have many PC behind a proxy with no autentification. I allow some domain to be reach. I want to know what is the domain needed by avast to update correctly.
avast spreads the load across many servers so updates are as quick as possible. Check the file C:\Program Files\Alwil Software\Avast4\Setup\servers.def (text file) for a current list, this is obviously subject to change.
So rather than give individual domain why not use some form of wildcard e.g. download*.avast.com and download.avast.com should cover them all? This assumes you can do this within your proxy.