Count or list active client connections to mosquitto server

Question

I just want to know how many clients are actively connected to my mosquitto server. Or even better, get a list of client ids connected to my mosquitto server. I read some documentation suggesting the topic $SYS/broker/clients/connected will give this information. But this command yielded no response and no results:

mosquitto_sub -h myserver.myserver.myserver -p 9500 -t $SYS/broker/clients/connected -u "my-user" -P "my-password" --capath /etc/ssl/certs/

(I replaced myserver.myserver.myserver and my-user and my-password with actual values.) I verified the connection is working because if I publish a message to the same topic, the message appears.

How can I get a list of clients with active connection to my mosquitto server? Or at least a numeric count of active connections?

Answer 1

Bah I figured it out. I need to put single quotes around the topic so that $SYS isn't interpreted as a variable. So like this:

mosquitto_sub -h myserver.myserver.myserver -p 9500 -t '$SYS/broker/clients/connected' -u "my-user" -P "my-password" --capath /etc/ssl/certs/

Answer 2

This seems to work:

netstat -ntp | grep ESTABLISHED.*mosquitto

Which in my case outputs:

tcp        0      0 10.42.0.2:1883          10.42.0.18:56553        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.19:54037        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.11:49321        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.15:48685        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.12:57691        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.13:56037        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.17:40679        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.16:39627        ESTABLISHED 448/mosquitto       
tcp        0      0 10.42.0.2:1883          10.42.0.14:33079        ESTABLISHED 448/mosquitto       

If one only cares about the total count:

netstat -natp | grep ESTABLISHED.*mosquitto | wc -l

prints

9

netstat arguments: -n to avoid getting host names for ip addresses (faster), -t for TCP instead of UDP and -p to display program names so we can filter using grep.