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Ubuntu DNS Configuration

Introduction​

Domain Name System (DNS) is a fundamental networking service that translates human-readable domain names (like example.com) into IP addresses (like 93.184.216.34) that computers use to identify each other on the network. Proper DNS configuration is essential for any Ubuntu system to navigate the internet or local networks efficiently.

In this guide, we'll walk through how to configure DNS settings on Ubuntu systems, understand the key configuration files, and troubleshoot common DNS issues. Whether you're setting up a workstation, server, or planning to run your own DNS server, this knowledge is crucial for maintaining reliable network connectivity.

DNS Configuration Files in Ubuntu​

Ubuntu uses several files and utilities to manage DNS configuration. Let's explore the most important ones:

/etc/resolv.conf​

This is the primary file that contains DNS resolver information, including:

  • Which nameservers to query
  • The domain search order
  • Various query options

A typical /etc/resolv.conf file might look like this:

bash
nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com
options timeout:2 attempts:3

In this example:

  • nameserver entries specify DNS servers to use (Google's public DNS in this case)
  • search specifies a domain to append to unqualified hostnames
  • options configures resolver behavior

Note: On modern Ubuntu systems, /etc/resolv.conf is often managed automatically by system services like NetworkManager, systemd-resolved, or resolvconf. Manual changes to this file may be overwritten.

/etc/hosts​

The /etc/hosts file provides static mappings between IP addresses and hostnames:

bash
127.0.0.1       localhost
127.0.1.1       ubuntu-desktop

# The following lines are desirable for IPv6 capable hosts
::1             localhost ip6-localhost ip6-loopback
fe00::0         ip6-localnet
ff00::0         ip6-mcastprefix
ff02::1         ip6-allnodes
ff02::2         ip6-allrouters

192.168.1.10    fileserver
192.168.1.11    printserver

This file is checked before DNS queries are sent, making it useful for:

  • Local hostname resolution
  • Overriding DNS entries
  • Blocking websites (by redirecting domains to 127.0.0.1)

/etc/nsswitch.conf​

The Name Service Switch configuration file determines the order in which different name resolution methods are used:

bash
# /etc/nsswitch.conf
hosts:          files dns mdns4_minimal [NOTFOUND=return]

This line tells the system to check the local files (like /etc/hosts) first, then query DNS servers, and finally use multicast DNS.

Configuring DNS on Ubuntu Desktop​

Ubuntu Desktop typically uses NetworkManager to handle network connections and DNS configuration. Here's how to configure DNS using the graphical interface:

Using the GUI​

  1. Click on the network icon in the top-right corner of your screen
  2. Select "Settings" or "Edit Connections"
  3. Choose your active connection and click the gear icon
  4. Navigate to the "IPv4" or "IPv6" tab
  5. Change the "Method" dropdown to "Automatic (DHCP) addresses only" to set DNS manually
  6. Enter your preferred DNS servers in the "DNS servers" field (e.g., "8.8.8.8, 1.1.1.1")
  7. Click "Apply"

Using NetworkManager via Command Line​

You can also use the nmcli command:

bash
# Show current connection details
nmcli device show | grep DNS

# Set DNS servers for a specific connection
nmcli connection modify "My Connection" ipv4.dns "8.8.8.8 8.8.4.4"

# Apply changes
nmcli connection up "My Connection"

Configuring DNS on Ubuntu Server​

On Ubuntu Server, you'll typically use different methods to configure DNS.

Netplan Configuration​

Modern Ubuntu Server installations use Netplan for network configuration. Create or edit a file in /etc/netplan/ (usually named something like 01-netcfg.yaml or 50-cloud-init.yaml):

yaml
network:
  version: 2
  ethernets:
    enp0s3:
      dhcp4: yes
      nameservers:
        addresses: [8.8.8.8, 1.1.1.1]
        search: [example.com, local.lan]

Apply the changes with:

bash
sudo netplan apply

Systemd-resolved​

Many Ubuntu systems use systemd-resolved for DNS resolution management. Configure it by editing /etc/systemd/resolved.conf:

bash
[Resolve]
DNS=8.8.8.8 1.1.1.1
FallbackDNS=9.9.9.9
Domains=example.com

Restart the service to apply changes:

bash
sudo systemctl restart systemd-resolved

Check the status:

bash
systemd-resolve --status

Setting Up a Local DNS Resolver with Bind9​

For more advanced setups, you might want to run your own DNS server using Bind9:

Installation​

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

Basic Configuration​

Edit the main configuration file /etc/bind/named.conf.options:

bash
options {
    directory "/var/cache/bind";
    
    // Forward DNS queries to Google's DNS
    forwarders {
        8.8.8.8;
        8.8.4.4;
    };
    
    // Allow recursive queries from local network only
    allow-recursion {
        127.0.0.1/8;
        192.168.1.0/24;
    };
    
    // Enable IPv6
    listen-on-v6 { any; };
    
    // Recommended security settings
    dnssec-validation auto;
    auth-nxdomain no;
};

Checking Configuration and Restarting​

bash
# Check configuration syntax
sudo named-checkconf

# Restart the service
sudo systemctl restart bind9

# Check status
sudo systemctl status bind9

Creating a Local Zone​

To create a local zone for your network, edit /etc/bind/named.conf.local:

bash
zone "local.lan" {
    type master;
    file "/etc/bind/db.local.lan";
};

