Description of problem:

mosquitto cannot accept more than 1014 clients on its default configuration.

When trying to register more, it'll complain:

1. journalctl -f -u mosquitto
   . . .
   Oct 06 15:17:12 F.Q.D.N mosquitto[47597]: 1665069432: Unable to accept new connection, system socket count has been exceeded. Try increasing "ulimit -n" or equivalent.

Version-Release number of selected component (if applicable):

Satellite 6.12.0 Snap 13

How reproducible:

Always

Steps to Reproduce:
1. Configure Satellite or capsule to use MQTT broker
2. Start registering systems against it
3. Wait until reaching 1015

Actual results:

Unable to accept new connection, system socket count has been exceeded. Try increasing "ulimit -n" or equivalent.

Expected results:

New client connected from $CLIENT_IP:$HIGH_PORT as $UUID (p2, c1, k30, u'$UUID').

Additional info:

I'm solving it with this:

cat /etc/systemd/system/mosquitto.service.d/limits.conf
[Service]
LimitNOFILE=10000

I've chosen 10000 because it's the "maximum number of clients" for a default Satellite installation according to the tuning guide.