About DNS Records
- How does DNS resolve?
- How do I register a domain name?
- What is a zone file?
1. How does DNS resolve?
Say you wanted to check out sampledomain.com. You start your browser, enter the URL (Universal Resource Locator) and hit
return. Soon after the resolver on your PC asks your local name server, "What's the IP address of www.sampledomain.com?"
This resolver-to-name server query has the "recursion desired" bit turned on in the DNS packet header.
A recursive query asks the name server to do all the work and just return an answer, not a referral to another name
server. Assuming your local name server doesn't have any www.sampledomain.com information cached already (and ignoring
any complicating firewall security measures), your local name server asks a root name server the same question it was
asked, but this time as a non-recursive query: "What's the IP address of sampledomain.com?"
The root name server responds with either the answer (doubtful) or a referral: "I don't know the answer, but here are the
NS records of the .com name servers--try asking them."
Referrals are a legitimate response to a non-recursive query and a name server knows what to do with them. A resolver, on
the other hand, is rather simple-minded and can't follow a referral--it needs the answer itself, thus the reason for its
recursive queries.
Your local name server now shifts its attention to the com name servers, asks them the same non-recursive query, and gets
a referral (more NS records) to the sampledomain.com name servers. Upon asking one of the sampledomain.com name servers
the same question, it receives an actual response instead of a referral.
Curiously enough, this response also has NS records in the authority section, as well as the address (A) record for
sampledomain.com in the answer section. Of course, including the sampledomain.com NS records in the response doesn't do
your name server much good, because it already knows them--the com name server already told it. But that's the way BIND
works.
Finally, your name server responds back to your PC's resolver with www.sampledomain.com's IP address, with the
"authoritative answer" flag bit in the DNS packet header set. This flag means that the answer came from an authoritative
name server, either directly or--in this case--indirectly via your name server. Your name server caches this A record and
if you ask for it again, the answer will be non-authoritative, since it now comes from your name server's cache. However,
the authority section will still have the sampledomain.com NS records filled in.
2. How do I register a domain name?
There are several organizations responsible for distributing domain names. Most countries have their own organization,
called an Network Information Center or NIC;s to handle the registration of international domain such as .uk (United
Kingdom) and .au (Australia). Use the following link to see a current listing of the international NIC's from
ICANN (The Internet Corporation for Assigned Names and Numbers).The oldest and most widely known U.S. NIC, responsible for the .com, .edu, and .org domains, is called InterNIC. InterNIC
is currently operated by a company called Network Solutions, headquartered in Herndon, Virginia.
In order to use a specific domain name, you must first register the domain name with the NIC agency responsible for that
domain. You in effect have to purchase the right to use a specific domain name. If you have not previously done this, a
Commercial RCN Sales Agent can process this for you when you sign up for RCN Web Hosting service.
A domain name registration shows contact, technical, and billing information for your domain, and defines a specific name
server, or group of name servers, that will contain the information on where that domain name should point. The name
servers that appear in a domain.s registration record are called authoritative, meaning they contain the most current and
correct name server information about the domain. For example, the domain registration for RCN.com looks like this:
Registrant:
RCN
196 Van Buren Street
Suite 300
Herndon, VA 20170
US
Domain Name: RCN.COM
Administrative Contact, Technical Contact:
RCN abuse@RCN.COM
105 Carnegie Center
Princeton, NJ 08540
US
877-726-7000 fax: 999 999 9999
Record expires on 29-Jun-2011.
Record created on 30-Jun-1997.
Database last updated on 19-Feb-2007 12:17:04 EST.
Domain servers in listed order:
AUTH1.DNS.RCN.NET 207.172.3.20
AUTH3.DNS.RCN.NET 207.172.3.21
AUTH4.DNS.RCN.NET 207.172.3.22
AUTH2.DNS.RCN.NET 207.172.11.14
The Registrant is the organization or individual that owns the domain. In the example above you can see this is RCN.
The Administrative Contact is in charge of how this domain name is managed. This is also listed as RCN.
The Technical Contact is in charge of managing the technical aspects of this domain, this is usually the ISP or hosting provider. This is listed above as our Erols
NOC (Network Operations Center) or domreg@erols.com.
The Billing Contact receives all billing inquiries for this domain from the NIC whom it is registered under. When your
domain is up for renewal, typically an email is sent to the email address of the Billing Contact, notifying them of their
need to renew the domain. In the example above, this is jennifer.ochtun@rcn.net.
A domain registration record also shows the name servers that are authoritative for the IP addresses that the domain
should point to. The four name servers in this record are auth1-4.dns.rcn.net.
You can view the domain registration record for any existing U.S. domain name by typing the following from a LINUX or UNIX command prompt:
whois -h whois.networksolutions.com <Hit Enter>
Domain registration records do not contain the IP information for the domain, only the host name. IP information is
located on the authoritative name servers.
3. What is a zone file?
Once an NIC agency has a domain name registered and has information about which name servers are authoritative for a
domain, the authoritative name servers need to have the information about the IP addresses that correspond to the domain
loaded into its DNS tables. This information is called a DNS Record, or zone file.
Although you will never be required to edit a DNS record directly, you may be interested in seeing what one looks like.
The following is a sample DNS record, showing the IP address associated with your NT/Win2k server, as well as the IP address
for your statistics server:
1 IN SOA ns2.nameserver.com. (
2 1998111301 ; Serial
3 43200 ; Refresh
4 3600 ; Retry
5 604800 ; Expire
6 86400 ) ; Minimum
7
8 IN NS ns2.nameserver.com.
9 IN NS ns3.nameserver.com.
10 IN NS ns1.nameserver.com.
11
12 IN MX 0 mail.mastermail.com.
13
14 IN A 192.168.1.1
15 www IN A 192.168.1.2
Note: The column of numbers (1-15) does not appear normally in a zone record. They are provided for illustration purposes
only.
Line 1: Shows the name server that is the SOA or Start of Authority for this domain.
Line 2: Shows the serial number for this zone record. Name servers compare the value of the serial number to check if any
modifications have been made to a zone file. If the serial value has increase, name servers will register this as an
indication that the zone file content has change and they should update their cached copy. Typically the format for the
serial number is YYYY/MM/DD/VV, but not always. The serial can be any number. For example, 1234 is a valid serial number.
The only requirement is that the value must be increased for other name servers to register a change.
Lines 8-10: List the primary, secondary and tertiary name servers.
Line 12: Indicates the host name of the server where all mail for this domain should be directed.
Line 14: Indicates the IP address that your unqualified domain name resolves to. Sampledomain.com is in example of an
unqualified domain name, note the missing www entry.
Line 15: Indicates the IP address that you fully qualified domain name resolves to. www.sampledomain.com is an example.
Note the www. prefix in the record above.
