OPNsense insists that DHCPv6 server is active when I've got it disabled

[nray]

I have been a long-time pfSense user and just migrated my home setup to OPNsense 25.7-amd64. A while back I had stopped using the pfSense DHCP and DNS servers and set up Technitium on my network, so I had no plans to use any of the DNS or DHCP features in OPNsense. I set up OPNsense and everything seemed to be going well until I tried to do DNS lookups of my LAN systems from my LAN, and they didn't look up. I checked a client DHCP lease and it had an IPv6 entry for a DNS server which I had never seen in my pfSense setup (see attached). When the OPNsense setup asked about configuring a DHCP or DNS server I had said no, and when I set up Technitium everything in that is IPv4, so this is very perplexing.

I went through all the OPNsense DHCP and DNS server settings, and I did find that that ISC DHCPv6 was set to "Enable DHCPv6 on LAN interface", which I had not configured during setup and don't remember it asking to enable that at any point. So I unchecked it, hit save, and then renewed a client DHCP lease. No change. So then I went to the LAN interface configuration in OPNsense, and tried to change the IPv6 Configuration type from "Track Interface" to "None", but that throws this error:

The following input errors were detected:
The DHCPv6 Server is active on this interface and it can be used only with a static IPv6 configuration. Please disable the DHCPv6 Server service on this interface first, then change the interface configuration.

I have verified that there is no DHCPv6 Server service active on this interface in the GUI, so now I am very confused. I went through all the settings, double-checked, and rebooted OPNsense, but I can't change the LAN IPv6 Configuration Type to "None", same error every time. For now I have resorted to changing the WAN IPv6 Configuration Type to "None", which finally cleared the IPv6 DNS server from my clients.

Even with the WAN IPv6 Configuration Type to "None", and no DHCPv6 server enabled anywhere in the GUI, I still cannot get the LAN IPv6 Configuration Type moved off of "Track Interface".

Are these bugs in OPNsense? What is the proper way to get things set up?

[jcr]

Hi,

I have exactly the same problem
My computers have opnsense as prefered dns 😕

Serveurs DNS. . .  . . . . . . . . . . : 2001:861:4080:bff3:215:5dff:fe96:a36
                                       192.168.100.1
                                       8.8.8.8

So, no resolution of my lan server.
my lan have a dns and a dhcp server configured juste with ipv4 (active directory domain)
All dhcp and dns are disable on my opnsense

I desactived ISC DHCPv6 lan and all work fine

[nray]

What is the best way for me to gather information regarding the true state of the DHCPv6 server so I can file a bug report?

[OPNenthu]

I think either DHCPv6 and/or IPv6 RAs are advertising the DNS server to your clients.

Do you intend to use IPv6?

If no, then disable IPv6 from all your interfaces.  Start with the LAN-side interfaces and disable it on WAN last.

Also go trough all of the available DHCP services in OPNsense (ISC, Dnsmasq, Kea) and make sure that DHCPv6 is not active on any of them and that no IPv6 ranges are defined in Dnsmasq.  Also disable the RA services from Dnsmasq (General->DHCP->Router Advertisements) and make sure that the default Router Advertisement service (outside of Dnsmasq, under Services->Router Advertisements) is not enabled.  If you don't see the Router Advertisements service listed in the menu that means it's not being used.  There's also an "Allow IPv6" setting under Interfaces->Settings, but I don't think that's needed and should probably stay checked except for special circumstances.

In case the IPv6 configuration is coming via RAs then DHCP release/renew will not clear it.  You'll need to reset your host's network connection.

If yes, then check out @meyergru's guide: https://forum.opnsense.org/index.php?topic=45822.0

You can specify your own DNS server in the IPv6 RAs using RDNSS in this case.

[nray]

I think either DHCPv6 and/or IPv6 RAs are advertising the DNS server to your clients.

Do you intend to use IPv6?

If no, then disable IPv6 from all your interfaces.  Start with the LAN-side interfaces and disable it on WAN last.

