router(host): allowlist only certain icmpv6 types

resources/chaosknoten/router/nftables/nftables.conf

@@ -39,7 +39,23 @@ table inet host {
         ct state established,related accept
 
 		ip protocol icmp accept
-		ip6 nexthdr icmpv6 accept
+        # ICMPv6
+        # https://datatracker.ietf.org/doc/html/rfc4890#autoid-24
+        # Allowlist consisting of: "Traffic That Must Not Be Dropped" and "Traffic That Normally Should Not Be Dropped"
+        # Error messages that are essential to the establishment and maintenance of communications:
+        icmpv6 type { destination-unreachable, packet-too-big } accept
+        icmpv6 type { time-exceeded } accept
+        icmpv6 type { parameter-problem } accept
+        # Connectivity checking messages:
+        icmpv6 type { echo-request, echo-reply } accept
+        # Address Configuration and Router Selection messages:
+        icmpv6 type { nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert } accept
+        # Link-Local Multicast Receiver Notification messages:
+        icmpv6 type { mld-listener-query, mld-listener-report, mld-listener-done, mld2-listener-report } accept
+        # SEND Certificate Path Notification messages:
+        icmpv6 type { 148, 149 } accept
+        # Multicast Router Discovery messages:
+        icmpv6 type { 151, 152, 153 } accept
 
         # Allow SSH access.
         tcp dport 22 accept comment "allow ssh access"