How to set up DHCP Dynamic DNS on Ubuntu

Jan 17, 2022

· 3 mins read

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#### In the video below, we show you how to configure Dynamic DNS (DDNS) in Linux using Ubuntu 20.04 LTS with Bind9 and ISC DHCP

We’ll create a key for our DHCP server to authenticate with, configure the DNS server to accept DDNS updates from the DHCP server and the DHCP server to send DDNS updates to the DNS server

Configuration example:

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Create Key File

Create a key file to keep the password separate from the main file

On the DNS server, switch to the bind folder

cd /etc/bind

Then create our key by running the following command

ddns-confgen -k dhcp1.templab.lan

Copy the key example and modify to suit, e.g.

key "dhcp1.templab.lan" {  
	algorithm hmac-sha256;  
	secret "/mAXOLTQUp8V9XzYnw88dkOkiDXBU6SNv/jEL3IgKVE=";  
};

I used the FQDN of the DHCP server to name this key, but it’s up to yourself as to what reference you want to use

Create a key file, paste the contents in and save this file

sudo nano dhcp1.key

Change the ownership if necessary

sudo chown root:bind dhcp1.key

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Move Zone Files

The bind server needs to create new files and modify the zone files when updates are received

For this reason, any zones requiring dynamic updates need to be moved to /var/lib/bind/

sudo mv db.templab.lan /var/lib/bind/
sudo mv db.172.16 /var/lib/bind/

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Update DNS Configuration

The DNS server configuration needs to be updated as the zone files have been moved

It needs to know where to find the key, where to find the zone files we’ve moved and be configured to allow updates from the DHCP server

First, make a backup copy of the file

sudo cp named.conf.local named.conf.local.old
sudo nano named.conf.local

And then apply our changes

include "/etc/bind/dhcp1.key";  
zone "templab.lan" {
	type master;
	file "/var/lib/bind/db.templab.lan";
	update-policy {
	grant dhcp1.templab.lan wildcard *.templab.lan A DHCID;
	};
};
zone "17.16.172.in-addr.arpa" {
	type master;
	file "/var/lib/bind/db.172.16";
	update-policy {
	grant dhcp1.templab.lan wildcard *.16.172.in-addr.arpa PTR;
	};
};

The update policies above allows a computer with the key to change host records of any name in the forward lookup zone, but only if these are type A or DHCID records

This is possible because we used the wildcard option

It can also update the reverse lookup zone, but only if these are PTR records

Check the DNS server configuration syntax

sudo named-checkconf

Then restart and check the bind9 status

sudo systemctl restart bind9
sudo systemctl status bind9

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Update DHCP Configuration

The DHCP server needs to know the key so we’ll create a new file and copy the key we created on the DNS server

cd /etc/dhcp  
mkdir ddns-keys  
sudo nano ddns-keys/dhcp1.key

It also needs updating to support DDNS, to tell it where to find the key, to enable DDNS using the standard style and also which zones to update, what the primary DNS server is and what key to use

First, make a backup copy of the file

sudo cp dhcpd.conf dhcpd.conf.old
sudo nano dhcpd.conf

And then apply our changes

include "/etc/dhcp/ddns-keys/dhcp1.key";
ddns-updates on;
ddns-update-style standard;
zone templab.lan. {
	primary 172.16.17.10;
	key dhcp1.templab.lan;
}
zone 16.172.in-addr.arpa. {
	primary 172.16.17.10;
	key dhcp1.templab.lan;
}

After saving the changes, restart and check the DHCP server status

sudo systemctl restart isc-dhcp-server
sudo systemctl status isc-dhcp-server

DNS should now be updated when IP addresses are leased or released

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Maintenance

Pause DDNS before making static DNS changes

sudo rndc freeze

Apply your changes, increment the serial number then resume

sudo rndc thaw

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Troubleshooting

If host entries aren’t being updated monitor syslog on both servers

sudo tail -f /var/log/syslog

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