Absolute Name: In DNS www.microsoft.com is not and absolute
name. And, although it isn't an absolute name it works for
most cases, except in DNS zone files. Absolute names are required
in DNS zone files because they are set up for easy administration.
What happens is if you have a name such as "www"
and you put it into your zone file like that, for, say, an
address record, when the DNS program reads it, the DNS program
automatically appends the domain name to the end, to make
"www.mydomain.com". This is the ease of administration
part. Now, say you wanted to put the whole thing in for and
address record, such as "www.mydomain.com". If you
used that exact syntax, you DNS server would read it into
memory, do its automation thing, and think it reads "www.mydomain.com.mydomain.com".
See what's wrong with that picture?
So How do I get around that? Easy one. Just append a period
to any name that is to be considered absolute, e.g. "www.mydomain.com.".
CNAME records: CNAME 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 188.8.131.52
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.)
Forwarder: A forwarder is used to signify that your server
has a forwarder specified on it, and it also sometimes refers
to the server the request are being forwarded to. What the
forwarder on does on your DNS server is forward all request
it cannot answer, out of its own cache or domains it hosts,
to the specified DNS server and asks it to get the information
for your server.
FQN: Fully Qualified Name. "www" or "mail"
is not an FQN. "www.microsoft.com" or "ftp.microsoft.com"
is an FQN. "microsoft.com" is a domain name, but
not necessarily an FQN.
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
Primary DNS servers can be secondary DNS servers for other
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: This is an alias record, for situations where you do
not necessarily want another A address for a host, but you
need to have more than one name point to the same place.
Reverse-Lookup: The process of obtaining a DNS host name and
domain name from and IP address.
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
It can also be primary DNS server, for a different zone than
it is secondary. And multiple zones apply here too.
Slave Server: Do not confuse this with a secondary server.
This is not a secondary server. This is a server that does
not store any zone files, primary or secondary, it merely
serves what it has in its cache, and queries other servers
for anything else. While there are reasons for having a server
like this, I can't think of any right now.
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. More
Refresh: This is the amount of time the secondaries will wait
before checking to see if they should get a new transfer from
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 isn't 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
TTL: A.K.A. Minimum TTL. This is how long the records from
this zone file will remain in another DNS server's 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.
Top-Level Domains: The Top-Level domains are "com",
"net", "edu", "mil", "int",
etc. There are also top level domains for each country, such
as "ca" (Canada), "de" (Germany), "be"
(Belgium), "jp" (Japan), "nu" (Niese).
These are collectively referred to in DNS as "."
(yes, that's just a period [dot]). The dot, ".",
is the absolute top of the domain name hierarchy. You may
notice if you look in your boot file, the cache record says
"cache . cache.dns" (for Bind). In NT DNS you will
have to look under HKLM/System/CurrentControlSet/Services/DNS/Zones,
and you will see a key called ".". This is the equivalent
of the boot file and specifies the cache file for top-level
domains, ALL top-level domains. In cases where the Root server
is not authoritative for a domain, such as Canada, "ca",
or Germany, "de", the Root server knows where the
top-level name servers are for that domain and will answer
the query as such. Then the DNS server who receives the query,
such as the ISP's DNS server, or your DNS server, will then
send the same query to the DNS server for that top-level domain
and get an answer back from that DNS server. This adds an
extra step that does not apply to the domains that the Root-Servers.net
Zone: The technically correct name for a section of a domain.
Zone and domain are used pretty much synonymous, but a zone
actually refers to a section of a domain. For instance, if
you have "mydomain.com", then you have the "mydomain"
section, or zone, of the "com" domain. If you have
the "customers.mydomain.com" zone, you have the
"customers" section of the "mydomain"
section of the "com" domain.
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