hostname problem

I have this problem:
Your Hostname is: localhost and it resolves to IP: (ERROR: You don't have a valid hostname set!)

To what and where i must change it?

And how to fix "rDNS/PTR = FAILED, check with your hosting provider!"?

Please follow the steps below

#####   Set Hostname & FQDN Permanently     #####

1. Change Hostname Temporary:
-----------------------------

# 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 .

What must be the hostname.
Did i need to make subdomain and then make it hostname or ?

I pointed out my hostname in my domain registrar, didnt create any subdomains.

What must be the hostname.
Did i need to make subdomain and then make it hostname or ?

A hostname must be a fully qualified domain name, which is your [servername].[domain].[tld] for example if you have a website called mywidgets.com and you want your server to be called shirley then your hostname would be:

shirley.mywidgets.com

The rDNS issue is probably caused by you not having a proper hostname.  Once your proper hostname has been set in linux then you CWP will need to be updated to include this and the rDNS error should go away within 4 - 24 hours.

this is my problem too, hostname problem same with this post and another problem is dns not resolve.
i am already set permanent hostname. but still not fixed.

I make subdomain and after change hostname with that subdomain that i make it, apache cant start and shows this error:
May 29 14:25:32 salespc apachectl: AH00558: httpd: Could not reliably determine the server's fully qualified domain name, using

subdomain.domain.net. Set the 'ServerName' directive globally to suppress this message
May 29 14:25:32 salespc apachectl: (98)Address already in use: AH00072: make_sock: could not bind to address [::]:8181
May 29 14:25:32 salespc apachectl: (98)Address already in use: AH00072: make_sock: could not bind to address 0.0.0.0:8181
May 29 14:25:32 salespc apachectl: no listening sockets available, shutting down
May 29 14:25:32 salespc apachectl: AH00015: Unable to open logs
May 29 14:25:32 salespc systemd: httpd.service: control process exited, code=exited status=1
May 29 14:25:32 salespc systemd: Failed to start Web server Apache.
May 29 14:25:32 salespc systemd: Unit httpd.service entered failed state.
May 29 14:25:32 salespc systemd: httpd.service failed.
May 29 14:25:34 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
29 14:27:46 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:27:47 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:70:f3:95:1a:e5:53:08:00 SRC=192.168.3.202 DST=255.255.255.255 LEN=29 TOS=0x00 PREC=0x00 TTL=128 ID=26531 PROTO=UDP SPT=35623 DPT=35622 LEN=9
May 29 14:27:48 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:27:56 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:27:58 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:27:59 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=01:00:5e:00:00:fb:d8:cb:8a:da:79:e7:08:00 SRC=192.168.3.200 DST=224.0.0.251 LEN=56 TOS=0x00 PREC=0x00 TTL=1 ID=304 PROTO=UDP SPT=5353 DPT=5353 LEN=36
May 29 14:28:07 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:08 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:18 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:18 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:28 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:29 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:38 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:70:f3:95:1a:e5:53:08:00 SRC=192.168.3.202 DST=255.255.255.255 LEN=29 TOS=0x00 PREC=0x00 TTL=128 ID=26548 PROTO=UDP SPT=35623 DPT=35622 LEN=9
May 29 14:28:38 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:61:06:36:08:00 SRC=192.168.3.98 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472
May 29 14:28:39 salespc kernel: Firewall: *UDP_IN Blocked* IN=ens33 OUT= MAC=ff:ff:ff:ff:ff:ff:00:da:7f:5f:06:36:08:00 SRC=192.168.3.95 DST=255.255.255.255 LEN=492 TOS=0x00 PREC=0x00 TTL=64 ID=0 DF PROTO=UDP SPT=3030 DPT=3001 LEN=472