Comment 8 for bug 1765765

@Guillermo
The OS shouldn't be trying to raise the interface as it's already configured during the boot process. You can't reconfigure the network interface while you're booting from the network :)
You need to modify /etc/network/interfaces on your boot server to set the interface to manual. See step 5 under 'creating your NFS installation' on this page
https://help.ubuntu.com/community/DisklessUbuntuHowto

Related to this, I also set 'net.ifnames=0' in the kernel boot line so the network devices are always named ethX rather than hardware-specific names. That's personal preference though.

@beta-tester
Strange. Without specifying 'ip=dhcp' perhaps it was sometimes getting a bootp or rarp response without a DNS server settings? Just a guess. If 'ip=X' isn't set, it defaults to any autoconfig protocol. I'd suggest setting the ip option specifically to dhcp if that's what you expect it to use.
https://www.kernel.org/doc/Documentation/filesystems/nfs/nfsroot.txt