Also go trough all of the available DHCP services in OPNsense (ISC, Dnsmasq, Kea) and make sure that DHCPv6 is not active on any of them and that no IPv6 ranges are defined in Dnsmasq.  Also disable the RA services from Dnsmasq (General->DHCP->Router Advertisements) and make sure that the default Router Advertisement service (outside of Dnsmasq, under Services->Router Advertisements) is not enabled.  If you don't see the Router Advertisements service listed in the menu that means it's not being used.  There's also an "Allow IPv6" setting under Interfaces->Settings, but I don't think that's needed and should probably stay checked except for special circumstances.

In case the IPv6 configuration is coming via RAs then DHCP release/renew will not clear it.  You'll need to reset your host's network connection.

If yes, then check out @meyergru's guide: https://forum.opnsense.org/index.php?topic=45822.0

You can specify your own DNS server in the IPv6 RAs using RDNSS in this case.

I don't intend to use IPv6 unless there is a good reason to (is there one?)

Like I said in my post, I haven't been able to disable IPv6 on the LAN side because no matter what, it says I have a DHCPv6 Server active, even though it is not enabled in the GUI. All DHCP services are off - ISC, Dnsmasq, and Kea. Services > Router Advertisements > [LAN] was set to "Assisted" and I just changed it to "Disabled". Interfaces > Settings does show "Allow IPv6" as checked and I've left it alone as you recommended.

Right now if I try to change my [LAN] interface IPv6 Configuration Type from "Track Interface" to "None" I still get "The DHCPv6 Server is active on this interface and it can be used only with a static IPv6 configuration. Please disable the DHCPv6 Server service on this interface first, then change the interface configuration." My [WAN] IPv6 Configuration Type is set to "None". Like I mentioned in my post, since I did that, client release/renew of DHCP cleared the DHCPv6 DNS server entry, which restored name lookups on my LAN via my Technitium server at least.

So back to my question - is there anything I can do via the command line to determine whether the DHCPv6 Server is still active despite it being disabled everywhere in the GUI? I'd just like to make an accurate bug report, and I need to know whether the DHCPv6 Server is still active and the GUI isn't accurately reflecting the state, or whether the DHCPv6 Server is inactive and the part of the GUI that determines whether it is active is broken. It is one of those two things, and it is preventing me from changing the IPv6 Configuration Type for my LAN.

[OPNenthu]

s there anything I can do via the command line to determine whether the DHCPv6 Server is still active

'ps aux' to list all the processes and 'sockstat -l' to see what's listening.

[nray]

s there anything I can do via the command line to determine whether the DHCPv6 Server is still active

'ps aux' to list all the processes and 'sockstat -l' to see what's listening.

