Network+ - Domain Names and DNS (Domain Name System)
Name Servers: Top-level domain (TLD) servers
.com .org .edu
Resolvers
A DNS client that requests information from DNS name servers
Name Servers: Recursive query
A query that demands a resolution or the answer "It can't be found." For example, the initial request the resolver makes to the local server is a recursive query.
Name Servers: Iterative query
A query that does not demand resolution. For example, when the local server issues queries to other servers, the other servers only provide information if they have it.
Name Servers: caching DNS server
A server that accesses public DNS data and caches the DNS information it collects. This server receives DNS queries from local network clients and works to resolve them by contacting other DNS servers for information.
Resource Records in a DNS Database: SPF (Sender Policy Framework)
A validation system that helps fight spam by identifying the email servers allowed to send email on behalf of a domain.
Resource Records in a DNS Database: DKIM (DomainKeys Identified Mail)
An authentication method that uses encryption to verify the domain name of an email's sender.
Name Servers: forwarding DNS server
An optional server that receives queries from local clients but doesn't work to resolve the queries. Typically, a forwarding server will maintain its own DNS cache from previous queries, and so it might already have the information the client needs.
Domain suffix: BIZ
Businesses
Domain suffix: COM
Commercial
Name servers
Computers that hold these databases, organized in a hierarchical structure
Namespace Databases
DNS namespace databases are stored on thousands of servers around the world, rather than being centralized on a single server or group of servers. In other words, DNS doesn't follow a centralized database model, but rather a distributed database model.
Domain suffix: EDU
Educational
Domain suffix: GOV
Government
Name Servers: Root servers
Hold information used to locate the TLD (top-level domain) servers. These TLD servers hold information about the authoritative servers owned by various organizations.
Resource Records in a DNS Database: CNAME (Canonical Name) record
Holds alternative names for a host. These names can be used in place of the canonical name, which is the complete and properly formatted name, such as www.mycompany.com.
Resource Records in a DNS Database: TXT (Text) record
Holds any type of free-form text. It might contain text designed to be read by humans regarding network, server, or accounting issues.
Resource Records in a DNS Database: AAAA (Address) record (called a "quad-A record")
Holds the name-to-address mapping for IPv6 addresses.
Resource Records in a DNS Database: MX (Mail Exchanger) record
Identifies an email server and is used for email traffic.
Resource Records in a DNS Database: SRV (Service) record
Identifies the hostname and port of a computer that hosts a specific network service besides email, such as FTP or SIP.
TTL (Time to Live)
In an actual DNS zone file, each resource record begins with a TTL (Time to Live) field that identifies how long the record should be saved in a cache on a server. Administrators can set the TTL based on how volatile is the DNS data (in other words, how often the administrator expects the IP addresses to change). TTL information is included in zone transfers.
DNS (Domain Name System or Domain Name Service)
In the mid-1980s was designed to associate computer names with IP addresses. DNS is an Application layer client-server system of computers and databases made up of these elements.
Resource Records in a DNS Database: NS (Name Server) record
Indicates the authoritative name server for a domain. It's mostly used for delegating subdomains to other name servers.
Namespace Databases: Authoritative server
Is the authority on computer names and their IP addresses for computers in their domains.
Name resolution
Is the process of discovering the IP address of a host when its FQDN is known.
Resource Records in a DNS Database:
Namespace databases are stored in DNS zone files, which keep information in various types of resource records
Domain suffix: NET
Network (such as an ISP)
Domain suffix: ORG
Noncommercial organization (such as a nonprofit agency)
Resource Records in a DNS Database: Reverse zone.
PTR records are usually created by ISPs and stored in a specially formatted reverse lookup zone file, or reverse zone.
TLD (top-level domain).
Recall that an FQDN is a host name and a domain name together, such as www.cengage.com. The last part of an FQDN (com in our example)
Domain suffix: ARPA
Reverse lookup domain (special Internet function)
Resource Records in a DNS Database: A (Address) record
Stores the name-to-address mapping for a host. This resource record provides the primary function of DNS—to match host names to IP addresses, using IPv4 addresses.
Name Servers: primary DNS server
The authoritative name server for the organization, which holds the authoritative DNS database for the organization's zones.
Name Servers: primary DNS server
The backup authoritative name server for the organization. When a secondary DNS server needs to update its database, it makes the request to the primary server for the update; this process is called a zone transfer.
Namespace Databases: DNS zone
The domains (for example, cengage.com and course.com) that the organization is responsible for managing
Namespace
The entire collection of computer names and their associated IP addresses stored in databases on DNS name servers around the globe.
Domain suffix: MIL
United States military organization
Domain suffix: INFO
Unrestricted use
Resource Records in a DNS Database: PTR (Pointer) record
Used for reverse lookups, which provide a host name when you know its IP address.
Name Servers: Authoritative servers
microsoft; amazon; google; craigslist; pbs; ua; mdc
Resource Records in a DNS Database: forward zone
that holds A records, in that the IP addresses must be stored in reverse—with the last octet listed first—plus the domain .in-addr.arpa.
