Oct 9, 2026, 8:12 AM- Oct 9, 2026, 8:12 AM
Replies
subnetspidersubnetspider@bsd.cafeOct 9, 2026, 12:00 PM@pertho I suppose its not DHCP that's doing this, but router advertisements? Windows for example will by default set the source IP it receives RAs from as DNS server.
- Oct 9, 2026, 12:03 PM
@subnetspider I think so? Not sure how to check though. My resolv.conf ends up looking like:
nameserver <the IPv4 address that I want>
nameserver fe80::de:ad:be:ef%if0 (link local of the router)
nameserver <the IPv6 address that I want>I don't want the middle entry of the link-local router address. I can manually specify the IPv4 and IPv6 addresses and that gets rid of it, but it negates the whole "things should be automated and working" bits of DHCP/DHCPv6.
- subnetspidersubnetspider@bsd.cafeOct 9, 2026, 12:53 PM
@pertho what OS are you on and how does the network configuration look like?
- Oct 9, 2026, 2:12 PM
@subnetspider One machine running Void Linux (no systemd, but using NetworkManager which writes directly to /etc/resolv.conf) and Mac OS X Tahoe 26.7.1.
Oddly enough, on Mac OS X, if I set IPv6 from automatic to link-local only, it stops sticking that link local address in DNS (scutil --dns) but that's expected because it effectively disables IPv6 except for just link local traffic.
- subnetspidersubnetspider@bsd.cafeOct 9, 2026, 6:09 PM
@pertho Have you tried to capture ICMPv6 messaged received on your machines network interface?
Example:
sudo tcpdump -vvvv -ttt -i igb0 icmp6 and 'ip6[40] = 134'
Just replace igb0 with your hosts interface and wait for a bit. If there is a router advertisement with a link-local address, you see where it comes from.
If there is none, I would do the same with DHCPv6 packets.
- Oct 9, 2026, 8:46 PM
@subnetspider Going to do that tomorrow morning. Cheers! 👍
- Oct 10, 2026, 10:47 AM
@subnetspider I get a router advertisement packet back with icmp6. It gives me a hop limit, a prefix info, mtu option, rdnss, and source link-address option. I think it's the 'rdnss' that is making it add the router to the DNS?
IP6 (flowlabel 0xd8f32, hlim 255, next-header ICMPv6 (58), payload length 104) fe80::eefc:2fff:fe8f:7c47 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 104
hop limit 64, Flags [managed, other stateful, home agent], pref medium, router lifetime 300s, reachable time 30000ms, retrans timer 0ms
prefix info option (3), length 32 (4): 2axx:xxxx:xxxx:xxxx::/64, Flags [onlink, auto], valid time 3600s, pref. time 3600s
mtu option (5), length 8 (1): 1488
rdnss option (25), length 40 (5): lifetime 60s, addr: fe80::eefc:2fff:fe8f:7c47 addr: ::
source link-address option (1), length 8 (1): ec:fc:2f:8f:7c:47
- subnetspidersubnetspider@bsd.cafeOct 10, 2026, 3:40 PM
@pertho Yes, the RDNSS option is what your OS uses to set it's IPv6 DNS server. Defending on the router, you should be able to disable it.
- Oct 10, 2026, 10:52 PM
@subnetspider I'll have to see what my options are. I've not seen any option for that though.
Oct 9, 2026, 2:52 PM@pertho if it's not coming from DHCPv6 / your DHCP daemon, it definitely seems like it could be coming from RAs. What's your router and it's configuration for router advertisements?
- Oct 9, 2026, 8:50 PM
@hugo I'm using some sort of rebranded Sagemcom WiFi 7 router my ISP gave me.
- Oct 9, 2026, 8:55 PM
@pertho gotcha. So, limited visibility into what the router itself is doing, yea?
But for DHCPv6, that's where you specified your intended DNS servers? In DHCPv6 configuration of the ISP-provided Sagemcom router? Is there anywhere in the admin UI for that Sagemcom device to configure the IPv6 router advertisements?
If not, we'd probably be at the point of doing packet captures for icmp6 on a machine on the network to try to see if the router is setting its own link local IPv6 address in RDNSS in the router advertisements.