pfSense

It's possible to silently crash the Router Advertisements daemon (radvd) with a DHCPv6 scope that is configured with four DNS servers. pfSense WebUI appears to accept this configuration and does not readily indicate that the daemon has failed or why.

Netgate Forums article: https://forum.netgate.com/topic/195340/radvd-crashes-with-4-dns-servers-in-dhcpv6-scope

Steps to reproduce:
1) Enter four DNS servers into a DHCPv6 scope.
2) Enable Router Advertisements for the subnet and check the option for "Mirror DHCPv6 DNS Configuration".
3) Save the configuration. It will appear to save OK, but radvd has crashed. Clients will lose the IPv6 RA in time.
4) Further changes can be made to Routing Advertisements in the pfSense WebUI, but the daemon crashes on each save.

The following error is emitted by radvd:

too many addresses in RDNSS section

It's been confirmed by Netgate that radvd only accepts three DNS servers.

Steps to resolve / work around:
1) Reduce the number of DNS servers in the scope to three or less. Save the configuration. Service will start running.

Expected behavior - There are a couple options:
1) Refuse to accept the configuration and output an error.
2) Accept the configuration and output a warning that only the first three DNS servers will be advertised.