---
status: open
opened: 2026-10-02
located-in: [mesh-catalog modules/dnsmasq, mesh-controller cmd/mesh-controller]
fixed-by:
amended-design:
---

# 202 — A module whose required setting nobody set is left out of the machine, and the resolver is the module it happened to

## What was observed

Running the controller's own test suite against the catalogue beside it, 2026-10-02.
`TestTheResolverIsToldEveryMachineOnTheNetworkAndToldAgainWhenOneLeaves` fails with *"the resolver
was not handed the machines"*. Composing the same machine by hand and listing what it receives
shows why: **dnsmasq contributes nothing at all.** Four resources are composed for that node, all
of them the overlay's. The resolver's package, its configuration, its service and the fact that
carries every machine's name are simply not there.

The cause is one line added to `dnsmasq`'s configuration earlier the same day: the addresses it
listens on beside the machine's own became an operator setting,
`listen-address=${setting:listen-addresses}`, with no default. A `${setting:…}` nothing sets is
refused, a module that cannot be composed is **left out** rather than failing the whole machine
([ADR 0163](../../02-DECISIONS/0163-taking-a-module-over-is-a-comparison.md)), and so a node
assigned the resolver is handed a declaration with no resolver in it.

The failing test is the symptom that surfaced it. The test is not what is wrong.

**Proven rather than inferred.** Composing the same machine a second time with
`listen-addresses` set to `127.0.0.1` and nothing else changed, every one of dnsmasq's eight
resources appears — `needs-broker`, `mesh-state`, `package`, `config`, `runtime-dns`, `runtime`,
`service` and `fact-node-zones`. The only difference between a machine with a resolver and a
machine without one is whether somebody set a value that did not exist yesterday.

## Why it matters beyond this instance

**Leaving a module out is right, and being quiet about it is not.** The rule exists so one
module's broken setting cannot stop a machine converging — a good rule. But the outcome here is a
machine that applies cleanly, reports current, and is missing its DNS resolver. Every name on that
machine then resolves through whatever was there before, or not at all, and nothing in the mesh
says the resolver was dropped. That is the shape
[issue 152](../152-a-nodes-plan-failure-silently-drops-its-routed-names/00-report.md) records for
routed names, here for a whole module.

**And a setting with no default is a definition that cannot be assigned.** Every other
`${setting:…}` in the catalogue names something that is genuinely particular to one installation —
a public domain, an issuer. "Which addresses besides my own do I answer on" has an obvious correct
default for every machine that is not a LAN gateway: none beside loopback. A definition that
refuses to compose until somebody sets a value most machines do not need is a definition that
breaks the next node to be assigned it, and genesis with it.

## What this does not claim

Whether the live machines are affected was not checked — those four have had the setting set, or
their resolvers would already be gone. The claim is about a machine assigned the resolver *from
now on*, and about the silence.

## Open questions

- Should the declaration say which modules it left out, where a person or the console can see it?
  `left_out` already travels to the host ([ADR 0163](../../02-DECISIONS/0163-taking-a-module-over-is-a-comparison.md));
  what is missing is anything that reads it back and says so.
- Should a `${setting:…}` be allowed a default in the definition — making "unset" mean "the
  default" rather than "refuse" — or is a setting with a default no longer the operator's value?
- Is leaving a module out ever right for a module a node is **assigned**, as opposed to one it
  merely pulls in? An assignment is somebody saying *this machine runs this*; silently not running
  it is the one answer nobody asked for.