Hi and thank you for your reply.

When I stop unbound and check for running processes there is no unbound running.

[2.4.4-RELEASE][admin@gateway.REDACTED.TLD]/root: ps ax | grep unbound 21735 0 S+ 0:00.00 grep unbound [2.4.4-RELEASE][admin@gateway.REDACTED.TLD]/root:

After stopping all DHCP servers the following processes are running:

[2.4.4-RELEASE][admin@gateway.REDACTED.TLD]/root: ps ax | grep dhcp 4049 - S 0:00.00 /bin/sh /var/etc/dhcp6c_wan_script.sh 56033 - Ss 618:49.04 /usr/sbin/syslogd -s -c -c -l /var/dhcpd/var/run/log -l /tmp/haproxy_chroot/var/run/log -P /var/run/syslog.pid -f /etc/syslog.conf 97216 - Ss 0:01.42 /usr/local/sbin/dhcp6c -d -c /var/etc/dhcp6c_wan.conf -p /var/run/dhcp6c_pppoe0.pid pppoe0 14705 0 S+ 0:00.00 grep dhcp [2.4.4-RELEASE][admin@gateway.REDACTED.TLD]/root:

The DHCP log keeps getting spammed by DHCP6 client:

Nov 5 17:12:53 dhcp6c 97216 Sending Solicit Nov 5 17:12:54 dhcp6c 97216 Sending Request Nov 5 17:12:54 dhcp6c 97216 dhcp6c Received REQUEST Nov 5 17:12:54 dhcp6c 97216 status code for NA-0: no addresses Nov 5 17:12:55 dhcp6c 97216 Sending Solicit Nov 5 17:12:57 dhcp6c 97216 Sending Request Nov 5 17:12:57 dhcp6c 97216 dhcp6c Received REQUEST Nov 5 17:12:57 dhcp6c 97216 status code for NA-0: no addresses Nov 5 17:12:58 dhcp6c 97216 Sending Solicit Nov 5 17:12:59 dhcp6c 97216 Sending Request Nov 5 17:13:00 dhcp6c 97216 dhcp6c Received REQUEST Nov 5 17:13:00 dhcp6c 97216 status code for NA-0: no addresses Nov 5 17:13:02 dhcp6c 97216 Sending Solicit Nov 5 17:13:03 dhcp6c 97216 Sending Request Nov 5 17:13:03 dhcp6c 97216 dhcp6c Received REQUEST Nov 5 17:13:03 dhcp6c 97216 status code for NA-0: no addresses

My WAN connection uses DHCP6 and I confimed IPv6 connectivity.
WAN has an address and IPv6 is routed as expected.

After killing

97216 - Ss 0:01.42 /usr/local/sbin/dhcp6c -d -c /var/etc/dhcp6c_wan.conf -p /var/run/dhcp6c_pppoe0.pid pppoe0

I lost IPv6 connectivity and the spamming of DHCP log by DHCP6 client stopped.
So I reconnected WAN and the spamming was back.

Nov 5 17:26:20 dhcp6c 97216 Start address release Nov 5 17:26:20 dhcp6c 97216 Sending Release Nov 5 17:26:20 dhcp6c 97216 remove an address 2003:REDACTED:d1d4/64 on igb0 Nov 5 17:26:20 dhcp6c 97216 dhcp6c Received RELEASE Nov 5 17:26:20 dhcp6c 97216 status code: success Nov 5 17:26:21 dhcp6c 97216 exiting Nov 5 17:30:56 dhcp6c 74412 failed to open /usr/local/etc/dhcp6cctlkey: No such file or directory Nov 5 17:30:56 dhcp6c 74412 failed initialize control message authentication Nov 5 17:30:56 dhcp6c 74412 skip opening control port Nov 5 17:30:57 dhcp6c 74510 Sending Solicit Nov 5 17:30:58 dhcp6c 74510 Sending Request Nov 5 17:30:58 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:30:58 dhcp6c 74510 add an address 2003:REDACTED:d1d4/64 on igb0 Nov 5 17:30:58 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:00 dhcp6c 74510 Sending Solicit Nov 5 17:31:01 dhcp6c 74510 Sending Solicit Nov 5 17:31:03 dhcp6c 74510 Sending Solicit Nov 5 17:31:07 dhcp6c 74510 Sending Solicit Nov 5 17:31:15 dhcp6c 74510 Sending Solicit Nov 5 17:31:32 dhcp6c 74510 Sending Solicit Nov 5 17:31:33 dhcp6c 74510 Sending Request Nov 5 17:31:33 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:33 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:35 dhcp6c 74510 Sending Solicit Nov 5 17:31:36 dhcp6c 74510 Sending Request Nov 5 17:31:36 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:36 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:37 dhcp6c 74510 Sending Solicit Nov 5 17:31:38 dhcp6c 74510 Sending Request Nov 5 17:31:38 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:38 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:40 dhcp6c 74510 Sending Solicit Nov 5 17:31:41 dhcp6c 74510 Sending Request Nov 5 17:31:41 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:41 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:43 dhcp6c 74510 Sending Solicit Nov 5 17:31:44 dhcp6c 74510 Sending Request Nov 5 17:31:44 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:44 dhcp6c 74510 status code for NA-0: no addresses Nov 5 17:31:46 dhcp6c 74510 Sending Solicit Nov 5 17:31:47 dhcp6c 74510 Sending Request Nov 5 17:31:47 dhcp6c 74510 dhcp6c Received REQUEST Nov 5 17:31:47 dhcp6c 74510 status code for NA-0: no addresses

@Gertjan said in DNS Resolver & DHCP Server are constantly restarting:

and thus dhcpleases should not run.
Or, it's that process that restart unbound - see your own logs.

dhcpleases was running because I enabled it again after disabling didn't change the behaiviour.

@Gertjan said in DNS Resolver & DHCP Server are constantly restarting:

Then restart unbound (resolver) and DHCP servers one by one - pause and observe behaviour in logs after each start.

After starting only unbound with DHCP Registration and Static DHCP disabled unbound gets restarted every time dhcp6c is logging "Sending Solicit"

So I checked my WAN settings and compared it to another pfSense firewall I am running with the same ISP (Deutsche Telekom Business).
Under DHCP6 Client Configuration there is an option called Request only an IPv6 prefix (Only request an IPv6 prefix, do not request an IPv6 address).
After enabling the checkbox the spamming of DHCP logs by DHCP6 client stopped and unbound is running without getting restarted.
DHCP servers are also running again with no issues.

I have no idea why it was working fine for 2+ years without the "Request only an IPv6 prefix" option checked.
Maybe the ISP changed some settings on their side.

Thank you very much @Gertjan for pointing me in the right direction.