Chapter 2. Setting up an unbound DNS server

Procedure

1. Install the unbound package:

   # yum install unbound

2. Edit the /etc/unbound/unbound.conf file, and make the following changes in the server clause:

   1. Add interface parameters to configure on which IP addresses the unbound service listens for queries, for example:

      interface: 127.0.0.1
      interface: 192.0.2.1
      interface: 2001:db8:1::1

      With these settings, unbound only listens on the specified IPv4 and IPv6 addresses.

      Limiting the interfaces to the required ones prevents clients from unauthorized networks, such as the internet, from sending queries to this DNS server.

   2. Add access-control parameters to configure from which subnets clients can query the DNS service, for example:

      access-control: 127.0.0.0/8 allow
      access-control: 192.0.2.0/24 allow
      access-control: 2001:db8:1::/64 allow

3. Create private keys and certificates for remotely managing the unbound service:

   # systemctl restart unbound-keygen

   If you skip this step, verifying the configuration in the next step will report the missing files. However, the unbound service automatically creates the files if they are missing.

4. Verify the configuration file:

   # unbound-checkconf
   unbound-checkconf: no errors in /etc/unbound/unbound.conf

5. Update the firewalld rules to allow incoming DNS traffic:

   # firewall-cmd --permanent --add-service=dns
   # firewall-cmd --reload

6. Enable and start the unbound service:

   # systemctl enable --now unbound

Verification

1. Query the unbound DNS server listening on the localhost interface to resolve a domain:

   # dig @localhost www.example.com
   ...
   www.example.com.    86400    IN    A    198.51.100.34
   
   ;; Query time: 330 msec
   ...

   After querying a record for the first time, unbound adds the entry to its cache.

2. Repeat the previous query:

   # dig @localhost www.example.com
   ...
   www.example.com.    85332    IN    A    198.51.100.34
   
   ;; Query time: 1 msec
   ...

   Because of the cached entry, further requests for the same record are significantly faster until the entry expires.