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USER    PID  %CPU %MEM  VSZ  RSS TT  STAT STARTED        TIME COMMAND
root    11 600.0  0.0    0    96  -  RNL  Sun11  30966:29.09 [idle]
root      0  0.0  0.0    0  896  -  DLs  Sun11      16:29.96 [kernel]
root      1  0.0  0.0 12324  996  -  ILs  Sun11      0:00.06 /sbin/init
root      2  0.0  0.0    0    96  -  WL  Sun11      1:02.24 [clock]
root      3  0.0  0.0    0  112  -  DL  Sun11      0:00.00 [crypto]
root      4  0.0  0.0    0    64  -  DL  Sun11      0:00.07 [cam]
root      5  0.0  0.0    0    16  -  DL  Sun11      0:00.00 [busdma]
root      6  0.0  0.0    0    16  -  DL  Sun11      2:13.48 [pf purge]
root      7  0.0  0.0    0    16  -  DL  Sun11      0:34.82 [rand_harvestq]
root      8  0.0  0.0    0    16  -  DL  Sun11      0:00.30 [enc_daemon0]
root      9  0.0  0.0    0    48  -  DL  Sun11      0:13.84 [pagedaemon]
root    10  0.0  0.0    0    16  -  DL  Sun11      0:00.00 [audit]
root    12  0.0  0.0    0  288  -  WL  Sun11      0:15.92 [intr]
root    13  0.0  0.0    0    48  -  DL  Sun11      0:00.00 [geom]
root    14  0.0  0.0    0    16  -  DL  Sun11      0:00.00 [sequencer 00]
root    15  0.0  0.0    0    80  -  DL  Sun11      0:10.22 [usb]
root    16  0.0  0.0    0    16  -  DL  Sun11      0:03.94 [acpi_thermal]
root    17  0.0  0.0    0    16  -  DL  Sun11      0:00.00 [vmdaemon]
root    18  0.0  0.0    0  144  -  DL  Sun11      0:13.16 [bufdaemon]
root    19  0.0  0.0    0    16  -  DL  Sun11      0:01.36 [vnlru]
root    20  0.0  0.0    0    16  -  DL  Sun11      1:18.91 [syncer]
root    385  0.0  0.0 28108 14744  -  Is  Sun11      0:00.82 /usr/local/bin/python3 /usr/local/opnsense/service/configd.py (python3.11)
root    387  0.0  0.1 92444 39788  -  I    Sun11      0:08.80 /usr/local/bin/python3 /usr/local/opnsense/service/configd.py console (python3.11)
root  1061  0.0  0.0 15340  4036  -  Is  Sun11      0:00.17 /sbin/devd
root  1069  0.0  0.0 14028  2332  -  Is  Sun11      0:00.00 /usr/sbin/moused -p /dev/ums0 -t auto -I /var/run/moused.ums0.pid
root  5931  0.0  0.0 14076  2584  -  Is  Tue07      0:00.02 dhclient: system.syslog (dhclient)
root  9766  0.0  0.0 14312  2608  -  I    22:00      0:00.00 /bin/sh /usr/local/opnsense/scripts/firmware/launcher.sh -u changelog cron
root  10149  0.0  0.0 14076  2648  -  Is  Tue07      0:00.07 dhclient: ix0 [priv] (dhclient)
root  11319  0.0  0.0 14312  2616  -  I    22:00      0:00.00 /bin/sh /usr/local/opnsense/scripts/firmware/changelog.sh cron
root  11527  0.0  0.0 27368 14196  -  S    Sun11      0:23.17 /usr/local/bin/python3 /usr/local/sbin/configctl -e -t 0.5 system event config_changed (python3.11)
root  15867  0.0  0.1 64304 34604  -  I    Sun11      0:01.09 /usr/local/bin/php-cgi
root  16464  0.0  0.0 13924  2432  -  SCs  Sun11      1:15.06 /usr/sbin/rtsold -aiu -p /var/run/rtsold.pid -A /var/etc/rtsold_script.sh -R /usr/local/opnsense/scripts/interfaces/rtsold_resolvconf.sh
root  17259  0.0  0.1 59804 31356  -  I    20:40      0:00.13 /usr/local/bin/php-cgi
root  17856  0.0  0.0 13924  2400  -  Is  Sun11      0:00.00 rtsold: rtsold.llflags (rtsold)
root  18092  0.0  0.0 13924  2404  -  Is  Sun11      0:00.00 rtsold: rtsold.script (rtsold)
root  18292  0.0  0.0 13852  2552  -  Is  Sun11      0:03.28 /usr/sbin/cron -s
root  18866  0.0  0.0 13648  2072  -  IC  22:00      0:00.00 sleep 26825
root  18979  0.0  0.0 13924  2396  -  Is  Sun11      0:00.00 rtsold: rtsold.sendmsg (rtsold)
root  19113  0.0  0.0 13924  2408  -  Ss  Sun11      1:11.11 rtsold: system.syslog (rtsold)
root  19510  0.0  0.0 20820  9608  -  Is  01:18      0:00.01 sshd-session: root [priv] (sshd-session)
root  22894  0.0  0.0 13800  2444  -  Is  Sun11      0:00.00 /usr/local/sbin/dhcp6c -c /var/etc/dhcp6c.conf -p /var/run/dhcp6c.pid
root  25597  0.0  0.0 28392 14784  -  S    Sun11      0:22.39 /usr/local/bin/python3 /usr/local/opnsense/scripts/syslog/lockout_handler (python3.11)
root  27578  0.0  0.0 24572  9608  -  I    Sun11      0:00.00 /usr/local/sbin/syslog-ng -f /usr/local/etc/syslog-ng.conf -p /var/run/syslog-ng.pid
