2026-09-14 12:59:31 +00:00
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 status: open
 opened: 2026-09-14
 located-in: []
 fixed-by:
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 # 047 — The firewall does not cover published container ports
 ## Symptom
 A machine running the mesh's firewall drops what it should drop and leaves every container port
 published on all addresses open to anything that can reach the machine.
 Rehearsed in the lab on three machines, one probing another, with a single change between the two
 measurements:
 | | ordinary port on the host | container port published on `0.0.0.0` |
 |---|---|---|
 | before the mesh's rules | reachable | reachable |
 | after the mesh's rules | **refused** | **still reachable** |
 Same prober, same ports, same second. The first column is the firewall working. The second is the
 question.
 ## Why this is the way it is
 It is deliberate, and the reasoning is written where the rules are generated: the mesh's table hooks
 `input` and `output` and **no forward chain**, because what a machine forwards is the container
 runtime's business — the runtime writes rules for the bridges it creates, and a second table
 dropping in `forward` would drop container traffic the runtime explicitly allowed, stopping every
 container on the node including the control plane.
 That reasoning is sound. The conclusion drawn from it is not the only one available, because traffic
 to a published port does not traverse `input` at all: it is redirected and then forwarded. So a
 default-drop `input` chain says nothing whatsoever about it, and the firewall is silent on the ports
 most likely to matter.
 ## Why it matters, measured rather than argued
 The anchor this mesh will be installed onto publishes **38 distinct container ports on all
 addresses** — a document store, three cache sentinels, mail transfer and retrieval, the forge's own
 SSH, and thirty more.
 Today those are filtered, by the system being replaced, which does it with rules that hook exactly
 the path this one leaves alone. So the cutover from the old firewall to this one **opens all
 thirty-eight at once, silently, on a machine with a public address**, while every check reports a
 firewall that is up and dropping by default.
 That is the worst shape a security fault can take: the mechanism is present, it reports success, and
 the thing it is believed to be doing is not the thing it does.
 ## How it would be checked
 Two probes from another machine, against a port on the host and a published container port, before
 and after the rules are loaded. Both must change together. The lab can do this — it is how the table
 above was produced — and nothing does it today.
 ## Open questions
 - Can the mesh filter forwarded traffic without owning the whole `forward` chain? A table that
  accepts by default and drops only what the mesh knows about would not stop the runtime's traffic,
  and hooking `forward` at all is the thing the current comment rejects.
 - Should a module's declared reach apply to its published ports? A module that says it listens to
  the mesh only, and is then reachable from everywhere because its port is published, is not doing
  what its manifest says.
 - Is the answer instead that the mesh should not publish on all addresses? A port bound to the
  private network needs no filtering, and most of these want exactly that.