my machine doesn't seems to accept incoming DNS service. I think it's the Redhat8.0 firewall. Is there a command to open the DNS port?

Small misconception, np.
A port is opened when an application requests a network connection.
So, to have UPD/53 "opened" you would need for instance to start named, the DNS server. You don't want that cuz you don't want to provide DNS records to other ppl.
What you want is some fw rules:

# Allow UDP packets in for DNS client from nameservers.
iptables -A INPUT -i $IFACE -p udp -s $NAMESERVER_1 --sport 53 -m state --state ESTABLISHED -j ACCEPT
iptables -A INPUT -i $IFACE -p udp -s $NAMESERVER_2 --sport 53 -m state --state ESTABLISHED -j ACCEPT
# Allow UDP packets to DNS servers from client.
iptables -A OUTPUT -o $IFACE -p udp -d $NAMESERVER_1 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o $IFACE -p udp -d $NAMESERVER_2 --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT

Where $IFACE is the network device with the public IP address on it, and $NAMESERVER_# are your ISP's namserver IP addresses.

I just reinstall my system and make sure the firewall accept DNS port. All of the named is working now. Thanks for ur reply though.

You certainly haven't gone short way.
a simple command like
iptables -L
iptables -L -t nat
can show you what's going on with your firewall
besides there is a gui program to manage your firewall