# 4.2 Desired state

_Peios / Peios technical reference / netd / Interfaces_

> What a joined interface should carry — link, MTU, addresses, routes and their metric — computed from its profile and what its network has offered; and what DOWN and IGNORE desire.

On every reconcile netd computes, for each interface, what it should look
like. That **desired state** is a link state, an MTU, a set of addresses
and a set of routes. It is derived from the interface's verdict, its
profile, and its clients' current state. During the first INIT-REBOOT it
also includes the provisional IPv4 kernel state described in §2.1; netd
keeps no general record of what it applied.

## 4.2.1 By verdict [*desired.by-verdict]

- Loopback, or an `IGNORE`d interface: there is no desired state, and
  netd plans nothing for the interface.
- `DOWN`: the link down, with no addresses and no routes.
- `JOIN`: the link up, and everything below.

## 4.2.2 The metric [*desired.metric]

Every route netd adds for an interface carries the interface's metric:
the profile's `Route.Metric` when it is set, otherwise 600 for a
`wireless` interface and 100 for anything else.

## 4.2.3 The MTU [*desired.mtu]

The profile's `Mtu.Value` when set. Otherwise, with `Mtu.Offered`, the
lease's MTU (DHCP option 26), else the routers' advertised MTU (§6.1).
Otherwise no MTU is desired and the link's MTU is left as it is.

## 4.2.4 IPv4 addresses [*desired.ipv4-addresses]

- every IPv4 entry of `Address.Static`, at its prefix;
- the lease's address at the lease's prefix (§5.3), when a lease is
  held;
- while the first INIT-REBOOT is pending, the requested address at its
  already-installed prefix (§2.1);
- the link-local address (§5.5) at prefix 16, only when the client has
  fallen back to one **and** no lease is held **and** the profile has no
  static address of either family.

The order in which they are listed, or written in `Address.Static`, has
no effect. netd adds the addresses an interface lacks in ascending
address order (§4.3), and the kernel makes an address primary only when
the interface holds no other address in the same subnet, so the first
one added in a subnet is its primary. Of two statics in one subnet written
`10.0.0.9/24, 10.0.0.3/24`, both added in one pass, `10.0.0.3` is the
primary.

While a lease that carried a broadcast address (option 28) is held, that
broadcast is the one netd sets on every IPv4 address it adds to the
interface: the lease's own address, and a static added in that time,
whatever its subnet (§4.3).

## 4.2.5 IPv4 routes [*desired.ipv4-routes]

- **The default route** goes via `Route.Gateway`'s IPv4 entry when it is
  set. Otherwise, with `Route.Offered`, it goes via the lease's gateway:
  the first router of option 3, or, when the lease carried classless
  routes, the gateway of the classless `0.0.0.0/0` route. With neither,
  there is no IPv4 default route.
- **Classless static routes** (option 121) with a prefix above zero are
  added, each via its gateway, only with `Route.Offered`.

During the first INIT-REBOOT, existing netd-marked IPv4 routes accompany
the provisional address when `Route.Offered` is enabled. The profile's
explicit gateway and current metric still take precedence (§2.1).

`Route.Gateway` is used as written; netd does not check that the gateway
is on a subnet the interface has. A gateway the kernel cannot reach makes
the route's add fail on every reconcile, logged each time (§11).

## 4.2.6 IPv6 addresses [*desired.ipv6-addresses]

1. every IPv6 entry of `Address.Static`, at its prefix, preferred, with
   a prefix route;
2. every address router discovery currently wants (§6.2): stable-privacy
   and temporary addresses, each deprecated or not, each with or without
   a prefix route according to the prefix's on-link flag.

## 4.2.7 IPv6 default route [*desired.ipv6-default-route]

Via `Route.Gateway`'s IPv6 entry when it is set. Otherwise, with
`Route.Offered`, via the default router router discovery has chosen
(§6.1). With neither, there is no IPv6 default route.

## 4.2.8 Not desired

netd never desires the kernel's own IPv6 link-local address, the
kernel's prefix routes, or any route without netd's protocol mark. Those
are outside what reconciliation compares (§4.3).