 <p>I feel your process(mosquitto) have hit the maximum number of open file descriptors limit. Check your max open files by <code>ulimit -n</code>. Then increase the limit to max number of connections expected by you. E.g. For 10k connection it would be <code>ulimit -n 10000</code></p> <p>A note on ulimit(<a href="http://ss64.com/bash/ulimit.html" rel="nofollow">1</a>). It is only set for the current terminal and for persistent changes you will need to edit config files as per your Linux flavor( <em>/etc/security/limits.conf</em> + <em>/etc/pam.d/common-session*</em> on Ubuntu ). </p>