Beyond the Basics of DNS

Forward and Reverse Lookup

Forward Lookup refers to the process of 'looking forward' from a hostname or domain name to lookup the IP address for it.

Reverse Lookup refers to the opposite process, finding the domain or hostname that relates to a known IP address

DNS servers maintain forward and reverse lookup zones, with directories which facilitate this process.
A forward lookup is used in the standard DNS queries described above. A reverse lookup is often used by e-mail servers to combat spam. When a message comes in, a server may also do a reverse loopup on the IP address the mail came from. If it doesn't match the domain name the e-mail claims to be coming from, the server may discard the message.

Caching

Once the computer or the DNS servers it has referred to have an IP for a domain or host name, it will 'cache' it, or hold the information for a period of time. This time will vary from system to system, but it is typically a fairly short time. The principal reason for the short time period is that IP addresses can change.

Fully Qualified Domain Name

A fully qualified domain name (FQDN) is the complete domain name for a specific computer, or host, on the Internet. The FQDN consists of two parts: the host name and the domain name. For example, an FQDN for a hypothetical mail server might be mymail.k2.com. The host name is mymail, and the host is located within the domain k2.com.

Understanding more about DNS Mapping

Domain name syntax

A domain name consists of one or more parts, technically called labels, that delimited by dots, such as example.com. The following list provides the basic outline of DNS name syntax:

The right-most label conveys the top-level domain; for example, the domain name www.example.com belongs to the top-level domain com.
The hierarchy of domains descends from right to left – with each label to the left indicating a further a subdivision, or subdomain of the domain to the right. For example: the label example specifies a subdomain of the com domain. The tree of subdivisions may have up to 127 levels.
Each label may contain up to 63 characters. The full domain name may not exceed a total length of 253 characters in its external dotted-label specification.
DNS names may technically consist of any character representable in an octet. However, the allowed formulation of domain names in the DNS root zone, and most other sub domains, uses a preferred format and character set. The characters allowed in a label are a subset of the ASCII character set, and includes the characters a through z, A through Z, digits 0 through 9, and the hyphen. This rule is known as the LDH rule (letters, digits, hyphen).
A host name is a domain name that has at least one IP address associated. For example, the domain names www.example.com and example.com are also host names, whereas the com domain is not.

DNS Resolvers

The client-side of the DNS is called a DNS resolver. It is responsible for initiating and sequencing the queries that ultimately lead to a full translation of the resource sought, e.g., the translation of a domain name into an IP address.

A DNS query may be either a non-recursive query or a recursive query:

A non-recursive query is one in which the DNS server provides a record for a domain for which it is authoritative itself, or it provides a partial result without querying other servers.
A recursive query is one for which the DNS server will fully answer the query (or give an error) by querying other name servers as needed. DNS servers are not required to support recursive queries.

The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers. Resolving usually entails searching in sequence through several name servers to find the needed information. However, some resolvers function more simply by communicating only with a single name server. These simple resolvers (called "stub resolvers") rely on a recursive name server to perform the work of finding information for them.

A DNS Example Record

A Resource Record (RR) is the basic data element in the domain name system. Each record has a type (A, MX, etc.), an expiration time limit, a class, and some type-specific data. Resource records of the same type define a resource record set. An example DNS configuration (with the most commonly used resource record types) is shown in the table below, with explanations of each of the record types in the following paragraphs:

A Records
example.com	69.90.142.25 (a primary server)
help.example.com	69.90.142.26
CNAME Records
vpn.example.com	Cr758341-a.ourisp.com
files.example.com	example.com
www.example.com	example.com
MX Records
example.com	example.com (see below for more information)

A Records / Host Records

The bread and butter behind the DNS system is the A Record. The A record (address record, or host record) maps a domain name to an IP address on the local network or on the Internet.

In this example, the network system is hosting example.com. Using a dynamic DNS tool, we could set our domain to be example.com and the IP address (69.90.142.25) will be automatically updated via dynamic DNS. For our vpn, we need to create a static A record with the IP address (69.90.142.26) associated with vpn.example.com.

So, we have two names mapped to IP addresses (A Records):

example.com - 69.90.142.25
help.example.com - 69.90.142.26

CNAME Records / Alias Records

CNAME Records (Canonical Name records) act as aliases for host names. Instead of mapping a domain name to an IP address (an A record) you can map a domain name to another domain name. In the example, you have:

files.example.com - example.com

www.example.com - example.com

vpn.example.com - Cr758341-a.ourisp.com

What are the advantages of CNAMEs? Multiple domain names can be mapped to one - sometimes dynamic - IP address. In our example, files.example.com and www.example.com will now be associated with example.com's IP address (a Dynamic DNS A record). In the case of the vpn, CNAMES gives the options of changing a not-so-easy-to-remember-super-long domain name into something better.

MX Records / Mail Records

MX Records (Mail eXchanger record) tells mail systems how to handle mail that is addressed to a particular domain. Like CNAME records, the MX record maps a domain name to another domain name.
In the example, we use our primary machine as a server for mail to xyx@example.com. Every MX record is tagged with a priority number. The MX record with the lowest number is the primary mail server. If the primary server is unavailable, the backup mail server (also called a “secondary mail server”) will queue the mail.
For a list of all the resource record types used in DNS lookups, see the Wikipedia article http://en.wikipedia.org/wiki/List_of_DNS_record_types

General Reference

DNS Basics