Fast DDS (the default ROS2 middleware in Jazzy) uses UDP multicast to discover other nodes, but transfers data via shared memory (/dev/shm) when both nodes are on the same machine. This avoids serialisation overhead for high-bandwidth topics like point clouds.

Splitting ROS nodes into their own containers is a natural way to isolate subsystems: one container per sensor driver, one for localisation, etc. With --net host, discovery works fine across containers. But rootless Podman gives each container its own IPC namespace, which means its own /dev/shm. One container writes data to shared memory and tells another to read it, but that memory segment doesn’t exist in the other container’s namespace. The read silently fails: ros2 topic list shows everything, ros2 topic echo hangs forever.

The fix is to share the IPC namespace between containers using --ipc shareable on the first container and --ipc container:<name> on the rest:

podman run --net host --ipc shareable --name my-publisher ...
podman run --net host --ipc container:my-publisher --name my-subscriber ...

podman-compose version support

The Docker Compose equivalent is:

services:
  publisher:
    ipc: shareable
  subscriber:
    ipc: service:publisher

This works with docker compose. In podman-compose v1.5.0, the ipc directive appeared in podman-compose config output but was not passed through to podman run, as documented in issue #1292.

Upstream has since merged support for the service-level ipc option, including service:<name>. If your installed version does not include that change, use direct podman run commands as above or Docker Compose. Check the resulting namespaces rather than relying on the rendered Compose configuration alone.

Diagnosing the problem

If you’re not sure whether IPC namespaces are shared, compare the namespace IDs:

podman exec container-a ls /proc/self/ns/ipc -la
# ipc:[4026533574]
podman exec container-b ls /proc/self/ns/ipc -la
# ipc:[4026533663]  ← different = broken

Different IDs show that the containers have separate IPC namespaces. If Fast DDS is trying to use shared memory between them, that is the mismatch to investigate; it does not by itself rule out communication over a network transport.