ADR 0140: the filter constrains what arrives from outside, and says nothing about a machine's own guests

Reading a converged machine's rendered rules showed the cause: the chain blocks
everything passing through and then allows the machine's own containers back by
listing their address ranges. 0137 made that list typeable and 0139 tried to
generate it; both refined a list that should not exist, because the mesh has no
position on a container reaching outward. Constrain what arrives from outside,
allow what did not, and let the machine report which links face outside — one
fact instead of a list. Ports keep following the modules unchanged.

The records check now allows one record to supersede several, and stops
requiring a withdrawn record's own citations to be live.

00-META/checks/records.py

@@ -164,6 +164,12 @@ def check_rests_on(failures, records):
                 # decision is exactly what as-is is for."
                 if rel(path).startswith("03-DESIGN/00-as-is/"):
                     continue
+                # A withdrawn record's citations are history. It instructs nobody -- every reader
+                # is sent to its superseder -- so what it was built on may itself be withdrawn.
+                # Refusing that would mean rewriting the lineage of a record whose reasoning is
+                # the thing the immutability rule protects.
+                if frontmatter(read(path)).get("status") == "superseded":
+                    continue
                 # An extension that supersedes legitimately names what it replaced.
                 this = ADR_FILE.match(os.path.basename(path))
                 supersedes = records[number]["front"].get("superseded-by", "")
@@ -241,13 +247,20 @@ def check_supersession_symmetry(failures, records):
             failures.add("supersession", rel(record["path"]), f"superseder does not exist: {by}")
             continue
         other = records[match.group(1)]
-        claims = os.path.basename(str(other["front"].get("supersedes", "")))
+        # `supersedes:` may name one record or several. One decision replacing two is a real
-        if claims != record["name"]:
+        # situation -- two records that built and refined the same wrong mechanism are withdrawn
+        # by the one record that removes it -- and a check that allows only one would force
+        # either a chain of pro-forma records or an unmarked supersession.
+        claimed = other["front"].get("supersedes", "")
+        if isinstance(claimed, str):
+            claimed = [claimed] if claimed else []
+        claims = [os.path.basename(str(entry)) for entry in claimed]
+        if record["name"] not in claims:
             failures.add(
                 "supersession",
                 rel(other["path"]),
                 f"ADR {number} says this supersedes it; this record does not say so "
-                f"(supersedes: {claims or 'absent'})",
+                f"(supersedes: {', '.join(claims) or 'absent'})",
             )
 
 

02-DECISIONS/0137-a-machine-says-which-networks-it-routes.md

@@ -1,10 +1,11 @@
 ---
 topic: what runs on it
-status: accepted
+status: superseded
 date: 2026-09-28
 deciders: jochen
 extends: 02-DECISIONS/0100-a-node-in-use-is-adopted-before-it-is-converged.md
 reconstructed: false
+superseded-by: 02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md
 ---
 
 # 137. A machine says which networks it routes

02-DECISIONS/0139-a-network-is-forwarded-because-a-module-declared-it.md

@@ -1,10 +1,11 @@
 ---
 topic: what runs on it
-status: accepted
+status: superseded
 date: 2026-09-28
 deciders: jochen
 reconstructed: false
 extends: 02-DECISIONS/0137-a-machine-says-which-networks-it-routes.md
+superseded-by: 02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md
 ---
 
