isc-dhcp-server overwrites /etc/default/isc-dhcp-server during update

[Milan Kerslager]

Today unattended upgrade of ISC DHCPD overwrite config file /etc/default/isc-dhcp-server and set wrong interface where daemon have to listen (eno2 instead of br0 as was set before update).

I see no backup file of original config file so I had to investigate where the problem was.

Update have to never overwrite config file and throw away previous version.

/var/log/apt/history.log:
Start-Date: 2021-05-28 06:17:41
Commandline: /usr/bin/unattended-upgrade
Upgrade: isc-dhcp-server:amd64 (4.4.1-2.1ubuntu5, 4.4.1-2.1ubuntu5.20.04.2)
End-Date: 2021-05-28 06:17:47

root@linux:~# ls -l /etc/default/isc-dhcp-server
-rw-r--r-- 1 root root 629 May 28 06:17 /etc/default/isc-dhcp-server

[Seth Arnold (seth-arnold)]

Hello Milan, I just tested an upgrade:

Unpacking isc-dhcp-server (4.4.1-2.1ubuntu5.20.04.2) over (4.4.1-2.1ubuntu5) ...

and my /etc/default/isc-dhcp-server modifications had been left in place.

The maintainer scripts will create a new one if the file cannot be read:
https://sources.debian.org/src/isc-dhcp/4.4.1-2.2/debian/isc-dhcp-server.postinst/#L33

(Debian sources, but Ubuntu's are very similar.)

Is it possible your old /etc/default/isc-dhcp-server could not be read?

Thanks

[Milan Kerslager (milanke)]

I had a working setup and it was broken by update.
The config file was modified/recreated/whatever.
I don't know what was exactly written in this file before update and no backup was created.

In any case, update must not break working config, especially on LTS version.