Please follow the steps below
##### Set Hostname & FQDN Permanently #####
1. Change Hostname Temporary:
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# show current hostname
[root@server1 ~]# hostname
server1.datahead.biz
# change hostname
[root@server1 ~]# hostname server1.datahead.biz
[root@localhost ~]# hostname
server1.datahead.biz
2. Change Hostname Permanently:
-------------------------------
[root@server1 ~]# hostnamectl set-hostname server1.datahead.biz
[root@server1 ~]# hostnamectl
Static hostname: server1.datahead.biz
Icon name: computer-vm
Chassis: vm
Machine ID: 04ca692cd07d41698530705c118b9b0d
Boot ID: 1e75261cdccc4fd2a412e9d95cbe5a25
Virtualization: xen
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-693.21.1.el7.x86_64
Architecture: x86-64
[root@server1 ~]# vi /etc/hostname
server1.datahead.biz
:x
[root@server1 ~]# vi /etc/hosts
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
103.56.209.100 server1.datahead.biz server1
:x
[root@server1 ~]# vi /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=server1.datahead.biz
OR
# Created by anaconda
HOSTNAME="server1.datahead.biz"
:x
rDNS is another issues , fix your hostname first , if you need help regarding dns , please reply .