPN connection is active using pf.conf. However, I lose Wi-Fi connectivity if the connection is broken by closing and opening the laptop lid or toggling Wi-Fi off and on again.
I'm on Mac OS 10.8.1.
I connect to the Web via Wi-Fi (from varying locations, including Internet cafés).
The OpenVPN connection is set up with Viscosity.
I have the following packet filter rules set up in /etc/pf.conf
# Deny all packets unless they pass through the OpenVPN connection
wifi=en1
vpn=tun0
block all
set skip on lo
pass on $wifi proto udp to [OpenVPN server IP address] port 443
pass on $vpn
I start the packet filter service with sudo pfctl -e and load the new rules with sudo pfctl -f /etc/pf.conf.
I have also edited /System/Library/LaunchDaemons/com.apple.pfctl.plist and changed the line <string>-f</string> to read <string>-ef</string> so that the packet filter launches at system startup.
This all seems to works great at first: applications can only connect to the web if the OpenVPN connection is active, so I'm never leaking data over an insecure connection.
But, if I close and reopen my laptop lid or turn Wi-Fi off and on again, the Wi-Fi connection is lost, and I see an exclamation mark in the Wi-Fi icon in the status bar. Clicking the Wi-Fi icon shows an "Alert: No Internet connection" message:
To regain the connection, I have to disconnect and reconnect Wi-Fi, sometimes five or six times, before the "Alert: No Internet connection" message disappears and I'm able to open the VPN connection again. Other times, the Wi-Fi alert disappears of its own accord, the exclamation mark clears, and I'm able to connect again. Either way, it can take five minutes or more to get a connection again, which can be frustrating.
Why does Wi-Fi report "No internet connection" after losing connectivity, and how can I diagnose this issue and fix it?
