The Nessus user interface uses port 8834. If not already open, open port 8834 by consulting your firewall vendor's documentation for configuration instructions.
If your Nessus server is configured on a host with 3rd-party firewall such as ZoneAlarm or Windows firewall, you must configure it to allow connections from the IP addresses of the clients using Nessus.
Nessus and FirewallD
Nessus can be configured to work with FirewallD. When Nessus is installed on RHEL 7, CentOS 7, and Fedora 20+ systems using firewalld, firewalld can be configured with the Nessus service and Nessus port.
To open the ports required for Nessus, use the following commands:
>> firewall-cmd --permanent --add-service=nessus
>> firewall-cmd --reload