I would like to know if it is possible in a standard MQTT bridge configuration with Mosquitto the dynamic subscription of topics from the MQTT clients to the remote broker.
That is, only if a client requests the subscription to a certain topic, it is Mosquitto who requests to the remote broker the subscription to that topic.
From what I have seen in the configuration it is possible to set it statically when starting the Mosquitto service but not dynamically according to the requirements of the MQTT clients who can decide at runtime to which topics they want to subscribe.
Is it possible in a Mosquitto bridge, a dynamic subscription scenario in which the subscription request of the clients is forwarded to the remote broker?
It is not possible, the broker does not support dynamically changing the bridge subscription.
It's also doing dynamic subscription forwarding won't work well in a situation where usernames/password and ACLs are implemented because the bridge connection acts as any other client connection. This means that the bridge connection can only subscribe/publish to topics allowed by the user assigned to the bridge.
Both brokers are likely to have very different sets of users and ACLs in place