3.3 KiB
topic, status, date, deciders, reconstructed, extends
| topic | status | date | deciders | reconstructed | extends |
|---|---|---|---|---|---|
| the mesh | accepted | 2026-09-22 | jochen | false | 02-DECISIONS/0100-a-node-in-use-is-adopted-before-it-is-converged.md |
101. A machine's own resolver does not make it in use
Context
ADR 0100 has a converged genesis refuse a machine in use. It defines in use as a container running, or a port listening on an address other than loopback that is not ssh's. That definition was written before anything was measured.
Measured on a freshly installed lab machine, running nothing but its operating system:
| Listening | Held by |
|---|---|
| TCP and UDP on every address, the link-local name resolution port | the system's name resolver |
| UDP on every address, the multicast name resolution port | the system's name resolver |
| UDP on a link-local address, the address-configuration client port | the system's network manager |
| TCP and UDP on loopback | the resolver's stub and the container runtime |
By ADR 0100's words, the resolver's TCP listener on every address makes every freshly
installed machine a machine in use. A converged genesis would refuse them all, and --adopted
would become the only way to raise anything. The refusal exists to catch a forgotten flag on a
working machine. Refusing an empty one defeats it, and teaches operators to pass the flag by
habit.
Considered Options
- Keep the words. Rejected: every fresh machine is refused.
- Count TCP only, ignore UDP. Rejected: the resolver listens on TCP too, and a machine that serves over UDP alone, a resolver or a tunnel, is in use.
- Ignore listeners held by the operating system's own network daemons, a short named list, on both protocols. Adopted.
Decision
A listener held by one of the operating system's own network daemons does not make a machine in use. The daemons are the ones the measurement found: the name resolver and the network manager, named in the installer's code beside that measurement. Everything else in ADR 0100's definition stands: a running container, or any other listener on an address other than loopback that is not ssh's, makes the machine in use, and genesis still names every one it counted.
A daemon is added to the list only with a measurement of a fresh machine that holds it.
Consequences
- A converged genesis on a fresh machine goes ahead, as it did before ADR 0100.
- A machine whose resolver is also serving other machines is not counted as in use by its resolver alone. It is one of the daemons that serves nobody on a fresh machine, and the one kind of service this lets through.
- The list is code, not configuration, so it changes by review.
How it is checked
A unit test in the installer feeds the listeners captured from the fresh machine, as the listening-socket tool printed them, and asserts the machine is not in use. The existing tests still assert that a serving machine is in use and that every container and listener is named. The adoption lab bed raises a converged genesis on a fresh machine and asserts it is not refused.