Merge pull request 'Issue 197: a physical link that is down is not filtered when it comes up' (#280) from jschoubben/every-physical-link-faces-outside into main

2026-10-02 10:22:40 +00:00
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+---
+status: resolved
+opened: 2026-10-02
+located-in: [mesh-host internal/outward (Links reported only the links carrying a default route)]
+fixed-by: mesh-host PR 66 (a link backed by a physical device is named outward, up or down), live 2026-10-02
+amended-design: []
+---
+
+# 197 — A physical link that is down is not filtered when it comes up
+
+## What was observed
+
+A sweep of every machine's filter on 2026-10-02. A laptop-class machine connected by its radio has a
+wired port that was unplugged. Its filter guarded the radio and the tunnel, and accepted everything
+arriving on any other link:
+
+```
+iifname != { "mesh0", "<radio>" } accept
+```
+
+The wired port was not in the list. Plugged in, everything arriving on it would have been accepted,
+every port of the machine open to whatever network the cable reached. That would last until the
+machine reported again and was pushed a new filter.
+
+## Why it matters
+
+**The filter's one rule about links fails open.** [ADR 0140](../../02-DECISIONS/0140-the-filter-constrains-what-arrives-from-outside.md)
+has the filter constrain what arrives from outside, and has the machine say which links face outside.
+Everything not named is treated as the machine's own, its containers and bridges. So a link the machine
+fails to name is not filtered at all. The host named only the links carrying a default route at the
+moment it reported. A cable plugged in later is the ordinary case for a laptop. A second wired network
+that never carries the default route, such as a direct link to a storage box, is never named at all.
+
+## Open questions
+
+- A virtual link that faces outside (a VPN client's interface, a USB tether that appears as a virtual
+  device) has no physical device behind it. It is named only while it carries the default route. Is
+  that enough?
+
+## Resolved (2026-10-02)
+
+Live on the affected machine after the host was delivered and one more push: its filter now guards the
+radio, the tunnel and the unplugged wired port, before anything is plugged into it.
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+# Diagnosis
+
+*2026-10-02.*
+
+**Located in `mesh-host` `internal/outward`.** `Links` read the kernel's routing tables and returned the
+interfaces carrying a default route. An unplugged port carries none, so it was never reported, and the
+controller rendered the filter around the links it was given.
+
+**The fix.** A link faces outside if it carries a default route **or** has a physical device behind it.
+The kernel lists every interface under `/sys/class/net`, with a `device` entry for one backed by
+hardware. A bridge, a veth, the tunnel and the loopback have none, so they stay the machine's own. The
+wired port is now reported up or down, and the filter guards it before anything is plugged in. Tested
+with a radio carrying the default route and an unplugged wired port beside a bridge, a veth, the docker
+bridge, the tunnel and the loopback: the two physical links are reported, nothing else.