 # 139. A network is forwarded because a module declared it

02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md

@@ -0,0 +1,146 @@
+---
+topic: what runs on it
+status: accepted
+date: 2026-09-28
+deciders: jochen
+reconstructed: false
+extends: 02-DECISIONS/0045-a-machine-firewall-is-the-sum-of-what-it-listens-on.md
+supersedes:
+  - 02-DECISIONS/0137-a-machine-says-which-networks-it-routes.md
+  - 02-DECISIONS/0139-a-network-is-forwarded-because-a-module-declared-it.md
+---
+
+# 140. The filter constrains what arrives from outside, and says nothing about a machine's own guests
+
+## Context
+
+The filter the mesh derives blocks traffic passing *through* a machine unless something allows it,
+because a container's published port is traffic passing through rather than traffic arriving at the
+machine itself ([ADR 0100](0100-a-node-in-use-is-adopted-before-it-is-converged.md),
+[issue 047](../04-ISSUES/047-the-firewall-does-not-cover-published-container-ports/00-report.md)).
+Having blocked all of it, the filter then had to let the machine's own containers reach outward again.
+It does that by listing the address ranges those containers sit on.
+
+As rendered on a converged workstation today:
+
+```
+policy drop
+ct state established,related accept
+ip saddr 172.16.0.0/12 accept
+ip saddr 192.168.128.0/17 accept
+ip saddr 10.0.0.0/8 accept
+ip saddr 192.168.16.0/20 accept
+... four more
+```
+
+Two of those ranges were constants in the control plane's source. The rest were typed by the operator
+after [ADR 0137](0137-a-machine-says-which-networks-it-routes.md), which existed to make the typing
+possible, because converging that workstation had cut every one of its containers off from the
+internet and nothing reported a fault
+([issue 137](../04-ISSUES/137-converging-a-machine-cut-off-its-own-guests/00-report.md)).
+
+**The list is the mistake, not its contents.** Every attempt to make it correct fails the same way.
+A constant describes one machine. A typed range goes stale, and cannot tell a network the mesh made
+from one a predecessor left behind — measured on the control-node, where six such ranges fall outside
+the constants and two of the six belong to services the mesh does not run
+([issue 141](../04-ISSUES/141-the-forward-chain-does-not-follow-the-modules/00-report.md)).
+[ADR 0139](0139-a-network-is-forwarded-because-a-module-declared-it.md) tried to generate the same
+list from the modules and put half the rule set on the machine to do it. Three records, one list, and
+the list should not exist.
+
+**Because the mesh has no policy about a container reaching outward.** What the filter is for is
+stated in [ADR 0045](0045-a-machine-firewall-is-the-sum-of-what-it-listens-on.md): which port is open,
+and to whom. That is about what arrives. A container of this machine's own opening a connection to
+something else is not a port being opened to anybody, and enumerating the addresses it might do so
+from is bookkeeping about the machine's internal plumbing, which the mesh neither owns nor can know.
+
+**The system being replaced never had this fault, and its rule says why.** The chain still protecting
+the control-node applies only to traffic arriving on that machine's outward link, and leaves
+everything else alone. The mesh's filter dropped that distinction and replaced it with a list of
+addresses.
+
+## Considered Options
+
+1. **Keep the list and generate it better** — from the modules' declared networks, or from what the
+   machine reports. Rejected: [ADR 0139](0139-a-network-is-forwarded-because-a-module-declared-it.md)
+   is that, and it puts part of the rule set on the machine, which makes the rule set partly the
+   machine's and the derivation advisory.
+2. **Name the guest links instead of their addresses, and allow only those.** Rejected as more than is
+   needed: it fails in the safe direction, but it is still a list that has to keep up with the
+   machine, and the thing it protects against — a container reaching outward — is not a thing the mesh
+   has a position on.
+3. **Do not block traffic passing through at all.** Rejected: that is
+   [issue 047](../04-ISSUES/047-the-firewall-does-not-cover-published-container-ports/00-report.md),
+   where a published port was reachable from anywhere because no rule mentioned it.
+4. **Constrain what arrives from outside, and nothing else.** Adopted.
+
+## Decision
+
+**The filter constrains traffic arriving from outside the machine, and says nothing about traffic that
+did not.** Traffic passing through the machine is allowed unless it arrived on one of the machine's
+outward links, in which case it is allowed only where a declared endpoint's reach admits it
