On OS X is it possible to override DHCP assigned DNS servers but still keeping them for fallback?

By default I would like to use the google public DNS servers and to fallback to intranet ones when these are failing.

Is this possible? ... I am looking for a setup that would not break when I move my laptop no other networks.

It would be acceptable if I found a solution specific to this wifi network connection (to do the trick only in this case, and to use defaults for others).

Solution 1:

You can use ipconfig getpacket to find the DNS server that DHCP recommended:

$ ipconfig getpacket en0 
...
domain_name_server (ip_mult): {192.168.3.2, 192.168.42.1}
...

So, assuming you are using the Wi-Fi adapter and its device is en0, you can:

#!/bin/bash

default_servers=$( ipconfig getpacket en0 | \
   perl -ne'/domain_name_server.*: \{(.*)}/ && print join " ", split /,\s*/, $1' )
networksetup -setdnsservers Wi-Fi 127.0.0.1 $default_servers

Solution 2:

Yes, you can add the Google DNS servers at the top of the list and follow those with your own (intranet servers).

• Go to System Preferences > Network
• Select your network interface from the list on the left
• Click the Advanced button on the right
• In the DNS tab of the dialog, click "+" to add or "-" to remove entries (you can also edit entries by double clicking or selecting and clicking on the address)
• Add the Google DNS addresses as the first two entries
• Add your intranet DNS servers below those
• Click OK
• Click Apply
• Close System Preferences

Solution 3:

There's also a way to do this from the command-line if you're a network administrator looking for a way to do this in a batch:

networksetup listallnetworkservices
# look for the correct network device here, probably "Wi-Fi"
sudo networksetup -setdnsservers Wi-Fi 8.8.8.8 8.8.4.4

Replace "Wi-Fi" with the correct network device. You may, instead, wish to use:

device=`networksetup listallnetworkservices | grep Wi-Fi`
sudo networksetup -setdnsservers "$device" 8.8.8.8 8.8.4.4