steward/docs/subnets.md

# Subnets, pools, reservations and leases

## Subnets and interfaces

A subnet is an IPv4 network Steward hands addresses out in, served on
one network interface:

```sh
steward subnet add 10.0.10.0/24 --interface eth0 --name office \
    --pool 10.0.10.100-10.0.10.199 --option routers=10.0.10.1
```

**The interface decides.** stewardd opens one socket on port 67 per
interface that has a subnet, bound to that interface. A request is
answered only from the subnets of the interface it came in on. A client on
`eth0.20` never gets an `eth0` address, even if it asks for one. It gets a
NAK, then an address of its own VLAN.

For VLANs, give the server an interface on each, such as `eth0.20`, or a
bridge such as `br-iot`, each with an address in its subnet. That address
is the server identifier clients see, and the one they renew their lease
with. `steward interface list` shows the server's interfaces with their
addresses and subnets.

An interface that doesn't exist yet is fine. The subnet gets a warning
(`interface_unknown`), and stewardd starts serving it within 30 seconds of
the interface appearing. `steward status` and the dashboard show each
interface's state; when one isn't served, they say why.

**Several subnets on one interface** form a shared network: two networks
on the same wire, the server with an address in each. Addresses come from
the first subnet's pools, then the next. Clients of every subnet are
answered from the interface's address in their own subnet, or its first
address.

A subnet can be **disabled**: its configuration and leases stay, but its
clients get no answers (`steward subnet disable NAME`).

## Pools

Pools are the ranges handed out to any client: `FIRST-LAST`, or a prefix,
`10.0.10.128/25`. A subnet may have several pools, or none if only clients
with a reservation should get addresses. Pools must be inside the subnet,
must not overlap, and can't include the network or broadcast address.

Pool sizes are shown as the addresses that can really be handed out.
Reserved addresses inside a pool don't count, since nobody else gets them.

## Reservations

A reservation fixes a client's address:

```sh
steward reservation add 10.0.10.20 aa:bb:cc:dd:ee:01 --hostname nas
steward lease reserve 10.0.10.57       # the address a client has now
```

* **The client** is known by its MAC address, or by its client identifier
  (option 61) when the reservation has one (`--client-id`; give `-` as the
  MAC). Some devices change their identifier but not their MAC address,
  and others the other way round. When unsure, use the MAC address.
* **The address picks the subnet**: the subnet whose network holds it. It
  may be inside a pool or outside one; either way, nobody else gets it.
* **The host name** is sent to the client (option 12).
* **Options** set on the reservation are for that client only, and win
  over the subnet's and the settings'.
* **A device on two VLANs** can have a reservation in each subnet. Within
  one subnet, a client has at most one.

A client with a reservation always gets that address, not a pool address.
If it asks for another one, for example after the reservation was added,
it gets a NAK that names its reserved address, and then the reservation.

## Options

Options are set at three levels. A reservation's win over its subnet's,
and a subnet's over the settings' (the options every subnet gets):

```sh
steward settings set --option domain-name-servers=10.0.10.2 --option domain-name=lan
steward subnet update office --option routers=10.0.10.1 --option ntp-servers=10.0.10.2
steward reservation update nas --option domain-name-servers=1.1.1.1
```

`--option NAME=VALUE` sets one and keeps the others; `--unset-option NAME`
removes one. [options.md](options.md) lists the names, and how to write
each kind of value. Any other option can be set by number, with its type:
`--option 224:string=hello`.

Every reply carries the subnet mask, the server identifier and, for a
lease, the lease time and the times to renew (half of it) and rebind (7/8
of it). With a parameter request list (most clients send one), the client
gets the options it asked for, in its order. The subnet mask, routers, DNS
servers, domain name and host name are sent even if it didn't ask. Without
a list, it gets every option set for it.

## Lease times

The settings have the default (12 hours, `steward settings set
--lease-time 1d`); a subnet can have its own (`--lease-time 1h`). Clients
renew halfway through. Short times let changes reach clients sooner;
long ones keep clients working longer if Steward is down.

## How an address is chosen

For a DISCOVER, Steward offers, in this order:

1. the client's reservation in a subnet of the interface;
2. the address the client had before, from its last lease, even if that
   ended, as long as it is still free and in a pool;
3. the address the client asks for (option 50), if it is free and in a
   pool;
4. a free pool address: one never handed out before, else the one that has
   been free the longest.

An address is free when no other client has an active lease on it, it
isn't held back, and it isn't offered to someone else in the last minute.
The server's own addresses are never offered. A fresh pool address is
pinged first: if something answers, it is held back and the next one is
tried.

When the client confirms with a REQUEST, the lease is saved, and only
then is the ACK sent.

## Leases

| State | Meaning |
|---|---|
| active | the client holds the address until the lease ends |
| expired | the lease ran out; the client gets the address back if it returns while it is free |
| released | the client gave the address back |
| held back | a client declined the address because another device answered for it, or it answered Steward's ping. It isn't handed out until the hold ends (a day; `--decline-hold`) |

`steward lease list` shows active and held-back leases; `--all` shows
the others too. Leases that ended more than 30 days ago are deleted.

`steward lease delete IP` frees an address at once. A client still using
it gets a NAK when it next renews, then asks for a new address.

## Authority

Steward is **authoritative** for its interfaces. A client that asks to
keep an address that doesn't belong on its interface, or that someone
else holds, gets a NAK and starts over at once, rather than keeping a
wrong address until its lease runs out. Turn this off (`steward settings
set --authoritative off`) only when another DHCP server serves the same
network. Requests for addresses Steward knows nothing about then get no
answer.

## Not in 0.1

Relayed requests (with `giaddr`) are ignored and logged once an hour: a
subnet must be on one of the server's own interfaces. DHCPv6, failover,
client classes and DNS names through Jarl are planned
([design.md](design.md)).