+([ADR 0138](0138-an-assignment-binds-an-endpoint-and-says-how-far-it-reaches.md)). A container of this
+machine's own reaching anywhere is not filtered, because the mesh has no position on it.
+
+**A machine says which of its links face outside.** One node-level fact, reported by the machine the
+way it already reports the kind of firewall it found and the tunnel it carries — not a setting, not a
+list of addresses, and not something anybody types. It does not change when a module is added or
+removed, which is what separates it from the list it replaces.
+
+**A machine that has reported no outward link is sent no filter.** Rendering a rule around a link
+whose name is not known produces a rule set that does not load, which is a machine filtering nothing
+while its unit reports success. The refusal happens in the control plane, where a person reads it, and
+the machine keeps the filter it already has.
+
+**No addresses of the machine's own networks appear in the filter.** The two constants are removed and
+`node networks` is removed with them, along with everything any machine was told to say through it.
+Ports continue to follow the modules exactly as before: a module assigned to a machine opens the port
+its assignment says it reaches on, and nothing about a network is said anywhere.
+
+## Consequences
+
+- **Three records collapse into one rule.** 0137 and 0139 are superseded. What 0137 was right about —
+  that converging a machine had silently cut off its own containers, and that nothing previewed it — is
+  answered by removing the cause rather than by giving the operator a way to compensate for it.
+- **Every machine already converged loses its declared ranges and keeps working**, because the traffic
+  those ranges allowed is now allowed by not having arrived from outside. The workstation's five ranges
+  and the laptop's one are deleted rather than migrated.
+- **A machine's test beds stop being a special case.** A bed's network is created while the machine
+  runs and was the case no list could cover; it is now covered by not being mentioned.
+- **A new fact travels in the report**, and the control plane refuses to compose a filter without it,
+  so the order of the roll-out matters: the machines report before the control plane depends on it.
+- **A machine with more than one outward link says so**, and a machine that acquires one while the mesh
+  is not looking is treated as internal until its next report. That window is the cost of this shape;
+  it is bounded by the report interval, and it exists on machines whose outward link changes, which
+  are the machines with nothing published to the outside.
+- **What got harder:** nothing in the declaration, and one more thing a machine must be able to work
+  out about itself. A machine that cannot say which link faces outside cannot be given a filter.
+
+## How it is checked
+
+- **A machine's own container reaches outward with no network named anywhere.** A bed converges a
+  machine carrying containers on several networks, none of them mentioned in any setting, and each
+  reaches out afterwards. This fails against the previous behaviour, where the same flip cut them off,
+  and that is how it is written.
+- **A port declared reachable from outside is reachable; one that is not, is not.** Probed from off the
+  machine's private network, for a published port and for an undeclared one, before and after the flip.
+- **A network created after the filter was composed needs no new filter.** A network is made on the
+  machine after its last declaration and a container on it reaches out, with nothing re-sent.
+- **No address of a machine's own networks appears in a rendered filter**, asserted on the text so a
+  range cannot creep back in.
+- **A machine that reports no outward link is sent no filter, and the refusal names it** — asserted in
+  the control plane, and that the machine's existing filter is left alone.
+- **A machine reporting two outward links has both constrained**, asserted per chain body so a rule
+  covering one and not the other cannot pass.
+
+## References
+
+- [ADR 0045](0045-a-machine-firewall-is-the-sum-of-what-it-listens-on.md) — what the filter is for
+- [ADR 0138](0138-an-assignment-binds-an-endpoint-and-says-how-far-it-reaches.md) — what admits traffic
+  arriving from outside
+- [ADR 0100](0100-a-node-in-use-is-adopted-before-it-is-converged.md) — why traffic passing through is
+  filtered at all
+- [ADR 0137](0137-a-machine-says-which-networks-it-routes.md),
+  [ADR 0139](0139-a-network-is-forwarded-because-a-module-declared-it.md) — superseded here
+- [issue 137](../04-ISSUES/137-converging-a-machine-cut-off-its-own-guests/00-report.md),
+  [issue 141](../04-ISSUES/141-the-forward-chain-does-not-follow-the-modules/00-report.md)

