IPv6 Neighbor Discovery

RA messages are sent with ICMPv6. These can carry options, so they’ve been extended to carry RDNSS information.

The router can say “Here is the DNS info”.

Terms

RA — Router Advertisement

In v6, routers can just advertise the prefix of the attached subnet and options like RDNSS.

RDNSS — Recursive DNS Server

Router Advertisements

From RFC 4861: Neighbor Discovery for IP version 6 (IPv6).

packet
  0-7: "Type"
  8-15: "Code"
  16-31: "Checksum"
  32-39: "Cur Hop Limit"
  40: "M"
  41: "O"
  42-47: "Reserved"
  48-63: "Router Lifetime"
  64-95: "Reachable Time"
  96-127: "Retrans Timer"
  128-159: "Options ..."

RDNS Server Option

From RFC 8106: IPv6 Router Advertisement Options for DNS Configuration

packet
  0-7: "Type"
  8-15: "Length"
  16-31: "Reserved"
  32-63: "Lifetime"
  64-127: "Addresses of IPv6 Recursive DNS Servers"

Packet Capture

Taken from my home router 13-July-2026, I’ve modified the IPs.

Frame 46: Packet, 174 bytes on wire (1392 bits), 174 bytes captured (1392 bits) on interface <removed>
Ethernet II, Src: Routerboardc_ef:69:14 (48:a9:8a:ef:69:14), Dst: IPv6mcast_01 (33:33:00:00:00:01)
Internet Protocol Version 6, Src: fe80::4aa9:8aff:feef:6914, Dst: ff02::1
Internet Control Message Protocol v6
    Type: Router Advertisement (134)
    Code: 0
    Checksum: 0xa34c [correct]
    [Checksum Status: Good]
    Cur hop limit: 0
    Flags: 0x00, Prf (Default Router Preference): Medium
    Router lifetime (s): 1800
    Reachable time (ms): 0
    Retrans timer (ms): 0
    ICMPv6 Option (Source link-layer address : 48:a9:8a:ef:69:14)
    ICMPv6 Option (Recursive DNS Server 2001:db8:12:15::6 2001:db8:12:15::7)
        Type: Recursive DNS Server (25)
        Length: 5 (40 bytes)
        Reserved
        Lifetime: 1800 (30 minutes)
        Recursive DNS Servers: 2001:db8:12:15::6
        Recursive DNS Servers: 2001:db8:12:15::7
    ICMPv6 Option (Recursive DNS Server fe80::4aa9:8aff:feef:6914)
        Type: Recursive DNS Server (25)
        Length: 3 (24 bytes)
        Reserved
        Lifetime: RDNSS address MUST no longer be used (0) (0 seconds)
        Recursive DNS Servers: fe80::4aa9:8aff:feef:6914
    ICMPv6 Option (Prefix information : 2001:db8:12:15::/64)
        Type: Prefix information (3)
        Length: 4 (32 bytes)
        Prefix Length: 64
        Flag: 0xc0, On-link Flag (L), Autonomous Address Configuration Flag (A)
            1... .... = On-link Flag (L): Set
            .1.. .... = Autonomous Address Configuration Flag (A): Set
            ..0. .... = Router Address Flag (R): Not set
            ...0 .... = DHCPv6-PD Preferred Flag (P): Not set
            .... 0000 = Reserved: 0
        Valid Lifetime: 2592000 (30 days)
        Preferred Lifetime: 604800 (7 days)
        Reserved
        Prefix: 2001:db8:12:15::

Wireshark

BPF:ip6 protochain 58

References

RFC 4861: Neighbor Discovery for IP version 6 (IPv6) | RFC Editor

RFC 8106: IPv6 Router Advertisement Options for DNS Configuration | RFC Editor

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