---
name: podman-netns-addressability-probe
description: "Runbook to answer \"can container X actually reach service Y\" on a rootless-podman/netavark host, instead of assuming localhost works everywhere; includes the probe commands and the false-positive traps."
---

# Podman netns addressability probe

Answer **"can this container actually dial that service?"** per *network namespace*,
not per host. On rootless podman + netavark, a bridge-net container cannot reach
host-netns loopback listeners at all, so `localhost:PORT` reasoning is wrong for it.

## Decision rule
- `NetworkMode=host` → the container **is** the host netns: dials everything on
  `127.0.0.1`, binds any free port.
- bridge/named network → own netns. Only **published** ports are reachable, via the
  network's **gateway IP**. Unpublished host listeners are **refused instantly (RST)**,
  not routed — even when the listener is bound `0.0.0.0`.

## Probe sequence (all read-only, run on the host)
1. Namespace: `podman inspect <name> --format '{{.HostConfig.NetworkMode}}'`
2. Owner of a listener: `sudo ss -ltnp 'sport = :<port>'` (host netns only — this is
   why a `0.0.0.0` listener can still be unreachable from a bridge net).
3. From inside the suspect container:
   `podman exec <name> cat /proc/net/route` → default route = gateway IP to dial.
4. Probe the candidate address **inside that netns**:
   `podman exec <name> sh -c 'timeout 2 bash -c "exec 3<>/dev/tcp/<gw-ip>/<port>"; echo rc=$?'`

## Interpretation (do not blur these)
- **instant refusal** = nothing listening in a reachable namespace (or rootlessport
  rejected an unpublished port map).
- **timeout** = no route to that address at all. `host.containers.internal`
  (`169.254.1.2`) is an alias on the **default `podman` net only**; containers on a
  named network time out against it rather than being refused.

## Probe-tool gotchas verified on this host (podman 5.x, netavark)
- `podman network inspect <net> --format '{{json .containers}}'` **errors**
  (`can't evaluate field containers in type interface {}`) — membership is not exposed
  there. List from the container side instead:
  `podman ps -a --format '{{.Names}} {{.Networks}}'`
- An **empty** `Networks` field in that output = host network or a shared namespace
  (`--network=container:<other>`), not "no network".
- Quadlet `Network=<name>.network` yields runtime network name **`systemd-<name>`**;
  probe with `podman network ls`, never by the `.network` filename.

## Fix options when a bridge-net container must reach a host-netns listener
(a) move it to `Network=host`, or (b) publish the port — (b) makes that port reachable
from every namespace **and the LAN**, so it is an exposure change needing explicit
operator approval, never a silent fix.

Repo source of truth for this fleet: `serverAppConfigs-docs/Architecture/Container
Network Namespaces.md` (commit `2e179b0`).