Domain Name System (DNS)

A server that hosts a website has to communicate using only IP addresses. DNS is how someone wishing to access the web content converts the website’s domain name, e.g. www.ericdweise.com. Gaining the IP address for the server hosting the website from a DNS server is called resolving.

DNS is a very distributed database. There are many servers that can resolve IP addresses. Usually a device is configured to connect to a root server which can direct your request to other servers.

Zones

Forward Lookup Zone

Given an FQND, the Forward lookup zone will provide an IP address for the host serving that site. ### Reverse Lookup Zone Given an IP address, the reverse lookup zone will provide a FQDN.

Primary and Secondary Zones

Primary zone is the first place to look for a record. Secondary zone takes over if the primary is down, but cannot add new records. The secondary zone will receive updates from the primary.

DNS Records

A DNS Server keeps a large list of IP addresses connected to their domain name. this is just a text file.

DNS Record Types

There are over 30 types of DNS records. This list will go over some of the most common ones.

NOTE: the semi-colon, ;, is the comment symbol.

Start of Authority (SOA)

An SOA record points at a server that is in charge of a forward lookup zone.

Name Server (NS)

This points at the name server for a particular domain

Address Records: A and AAAA records

• A Records are used for IPv4 addresses
• AAAA Records are used for IPv6 addresses

An example A record is

example.com    IN A    123:45:67:89

Canonical Name Record: CNAME

A CNAME record is used to associate several domain names with one device, i.e: one IP address. The first domain name will be set up as an A or AAAA record, but any subsequent domains will alias

If we wanted to add the www, mail, and ftp subdomains to the example.com and want these to live on the same physical server (i.e. share the same IP address) as example.com, then the CNAME record would look like this:

; Alias (canonical) names
    www IN CNAME example.com.
    ftp IN CNAME example.com.
    mail IN CNAME example.com.

Service Records (SRV)

These records are used to find particular services on a network. Examples of Services that might need a SRV record might be a VoIP controller, or an instant messaging server.

An example of a SRV record directing to a network’s Microsoft Active Directory Controller is shown below.

; Service records
; _service._proto.name. TTL class SRV priority weight port target
 _ldap._tcp.domain.com. 300 IN    SRV 10       60     389  s1.domain.com.

#### Mail Exchange Record: MX This is an additional record in the DNS file that specifies the domain name of the mail server. The server still needs to have an A Record (or CNAME record) in the file.

IN MX mail.example.com

Pointer Records

These are reverse look up records. They allow a user to find a domain name given an IP address. They are added to the reverse map zone file.

123.45.67.89    IN    PTR   example.com.

Text Records

These are not records, per se, but they offer useful information. They include things like: - Sender Policy Framework (SPF), which authorize an external host to send mail from a given domain - Domain Keys Identified Mail (DKIM), are public key parts of a digital signing framework so that a mail recipient can verify that the mail came from the private key holder.

Common Name Servers