root  28327  0.0  0.0 45724 14512  -  Ss  Sun11      0:36.99 /usr/local/sbin/syslog-ng -f /usr/local/etc/syslog-ng.conf -p /var/run/syslog-ng.pid
root  29000  0.0  0.0 23836 11240  -  S    Sun11      0:09.05 /usr/local/sbin/lighttpd -f /usr/local/etc/lighttpd_webgui/lighttpd.conf
root  29304  0.0  0.1 54044 27880  -  Is  Sun11      0:00.01 /usr/local/bin/php-cgi
root  30120  0.0  0.1 54044 27884  -  Is  Sun11      0:00.01 /usr/local/bin/php-cgi
root  30881  0.0  0.1 54044 27888  -  Is  Sun11      0:00.01 /usr/local/bin/php-cgi
root  30885  0.0  0.1 54044 27876  -  Is  Sun11      0:00.01 /usr/local/bin/php-cgi
root  31260  0.0  0.1 59740 33484  -  I    Sun11      0:00.27 /usr/local/bin/php-cgi
root  31709  0.0  0.1 54108 30000  -  I    Sun11      0:00.23 /usr/local/bin/php-cgi
root  31825  0.0  0.1 59740 33380  -  I    Sun11      0:00.03 /usr/local/bin/php-cgi
root  32353  0.0  0.1 54044 27888  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  33135  0.0  0.1 54044 27888  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  33755  0.0  0.1 59740 33624  -  I    Sun11      0:00.51 /usr/local/bin/php-cgi
root  34134  0.0  0.1 59740 33492  -  I    Sun11      0:00.43 /usr/local/bin/php-cgi
root  34243  0.0  0.1 54044 27888  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
_dhcp 34747  0.0  0.0 14080  2716  -  ICs  Tue07      0:00.03 dhclient: ix0 (dhclient)
root  34975  0.0  0.1 54044 27888  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  35669  0.0  0.1 54044 27888  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  36756  0.0  0.1 54044 27892  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  37327  0.0  0.1 54044 27892  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  37432  0.0  0.1 54044 27892  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  37689  0.0  0.1 60012 34448  -  I    Sun11      0:00.22 /usr/local/bin/php-cgi
root  38994  0.0  0.0 20276  8056  -  Is  Sun11      0:00.01 sshd: /usr/local/sbin/sshd [listener] 0 of 10-100 startups (sshd)
root  39656  0.0  0.1 57900 31808  -  I    Sun11      0:01.10 /usr/local/bin/php-cgi
root  40089  0.0  0.1 54044 27880  -  I    Sun11      0:00.00 /usr/local/bin/php-cgi
root  41073  0.0  0.0 20820  9880  -  S    01:19      0:00.03 sshd-session: root@pts/0 (sshd-session)
root  44513  0.0  0.0 13756  2184  -  Ss  Sun11      1:06.97 /usr/sbin/powerd -b hadp -a hadp -n hadp
root  49625  0.0  0.1 62060 36256  -  I    20:41      0:00.34 /usr/local/bin/php-cgi
root  60116  0.0  0.0 14044  2560  -  Ss  Tue07      0:14.70 /usr/local/sbin/filterlog -i pflog0 -p /var/run/filterlog.pid
root  60772  0.0  0.1 64032 34268  -  I    Sun11      0:00.66 /usr/local/bin/php-cgi
root  70558  0.0  0.0 14108  2924  -  Ss  Tue07      0:06.95 /usr/local/sbin/miniupnpd -f /var/etc/miniupnpd.conf -P /var/run/miniupnpd.pid
root  73029  0.0  0.0 24116  8436  -  Ss  Tue07      0:05.65 /usr/local/sbin/ntpd -g -c /var/etc/ntpd.conf
root  74742  0.0  0.0 13780  2224 u0  Is+  Sun11      0:00.00 /usr/libexec/getty 3wire.115200 ttyu0
root  71039  0.0  0.0 13780  2232 v0  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv0
root  71516  0.0  0.0 13780  2232 v1  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv1
root  72110  0.0  0.0 13780  2220 v2  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv2
root  72405  0.0  0.0 13780  2224 v3  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv3
root  73149  0.0  0.0 13780  2228 v4  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv4
root  73266  0.0  0.0 13780  2224 v5  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv5
root  73910  0.0  0.0 13780  2220 v6  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv6
root  74352  0.0  0.0 13780  2228 v7  Is+  Sun11      0:00.00 /usr/libexec/getty Pc ttyv7
root  41267  0.0  0.0 14312  2708  0  Is  01:19      0:00.00 /bin/sh /usr/local/sbin/opnsense-shell
root  51157  0.0  0.0 17352  4276  0  S    01:19      0:00.02 /bin/csh
root  54081  0.0  0.0 14384  3020  0  R+  01:19      0:00.00 ps aux

