Zone Files & Authoritative Lookups

A zone file lists the records an authoritative server holds for a zone:

$TTL 86400
$ORIGIN example.com.

@           SOA   ns1.example.com. admin.example.com. (
                  2024010101  ; serial
                  3600        ; refresh
                  600         ; retry
                  86400       ; expire
                  300 )       ; minimum
@           NS    ns1.example.com.
@           NS    ns2.example.com.
ns1         A     192.0.2.10
ns2         A     192.0.2.11
@           A     192.0.2.1
www         A     192.0.2.1
api         A     192.0.2.20
www         AAAA  2001:db8::1
mail        MX    10 mailhost.example.com.
mailhost    A     192.0.2.30
@           TXT   "v=spf1 mx -all"

The @ symbol means "the zone's apex" (example.com.). A bare hostname (www) means www.example.com.. Trailing dots distinguish FQDN from relative names.

Authoritative lookup:

def lookup(name, type):
    records = zone[name]
    matching = [r for r in records if r.type == type]
    return matching   # may be 0, 1, or many records

Special cases:

• A query for example.com A returns the apex A records.
• A query for *.example.com A (wildcard) matches anything not explicitly listed (some zones use this).
• A CNAME response means "follow this alias" — the resolver should chase it.

Authoritative servers don't recurse. They only answer for zones they're authoritative for. If they don't know, they return RCODE=3 NXDOMAIN.