Building an Ubuntu Router Part 5 - Dns Server

DNS server

Install bind9

sudo apt-get install bind9 -y

Edit file named.conf.options

sudo cp /etc/bind/named.conf.options{,.bak}
sudo nano /etc/bind/named.conf.options

These are the servers that BIND will ask to resolve domains when it doesn’t know the answer. We’ll just use Cloudflare’s public DNS servers:

forwarders {
    1.1.1.1;
    1.0.0.1;
};

Restart bind9

sudo systemctl restart bind9

Update service dhcp

If the installation was on the same computer, it is not necessary to update dhcp.

sudo nano /etc/dhcp/dhcpd.conf

Update the domain-servers that will assign the dhcp service.

option domain-name-servers 128.0.0.2;

In this exercise another machine of the same network will be the server and has the ip 128.0.0.2.

You can customize a domain for your router, printer or any device, on the internet you will find many manuals how to do it with bind9.

---

Suggestion

Reserve a static ip to the server dns in the same file dhcpd.conf.

host ServerDNS {
        hardware ethernet 08:00:27:D9:A9:1A;
        fixed-address 128.0.0.2;
}

---

Force use DNS

Case 1: Based on the design shown above.

All clients with the ip ending .3 up to.30 will be forced to use the 128.0.0.2 dns.

sudo iptables -t nat -A PREROUTING -i enp0s8 -p udp -m iprange --src-range 128.0.0.3-128.0.0.30 --dport 53 -j DNAT --to 128.0.0.2
sudo iptables -t nat -A PREROUTING -i enp0s8 -p tcp -m iprange --src-range 128.0.0.3-128.0.0.30 --dport 53 -j DNAT --to 128.0.0.2

Note:

If the user manually changes their dns (example 8.8.8.8), I will no longer have an Internet connection.

Case 2: If the design of the network was the following:

Run this command in server dhcp:

sudo iptables -t nat -A PREROUTING -i enp0s8 -p udp --dport 53 -j DNAT --to 192.168.0.3
sudo iptables -t nat -A PREROUTING -i enp0s8 -p tcp --dport 53 -j DNAT --to 192.168.0.3

Note:

If the user manually changes his dns (example 8.8.8.8), he will still have an internet connection, but he would still be going through our dns.

Case 3: If the installation was on the same computer, where the dhcp is installed.

Run this command in server dhcp:

sudo iptables -t nat -A PREROUTING -i enp0s8 -p udp --dport 53 -j DNAT --to 10.55.15.58
sudo iptables -t nat -A PREROUTING -i enp0s8 -p tcp --dport 53 -j DNAT --to 10.55.15.58

Note:

If the user manually changes his dns (example 8.8.8.8), he will still have an internet connection, but he would still be going through our dns.

Remember to save iptables changes

sudo dpkg-reconfigure iptables-persistent

You will need to answer yes to the questions.