6.3 DNS and DHCPv6

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6.3.1 RDNSS and DNSSL #

Each server of an RDNSS option, and each domain of a DNSSL option, is held for the option's lifetime from the moment the advertisement arrived. 0xffffffff is forever, and a lifetime of zero removes that server or domain at once. A server or domain advertised again has its lifetime replaced.

6.3.2 Stateless DHCPv6 #

Peios gets IPv6 addresses by SLAAC alone. DHCPv6 is used for configuration only: an INFORMATION-REQUEST (RFC 8415 §18.2.6) for DNS servers and search domains. The stateful address exchange is not implemented, and an advertisement's M flag is treated as O.

6.3.2.1 When it runs #

A DHCPv6 client starts on an interface once its router-discovery engine has seen an advertisement with M or O (§6.1), provided the interface has a non-tentative link-local address and a MAC, and its socket opens. netd logs interface <name>: dhcpv6 information request. It stops with router discovery.

6.3.2.2 The socket #

UDP, bound with SO_BINDTODEVICE to the interface and to its link-local address on port 546, so two interfaces' clients never share a port. Messages go to ff02::1:2 port 547.

6.3.2.3 The request #

An INFORMATION-REQUEST (type 11) with a random 3-byte transaction id and these options:

The first is sent when the exchange begins. Retransmissions follow after 1 s, doubling up to 3600 s, each with a jitter of ±10%, and never stop until a reply is accepted.

6.3.2.4 The reply #

A datagram is accepted only while an exchange is running, and only when:

• it is a REPLY (type 7) with the exchange's transaction id;
• it carries a client identifier equal to the machine's DUID;
• it carries a non-empty server identifier.

Anything else is ignored, and retransmission continues. From an accepted reply, option 23 gives the DNS servers — when its length is a whole number of addresses, up to 16, with unspecified, multicast and loopback addresses dropped — and option 24 the search domains, read as a DNSSL's are (§6.1).

6.3.2.5 Refresh #

After a reply, the client asks again after the reply's information refresh time (option 32), or one day when there is none, but never sooner than 600 s. Each refresh is a new exchange with a new transaction id.

An answer that differs from the last logs dhcpv6 answered and leads to a reconcile and a publish. An identical one changes nothing.

6.3.3 Where these go #

Advertised and DHCPv6 servers and domains reach name resolution only through the profile's Dns.Offered, including link-local servers with the interface supplied by their DNS scope (§8.3). They are always part of what a network has shown for its identity and its record (§7).

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