I believe I have reproduced this bug on my system (system would hang for 5 mins waiting for DHCP address). If I manually ran dhclient after the system cam up it would work fine.
I first saw this issue after upgrading systemd to 230-7~bpo8+2, ifupdown to 0.8.13~bpo8+1 and udev 230-7~bpo8+2f rom jessie-backports. My topology is dhcp on a bridge with eth0, and tap0, and a network namespace interface. The system would boot fine if I didn't rely on DHCP for addressing of br0. $ journalctl -u networking Dec 01 19:46:26 server dhclient[3951]: Listening on LPF/br0/00:1d:7d:02:3c:4d Dec 01 19:46:26 server ifup[3779]: Listening on LPF/br0/00:1d:7d:02:3c:4d Dec 01 19:46:26 server ifup[3779]: Sending on LPF/br0/00:1d:7d:02:3c:4d Dec 01 19:46:26 server ifup[3779]: Sending on Socket/fallback Dec 01 19:46:26 server ifup[3779]: DHCPREQUEST on br0 to 255.255.255.255 port 67 Dec 01 19:46:26 server dhclient[3951]: Sending on LPF/br0/00:1d:7d:02:3c:4d Dec 01 19:46:26 server dhclient[3951]: Sending on Socket/fallback Dec 01 19:46:26 server dhclient[3951]: DHCPREQUEST on br0 to 255.255.255.255 port 67 Dec 01 19:46:26 server dhclient[3951]: DHCPACK from 192.168.0.1 Dec 01 19:46:26 server ifup[3779]: DHCPACK from 192.168.0.1 Dec 01 19:51:26 server systemd[1]: networking.service: Start operation timed out. Terminating. Dec 01 19:51:26 server systemd[1]: Failed to start Raise network interfaces. Dec 01 19:51:26 server systemd[1]: networking.service: Unit entered failed state. Dec 01 19:51:26 server systemd[1]: networking.service: Failed with result 'timeout'. I was able to fix it by downgrading udev, systemd and ifupdown to jessie. --Brian
