Get the error 'Couldn't resolve host 'gitlab.com'' when the firewall is on

Hi, I just configured a runner on my own server. That server have a firewall running. When the firewall is on, the runner fails throwing the following error :

fatal: unable to access 'https://gitlab-ci-token:xxxxxxxxxxxxxxxxxxxx@gitlab.com/leafycode/gitbook-test.git/': Couldn't resolve host 'gitlab.com'

But already the firewall allow outgoing and incoming on port 80 and 443 (Also allow outgoing only for 9418). When the firewall is off the runner works without error. Any idea what's happening?