02-DECISIONS/README.md

@@ -217,9 +217,10 @@ python3 00-META/checks/index.py            fail if stale
 - **0133** — [A module owns its migrations, and the mesh owns when they run](0133-a-module-owns-its-migrations-and-the-mesh-owns-when-they-run.md)  *(superseded)*
 - **0135** — [A module version prepares its state before it runs](0135-a-module-version-prepares-its-state-before-it-runs.md)
 - **0136** — [A step gates its module, not the machine](0136-a-step-gates-its-module-not-the-machine.md)
-- **0137** — [A machine says which networks it routes](0137-a-machine-says-which-networks-it-routes.md)
+- **0137** — [A machine says which networks it routes](0137-a-machine-says-which-networks-it-routes.md)  *(superseded)*
 - **0138** — [An assignment binds an endpoint and says how far it reaches](0138-an-assignment-binds-an-endpoint-and-says-how-far-it-reaches.md)
-- **0139** — [A network is forwarded because a module declared it](0139-a-network-is-forwarded-because-a-module-declared-it.md)
+- **0139** — [A network is forwarded because a module declared it](0139-a-network-is-forwarded-because-a-module-declared-it.md)  *(superseded)*
+- **0140** — [The filter constrains what arrives from outside, and says nothing about a machine's own guests](0140-the-filter-constrains-what-arrives-from-outside.md)
 
 ### How it is built
 

03-DESIGN/01-to-be/08-connectivity.md

@@ -10,7 +10,7 @@ code:
 updated: 2026-09-28
 decisions:
   - 02-DECISIONS/0138-an-assignment-binds-an-endpoint-and-says-how-far-it-reaches.md
-  - 02-DECISIONS/0139-a-network-is-forwarded-because-a-module-declared-it.md
+  - 02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md
   - 02-DECISIONS/0104-a-provision-may-be-answered-by-an-adapter-to-the-predecessor.md
   - 02-DECISIONS/0106-the-bus-is-nats.md
   - 02-DECISIONS/0105-the-mesh-adopts-the-predecessors-tunnel-in-place.md
@@ -627,41 +627,49 @@ the found firewall reloads and reachable from a container on the node, that a ma
 enrols through the openings before and after a reload and a reboot, and that after the flip the
 declared port is open and the undeclared one closed.
 
-### The networks it forwards are the ones its modules declared
+### It filters what arrives from outside, and not what the machine's own guests send
 
 *2026-09-28, preparing the control-node's convergence —
 [issue 141](../../04-ISSUES/141-the-forward-chain-does-not-follow-the-modules/00-report.md), settled by
+[ADR 0140](../../02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md), which replaces
+[ADR 0137](../../02-DECISIONS/0137-a-machine-says-which-networks-it-routes.md) and
 [ADR 0139](../../02-DECISIONS/0139-a-network-is-forwarded-because-a-module-declared-it.md).*
 
-The forward chain denies by default, so a machine's own guests have to be allowed back in. Until now
+Traffic passing through a machine is filtered, because a container's published port is traffic passing
-that was two ranges named in the control plane and, since
+through rather than traffic arriving at the machine itself. Having blocked it, the filter then had to
-[ADR 0137](../../02-DECISIONS/0137-a-machine-says-which-networks-it-routes.md), a list a machine could
+let the machine's own containers reach outward again — and it did that by listing the address ranges
-add to. On the machine measured here, six of its container networks fell outside those ranges — and
+they sit on. Two of those ranges were constants in this repository's code, and the rest were typed by an
-four of the six were networks the mesh's own modules had declared and the host had created, while two
+operator after the flip had already cut a workstation's containers off from everything.
-were the predecessor's leftovers. A range wide enough to keep the four keeps the two: a filter widened
-by hand to protect what should not be there.
 
