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DNS (Domain Name System) translates human-readable hostnames like example.com into IP addresses your system can route traffic to. On Linux, name resolution follows a layered order: the system checks /etc/hosts for local overrides first, then queries the DNS resolvers listed in /etc/resolv.conf (or managed by systemd-resolved on modern Ubuntu). Understanding this stack lets you override specific names locally, point your system at fast public resolvers, and quickly diagnose why a hostname is not resolving as expected.

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Configuring /etc/resolv.conf

/etc/resolv.conf tells the system which DNS servers to query and which domain suffixes to append to short names. 
nameserver 8.8.8.8
nameserver 8.8.4.4
search example.com
On systems running systemd-resolved, /etc/resolv.conf is typically a symlink to a file managed automatically by the service. Editing it directly may result in your changes being overwritten. Configure DNS through Netplan or resolvectl instead to ensure persistence.

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systemd-resolved

Most modern Ubuntu installations use systemd-resolved as the local DNS stub resolver. It listens on 127.0.0.53 and caches responses to improve lookup speed. 
resolvectl status                  # show DNS config per interface
resolvectl query example.com       # query a name
sudo systemctl restart systemd-resolved
To set static DNS servers persistently, add a nameservers block to your Netplan configuration: 
nameservers:
  addresses:
    - 1.1.1.1
    - 1.0.0.1
Then run sudo netplan apply for the change to take effect.

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Local Overrides with /etc/hosts

/etc/hosts is checked before any DNS query is made. Use it to define local hostname mappings or override public DNS for specific names. 
127.0.0.1   localhost
192.168.1.10 myserver.local myserver
/etc/hosts is ideal for development environments where you want to point a domain to a local server without touching DNS. Add an entry like 127.0.0.1 app.local and your browser will resolve it immediately without any DNS changes.

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DNS Lookup Tools

dig is the most powerful DNS query tool available on Linux. It gives you full control over record type, resolver, and output format.
dig example.com                    # full DNS query
dig example.com A                  # A record
dig example.com MX                 # MX record
dig @8.8.8.8 example.com          # use specific resolver
dig +short example.com             # IP only

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Reverse DNS Lookups

Reverse DNS maps an IP address back to a hostname. This is useful for auditing logs, verifying mail server identity, and network diagnostics. 
dig -x 8.8.8.8
host 8.8.8.8

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Flushing the DNS Cache

If you have recently changed a DNS record or /etc/hosts and need the system to pick up the new value immediately, flush the local cache: 
sudo resolvectl flush-caches

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Common DNS Record Types

RecordPurpose
AMaps a hostname to an IPv4 address
AAAAMaps a hostname to an IPv6 address
CNAMEAlias that points one hostname to another
MXSpecifies mail servers for a domain
TXTStores arbitrary text; used for SPF, DKIM, and domain verification
NSIdentifies the authoritative nameservers for a domain
PTRReverse lookup — maps an IP address to a hostname
Use 1.1.1.1 (Cloudflare) or 8.8.8.8 (Google) as your upstream resolvers for fast, reliable public DNS. Cloudflare’s 1.1.1.1 is consistently among the fastest resolvers globally and supports DNS-over-HTTPS and DNS-over-TLS for added privacy.