6.1 Router discovery

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6.1.1 The kernel's RA handling is off #

netd writes 0 to /proc/sys/net/ipv6/conf/<name>/accept_ra for all, for default, and for every interface listed there, at startup before any link is brought up (§2.1). It writes it again for each non-loopback interface the first time it sees it. A missing file — a kernel without IPv6 — is a warning only for all.

The kernel still creates link-local addresses and answers neighbour solicitations. That is the protocol, not policy. Everything an advertisement could configure is netd's to decide.

6.1.2 When router discovery runs #

On every full pass, for each joined interface:

• it wants router discovery when its profile has Address.Offered and IPv6 in its families;
• it can run it when it is up with carrier and the kernel's link-local address on it has finished duplicate address detection. A tentative address cannot source a solicitation.

Running discovery that is no longer wanted or possible is stopped (ipv6 stopping): the engine and any DHCPv6 client are discarded, and the next reconcile removes their addresses and route. Nothing is sent on the wire, because neither protocol has a goodbye. An interface that wants and can run it starts it (soliciting routers), provided its ICMPv6 socket opens — otherwise netd logs no icmpv6 socket and tries again on the next pass.

6.1.3 The socket #

One raw ICMPv6 socket per interface, bound to it with SO_BINDTODEVICE, filtered to receive only type 134 (router advertisement), sending with a multicast hop limit of 255, and asking for each received packet's hop limit. The kernel computes and checks ICMPv6 checksums.

6.1.4 Solicitation #

A router solicitation is type 133, code 0, with a source link-layer address option carrying the interface's MAC, sent to ff02::2 with hop limit 255.

The first is sent as soon as discovery starts. The interval is 4 s for the next two, then doubles before each later one, up to 3600 s. Each interval carries a jitter of ±10%. So solicitations go at about 0, 4, 8, 16, 32, 64, … s.

Soliciting stops when an advertisement arrives with a router lifetime above zero. It begins again, from the first interval, as soon as every default router has expired.

6.1.5 Which advertisements count #

An advertisement is dropped unless it arrived with hop limit 255 and from a link-local source (RFC 4861 §6.1.2). A lower hop limit has crossed a router, and a non-link-local source is not a router on this link.

It is then decoded. The whole advertisement is refused when:

• it is shorter than 16 bytes, or its type is not 134 or its code not 0;
• an option has length zero, or runs past the end of the message.

Within an accepted advertisement, an option that does not parse is skipped and the rest stand:

Any other option is ignored. So are the advertisement's hop limit, reachable time and retransmission timer.

6.1.6 What an advertisement does #

• Router lifetime above zero: the source becomes, or stays, a default router until that many seconds from now. Zero: the source is removed as a default router. Its prefixes, servers and domains stay.
• M or O flag: stateless DHCPv6 is wanted (§6.3). Once set it stays set until discovery restarts; a later advertisement without the flags does not withdraw it.
• MTU option: remembered as the link's advertised MTU. It is used only with Mtu.Offered, only when no lease gave one, and only when it is at least 68 (§4.2, §4.3).
• Prefixes: §6.2. RDNSS and DNSSL: §6.3.

Whenever what the interface should carry changes — an address, its deprecation, the default router, the servers, the domains, the MTU, the DHCPv6 flag — netd runs a full pass. That includes a change caused by time alone: a lifetime running out is a change.

6.1.7 The default router #

Of the routers whose lifetime has not run out, netd uses the one with the numerically lowest link-local address, so the choice is stable while the set is. Router preference (RFC 4191) is not read.

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