OSX: suddenly cannot get onto the network / internet

I had a problem where without warning I couldn't get onto the network, internet WiFi, anything. DHCP just wasn't getting an address.

Using my phone to search Google, it seemed the answer was to download the combo updater and run it. Well yeah like I'm going to download a 1Gb file on my phone.

Anyway, turns out that there's a much simpler way: clear your caches

In the finder do shift-apple-g and then type

/Library/Caches

and OK.


When this folder opens, delete EVERYTHING! You might need to supply your password.

Then, same again:

In the finder do shift-apple-g and then type

~/Library/Caches - spot the "~" ?

and OK.


Again, delete EVERYTHING!

and reboot.

Explanation:

If the Macs battery goes completely, the clock gets reset. Even after you reset the clock, the certificates and the firewall get out of sync and suddenly all your applications, including DHCP no longer have network access.

Re-applying the combo updater also resets the firewall, which is why that works.