ADR 0169: the firewall seat serves its verbs, and a foreign rule set is removed through one of them

Designs 33 and 08 revised. The first node-scoped seat with verbs: rules, reload, remove; the nftables module holds it from a runtime with NET_ADMIN, the first container to declare a capability.
2026-10-02 13:27:03 +02:00
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+---
+topic: the mesh
+status: accepted
+date: 2026-10-02
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0168-a-converged-machine-is-filtered-by-the-mesh-alone.md
+---
+
+# 169. The firewall seat serves its verbs, and a foreign rule set is removed through one of them
+
+## Context
+
+[ADR 0168](0168-a-converged-machine-is-filtered-by-the-mesh-alone.md) made the mesh say truthfully
+what filters a converged machine, and left the removal of what it did not write to the operator's
+hand. The first time that hand was needed — two machines, five rule sets a predecessor and a
+retired front end had left — there was no mesh way to lend it: the packet filter is a seat
+([ADR 0121](0121-a-system-seat-is-named-for-its-scope-and-modules-define-their-own.md)), a seat's
+holder serves its verbs ([ADR 0154](0154-the-meshs-own-verbs-are-the-controller-seats-tools.md),
+[ADR 0159](0159-a-tool-call-names-the-machine-and-a-holder-serves-its-seats-verbs.md)), and the
+firewall seat declared none. The only remaining path was a shell on the machine, which is the path
+the mesh exists to replace, and which the operator's own tooling rightly refused to an agent.
+
+A seat's verbs are the contract every holder implements, whatever filter it speaks. What a person
+asks a machine's packet filter is the same whether nftables, a front end or a legacy filter answers:
+what are the rules, reload the mesh's own, remove this thing the mesh did not write. What differs by
+filter is the holder's own business and may be its own tools beside the seat's.
+
+## Decision
+
+**1. The `node-packet-filter` seat serves three verbs**, and a module that claims it serves all
+three or is refused the claim, as with every seat:
+
+- `rules` — the packet filter as the machine enforces it now: the nftables ruleset, and the legacy
+  filter's listings where that tool exists; narrowed to one table or chain when asked. Read-only.
+- `reload` — load the mesh's own filter again from the file the mesh writes, and answer with the
+  mesh's table as loaded. The holder's own act on the holder's own rules.
+- `remove` — remove one rule set the mesh did not write, named exactly as the host reports it under
+  ADR 0168 (`chain HAL-MESH-ONLY (iptables-legacy)`, `table ip6 filter, chain DOCKER-USER`), and
+  answer with what was done. It refuses the mesh's own tables, the container runtime's own chains,
+  a built-in chain other than the runtime's user chain, and any chain of a found firewall that is
+  in force. The runtime's user chain is emptied back to its one return; another chain loses the
+  jumps into it, is flushed and deleted; a table of the machine's own is deleted whole. Each is an
+  operator's act, by name, on one thing the mesh reported — never a flush, never a rule the mesh
+  itself marked.
+
+**2. A holder may serve its own tools beside the seat's.** The nftables module keeps its reading of
+the mesh's table as its own tool, and a holder speaking a filter with specifics of its own may add
+tools for them; the seat's three are what every holder owes.
+
+**3. A container may ask for a capability.** Serving `remove` and `reload` needs the machine's
+network namespace and the right to change its packet filter; a holder's runtime declares
+`capabilities: ["NET_ADMIN"]` on its container and runs on the machine's network. The host grants
+exactly the capabilities declared, names them in the container's spec so a change recreates it, and
+refuses a name that is not a capability's. A privileged container stays undeclarable.
+
+**4. ADR 0168's "by hand" is read as "by the operator, through the seat".** Removing what the mesh
+reports as *other* is still the operator's act and is still never the mesh's own doing; the verb is
+how the act reaches the machine, recorded on the bus like every other, instead of a shell.
+
+## Consequences
+
+- The seat's row gains the three verbs; a mesh that already runs widens its row at the next
+  controller start. The nftables module claims them and gains a runtime — a tool server with the
+  packet filter's tools in its image, on the machine's network, with `NET_ADMIN`.
+- The host's container vocabulary grows by `capabilities`; an older host refuses a declaration that
+  carries it, so the host rolls before the module.
+- The two machines of this mesh that ADR 0168 found not filtered by the mesh alone are cleaned
+  through `remove`, and read *the mesh alone* afterwards; `status` returns to well without a hand on
+  either machine.
+
+## How this is checked
+
+| Rule | Checked by |
+|---|---|
+| The seat declares the three verbs; a claim that serves fewer is refused by name | the catalogue's seat tests |
+| `remove` refuses the mesh's tables, the runtime's chains, a built-in chain and an active front end's chains, and removes a user chain with its jumps, empties the user chain, deletes an own table | the module's tests over a fake command runner, with the shapes the host reported live |
+| A container's capabilities reach the runtime and its spec; an unknown name is refused | host tests |
+| Live | `node-packet-filter.remove@<node>` on the home server and the control node; `node show` reads *the mesh alone* on both; `status` is well |
+
+## References
+
+- [ADR 0168](0168-a-converged-machine-is-filtered-by-the-mesh-alone.md), [ADR 0154](0154-the-meshs-own-verbs-are-the-controller-seats-tools.md), [ADR 0159](0159-a-tool-call-names-the-machine-and-a-holder-serves-its-seats-verbs.md), [ADR 0121](0121-a-system-seat-is-named-for-its-scope-and-modules-define-their-own.md)
+- [Design 33 — The tools the mesh answers](../03-DESIGN/01-to-be/33-the-tools-the-mesh-answers.md), [Design 08 — Connectivity](../03-DESIGN/01-to-be/08-connectivity.md)
@@ -180,6 +180,7 @@ python3 00-META/checks/index.py            fail if stale
 - **0167** — [A membership carries what its module receives, and who the mesh is](0167-a-membership-carries-what-its-module-receives-and-who-the-mesh-is.md)
 - **0168** — [A converged machine is filtered by the mesh alone, and the host says what else refuses](0168-a-converged-machine-is-filtered-by-the-mesh-alone.md)
 - **0169** — [A machine joins through the tunnel, and the bus is never public](0169-a-machine-joins-through-the-tunnel-and-the-bus-is-never-public.md)
+- **0169** — [The firewall seat serves its verbs, and a foreign rule set is removed through one of them](0169-the-firewall-seat-serves-its-verbs.md)

