auto_id_prefix behaves inconsistently based on host port mapping in Docker

[hn05012]

I am experiencing an inconsistency in the behavior of the auto_id_prefix configuration. When running Mosquitto in Docker, the prefix is applied to server-assigned client IDs only when the host port mapped to the container's port 1883 is not 1883. If the host port is 1883, the client IDs are prefixed with auto- instead of the specified prefix.

Steps to reproduce:

1. Define a docker compose file:

services:
mosquitto-1:
image: eclipse-mosquitto:latest
ports:
- 1883:1883
- 8883:8883
- 9001:9001
volumes:
- ./mosquitto/mosquitto_1/mosquitto_1.conf:/mosquitto/config/mosquitto.conf

mosquitto-2:
image: eclipse-mosquitto:latest
ports:
- 1884:1883
- 8884:8883
- 9002:9001
volumes:
- ./mosquitto/mosquitto_2/mosquitto_2.conf:/mosquitto/config/mosquitto.conf

mosquitto-3:
image: eclipse-mosquitto:latest
ports:
- 1885:1883
- 8885:8883
- 9003:9001
volumes:
- ./mosquitto/mosquitto_3/mosquitto_3.conf:/mosquitto/config/mosquitto.conf

2. configure the brokers:

listener 1883
auto_id_prefix
allow_anonymous true

3. Start the containers

4. Connect a client with empty client id:

• Host port 1883
• Host port 1884
• Host port 1885

Expected Behavior:
The auto_id_prefix should consistently apply the specified prefix regardless of the host port mapping.

Actual Behavior:
When the host port is mapped to 1883, the prefix defaults to auto-