Publish to topic from file

Hello, I am quite new to ROS. I have a logfile (format: .pcapng) and I want to write a node which receives the logged udp-packets (as a simulation; later, the node should receive these packets from a CAN). My problem is the following: I know how to write a subscriber-node, but how can I make the file publishing the packets (as messages) to a topic? I have read something about ROS bags, but I am not sure whether they are the way to go. I would prefer some kind of programm with which I could play the file, so that I simply had to write the subscriber node. Thanks!