Bug#789247: di-netboot-assistant: broken deb.d.o urls
It looks like the deb.debian.org URLs in di-sources.list need to be updated
E: Can't download 'stretch' for 'amd64'
The URL should have a leading 'debian/' in the path, ie
I don't know if this is just some mirrors or everywhere, but not having
it resulted in an error for me (it resolved to cdn-aws.deb.debian.org)