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atlas bcd4fbd918 docs(swarm): cut the CA material into its own page

Second split under docs/swarm/, following the shape docs/turn-loop/ and
docs/web-ui/ already use.

The README keeps the `## Swarm CA` heading rather than deleting it: an
existing cross-reference targets that anchor, and a heading is the
cheapest thing to preserve. It now carries the one-paragraph summary and
a pointer, so a reader who lands on the anchor still arrives somewhere
that answers the question.

The CA page also absorbs three things that were true but undocumented —
why the trust bundle rather than the bare intermediate (openssl will not
terminate a chain at a non-self-signed anchor without -partial_chain),
that autoConfigure derives from enableAllLocalDefaults, and that the
name constraint excludes both IP families because a permitted-DNS-only
constraint says nothing about IP SANs.

2026-08-05 18:34:39 +02:00

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Swarm CA

A hive's internal TLS chains to a swarm root CA: the root signs each hive's own CA, and that hive CA signs the gateway leaf. A peer that trusts the root once validates every hive in the swarm, present and future, instead of being pinned to each one by hand.

That is the whole point of the hierarchy — it turns per-peer trust from O(n²) hand-pinning into one anchor per swarm.

Two provisioning modes, one structure

What differs is who puts the artifacts on disk:

	swarm root	this hive's CA
default	operator-provided	operator-provided, else self-signed as before
`autoConfigure = true` (all on one host)	generated by `swarm-ca.service` on first boot	issued by `hive-tls-ca.service` under the root

services.hyperhive.swarm.ca.autoConfigure selects between them, and is off by default: a swarm's services and its hives can live on different hosts, and a host cannot tell whether it is the one holding the root, so setting the swarm CA up is an operator action rather than something a host assumes. Turn it on for an all-on-one-host deployment and the hierarchy costs no configuration.

It defaults from services.hyperhive.enableAllLocalDefaults, the single switch that says "this box is the whole deployment".

A hive given neither artifact keeps the self-signed CA it has always had. It serves TLS exactly as before and simply isn't part of a swarm's trust hierarchy — the right outcome for a hive nobody has federated yet. Only autoConfigure issues a hive sub-CA, because only that case can: signing one needs the root's private key.

Moving the swarm CA onto its own host is then a matter of moving services.hyperhive.swarm.ca.stateDir and leaving autoConfigure off — there is no second code path to switch to.

Constraints on the material

The root's private key never reaches the nix store: the store is world-readable and content-addressed, so a key committed to a flake is a key published to everyone who builds it. Only certificates are distributed.

Each hive CA is name-constrained (X.509 nameConstraints) to that hive's own domain, so a hive CA that leaks can only mint names inside its own subdomain — enforced by every verifier rather than by convention. The constraint excludes both IP families as well, since a permitted-DNS-only constraint says nothing about IP SANs.

The root is issued with pathlen:1: it may sign hive CAs, and those may sign leaves, and the chain stops there.

What to hand a peer

hivectl peer-config prints the cp line. The file is <tls.stateDir>/trust-bundle.pem — the hive CA plus the swarm root — not ca.pem.

The distinction is load-bearing rather than cosmetic: once a hive CA is an intermediate, it is no longer something a verifier can build a chain to. OpenSSL will not terminate a chain at a trusted non-self-signed certificate without -partial_chain, so handing a peer the bare intermediate produces a verification failure that reads like a bad cert rather than like a missing anchor. The bundle carries both, so the same file works whichever mode issued it.

On a hive that predates the swarm root the bundle is just that hive's self-signed CA, so the recipe does not change.

⚠️ Consumers must read trust-bundle.pem, never ca.pem directly. A consumer that reads ca.pem works fine on a hive that has always been self-signed and breaks the moment that hive adopts the hierarchy — so the failure is invisible on the deployment you are most likely to test on.

Adopting the hierarchy on an existing hive

An existing ca.pem is never re-rooted automatically — swapping it would break every consumer that already trusts it, and agents only pick up new trust when their container restarts. To adopt, delete ca.pem + ca-key.pem under tls.stateDir and restart hive-tls-ca.service; the CA is re-issued under the root and the leaf re-signed. Until then the hive serves TLS exactly as before and is simply not part of the swarm's trust hierarchy.

Making that adoption a first-class, non-disruptive operation — rather than a documented rm — is tracked separately; the mechanism it needs (carrying the previous CA in the trust bundle across the overlap) already exists.

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