Tools/NetworkManager/Integration

Kernel

NM doesn't get notified about router advertisements and flag changes → we ask kernel upon any received route
NM doesn't get notified about gateways when accept_ra_defrtr=0
Kernel doesn't cache RDNSS and DNSSL, so we have to send router solicitation on NetworkManager start
Kernel doesn't observe RDNSS and DNSSL expiration time, so we have to manually send router solicitations to renew it in some configurations
NM can't trigger kernel router solicitation
Manually sent router solicitations give EADDRNOTAVAIL (link-local is there according to netlink)
Kernel doesn't treat dhcpv6 replies as ESTABLISHED, this is worked around by firewalld
Kernel autoconfigures link-local (and possibly also global) IPv6 addresses for bridge/bond members (I haven't tested this)

NetworkManager doesn't work well with dnssec-trigger as unbound DNS server will not receive the list of recursive nameservers and will thus resolve using the global DNS systems. This is incompatible with VPNs with their own private DNS zones.
dnssec-triggerd leaves broken /etc/resolv.conf on exit

NetworkManager supports firewall zones for firewalld.