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+---
+status: open
+opened: 2026-09-14
+located-in: []
+fixed-by:
+amended-design:
+---
+
+# 052 — The firewall closes the port the mesh runs on
+
+## Symptom
+
+A one-node mesh with the packet filter assigned generates this input chain:
+
+```
+policy drop
+ct state established,related accept
+iif "lo" accept
+icmp / icmpv6 accept
+<mesh address> tcp dport 22   accept      ssh
+<mesh address> tcp dport 5000 accept      the registry
+<mesh address> tcp dport 20000 accept
+udp dport 51820 accept                    the overlay
+```
+
+**The substrate broker's port is not there.** Every machine dials it to enrol and to receive every
+declaration it is ever sent. Nothing opens it.
+
+The rules are generated from what modules declare they listen on. The broker is not a module — it
+is raised by the installer from the bundle — so it declares nothing, and the generator has nothing
+to generate from.
+
+## Why this matters
+
+**It is invisible on the mesh where it is first assembled and fatal on the next one.** A mesh of one
+never dials its own broker across the network, so the missing rule changes nothing and the firewall
+looks correct. The first machine that tries to join is refused at the packet filter, during
+enrolment, before the mesh can report anything about it — and the cause is several steps from the
+symptom.
+
+**It would have been met during the migration, not before it.** The intended order is to raise the
+anchor, assign its modules, then join the other machines. Assigning the firewall before the second
+machine enrols is both the natural order and the one that breaks.
+
+**It is [051](../051-the-mesh-cannot-update-what-it-depends-on/00-report.md) in a second place.** That
+issue records that the substrate cannot be updated because the mesh holds no record of it. The same
+absence means the firewall cannot know the substrate exists. Anything else generated from what
+modules declare has the same hole: the store, the broker, and their ports are outside every such
+computation.
+
+**The store is the same shape and has not been checked.** It binds on the machine and is reached by
+the control plane; whether that survives a default-drop policy has not been established here.
+
+## Open questions
+
+- **Should the substrate declare its listens** — which means the substrate being something the mesh
+  holds a record of, i.e. 051's adoption?
+- **Or should the firewall have a floor** that is not derived from modules at all, the way ssh
+  already is? Ssh is in the rules for exactly this reason: a machine nobody can reach is a machine
+  nobody can repair, and that is not a thing any module declares. The broker is arguably the same
+  class of fact — the mesh cannot manage a machine it cannot talk to.
+- **What else is in that class?** Whatever the answer, the question "which ports must be open for
+  the mesh itself to work" should be answerable from one place rather than assembled from modules
+  that happen to exist.
+
+## How this would be checked
+
+| Rule | Checked by |
+|---|---|
+| A machine with the firewall assigned can still be joined | The packet filter is assigned to an anchor, and a second machine enrols afterwards. |
+| The mesh's own ports are open | The generated ruleset is compared against the substrate's own bindings, not only against module declarations. |