Here’s a bug in ROS 6.0rc5. I’m using a RB750.
My ISP is advertising DNS servers to me using DHCPv6. My router learns them just fine. I’ve also set it up to advertise DNS servers to the LAN in Router Advertisement messages:
[admin@MikroTik] > /ip dns print
servers:
dynamic-servers: 2a02:fe0:1:2:1::1:110,2a02:fe0:1:2:1::1:111
allow-remote-requests: yes
max-udp-packet-size: 4096
query-server-timeout: 2s
query-total-timeout: 10s
cache-size: 2048KiB
cache-max-ttl: 1w
cache-used: 41KiB
[admin@MikroTik] > /ipv6 nd print
Flags: X - disabled, I - invalid, * - default
0 * interface=all ra-interval=3s-4s ra-delay=3s mtu=unspecified
reachable-time=unspecified retransmit-interval=unspecified
ra-lifetime=2m hop-limit=unspecified advertise-mac-address=yes
advertise-dns=yes managed-address-configuration=no
other-configuration=no
However, there is no rdnss option included in the Router Advertisements the router emits:
lan-host$ sudo tcpdump -c 1 -i eth0 -v 'icmp6 and ip6[40] == 134'
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
19:31:49.277181 IP6 (class 0xc0, hlim 255, next-header ICMPv6 (58) payload length: 56) fe80::d6ca:6dff:fe4c:47bf > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 56
hop limit 0, Flags [none], pref medium, router lifetime 120s, reachable time 0ms, retrans time 0ms
source link-address option (1), length 8 (1): d4:ca:6d:4c:47:bf
prefix info option (3), length 32 (4): 2a02:fe0:cf16:c0::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
If I instead manually configure the name servers:
[admin@MikroTik] > /ip dns set servers=2a02:fe0:1:2:1::1:110,2a02:fe0:1:2:1::1:111
…it immediately starts working:
lan-host$ sudo tcpdump -c 1 -i eth0 -v 'icmp6 and ip6[40] == 134'
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
19:33:36.022856 IP6 (class 0xc0, hlim 255, next-header ICMPv6 (58) payload length: 96) fe80::d6ca:6dff:fe4c:47bf > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 96
hop limit 0, Flags [none], pref medium, router lifetime 120s, reachable time 0ms, retrans time 0ms
source link-address option (1), length 8 (1): d4:ca:6d:4c:47:bf
rdnss option (25), length 40 (5): lifetime 6s, addr: 2a02:fe0:1:2:1:0:1:110 addr: 2a02:fe0:1:2:1:0:1:111
prefix info option (3), length 32 (4): 2a02:fe0:cf16:c0::/64, Flags [onlink, auto], valid time 2592000s, pref. time 604800s
Conclusion: /ipv6 nd advertise-dns=yes ignores any dynamically learnt DNS servers.