25 Domain Name System Part 3 – Dynamic DNS

In Chapter 23, we manually configured our DNS entries with the IP addresses that were statically assigned to our client devices. However, it is uncommon to use static IP assignments, especially in networks with hundreds or even thousands of clients! If we can get our DHCP and ADNS servers to communicate with each other, then IPs can automatically be assigned and our zone files can automatically be populated. This practice is called Dynamic DNS (DDNS).

This lab will expand on the work in Chapter 23 and Chapter 24 to learn how to configure and manage DDNS in a small network environment.

Estimated time for completion: 45 minutes

learning objectives

• Learn how DDNS works on an enterprise network
• Learn how to work with multiple services (DHCP and DNS) on one network

prerequisites

• Chapter 21 – DHCP Relay
• Chapter 24 – Domain Name System Part 2

deliverables

• Four (4) screenshots
• BLUE1 (PC1) successfully pinging BLUE2 (PC2) by name
• BLUE2 (PC2) successfully pinging BLUE3 (PC3) by name
• BLUE1 (PC1) successfully performing a DNS query and resolution with the NS1 server via Wireshark monitoring
• Live stream of NS1 reports of the DHCP handshake and zone logs when (BLUE3) PC3 is rebooted

resources

• BIND9 Administrator Reference Manual – https://bind9.readthedocs.io/en/v9.18.16/
• MikroTik RouterOS Documentation – https://help.mikrotik.com/docs/display/ROS/RouterOS
• Archy’s Blog – Dynamic DNS with BIND and ISC-DHCP – https://archyslife.blogspot.com/2018/02/dynamic-dns-with-bind-and-isc-dhcp.html
• Configuring Dynamic DNS with BIND9 – https://doncrawley.com/soundtraining.net/files/configuringdynamicdnswithbind9.pdf

Contributors and testers

Mathew J. Heath Van Horn, PhD, ERAU-Prescott
Kyle Wheaton, Cybersecurity Student, ERAU-Prescott

Phase I – Building the Network Topology

This lab is an extension of the previous two chapters. If you have not completed them yet, it is recommended that you do so first before continuing. By the end your network should look like the following:

1. Preliminary step in VirtualBox
   1. Create/clone a fresh copy of the Linux Server VM with ISC’s DHCP server installed
2. Start GNS3
   1. Save the previous lab as a new project: LAB_11
3. Reuse the network that you built in the previous chapter
   1. Configure the router to act as a DHCP relay for the Blue subnet (Chapter 21, Phase II, Step 1)
   2. Add a DHCP server to the Red network – Ubuntu server (DHCP1)

      NOTE: In reality, both the DHCP and DNS services can operate from the same machine; however, we are splitting them into two separate devices for clarity.

   3. Add a switch to the Red network – Ethernet switch
   4. Connect NS1 and DHCP1 to the switch so that they are on the same LAN
   5. Remove the static IP addresses from Tiny Core Linux clients by commenting out the rules for static IP assignment

      > vi /opt/bootlocal.sh

       

      

      NOTE: Make sure that each client device keeps its unique hostnames!

4. Label and organize your network as necessary

1. Start NS1 and login
2. Generate a new Remote Name Daemon Control (RNDC) key to use to communicate with DHCP1

   NOTE: Ensure that you have the bind9-utils package installed!

   > rndc-confgen -a -b 512

   Switch	Description
   -a	Automatically generate the RNDC file in the /etc/bind directory.
   -b	Set the size of the key as a value between 1 and 512 bits.

3. Modify the file permissions of rndc[.]key
   1. Change the owner and group from root to the bind user

      > chown bind:bind /etc/bind/rndc.key

   2. Change the file permissions to give read access for the group

      > chmod 640 /etc/bind/rndc.key

4. Modify the named[.]conf[.]local file

   > vi /etc/bind/named.conf.local

   1. At the beginning of the file, before the zone declarations, include the RNDC key

      include “/etc/bind/rndc.key”;

       

      

   2. Within each zone declaration, add the following directive to allow the zones to be updated with the key

      allow-update { key rndc-key; };

       

      

   3. Save and exit the editor
   4. Verify the configuration settings

      > named-checkconf

5. Add a new static entry in the red[.]net zone files for DHCP1
   1. Modify the forward lookup zone file

      > vi /var/lib/bind/db.red.net

       

      

   2. Modify the reverse lookup zone file

      > vi /var/lib/bind/db.red.net.rev

       

      

6. Remove all client static entries in the blue[.]net zone files
   1. Modify the forward lookup zone file

      > vi /var/lib/bind/db.blue.net

       

      

   2. Modify the reverse lookup zone file

      > vi /var/lib/bind/db.blue.net.rev

       

      

7. Restart the DNS server and verify that it is running

   > systemctl restart named.service

   > systemctl status named.service

   NOTE: Errors at this stage are likely due to incorrect permissions on the RNDC file. Ensure that both the owner (bind) and group (bind) can read the file.

