DNS Terms

 

Frequently used terms in DNS troubleshooting and administration circles.

   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              192.1.1.1
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 zones.

   Primary DNS servers can be secondary DNS servers for other zones.

   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: 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 zones.

   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

   Refresh: This is the amount of time the secondaries 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 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 primary.

   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 handle.

   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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