ROS2 foxy: Can't discover nodes from other machines

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Hi,

I'm currently looking into transitioning a ROS1 setup to ROS2. However, I can't get two machines to reliably talk to each other.

Setup

My test setup consists of a laptop (192.168.2.105) running Ubuntu 20.04 + ROS2 foxy and a Raspberry Pi 4b 4GB (192.168.2.185), also running Ubuntu (Server 64bit) 20.04 + foxy. For both installs I followed this tutorial. As far as I can tell, everything went smoothly. The suggested example (talker + listener) run just fine locally. The systems are connected via wifi.

Problem

However, when I try to run the talker on the RPi and the listener on my laptop, I don't receive any output from the listener.

I run this to start the nodes:

RPi:

.bashrc:

source /opt/ros/foxy/setup.bash
source ~/dev_ws/install/setup.bash
export ROS_DOMAIN_ID=1
source /usr/share/colcon_cd/function/colcon_cd.sh
export _colcon_cd_root=~/ros2_install

ros2 run py_pubsub talker

Output:

[INFO] [1614045051.113840842] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [1614045051.546117451] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [1614045052.046084469] [minimal_publisher]: Publishing: "Hello World: 2"

Laptop

.bashrc:

source /opt/ros/foxy/setup.bash
source ~/dev_ws/install/setup.bash
export ROS_DOMAIN_ID=1
source /usr/share/colcon_cd/function/colcon_cd.sh
export _colcon_cd_root=~/ros2_install

ros2 run py_pubsub listener

Output:

No output, stuck in waiting for message...

Running ros2 node list shows only the node locally available. Running ros2 doctor gives the following print, that I would consider as OK (aside from the issue of this topic):

/opt/ros/foxy/lib/python3.8/site-packages/ros2doctor/api/package.py: 112: UserWarning: rqt_reconfigure has been updated to a new version. local: 1.0.6 < required: 1.0.7
/opt/ros/foxy/lib/python3.8/site-packages/ros2doctor/api/package.py: 124: UserWarning: Cannot find required versions of packages:  py_pubsub my_package
/opt/ros/foxy/lib/python3.8/site-packages/ros2doctor/api/topic.py: 56: UserWarning: Subscriber without publisher detected on /chatter.

All 4 checks passed

Possible solutions

This is apparently a common issue as there are many related questions. However, none of the suggested solutions solved my issue. Below I will list every suggestion I could find and tested myself as a possible help to others:

• Troubleshooting

Check multicast communication:

Machine 1: ros2 multicast receive

Machine 2: ros2 multicast send

When sending once, the receiver does nothing. When sending for another time, the receiver reliably gets the message. Both directions (RPi <-> laptop) behave the same. I have tried waiting for a while after sending the first message, but this does not seem to be an issue of high latency.

Configure firewall

sudo ufw allow in proto udp to 224.0.0.0/4

sudo ufw allow in proto udp from 224.0.0.0/4

ifconfig

Multicast is listed in both wifi adapters.

• Post1

Check the network by pinging IP:

ping 192.168.2.185

PING 192.168.2.185 (192.168.2.185) 56(84) bytes of data.    
64 bytes from 192.168.2.185: icmp_seq=1 ttl=64 ...