4.1 Identity and carrier

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4.1.1 Kind #

netd classifies each kernel link once per dump:

A bridge, a veth or a TAP device has link type Ethernet and is wired.

4.1.2 Bus path and driver #

The bus path comes from sysfs. netd resolves /sys/class/net/<name>/device and walks up from it to the nearest ancestor whose name is a PCI address (dddd:bb:dd.f), and names it pci-<address>. A device with no PCI ancestor is named by the device's own directory name. A link with no device (a virtual device) has no bus path. A virtio NIC sits one level below its PCI function, so it is named by the function.

The driver is the name the device/driver link points at (virtio_net, for example), and empty when there is none.

Both are read once, when netd first sees the link.

4.1.3 The interface id #

The interface id is a SHA-1 digest of the bytes peios-netd-ifid|, then the bus path if there is one, then |, then the six MAC bytes — or, for a link with no 6-byte hardware address, its name. The first 16 bytes of the digest are stamped as a version 5, RFC 4122 variant UUID and written in lower-case hyphenated form.

So the same card in the same slot has the same id on every boot, however the kernel names it, and a renamed link keeps its id. A virtual device has no bus path, so its id follows its MAC alone.

The id is the key of the interface's record under Interfaces\ (§8.1), the Interface.Id fact, the key of the RFC 7217 address derivation (§6.2), and the basis of its DHCP IAID (§5.7).

4.1.4 Appearing and disappearing #

When netd first sees a link, it reads the link's identity, logs interface <name> (<path>) is <id>, and, unless it is the loopback, writes accept_ra = 0 for it (§6.1).

When a link disappears from the kernel's dump, netd drops it and everything it held: its clients, lease, network and judgement. Nothing is sent on the wire. Its record under Interfaces\ stays.

4.1.5 Carrier #

netd counts an interface as having carrier when it is administratively up and the kernel reports IFF_LOWER_UP. When an interface that had carrier loses it, netd logs interface <name>: carrier lost and:

• stops its DHCPv4 client: a RELEASE is sent if a lease was held, and the lease and any link-local address are forgotten (§5.4);
• stops router discovery and DHCPv6;
• forgets the network identified on it (§7.1), so Status Network is cleared and the Network.* facts go absent, for netd's own judgement and the kernel's packet layers alike.

The next reconcile then removes the leased and autoconfigured addresses and the routes that depended on them, because nothing desires them any more. Static addresses stay.

When carrier returns, the pass starts the clients again from scratch (§5.1, §6.1), and the DHCPv4 client asks first for the address the network last gave (§5.7).

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