Set Up Local DNS Resolver on Ubuntu 20.04 with BIND9

This tutorial will be showing you how to set up a local DNS resolver on Ubuntu 20.04, with the widely-used BIND9 DNS software. A DNS resolver is known by many names, some of which are listed below. They all refer to the same thing.

• full resolver (in contrast to stub resolver)
• DNS recursor
• recursive DNS server
• recursive resolver

Also be aware that A DNS server can also called a name server. Examples of DNS resolver are 8.8.8.8 (Google public DNS server) and 1.1.1.1 (Cloudflare public DNS server). The OS on your computer also has a resolver, although it’s called stub resolver due to its limited capability. A stub resolver is a small DNS client on the end-user’s computer that receives DNS requests from applications such as Firefox and forward requests to a recursive resolver. Almost every resolver can cache DNS response to improve performance, so they are also called caching DNS server.

Why Run Your Own Local DNS Resolver

Normally, your computer or router uses your ISP’s DNS resolver to query DNS names. Running your own local DNS resolver can speed up DNS lookups, because

1. The local DNS resolver only listens to your DNS requests and does not answer other people’s DNS requests, so you have a much higher chance of getting DNS answers directly from the cache on the resolver.
2. The network latency between your computer and DNS resolver is eliminated (almost zero), so DNS queries can be sent to root DNS servers more quickly.

If you run a mail server and use DNS blacklists (DNSBL) to block spam, then should run your own DNS resolver, because some DNS blacklists such as URIBL refuse requests from public DNS resolvers. If you run your own VPN server on a VPS (Virtual Private Server), it’s also a good practice to install a DNS resolver on the same VPS.

You may also want to run your own DNS resolver if you don’t like your Internet browsing history being stored on a third-party server.

If you own a website and want your own DNS server to handle name resolution for your domain name instead of using your domain registrar’s DNS server, then you will need to set up an authoritative DNS server, which is different from a DNS resolver. BIND can act as an authoritative DNS server and a DNS resolver at the same time, but it’s a good practice to separate the two roles on different boxes. This tutorial shows how to set up a local DNS resolver and because it will be used on local host/local network, no encryption (DNS over TLS or DNS over HTTPS) is needed. Setting up a DoT or DoH server will be discussed in a future article.

Set Up Local DNS Resolver on Ubuntu 20.04 with BIND9

BIND (Berkeley Internet Name Domain) is an open-source DNS server software widely used on Unix/Linux due to it’s stability and high quality. It’s originally developed by UC Berkeley, and later in 1994 its development was moved to Internet Systems Consortium, Inc (ISC).

Run the following command to install BIND 9 on Ubuntu 20.04, from default repository. BIND 9 is the current version and BIND 10 is a dead project.

sudo apt update
sudo apt install bind9 bind9utils bind9-doc bind9-host

Check version.

named -v

Sample output:

BIND 9.16.1-Ubuntu (Stable Release) 

To check the version number and build options, run

named -V

By default, BIND automatically starts after installation.You check its status with:

systemctl status bind9

If it’s not running, then start it with:

sudo systemctl start bind9

And enable auto start at boot time:

sudo systemctl enable bind9

The BIND server will run as the bind user, which is created during installation, and listens on TCP and UDP port 53, as can be seen by running the following command:

sudo netstat -lnptu | grep named

Usually DNS queries are sent to the UDP port 53. The TCP port 53 is for responses sizes larger than 512 bytes.

The BIND daemon is called named. (A daemon is a piece of software that runs in the background.) The named binary is installed by the bind9 package and there’s another important binary: rndc, the remote name daemon controller, which is installed by the bind9utils package. The rndc binary is used to reload/stop and control other aspects of the BIND daemon. Communication is done over TCP port 953.

For example, we can check the status of the BIND name server.

sudo rndc status

Configurations for a Local DNS Resolver

/etc/bind/ is the directory that contains configurations for BIND.

• named.conf: the primary config file which includes configs of three other files.
• db.127: localhost IPv4 reverse mapping zone file.
• db.local: localhost forward IPv4 and IPv6 mapping zone file.
• db.empty: an empty zone file

The bind9 package on Ubuntu 20.04 doesn’t ship with a db.root file, it now uses the root hints file at /usr/share/dns/root.hints. The root hints file is used by DNS resolvers to query root DNS servers. There are 13 groups of root DNS servers, from a.root-servers.net to m.root-servers.net.

Out of the box, the BIND9 server on Ubuntu provides recursive service for localhost and local network clients only. Outside queries will be denied. So you don’t have to edit the configuration files. To get you familiar with BIND 9 configurations, I will show you how to enable recursion service anyway.

The main BIND configuration file /etc/bind/named.conf sources the settings from 3 other files.

• /etc/bind/named.conf.options
• /etc/bind/named.conf.local
• /etc/bind/named.conf.default-zones

To enable recursion service, edit the first file.

sudo nano /etc/bind/named.conf.options

In the options clause, add the following lines. Replace IP addresses in the allow-recursion statement with your own local network addresses.

