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Describe the bug

rtadvd should probably use /64 prefixes derived from the delegated /62 for client subnets, but rtadvd.inc passing the /62 length directly into the prefix configuration.

To Reproduce

Steps to reproduce the behavior:

[Interface]→[WAN]

• IPv4 Configuration Type: PPPoE
• IPv6 Configuration Type: DHCPv6
• DHCPv6 client configuration:
  • Prefix delegation size: 62
  • Request prefix only: Selected
  • Send prefix hint: Selected
  • Optional prefix ID: (Blank)
  • Optional interface ID: (Blank)

[Interface]→[LAN]

• IPv6 Configuration Type: Track Interface
• Track IPv6 Interface:
  • Parent interface: WAN
  • Assign prefix ID: 0
  • Optional interface ID: (Blank)
  • Manual configuration: Selected (Allow manual adjustment of DHCPv6 and Router Advertisements)

[Services]→[ISC DHCPv6]→[LAN]

• Enable: Unchecked (Enable DHCPv6 server on LAN interface)

[Services]→[Router Advertisements]→[LAN]

• Router Advertisements: Assisted
• Source Address: Automatic
• Advertise Default Gateway: Selected

/var/etc/radvd.conf generated by rtadvd.inc:

# Automatically generated, do not edit
# Generated RADVD config for manual assignment on lan
interface igb5 {
        AdvSendAdvert on;
        MinRtrAdvInterval 200;
        MaxRtrAdvInterval 600;
        AdvLinkMTU 1492;
        AdvDefaultPreference high;
        AdvManagedFlag off;
        AdvOtherConfigFlag on;
        prefix 240e:3b4:d3:63b0::/62 {
                DeprecatePrefix on;
                AdvOnLink on;
                AdvAutonomous on;
        };
        RDNSS 240e:3b4:d3:63b0:2e2:69ff:fe3b:fe2f {
        };
};

Expected behavior

Generate '/64' prefix in radvd.conf:

# Automatically generated, do not edit
# Generated RADVD config for manual assignment on lan
interface igb5 {
        AdvSendAdvert on;
        MinRtrAdvInterval 200;
        MaxRtrAdvInterval 600;
        AdvLinkMTU 1492;
        AdvDefaultPreference high;
        AdvManagedFlag off;
        AdvOtherConfigFlag on;
        prefix 240e:3b4:d3:63b0::/64 {
                DeprecatePrefix on;
                AdvOnLink on;
                AdvAutonomous on;
        };
        RDNSS 240e:3b4:d3:63b0:2e2:69ff:fe3b:fe2f {
        };
};

Describe alternatives you considered

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Environment

OPNsense 25.7.9-amd64
FreeBSD 14.3-RELEASE-p5
OpenSSL 3.0.18