Failed to start firewalld.service: Unit is masked [How to Solve]

Centos7 failed to start firewall: Failed to start firewalld.service: Unit is masked.

Solution:

The error “Failed to start firewalld.service: Unit is masked.” appears when you run “systemctl start firewalld”. The service needs to be unlocked:

systemctl unmask firewalld.service
systemctl start firewalld

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