 // hide version number from clients for security reasons.
 version "not currently available";

 // optional - BIND default behavior is recursion
 recursion yes;

 // provide recursion service to trusted clients only
 allow-recursion { 127.0.0.1; 192.168.0.0/24; 10.10.10.0/24; };

 // enable the query log
 querylog yes;

Save and close the file. Then test the config file syntax.

sudo named-checkconf

If the test is successful (indicated by a silent output), then restart BIND9.

sudo systemctl restart bind9

If you have UFW firewall running on the BIND server, then you need to open port 53 to allow LAN clients to send DNS queries.

sudo ufw allow in from 192.168.0.0/24 to any port 53

This will open TCP and UDP port 53 to the private network 192.168.0.0/24. Then from another computer in the same LAN, we can run the following command to query the A record of google.com. Replace 192.168.0.102 with the IP address of your BIND resolver.

dig A google.com @192.168.0.102

Now on the BIND resolver, check the query log with the following command.

sudo journalctl -eu bind9

This will show the latest log message of the bind9 service unit. I found the following line in the log, which indicates that a DNS query for google.com’s A record has been received from port 57806 of 192.168.0.103.

named[1162]: client @0x7f4d2406f0f0 192.168.0.103#57806 (google.com): query: google.com IN A  E(0)K (192.168.0.102)

Setting the Default DNS Resolver on Ubuntu 20.04 Server

Systemd-resolved provides the stub resolver on Ubuntu 20.04. As mentioned in the beginning of this article, a stub resolver is a small DNS client on the end-user’s computer that receives DNS requests from applications such as Firefox and forward requests to a recursive resolver.

The default recursive resolver can be seen with this command.

systemd-resolve --status

Hint: If the above command doesn’t quit immediately, you can make it quit by pressing the Q key.

As you can see, BIND isn’t the default. If you run the following command on the BIND server,

dig A facebook.com

This DNS query can’t be found in BIND log. Instead, you need to explicitly tell dig to use BIND.

dig A facebook.com @127.0.0.1

To set BIND as the default resolver, open the systemd-resolved configuration file.

sudo nano /etc/systemd/resolved.conf

In the [Resolve] section, add the following line. This will set a global DNS server for your server.

DNS=127.0.0.1

Save and close the file. Then restart systemd-resolved service.

sudo systemctl restart systemd-resolved

Now run the following command to check the default DNS resolver.

systemd-resolve --status

Now perform a DNS query without specifying 127.0.0.1.

dig A facebook.com

You will see the DNS query in BIND log, which means BIND is now the default recursive resolver.

Configure Per-Link DNS Server on Ubuntu 20.04

You can also configure per-link DNS server, which will override the global DNS server, in files under /etc/systemd/network/ directory. List files in this directory.

ls /etc/systemd/network/

Sample output:

05-eth0.network  99-default.link

As you can see, I have two link configuration files. The 05-eth0.network file is for my main network interface, so I edit this file.

sudo nano /etc/systemd/network/05-eth0.network

Your filename might be different. If there are no files under this directory, that means all your links (aka network interfaces) are using the global network configuration, so you don’t need to configure per-link DNS server.

Comment out the default DNS and Domain entry, and add your own DNS entry.

DNS=127.0.0.1

Save and close the file. Then restart systemd-resolved service.

sudo systemctl restart systemd-resolved

Note that some Ubuntu servers might be using netplan to configure per-link networking. In this case, you need to configure DNS server in the .yaml file under /etc/netplan/ directory. List files in this directory.

ls /etc/netplan/

Sample output:

01-netcfg.yaml

So I edit this file.

sudo nano /etc/netplan/01-netcfg.yaml

Set the DNS server address in the nameservers section.

      nameservers:
        search: [ invalid ]
        addresses:
                - 127.0.0.1

Save and close the file. Then apply the change.

sudo netplan apply

Troubleshooting

Check the content of /etc/resolv.conf.

cat /etc/resolv.conf

As you can see, 127.0.0.1 (BIND) is default DNS resolver. If you see a different value, that means BIND is still not your default DNS resolver. You can use the resolveconf utility to set BIND as default resolver.

Install the resolvconf package

sudo apt install resolvconf

Then start the named-resolvconf service.

sudo systemctl start named-resolvconf.service

Enable auto-start at boot time.

sudo systemctl enable named-resolvconf.service

Now check the /etc/resolv.conf file again. BIND should be the default DNS resolver on your Ubuntu server now.

cat /etc/resolv.conf

Note that some hosting providers like Linode may use a network helper to auto-generate the /etc/resolv.conf file. To change the default DNS resolver, you need to disable that network helper in the hosting control panel.

Setting Default DNS Resolver on Client Computers

On Ubuntu desktop, you can follow the above instructions to set the default DNS resolver, but remember to replace 127.0.0.1 with the IP address of BIND server. The steps of setting default DNS resolver on MacOS and Windows can be found on the Internet.

How to Disable IPv6 in BIND

If you don’t use IPv6 in your network, then it’s a good idea to turn off IPv6 in BIND, otherwise, there will be a lot of errors about IPv6 in BIND log like below.

network unreachable resolving 'mirrors.fedoraproject.org/A/IN': 2001:4178:2:1269:dead:beef:cafe:fed5#53
network unreachable resolving 'mirrors.fedoraproject.org/AAAA/IN': 2001:4178:2:1269:dead:beef:cafe:fed5#53
network unreachable resolving 'mirrors.fedoraproject.org/A/IN': 2610:28:3090:3001:dead:beef:cafe:fed5#53
network unreachable resolving 'mirrors.fedoraproject.org/AAAA/IN': 2610:28:3090:3001:dead:beef:cafe:fed5#53

To disable IPv6 in BIND on Ubuntu, simply open the /etc/default/bind9 file

sudo nano /etc/default/named

Add “-4” to the OPTIONS.

OPTIONS="-u bind -4"

Save and close the file. Then restart BIND and you are done.

sudo systemctl restart bind9

Conclusion

I hope this tutorial helped you set up a local DNS resolver on Ubuntu 20.04 with BIND9. As always, if you found this post useful, then subscribe to our free newsletter to get more tips and tricks. Take care 🙂