Package: openssh-server Version: 1:7.9p1-10 After upgrading from stretch to buster, I noticed that the DHCP client (isc-dhcp-client, 4.4.1-2) exits after initially assigning an IP address. Further investigation showed, that "ifup@ens3.service" exited abnormally. The reason for this seems to be that the directory "/etc/network/if-up.d" is missing. As I keep my /etc under git control (using etckeeper), I was able to identify the removal of of "/etc/network/if-up.d/openssh-server" as the last action taking place which at least involved files in the if-up.d directory. On this machine, openssh-server was the only file in /etc/network/if-up.d, so I suppose that some script executed during the installation/upgrade of openssh-server removed the complete directory.
Recreating the directory resolves the issue and "ifup@ens3.service" works and terminates properly. The following is the output obtained from systemctl status: ifup@ens3.service - ifup for ens3 Loaded: loaded (/lib/systemd/system/ifup@.service; static; vendor preset: enabled) Active: failed (Result: exit-code) since Tue 2019-08-27 21:18:51 CEST; 9s ago Process: 6510 ExecStart=/bin/sh -ec ifup --allow=hotplug ens3; ifquery --state ens3 (code=exited, status=1/FAILURE) Main PID: 6510 (code=exited, status=1/FAILURE) Aug 27 21:18:51 hostname sh[6510]: DHCPREQUEST for 10.X.X.X on ens3 to 255.255.255.255 port 67 Aug 27 21:18:51 hostname dhclient[6518]: DHCPREQUEST for 10.X.X.X on ens3 to 255.255.255.255 port 67 Aug 27 21:18:51 hostname dhclient[6518]: DHCPACK of 10.X.X.X from 10.X.X.X Aug 27 21:18:51 hostname sh[6510]: DHCPACK of 10.X.X.X from 10.X.X.X Aug 27 21:18:51 hostname dhclient[6518]: bound to 10.X.X.X -- renewal in 3089 seconds. Aug 27 21:18:51 hostname sh[6510]: bound to 10.X.X.X -- renewal in 3089 seconds. Aug 27 21:18:51 hostname sh[6510]: run-parts: failed to open directory /etc/network/if-up.d: No such file or directory Aug 27 21:18:51 hostname sh[6510]: ifup: failed to bring up ens3 Aug 27 21:18:51 hostname systemd[1]: ifup@ens3.service: Main process exited, code=exited, status=1/FAILURE Aug 27 21:18:51 hostname systemd[1]: ifup@ens3.service: Failed with result 'exit-code'.