Here is what I have run for verification.
graylog2v2:~$ sudo systemctl status graylog-server
● graylog-server.service - Graylog server
Loaded: loaded (/usr/lib/systemd/system/graylog-server.service; enabled; vendor preset: enabled)
Active: active (running) since Wed 2018-08-29 10:40:45 EDT; 4min 52s ago
Docs: http://docs.graylog.org/
Main PID: 1199 (graylog-server)
Tasks: 30 (limit: 4915)
CGroup: /system.slice/graylog-server.service
├─1199 /bin/sh /usr/share/graylog-server/bin/graylog-server
└─1207 /usr/bin/java -Xms4g -Xmx4g -XX:NewRatio=1 -server -XX:+ResizeTLAB -XX:+UseConcMarkSweepGC -XX:+CMSConcurrentMTEnabled -XX:+CMSCl
graylog2v2:~$ curl -i -H 'Accept: application/json' http://graylog2.corp.com
curl: (7) Failed to connect to graylog2.corp.com port 80: Connection refused
graylog2v2:~$ curl -i -H 'Accept: application/json' http://graylog2.corp.com:12900
curl: (7) Failed to connect to graylog2.corp.com port 12900: Connection refused
graylog2v2:~$ curl -i -H 'Accept: application/json' http://graylog2.corp.com:12900/
curl: (7) Failed to connect to graylog2.corp.com port 12900: Connection refused
graylog2v2:~$ curl -i http://graylog2.corp.com:9000/
curl: (7) Failed to connect to graylog2.corp.com port 9000: Connection refused
graylog2v2:~$ ^C