8. Start and enable Systemd’s time synchronization daemon

   > systemctl start systemd-timesyncd.service

   > systemctl enable systemd-timesyncd.service

1. Start DHCP1 and login – Refer to (Chapter 23, Phase III, Step 3)
   1. Modify the machine’s hostname to dhcp1
      

      > hostnamectl hostname dhcp1

   2. Assign a static IP address, default gateway, primary DNS server, and local domain name

      NOTE: This example uses the following information:
      – IP Address: 89.30.80.34/29
      – Local Domain: red.net
      – Nameservers: 89.30.80.35
      – Gateway: 89.30.80.33

      1. Verify that the name server information is correctly pointing towards ns1 (89.30.80.35) and that the current domain is red[.]net
         

         > resolvectl status

         

2. Having all elements in a network synchronized to the same time is critical.  SNTP clients provide this service.  Linux uses Systemd’s timesync daemon to provide SNTP packets.  Start it and enable (start on boot) the service by typing

   > systemctl start systemd-timesyncd.service

   > systemctl enable systemd-timesyncd.service

   1. Transfer the RNDC key file from NS1 to the primary user’s home directory in DHCP1

      NOTE: The default user on my machines is iako; be sure to change the following commands as necessary.

      1. Login to the NS1 terminal

         > ssh iako@89.30.80.35

         NOTE: You should see the hostname change to ns1 when you login successfully.

      2. Switch to the root user

         > sudo su

      3. Use the Secure Copy (SCP) command to make a copy of rndc[.]key to the primary user of DHCP1

         > scp /etc/bind/rndc.key iako@89.30.80.34:/home/iako

      4. Exit from root

         > exit

      5. Exit the SSH session

         > exit

      6. Verify you see the RNDC key in your home directory

         > ls -l ~

   2. Modify the permissions for the RNDC key file
      1. Ensure that the owner and group are both root
         

         > chown root:root ~/rndc.key

      2. Change the permissions to remove read access from others

         > chmod 640 ~/rndc.key

      3. Move the file to the DHCP configuration directory

         > mv ~/rndc.key /etc/dhcp/ddns-keys

         NOTE: If the ddns-keys folder does not exist, make it with the following permissions:

         > mkdir /etc/dhcp/ddns-keys

         > chown root:dhcpd /etc/dhcp/ddns-keys

         > chmod 710 /etc/dhcp/ddns-keys

   3. Modify the DHCP daemon configuration file to support both subnets and their domains

      > vi /etc/dhcp/dhcpd.conf

      1. Include global parameters that apply to all subnets, including the new RNDC key file
         

         

      2. Add the forward lookup zones for the subnets included in our BIND9 server
         

         

      3. Add the reverse lookup zones for the subnets included in our BIND9 server
         

         

      4. Add the DHCP subnet directives for both the Red and Blue networks
         

         

   4. Restart the DHCP service
   5. Ensure that each client in blue[.]net is able to receive an IP address and that its domain information is correct
      1. Login to the terminal of PC1
      2. Verify it was assigned an IP address

         > ifconfig

         

      3. Verify its DNS information is correct

         > cat /etc/resolv.conf

         

      4. Repeat for the other two client devices
   6. Troubleshoot as necessary before proceeding to the next section

1. Start a Wireshark capture between NS1 and the switch
2. Test the dynamic DNS updates
   1. In Wireshark, filter for DNS
      

      

   2. In the NS1 terminal, start monitoring the system log

      > tail -f /var/log/syslog

       

      

   3. Reboot PC1 and watch the logs for the DHCP handshake and zone file mapping
      

      

   4. In the Wireshark window, you should see *DNS Update* packets
      

      

   5. Repeat for the other two PC clients

      NOTE: You can request new IP address leases in Tiny Core with the following command:

      > sudo udhcpc

3. After about 15 minutes, BIND9 will populate the db files with the updated host/IP records

   NOTE: By default, this information is stored in a journal file (.JNL) which is located in the same directory as the zone files. The main database files are not frequently updated to increase efficiency.

End of Lab