[Bug] MicroXRCE Agent fails to connect

[Apoorv-1009]

Describe the bug

Me and my friend are using the PX4 autopilot with ROS2 for a project, and initially we were getting each others topics when running ros2 topic list, so we then added ROS_LOCALHOST_ONLY=1 in our zshrc files. But recently, our MicroXRCE agent is giving small issues:
Running make px4_sitl gazebo-classic, the MicroXRCE agent connects, but running ros2 topic list does not give any topics
Running ./Tools/simulation/gazebo-classic/sitl_multiple_run.sh -m iris -n 1, the MicroXRCE agent simply does not connect, and so we dont get topics
May I know any possible reason for this problem? Thanks

To Reproduce

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Expected behavior

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Screenshot / Media

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Flight Log

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Software Version

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Flight controller

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Vehicle type

Multicopter

How are the different components wired up (including port information)

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Additional context

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[DominicMarti]

Hi @Apoorv-1009

Have you tried to start the agent independently of ROS2 interface to isolate the error sources? You can start the agent with

MicroXRCEAgent udp4 -p 8888

And in another terminal start your simulation: make px4_sitl gazebo-classic

Then you should see if the publisher and subscribers are set up properly in the agent terminal. If that works, then there might be an issue with the localhost flag.

Side Note: We had similar issues on 1.14 and corresponding MicroXRCE Agent versions, but not in Gazebo. We found the problem to be that the pixhawk would not start sending messages through the interface until it itself received a message. One way you can try to circumvent this is by sending heart beats from ROS2 through the interface on startup for a couple of seconds, just to make sure the pixhawk receives messages and starts sending as well.

Cheers!