DNS

Domain Name Service. Bind a name to IP-addresses.

Server side

Zone transfer between master and slave server is on TCP port 53.

Disable resolving IPv6 addresses by editing /etc/default/bind9 to have OPTIONS="-4 -u bind"

Client side

Clients query the nameserver on UDP port 53.

host <hostname or IPaddress>

Resolve hostname to IPadddres or IPaddress to hostname

host <host or IP> <nameserver>

Resolve using the specified nameserver

dig <hostname> @<nameserver>

Resolve hostname with extra info from the optinoal <nameserver>

/etc/resolv.conf

nameserver <dns-server1 IP>
nameserver <dns-server2 IP>
options <interface>
search <default search domain>

If the systemd-resolved is active

/etc/resolv.conf has 127.0.0.53 as nameserver

resolvectl status

Show which nameservers are used by the systemd-resolved