How to connect paho-mqtt example project to a local mosquitto broker

I am running Riot natively on a Linux machine and most things work fine. However, when I run the example project paho-mqtt I couldn’t connect to my local mosquito broker. I can successfully ping -6 to the riot program from the Linux machine by specifying the ipv6 address and tap interface.
The mosquitto broker cannot receive messages sent from the riot project. Does anyone know how to resolve this?
Thanks in advance!

Have you an ULA or global IP on your interface? And if that is the case, have you connected Wireshark and checked what packets go over the interface?