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USER    COMMAND    PID  FD  PROTO  LOCAL ADDRESS        FOREIGN ADDRESS
root    sshd-sessi 41073 9  stream (not connected)
root    sshd-sessi 19510 9  stream (not connected)
root    php-cgi    49625 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    17259 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    ntpd      73029 20  udp6  *:123                *:*
root    ntpd      73029 21  udp4  *:123                *:*
root    ntpd      73029 22  udp6  fe80::4adf:37ff:fee4:d26c%ix0:123 *:*
root    ntpd      73029 23  udp4  24.62.111.35:123      *:*
root    ntpd      73029 24  udp4  10.0.0.1:123          *:*
root    ntpd      73029 25  udp6  fe80::4adf:37ff:fee4:d26d%ix1:123 *:*
root    ntpd      73029 26  udp6  ::1:123              *:*
root    ntpd      73029 27  udp6  fe80::1%lo0:123      *:*
root    ntpd      73029 28  udp4  127.0.0.1:123        *:*
root    miniupnpd  70558 6  tcp6  *:2189                *:*
root    miniupnpd  70558 8  tcp4  *:2189                *:*
root    miniupnpd  70558 9  udp4  *:1900                *:*
root    miniupnpd  70558 10  udp4  10.0.0.1:16429        *:*
root    miniupnpd  70558 12  udp4  10.0.0.1:5351        *:*
root    php-cgi    15867 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    60772 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    php-cgi    40089 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    39656 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    37689 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    37432 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    php-cgi    37327 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    php-cgi    36756 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    php-cgi    35669 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    34975 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    34243 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    34134 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    33755 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    33135 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    php-cgi    32353 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    php-cgi    31825 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    php-cgi    31709 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    php-cgi    31260 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    php-cgi    30885 0  stream /var/lib/php/tmp/php-fastcgi.socket-3
root    php-cgi    30881 0  stream /var/lib/php/tmp/php-fastcgi.socket-2
root    php-cgi    30120 0  stream /var/lib/php/tmp/php-fastcgi.socket-1
root    php-cgi    29304 0  stream /var/lib/php/tmp/php-fastcgi.socket-0
root    lighttpd  29000 7  tcp4  *:443                *:*
root    lighttpd  29000 8  tcp6  *:443                *:*
root    lighttpd  29000 9  tcp4  *:80                  *:*
root    lighttpd  29000 10  tcp6  *:80                  *:*
root    sshd      38994 6  tcp6  fe80::4adf:37ff:fee4:d26d%ix1:22 *:*
root    sshd      38994 7  tcp6  2601:18a:8080:f8:4adf:37ff:fee4:d26d:22 *:*
root    sshd      38994 8  tcp4  10.0.0.1:22          *:*
root    sshd      38994 9  tcp6  fe80::1%lo0:22        *:*
root    sshd      38994 10  tcp6  ::1:22                *:*
root    sshd      38994 11  tcp4  127.0.0.1:22          *:*
root    dhcp6c    22894 6  udp6  *:546                *:*
root    syslog-ng  28327 20  dgram  /var/run/log <-
root    syslog-ng  28327 21  dgram  /var/run/logpriv
root    syslog-ng  28327 22  dgram  /var/dhcpd/var/run/log
root    syslog-ng  28327 23  dgram  /var/unbound/var/run/log
root    syslog-ng  28327 27  stream /var/db/syslog-ng.ctl
root    devd        1061 6  stream /var/run/devd.pipe
root    devd        1061 7  seqpac /var/run/devd.seqpacket.pipe
root    python3.11  387 3  stream /var/run/configd.socket
root    python3.11  387 9  stream /var/run/configd.socket
root    python3.11  387 10  stream /var/run/configd.socket
root    python3.11  387 13  stream /var/run/configd.socket
root    python3.11  387 16  stream /var/run/configd.socket
root    python3.11  387 18  stream /var/run/configd.socket
root    python3.11  387 25  stream /var/run/configd.socket

