3.1 Subscription

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The channel is the subscribe request of PSPU §6.9, on netd's control socket. This chapter describes resolvd's end of it; what netd puts in a snapshot and when it sends one is described in the netd TRM.

3.1.1 Connecting #

When a connection attempt is due (§3.2), resolvd:

connects to /run/netd/control.sock;
sets a two-second write timeout and writes the subscribe request in netd's framing — a four-byte little-endian length and a MessagePack map;
switches the socket to nonblocking mode.

The request needs netd's NETWORK_QUERY right for resolvd's account; whether that is granted is netd's decision.

3.1.1.2 A failed attempt #

A failure at any of the three steps is a failed attempt, and the next one is scheduled by the backoff of §3.2.

3.1.1.3 A successful attempt #

On success resolvd logs subscribed to netd at info level, discards anything buffered from an earlier connection, and waits for the first snapshot as an ordinary read on the channel. Only a successfully decoded snapshot resets its backoff (§3.2); connection success alone does not.

3.1.1.4 Nothing else is written to netd #

resolvd writes nothing more on the channel after the request.

3.1.2 Reading the stream #

When the channel is readable, resolvd reads everything available, in 8 192-byte chunks, and then takes every complete frame from what it holds, in order:

Frame	What resolvd does
A `snapshot` reply	Applies it (§3.3), once the whole read has been taken. Several in one read are applied one after another, so the last wins.
An error reply	Logs `netd refused the subscription: <message>`, and drops the channel once the whole read has been taken.
Any other well-formed reply	Ignores it.
A payload that does not decode	Logs `netd sent something unreadable: <error>`, and drops the channel once the whole read has been taken.
A length above 65 536 bytes	Stops taking frames: the rest of the buffer is discarded and the channel dropped.

3.1.2.1 Frames behind an error are still taken #

An error reply or an unreadable payload does not stop the frames behind it. Every complete frame already read is still taken, in order, and any snapshot among them is applied. The same holds when the read ended at end of file or a read error: the frames read before it are all taken. Only a length above 65 536 stops the taking, and the frames before it have been taken by then.

3.1.2.2 The channel is dropped before the read's snapshots are applied #

When a read ends with the channel to be dropped, it is dropped — and lost the netd channel; reconnecting logged — after every frame has been taken and before any snapshot from that read is applied. In the log, the lost … line therefore comes before the netd: … summary lines (§3.3) of the snapshots read with it, and status then shows netd as false together with the scopes of those snapshots, until the channel is reconnected.

3.1.2.3 A partial frame #

A partial frame stays buffered until the rest arrives.

3.1.2.4 End of file or a read error #

End of file, or any read error other than EINTR or EAGAIN, drops the channel. EINTR is retried, and EAGAIN ends the read.

3.1.2.5 Dropping the channel #

Dropping the channel closes the socket, discards the buffer, logs lost the netd channel; reconnecting at warn level — once, however many of the causes above occurred in the read — and schedules a reconnection (§3.2).

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