# 7.1 Identification

_Peios / Peios technical reference / netd / Networks_

> How netd decides which network is on the other side of an interface — the signals, the identity derived from them, when identification runs, and what a newly identified network does.

netd has to say "this is the same network as last time" with nothing on
the wire that says so. The identity is a guess from what the network has
offered, recorded with every signal it was made from, so that a later
proof can replace the guess without changing the record's key.

## 7.1.1 Signals [*netid.signals]

What a joined interface's network has shown, at the moment of the pass:

| Signal | From |
|---|---|
| kind | the interface's `Interface.Kind` |
| server | the DHCPv4 lease's server identifier |
| subnet | the lease address masked to the lease prefix |
| gateway | the lease's gateway (§5.3) |
| router | the default router router discovery chose (§6.1) |
| prefixes | the subnet, then every non-deprecated autoconfigured address's prefix, each once |
| DNS servers | the lease's, then the routers' RDNSS servers, link-local ones included |

## 7.1.2 The identity [*netid.derivation]

- With a lease (a server and a subnet), the basis is
  `dhcp:<server>|<subnet>/<prefix>`.
- Otherwise, with a default router and at least one autoconfigured
  prefix, the basis is `ra:<router>|<prefix>/<length>`, using the first
  prefix.
- Otherwise there is no identity yet: nothing has been offered, or the
  interface is static only.

The network id is a SHA-1 digest of the bytes `peios-netd-network|`, the
kind, `|`, and the basis, stamped and written as a version 5 UUID exactly
as an interface id is (§4.1).

So two leases from the same server in the same subnet are the same
network, whatever the address. A different server, a different subnet,
or a different kind of interface is a different network. An IPv4 lease
always wins over the IPv6 basis, so a dual-stack network is identified by
its DHCPv4 server. [*netid.same-server-and-subnet-same-network]

## 7.1.3 When [*netid.when]

Identification is the first step of every full pass (§2.2). It runs only
on a joined interface that is up with carrier, and only when the signals
yield an identity.

For each identity it creates or refreshes the network's record (§7.2) and
reads back what the operator wrote on it. While a lease is held, it
writes the lease address to the record's `RequestedAddress` (§5.7).

## 7.1.4 A newly identified network [*netid.new-network-rejudges]

When the identity differs from the network the interface already had —
including when it had none — netd logs `interface <name>: network
<name or id>`, with ` (trust <trust>)` when the record has one. It then
marks the interface for re-judgement, so the same pass judges it with the
`Network.*` facts present (§3.3). A rule conditioned on `Network.Name`
or `Network.Trust` takes effect then. If it moves the interface to
another profile, the interface's clients restart under that profile
(§3.3).

The network an interface stands on is kept while it has carrier, even
when a profile switch restarts the clients that identified it. A switch
that its own identification triggered would otherwise forget the network
and switch back. Losing carrier forgets it (§4.1).
[*netid.sticky-while-carrier]

The interface's `Status Network` names the identified network (§8.1). The
kernel reads it to give the packet layers their `Network.*` facts
(PKM §6.5), so it never names a network the link is not on.