Tested on OpenBSD 6.4

Configure nsd(8) on OpenBSD

Install two VMs in two different networks.
For example, OpenBSD.Amsterdam and Vultr.

Let's pick arbitrary names for them:

ns1.example.com
ns2.example.com

Edit nsd.conf(5) on ns1,
create a zone file for example.com,
copy nsd.conf and example.com.zone to ns2,
enable and start nsd(8) on both servers.

# cat > /var/nsd/etc/nsd.conf << EOF
server:
  database: ""

remote-control:
  control-enable: yes
  control-interface: /var/run/nsd.sock

zone:
  name: example.com
  zonefile: master/%s.zone
EOF
#
# cat > /var/nsd/zones/master/example.com.zone << EOF
$ORIGIN             example.com.
$TTL    300
@       3600  SOA   ns1.example.com. hostmaster.example.com. (
        2018121401  ; serial YYYYMMDDnn
        1440        ; refresh
        3600        ; retry
        604800      ; expire
        300 )       ; minimum TTL
@             NS    ns1.example.com.
@             NS    ns2.example.com.
ns1           A     46.23.88.178
ns2           A     140.82.28.210
@             MX    10 smtp.example.com.
@             MX    20 smtp.example.com.
@             A     46.23.88.178
www           A     46.23.88.178
EOF
#
# rcctl enable nsd
# rcctl start nsd
nsd (ok)
# dig +short example.com NS @127.0.0.1
ns1.example.com.
ns2.example.com.
#

Update nameservers ns1.example.com and their IP addreses (for glue records) at your domain registrar. Your mail server should accept mail for hostmaster@example.com.

Verify your setup with zonemaster.net.

Update zone

Edit the zone file and increment the serial on ns1, copy the zone file to ns2, reload nsd(8) on ns1 and ns2.

# rcctl reload nsd
nsd(ok)
#