Over the last few months, I upgraded my Debian machines from bullseye to bookworm. The process was uneventful, but I ended up reconfiguring several things afterwards in order to modernize my upgraded machines.

Logcheck

I noticed in this release that the transition to journald is essentially complete. This means that rsyslog is no longer needed on most of my systems:

apt purge rsyslog

Once that was done, I was able to comment out the following lines in /etc/logcheck/logcheck.logfiles.d/syslog.logfiles:

#/var/log/syslog
#/var/log/auth.log

I did have to adjust some of my custom logcheck rules, particularly the ones that deal with kernel messages:

--- a/logcheck/ignore.d.server/local-kernel
+++ b/logcheck/ignore.d.server/local-kernel
@@ -1,1 +1,1 @@
-^\w{3} [ :[:digit:]]{11} [._[:alnum:]-]+ kernel: \[[0-9. ]+]\ IN=eno1 OUT= MAC=[0-9a-f:]+ SRC=[0-9a-f.:]+
+^\w{3} [ :[:digit:]]{11} [._[:alnum:]-]+ kernel: (\[[0-9. ]+]\ )?IN=eno1 OUT= MAC=[0-9a-f:]+ SRC=[0-9a-f.:]+

Then I moved local entries from /etc/logcheck/logcheck.logfiles to /etc/logcheck/logcheck.logfiles.d/local.logfiles (/var/log/syslog and /var/log/auth.log are enabled by default when needed) and removed some files that are no longer used:

rm /var/log/mail.err*
rm /var/log/mail.warn*
rm /var/log/mail.info*

Finally, I had to fix any unescaped | characters in my local rules. For example error == NULL || \*error == NULL must now be written as error == NULL \|\| \*error == NULL.

Networking

After the upgrade, I got a notice that the isc-dhcp-client is now deprecated and so I removed if from my system:

apt purge isc-dhcp-client

This however meant that I need to ensure that my network configuration software does not depend on the now-deprecated DHCP client.

On my laptop, I was already using NetworkManager for my main network interfaces and that has built-in DHCP support.

Migration to systemd-networkd

On my backup server, I took this opportunity to switch from ifupdown to systemd-networkd by removing ifupdown:

apt purge ifupdown
rm /etc/network/interfaces

putting the following in /etc/systemd/network/20-wired.network:

[Match]
Name=eno1

[Network]
DHCP=yes
MulticastDNS=yes

and then enabling/starting systemd-networkd:

systemctl enable systemd-networkd
systemctl start systemd-networkd

I also needed to install polkit:

apt install --no-install-recommends policykit-1

in order to allow systemd-networkd to set the hostname.

In order to start my firewall automatically as interfaces are brought up, I wrote a dispatcher script to apply my existing iptables rules.

Migration to predictacle network interface names

On my Linode server, I did the same as on the backup server, but I put the following in /etc/systemd/network/20-wired.network since it has a static IPv6 allocation:

[Match]
Name=enp0s4

[Network]
DHCP=yes
Address=2600:3c01::xxxx:xxxx:xxxx:939f/64
Gateway=fe80::1

and switched to predictable network interface names by deleting these two files:

• /etc/systemd/network/50-virtio-kernel-names.link
• /etc/systemd/network/99-default.link

and then changing eth0 to enp0s4 in:

• /etc/network/iptables.up.rules
• /etc/network/ip6tables.up.rules
• /etc/rc.local (for OpenVPN)
• /etc/logcheck/ignored.d.*/*

Then I regenerated all initramfs:

update-initramfs -u -k all

and rebooted the virtual machine.

Giving systemd-resolved control of /etc/resolv.conf

After reading this history of DNS resolution on Linux, I decided to modernize my resolv.conf setup and let systemd-resolved handle /etc/resolv.conf.

I installed the package:

apt install systemd-resolved

and then removed no-longer-needed packages:

apt purge resolvconf avahi-daemon

I also disabled support for Link-Local Multicast Name Resolution (LLMNR) after reading this person's reasoning by putting the following in /etc/systemd/resolved.conf.d/llmnr.conf:

[Resolve]
LLMNR=no

I verified that mDNS is enabled and LLMNR is disabled:

$ resolvectl mdns
Global: yes
Link 2 (enp0s25): yes
Link 3 (wlp3s0): yes
$ resolvectl llmnr
Global: no
Link 2 (enp0s25): no
Link 3 (wlp3s0): no

Note that if you want auto-discovery of local printers using CUPS, you need to keep avahi-daemon since cups-browsed doesn't support systemd-resolved. You can verify that it works using:

sudo lpinfo --include-schemes dnssd -v

Dynamic DNS

I replaced ddclient with inadyn since it doesn't work with no-ip.com anymore, using the configuration I described in an old blog post.

chkrootkit

I moved my customizations in /etc/chkrootkit.conf to /etc/chkrootkit/chkrootkit.conf after seeing this message in my logs:

WARNING: /etc/chkrootkit.conf is deprecated. Please put your settings in /etc/chkrootkit/chkrootkit.conf instead: /etc/chkrootkit.conf will be ignored in a future release and should be deleted.

ssh

As mentioned in Debian bug#1018106, to silence the following warnings:

sshd[6283]: pam_env(sshd:session): deprecated reading of user environment enabled

I changed the following in /etc/pam.d/sshd:

--- a/pam.d/sshd
+++ b/pam.d/sshd
@@ -44,7 +44,7 @@ session    required     pam_limits.so
 session    required     pam_env.so # [1]
 # In Debian 4.0 (etch), locale-related environment variables were moved to
 # /etc/default/locale, so read that as well.
-session    required     pam_env.so user_readenv=1 envfile=/etc/default/locale
+session    required     pam_env.so envfile=/etc/default/locale

 # SELinux needs to intervene at login time to ensure that the process starts
 # in the proper default security context.  Only sessions which are intended

I also made the following changes to /etc/ssh/sshd_config.d/local.conf based on the advice of ssh-audit 2.9.0:

-KexAlgorithms curve25519-sha256@libssh.org,curve25519-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha256
+KexAlgorithms curve25519-sha256@libssh.org,curve25519-sha256,sntrup761x25519-sha512@openssh.com,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512