[Submerge009]

Encountered the same issue. Looks like some sort of bug. If Service->ISC DHCPv6->[LAN] "Enable DHCPv6 server on LAN interface" is UNCHECKED FIRST, then Interfaces->[LAN] "IPv6 Configuration Type" cannot be set from "Track Interface" to "None" with the error "The DHCPv6 Server is active on this interface and it can be used only with a static IPv6 configuration. Please disable the DHCPv6 Server service on this interface first, then change the interface configuration.". However, based on the GUI the DHCPv6 Server is not actually running. In such a state XML configuration file shows:

<dhcpdv6>
  <lan>
    <ramode>disabled</ramode>
    <enable>-1</enable>
  </lan>
</dhcpdv6>

However, if the Service->ISC DHCPv6->[LAN] "Enable DHCPv6 server on LAN interface" is left CHECKED, then changing Interfaces->[LAN] "IPv6 Configuration Type" to "None" is possible without any error. After these changes XML configuration file shows:

<dhcpdv6>
  <lan>
    <ramode>disabled</ramode>
  </lan>
</dhcpdv6>

[OPNenthu]

That is strange. I'm not sure why that happens, but at least it seems there's a solution: enable ISC DHPv6, then disable Track Interface, then disable ISC DHCPv6.

Maybe @franco or @Monviech can explain this.  I migrated off of ISC already before 25.7 and didn't notice anything like this although I was never using DHCPv6 on LAN.

[Submerge009]

Seems like the issue has been present since at least OPNsense 25.1.9_2 based on https://github.com/opnsense/core/issues/8838

The solution is more like enable ISC DHCPv6, then disable "Track Interface" and that is it because after setting Interfaces->[LAN] "IPv6 Configuration Type" to "None", [LAN] under Services->ISC DHCPv6 no longer exists, so no way to disable the flag. Maybe that is by design? Since DHCPv6 server is not running according to the GUI.
Although the XML configuration stays the same as if DHCPv6 was enabled.

[nray]

That is strange. I'm not sure why that happens, but at least it seems there's a solution: enable ISC DHPv6, then disable Track Interface, then disable ISC DHCPv6.

Maybe @franco or @Monviech can explain this.  I migrated off of ISC already before 25.7 and didn't notice anything like this although I was never using DHCPv6 on LAN.

I can confirm this is a successful workaround, specifically:

1. Enabled ISC DHCPv6 for my [LAN] interface
2. Under the [LAN] interface I could then change "IPv6 Configuration Type" from "Track Interface" to "None" and save and apply the changes
3. Going back to ISC DHCPv6 there was no longer a [LAN] interface there (only "Leases", which is empty), so there is nothing further to disable