Comment 1 for bug 974284

The problem is actually more prevalent than just boot time. In fact, if the DHCP server goes away at any point and the lease expires, dhclient will timeout and exit without ever retrying again. The result is that the system in question basically falls off the network, with the only way to recover being manual intervention. To be clear, this means recovery requires physical access or remote management capabilities.

It's straightforward to reproduce the problem. Configure an Ubuntu system as a DHCP client and set the lease time on the server to something short, like 60 seconds. After the system has successfully booted and grabbed an IP, shut down the DHCP server. Within 2 minutes, the dhcp client will timeout, lose it's IP address and drop off the network with no chance of recovering automatically.