-**A network is forwarded because a module declared it.** The forward chain forwards the networks of the
+**The list was the mistake, not its contents.** A constant describes one machine. A typed range goes
-modules assigned to that node and, by default, nothing else; a module unassigned stops being forwarded
+stale and cannot tell a network the mesh made from one a predecessor left behind — on the control-node,
-at the next reconcile. The control plane says which networks, by name, and the host — which created
+six ranges fall outside the constants and two of the six belong to services the mesh does not run. The
-them — renders their addresses, because the runtime allocates the subnet and the host is what knows it.
+attempt to generate the list from the modules put half the rule set on the machine and made the
-That keeps §4's shape and this document's: the mesh decides, the host applies.
+derivation advisory. Three records, one list.
 
-**This is derivation from the declaration, not from the machine.** 0137 rejected reading the machine,
+**And the mesh has no position on a container reaching outward.** §4 exists to say which port is open
-for two reasons that do not apply here: a network created between two declarations is already named in
+and to whom, which is about what arrives. A container of this machine's own opening a connection
-the one that asked for it, and a network nobody declared is never forwarded however it appeared. What
+somewhere is not a port opened to anybody, and the addresses it might do that from are the machine's
-0137's mechanism keeps is the case it was right for — guests no module declares, a test bed's range —
+internal plumbing, which the mesh neither owns nor can know.
-added to the derived set and never replacing it.
 
-The runtime's own default bridge, which a container attaches to when it names no module network, is the
+So the filter constrains what arrives from **outside** the machine and says nothing about what did not.
-runtime's and not a module's, so the host renders it from what the runtime reports. With that, no range
+Traffic passing through is allowed unless it came in on one of the machine's outward links, and then
-is named in the control plane at all.
+only where a declared endpoint's reach admits it (§6). The machine says which of its links face
+outside — one fact it reports, like the kind of firewall it found and the tunnel it carries, not a
+setting and not a list of addresses. It does not change when a module is added or removed, which is the
+whole difference from what it replaces. A machine that has reported no outward link is sent no filter
+at all, and keeps the one it has, because a rule written around a link with no name is a rule set that
+does not load — a machine filtering nothing while its unit reports success.
 
-*How it is checked:* a node with two modules declaring networks renders rules for exactly those two and
+Ports go on following the modules exactly as before: assign a module to a machine and the port its
-none for a third present on the machine that nothing declared — which fails against forwarding by
+assignment says it reaches on opens. Nothing about a network is said anywhere, by anybody.
-range, and is how it was written; unassigning one removes its rule at the next reconcile; the default
+
-bridge is asserted for a runtime whose bridge is somewhere other than the old constant named; and the
+*How it is checked:* a bed converges a machine carrying containers on several networks, none of them
-guests of a declared network keep address and name service, per chain body.
+named in any setting, and each reaches outward afterwards — which fails against the previous behaviour,
+where the same flip cut them off, and is how it was written; a network made *after* the last declaration
+needs no new filter; a declared port is reachable from off the private network and an undeclared one is
+not; no address of a machine's own networks appears in a rendered filter, asserted on the text; and a
+machine reporting no outward link is refused in the control plane with its existing filter left alone.
 
 ## 5 — Certificates
 

04-ISSUES/141-the-forward-chain-does-not-follow-the-modules/00-report.md

@@ -61,6 +61,18 @@ module declares — a test bed's pool — which is a much smaller residue than t
 has to know the runtime's allocations to tell whether that line is sufficient. On the machine
 measured here it read as though nothing needed saying.
 
+## What was decided
+
+*2026-09-28, later the same day.* The answer is not a better list. The question in the first open
+item below — should the chain be derived from the networks the modules declare — was answered *no*,
+after a converged machine's rendered rules were read: the chain blocks everything passing through the
+machine and then allows its own guests back by listing their addresses. Every route to a correct list
+fails, because the mesh has no position on a container reaching outward in the first place. The filter
+now constrains what arrives from **outside** the machine and says nothing about what did not, and a
+machine says which of its links face outside — one reported fact instead of a list. See
+[ADR 0140](../../02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md), which
+supersedes both 0137 and the first attempt at answering this.
+
 ## Open questions
 
 - Should the forward chain be derived from the network resources the node's modules declare, with the