1 SOA: A.K.A. SOA Record: Start Of Authority.
This is the first record in a zone file, the one that usually reads:
@ IN SOA NS1.mydomain.com. root.mydomain.com. (
1996050101 ; Serial [yyyyMMddNN]
21600 ; Refresh [6h]
3600 ; Retry [1h]
691200 ; Expire [8d]
86400) ; TTL [1d]
This designates that server as the Primary DNS server for the zone. Explanations of all the numbers are as follows:
Serial Number: This is the version of the file. The version of the file must be incremented each time the file is updated so that the secondary knows when to update its files.
Refresh: This is the amount of time the secondary servers will wait before checking to see if they should get a new transfer from the primary.
Retry: This it the amount of time the secondary will wait before trying to contact the primary again if the primary is not available when the secondary attempts to contact it. This is so that if the primary is down, the secondary does not panic and saturate the network attempting to contact the primary. This is not a big deal if you have one secondary, but if you have several, there is a very real possibility of saturating a network segment trying to contact a primary.
Expire: The length of time the secondary DNS server will keep the DNS records for a given zone, if it cannot contact it primary. TTL: A.K.A. Minimum TTL. This is how long the records from this zone file will remain in another DNS server cache before the caching DNS server deletes the record and queries the primary or secondary again for a copy of the record. This is so that if a DNS record changes, other DNS servers on the Internet will eventually get the latest version of that record.
2. Primary DNS Server:
This is the DNS server that maintains the master zone information/file for a domain. All changes to domain information take places here and get propagated to the secondary servers at the Refresh interval, as specified in the actual zone information file. Primary DNS servers can be primary DNS servers for multiple zones. Primary DNS servers can be secondary DNS servers for other zones.
3. Secondary DNS Server:
This is a DNS server that backs up a primary DNS server for a zone. You are required, by Internic/Network Solutions, to have at least one, but it is recommended that you have more, if resources permit. A secondary DNS server can be a secondary DNS server for many zones. It can also be primary DNS server, for a different zone than it is secondary. And multiple zones apply here too.
4. Record Types:
A: This is an address record, The most basic DNS record, for translating a name to an address.
MX: Mail transfer records, for directing mail across the Internet.
CNAME: These records are merely an alias name for another DNS name. This is mainly done to save admin time in that if you have "mail.domain.com" and you want to have other names for it but do not want to have to change the IP address for several records, if that time ever comes, you use a CNAME record. Example and record format to follow:
mail IN A 126.96.36.199
mailhost IN CNAME mail
pop3 IN CNAME mail
If you have to change the IP address you only have to modify one record instead of 3. Another reason is that reverse lookup only allows one name per IP address, so you would set up the most relevant name in the record for reverse lookup and use CNAMEs for the rest. (This is not mandatory. You can still use A records with the most relevant name on the reverse lookup record with no ill affects.)