Boot delay when Ethernet cable is not connected

Image used: 2022.01.21 Bookworm (Tested) for Pi 3B+

There seems to be a long delay during boot if an Ethernet cable is not connected to the Pi. It looks like DHCP is being attempted on eth0 and it takes a while to time out. Running systemd-analyze blame on my Pi shows that networking.service takes 1 minute 2 seconds to complete because of this.

One possible fix is to change auto eth0 to allow-hotplug eth0 in /etc/network/interfaces.d/eth0. With this change, the boot process is much faster without a cable connected. The Ethernet connection still works if a cable is plugged in either before boot or after boot has completed.

This change could cause unintended problems on some Pis though. If a cable is connected before boot, it's now possible to reach the login prompt and log in before the DHCP request has completed. If some services expect the network connection to be up before boot has finished, this could cause problems.