2.1 Startup

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2.1.1 The service definition #

The netd package ships its service definition as a registry seed, netd-service.reg, applied by an image that names it. The definition:

A second seed, netd-default-profile.reg, ships the baseline policy: the profile default and the rule wired (§3.1). It is separate so an image can run netd without agreeing that every wired interface joins whatever it is plugged into.

2.1.2 The startup sequence #

netd does these steps in this order:

1. Opens rtnetlink: one socket for requests and dumps, and a second subscribed to the link, IPv4 and IPv6 address, and IPv4 and IPv6 route multicast groups. Failure is fatal.
2. Opens the control socket at /run/netd/control.sock, removing a stale one first (§9.1). Failure is fatal.
3. Opens the udp/68 absorber (§5.6). Failure is a warning: netd runs without it, and a DHCP server's unicast reply can draw an ICMP port-unreachable from the kernel.
4. Switches the kernel's RA processing off on every interface that exists, and in all and default (§6.1). This happens before any link is brought up.
5. Reads the configuration (§2.3) and builds a generation from it (§3.2). A generation that fails to build at startup is logged as refused, and netd runs with an empty policy, so the backstop ignores every interface until a good generation is written.
6. Arms the registry watch on Machine\System\Network. Failure is a warning, and the configuration is then read once, never again.
7. Dumps the kernel's state (links, addresses and routes, both families). Failure is fatal.
8. Runs the first pass (§2.2): every interface is judged, every static address applied, every client that is due started.
9. Sends READY=1 on the notify socket.

Readiness is reported after the first pass, not after any lease arrives. A lease is a network event, not a startup one. A service that needs an address waits on a readiness level (§8.2), not on netd being ready.

A tie between two rules is not checked at startup: the startup build refuses a generation only for the build errors of §3.2. A tie present at startup is judged per interface as a conflict (§3.3), so the interfaces it covers are ignored with a warning rather than the generation being refused.

If Machine\System\Network does not exist when netd starts, the configuration is empty and the watch cannot be armed, so a key created later is never read; a restart of netd picks it up.

2.1.3 Restart #

netd re-derives everything from the registry and the kernel. Its DHCP clients start again, and a client remembers nothing across a restart: each one begins with an INIT-REBOOT request for the address its network last leased (§5.7), and holds no lease until that request is answered.

During an interface's first DHCP start in this process, netd preserves its requested IPv4 address at the prefix already installed in the kernel. It also preserves its netd-marked IPv4 routes while Route.Offered allows them; an explicit Route.Gateway takes precedence and the current profile's metric applies. This provisional state lasts only while INIT-REBOOT is pending. An unchanged ACK replaces it with the confirmed lease without deleting the address or route, so IPv4 readiness stays in place.

A NAK or two unanswered requests end revalidation and discard the provisional state as the client enters fresh discovery (§5.2). Carrier loss or a changed profile clears it too. Later client restarts in the same process do not adopt kernel state again. A requested address that is not already installed is never added merely because it was remembered.

This is not a restored lease: no lease duration, expiry, DNS servers or other offered information is inferred from the installed address. Status still reports no lease until the ACK. DHCP-provided DNS information and IPv6 router-discovery state must be reacquired; this does not promise that every aspect of a daemon restart is invisible. IPv6 addresses and routes from advertisements remain desired only after discovery hears the routers again (§6.1). Static addresses are desired from the first pass and stay.

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