In the Router Advertisements configuration, I tried to change AdvRDNSSLifetime and AdvDNSSLLifetime to 43200.
When I did this I get a Red Error:
The following input errors were detected:
AdvRDNSSLifetime must be between 600 and 1200 seconds.
AdvDNSSLLifetime must be between 600 and 1200 seconds.
My Maximum Interval is set to 600 so I think it is tied to that.
According to RFC 8106, IPv6 Router Advertisement Options for DNS Configuration:
Paragraph 5.1 Recursive DNS Server Option
Lifetime 32-bit unsigned integer. The maximum time in seconds
(relative to the time the packet is received) over which
these RDNSS addresses MAY be used for name resolution.
The value of Lifetime SHOULD by default be at least
3 * MaxRtrAdvInterval, where MaxRtrAdvInterval is the
maximum RA interval as defined in [RFC4861]. A value of
all one bits (0xffffffff) represents infinity. A value
of zero means that the RDNSS addresses MUST no longer
be used.
So it should be AT LEAST 1800 and the error message seems to be enforcing a maximum of 1200 seconds.
further in the a paragraph:
The rationale of the default value of the Lifetime field is as
follows. The Router Lifetime field, set by AdvDefaultLifetime,
has the default of 3 * MaxRtrAdvInterval as specified in
[RFC4861], so such a default or a larger default can allow for
the reliability of DNS options even under the loss of RAs on
links with a relatively high rate of packet loss. Note that
the ratio of AdvDefaultLifetime to MaxRtrAdvInterval is the
number of unsolicited multicast RAs sent by the router. Since
the DNS option entries can survive for at most three
consecutive losses of RAs containing DNS options, the default
value of the Lifetime lets the DNS option entries be resilient
to packet-loss environments.
Same at Paragraph 5.2 DNS Search List Option
Lifetime 32-bit unsigned integer. The maximum time in seconds
(relative to the time the packet is received) over which
these DNSSL domain names MAY be used for name resolution.
The Lifetime value has the same semantics as the
semantics for the RDNSS option. That is, Lifetime SHOULD
by default be at least 3 * MaxRtrAdvInterval. A value of
all one bits (0xffffffff) represents infinity. A value
of zero means that the DNSSL domain names MUST no longer
be used.
I think this may be left over from the old RFC 6106, since in the Appendix A. Changes from RFC 6106
o This document allows a higher default value of the lifetime of the
DNS RA options than RFC 6106 in order to avoid the frequent expiry
of the options on links with a relatively high rate of packet
loss; at the same time, this document also makes additional
clarifications. The lifetime's lower bound of
2 * MaxRtrAdvInterval was shown to lead to the expiry of these
options on links with a relatively high rate of packet loss. To
avoid this problem, this revision relaxes the lower bound and sets
a higher default value of 3 * MaxRtrAdvInterval.
Looks like this is already being addressed:
https://github.com/opnsense/core/pull/4893