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After some trial and error I got it to work and wanted to share here as various pieces online had parts but not together. Some searches even suggested to disable IPv6.
I am using a 3rd party DNS provider and my issue was that the IPv6 DNS was coming from ISP and taking over. IPv4 was being respected but not IPv6.
System:Settings:General already had "Allow DNS server list to be overridden by DHCP/PPP on WAN" unchecked.
I am using Spectrum so here are the settings that worked for me.
WAN interface:
IPv6 Configuration Type: DHCPv6
Prefix delegation size: 56
Request prefix only: checked
Request DNS configuration: checked (unchecking this seems to stop the ISP from providing an IPv6 altogether)
Send prefix hint: checked
LAN interface
Track Interface (legacy)
Parent interface: WAN int name
Assign Prefix ID: using VLAN ID in hex
Reserved prefix range: 1 (beats my understanding)
Allow manual adjustment of DHCPv6 and Router Advertisements: checked
Services: Router Advertisements
Added an entry for LAN interface
Mode: Stateless
Recursive DNS Servers: IPV6 DNS servers
Add additional advertisements for interfaces that need it. The above was done on OPNsense 26.7.3_8.
I am using a 3rd party DNS provider and my issue was that the IPv6 DNS was coming from ISP and taking over. IPv4 was being respected but not IPv6.
System:Settings:General already had "Allow DNS server list to be overridden by DHCP/PPP on WAN" unchecked.
I am using Spectrum so here are the settings that worked for me.
WAN interface:
IPv6 Configuration Type: DHCPv6
Prefix delegation size: 56
Request prefix only: checked
Request DNS configuration: checked (unchecking this seems to stop the ISP from providing an IPv6 altogether)
Send prefix hint: checked
LAN interface
Track Interface (legacy)
Parent interface: WAN int name
Assign Prefix ID: using VLAN ID in hex
Reserved prefix range: 1 (beats my understanding)
Allow manual adjustment of DHCPv6 and Router Advertisements: checked
Services: Router Advertisements
Added an entry for LAN interface
Mode: Stateless
Recursive DNS Servers: IPV6 DNS servers
Add additional advertisements for interfaces that need it. The above was done on OPNsense 26.7.3_8.