Then create the zone file /etc/bind/db.local.lan:

bash
$TTL    604800
@       IN      SOA     ns1.local.lan. admin.local.lan. (
                  3     ; Serial
             604800     ; Refresh
              86400     ; Retry
            2419200     ; Expire
             604800 )   ; Negative Cache TTL
;
@       IN      NS      ns1.local.lan.
ns1     IN      A       192.168.1.10
web     IN      A       192.168.1.20
db      IN      A       192.168.1.30

Don't forget to restart Bind9 after making changes:

bash
sudo systemctl restart bind9

Troubleshooting DNS Issues​

When you encounter DNS problems, these tools can help diagnose the issues:

Using dig​

The dig command is excellent for testing DNS resolution:

bash
# Basic query
dig example.com

# Query a specific DNS server
dig @8.8.8.8 example.com

# Query for a specific record type
dig example.com MX

Example output:

; <<>> DiG 9.16.1-Ubuntu <<>> example.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31245
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;example.com.			IN	A

;; ANSWER SECTION:
example.com.		86400	IN	A	93.184.216.34

;; Query time: 28 msec
;; SERVER: 127.0.0.53#53(127.0.0.53)
;; WHEN: Wed Feb 15 09:32:42 UTC 2023
;; MSG SIZE  rcvd: 56

Using host and nslookup​

These utilities provide simpler alternatives to dig:

bash
# Basic host query
host example.com

# Basic nslookup query
nslookup example.com

Checking systemd-resolved Status​

If your system uses systemd-resolved:

bash
# Check status
systemd-resolve --status

# Flush the DNS cache
sudo systemd-resolve --flush-caches

Checking DNS Response Time​

bash
# Use +stats option with dig to see query time
dig +stats example.com

Common Issues and Solutions​

  1. Unable to resolve any domains

    • Check internet connectivity
    • Verify DNS server addresses
    • Check firewall rules (UDP port 53)

  2. Some domains resolve, others don't

    • Try alternative DNS servers
    • Clear the DNS cache
    • Check for entries in /etc/hosts that might be outdated

  3. Slow DNS resolution

    • Try using DNS servers closer to your geographical location
    • Consider using a local caching DNS resolver
    • Check for network congestion

DNS Security Considerations​

When configuring DNS, keep these security best practices in mind:

Use Secure DNS Providers​

Consider using DNS providers that offer security features:

  • Cloudflare (1.1.1.1)
  • Quad9 (9.9.9.9)
  • Google Public DNS (8.8.8.8)

Enable DNSSEC​

DNSSEC adds authentication to DNS responses, helping prevent DNS spoofing attacks:

bash
# In /etc/bind/named.conf.options for Bind9
options {
    dnssec-validation auto;
    dnssec-enable yes;
}

Use DNS over HTTPS (DoH) or DNS over TLS (DoT)​

These protocols encrypt DNS traffic, preventing eavesdropping:

bash
# Install stubby for DNS over TLS
sudo apt install stubby

# Configure DoH in Firefox
# Navigate to Settings > Network Settings > Enable DNS over HTTPS

Real-World Applications​

Let's explore some practical applications of DNS configuration in Ubuntu:

Setting Up a Development Environment​

For web development, it's common to use local domain names. Edit your /etc/hosts file:

bash
127.0.0.1   myproject.local
127.0.0.1   api.myproject.local
127.0.0.1   admin.myproject.local

Creating a Multi-Server Environment​

In a larger setup with multiple servers, running your own DNS server can simplify management:

Running a Caching DNS Server​

Improve performance by setting up a caching DNS server for your network:

bash
# In /etc/bind/named.conf.options
options {
    directory "/var/cache/bind";
    
    forwarders {
        8.8.8.8;
        1.1.1.1;
    };
    
    // Define cache size
    max-cache-size 256M;
    
    // Cache tuning
    max-cache-ttl 86400;  // 1 day
    max-ncache-ttl 3600;  // 1 hour
    
    allow-query { 192.168.1.0/24; localhost; };
    recursion yes;
};

Summary​

In this guide, we've covered:

  • The fundamentals of DNS and its importance in Ubuntu networking
  • Key configuration files like /etc/resolv.conf, /etc/hosts, and /etc/nsswitch.conf
  • How to configure DNS on both Ubuntu Desktop and Server systems
  • Setting up a local DNS server with Bind9
  • Tools and techniques for troubleshooting DNS issues
  • Security considerations for DNS configuration
  • Real-world applications of DNS in various environments

DNS configuration is a critical skill for any Ubuntu system administrator or power user. Properly configured DNS ensures reliable network connectivity and can enhance security, performance, and management of your systems.

Additional Resources​

Here are some useful resources to deepen your understanding of DNS configuration:

  • Ubuntu Server Guide - DNS section
  • BIND 9 Administrator Reference Manual
  • RFC 1034 and RFC 1035 (DNS specification documents)
  • dig, host, and nslookup man pages (man dig, man host, man nslookup)

Practice Exercises​

  1. Configure your Ubuntu system to use Cloudflare's DNS servers (1.1.1.1 and 1.0.0.1)
  2. Set up local domain names for your development projects in /etc/hosts
  3. Install and configure a caching DNS server with Bind9
  4. Create a local DNS zone for your home or lab network
  5. Use dig and other tools to troubleshoot DNS resolution for various domains

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