 ### Its tiers, from the bottom up

@@ -9,6 +9,7 @@ code:
  - mesh-host internal/apply (the service that reflects a rule set)
 updated: 2026-10-02
 decisions:
+  - 02-DECISIONS/0169-the-firewall-seat-serves-its-verbs.md
  - 02-DECISIONS/0169-a-machine-joins-through-the-tunnel-and-the-bus-is-never-public.md
  - 02-DECISIONS/0168-a-converged-machine-is-filtered-by-the-mesh-alone.md
  - 02-DECISIONS/0167-a-membership-carries-what-its-module-receives-and-who-the-mesh-is.md
@@ -766,6 +767,11 @@ tests over a fixture report check the recording, the preview's fates, the status
 predicate. Live: the home server's record names the predecessor's chain as *other* and `status`
 names the machine until the chain is removed by hand.

+*2026-10-02, [ADR 0169](../../02-DECISIONS/0169-the-firewall-seat-serves-its-verbs.md):* removing what
+the host reports as *other* is reached through the packet filter seat's `remove` verb, an operator's act
+by name on the bus; the seat also serves `rules` and `reload`, and its holder's runtime declares the
+`NET_ADMIN` capability on the machine's network. See design 33.
+
 ## 5 — Certificates

 **Two authorities, kept separate on purpose.**
@@ -2,8 +2,9 @@
 layer: to-be
 status: implemented
 code: [mesh-controller, mesh-tools]
-updated: 2026-10-01
+updated: 2026-10-02
 decisions:
+  - 02-DECISIONS/0169-the-firewall-seat-serves-its-verbs.md
  - 02-DECISIONS/0160-the-mesh-issues-an-assignments-subjects-and-a-runtime-serves-what-it-is-issued.md
  - 02-DECISIONS/0159-a-tool-call-names-the-machine-and-a-holder-serves-its-seats-verbs.md
  - 02-DECISIONS/0154-the-meshs-own-verbs-are-the-controller-seats-tools.md
@@ -169,6 +170,18 @@ either way.
 What stays as designed and not built: which verbs any *other* seat serves, and §3 for module-declared
 seats' schemas beyond the names their manifests already list.

+## The firewall seat's verbs, 2026-10-02
+
+[ADR 0169](../../02-DECISIONS/0169-the-firewall-seat-serves-its-verbs.md). The first node-scoped seat
+to carry verbs: `node-packet-filter` serves `rules` (the filter as the machine enforces it, nftables
+and legacy), `reload` (the mesh's own filter from its file) and `remove` (one rule set the mesh did
+not write, named as the host reports it under ADR 0168; refusing the mesh's tables, the runtime's
+own chains, a built-in chain and an active found firewall's). Every holder serves all three; the
+nftables module does so from a runtime on the machine's network with `NET_ADMIN`, which is the first
+container to declare a capability. Removing a predecessor's rule set is an operator's act reached
+through the seat, recorded on the bus, instead of a shell on the machine. *How it is checked:* ADR
+0169's table.
+
 ## What this does not settle

 - Which verbs each seat should serve. That is a decision per seat, and the reason to do it slowly: a