Point domain to unmanaged VPS/Dedicated server or add any DNS record in Ubuntu/CentOS
To serve DNS records we need to install DNS server package like BIND9
Update Package manager “apt-get update” or CentOS “yum update”
Install BIND9
apt-get install bind9
or CentOS
yum install bind9
Configure and Create Zone File for domains to add records
open file name.conf.local and insert zone file name for domain
cd /etc/bind/
vi name.conf.local
zone "abc.com" {
        type master;
        allow-transfer {none;};
        file "/etc/bind/pri.abc.com";
};
zone "zyx.com" {
        type master;
        allow-transfer {none;};
        file "/etc/bind/pri.xyz.com";
};
save and close file.
Create Zone File for domain
touch pri.abc.com touch pri.xyz.com
To create name server ns1.abc.com and ns2.abc.com. added NS and A records. replace 000.000.000.000 with server IP
vi pre.abc.com
$TTL        3600
@       IN      SOA     host.abc.com. info.abc.com. (
                        2015062803       ; serial, todays date + todays serial #
                        7200              ; refresh, seconds
                        540              ; retry, seconds
                        604800              ; expire, seconds
                        86400 )            ; minimum, seconds
;
abc.com. 3600 A        000.000.000.000
mail 3600 A        000.000.000.000
sv4 86400 A        000.000.000.000
sv4 86400 A        000.000.000.000
www 3600 A        000.000.000.000
abc.com. 3600      MX    10   mail.abc.com.
abc.com. 3600      NS        ns1.abc.com.
abc.com. 3600      NS        ns2.abc.com.
Now add ns1.abc.com and ns2.abc.com as child nameserver in domain register panel and also add in nameserver address in domain register.
To add another domain to same vps
vi pri.xyz.com
$TTL        3601
@       IN      SOA     host.abc.com. info.abc.com. (
                        2015062813       ; serial, todays date + todays serial #
                        7200              ; refresh, seconds
                        540              ; retry, seconds
                        604800              ; expire, seconds
                        86400 )            ; minimum, seconds
;
mail 3600 A        000.000.000.000
xyz.com. 3600 A        000.000.000.000
www 3600 A        000.000.000.000
xyz.com. 3600      NS        ns1.abc.com.
xyz.com. 3600      NS        ns1.abc.com.
service bind9 restart