DNS

DNS is Domain Name System.

DNS is a protocol (of OSI application layer).


-Definition
DNS resolves a name to an IP address.
*On internet: DNS resolves a domain name to a public IP address;
*On LAN, MAN and WAN: DNS resolves a hostname to an IP address (and with Windows operating system, this resolution uses the Active Directory)


-DNS table
The DNS table has IP addresses, domain names or hostnames, and the associations between them.


-DNS is useful because it’s easier to remember a domain name than an IP address
->Example: it’s easier to remember www.google.com than 43.245.201.230
->Exercise: go to the DOS window, write ping www.yahoo.com, copy the displayed IP address (for example 87.248.100.2150, then paste it in your internet browser bar and press enter: Yahoo webpage will appear.
->Remark: each website located on internet has an IP address and each host (computer, server, tablet...) connected to a network (LAN, MAN, WAN, internet) has also an IP address.


-In Active Directory, there is "DNS administrators" group.
A user must be member of this group to be able to modify the DNS configuration.

-On a DNS server, there are "DNS event" logs.
Here are some errors and problems with DNS that you can analyze and try to solve.


-DNS server uses DNS protocol.


-Examples of DNS servers:
Windows Server 2003 can be a DNS server;
Windows Server 2012 can be a DNS server;
Windows Server 2016 can be a DNS server.
Windows Server 2019 can be a DNS server.


-13 DNS servers manage internet. Each DNS server has a DNS table updated regularly.
And 12 independent international organizations control these 13 DNS servers.


-Example of free and public DNS server:
Provider: Google
Primary DNS: 4.4.4.4
Secondary DNS: 8.8.8.8

Google also offers IPv6 versions:
Primary DNS: 2001:4860:4860::8888
Secondary DNS: 2001:4860:4860::8844


-In 2008, some DNS servers have been attacked. Then, a security protocol was created: DNSSEC.


-TTL DNS is Time To Live DNS
TTL DNS is the time (in seconds) during which a DNS server can keep the information in cache (DNS table).
Information are domain names and IP addresses.
When TTL DNS time expires, the DNS server has to update his information.


-Port 53 and UDP:
DNS uses the User Datagram Protocol (UDP) on port number 53 to serve requests.


-NSLOOKUP
The DOS command nslookup gives information about DNS server (hostname and IP address). Example:
nslookup www.wikipedia.org
Server : dns1.proxad.net
Address: 212.27.40.240


-The following DOS commands are used to solve some DNS problems:

Ipconfig /displaydns (this command displays the contents of the DNS Resolver Cache)

Ipconfig /flushdns (this command purges the DNS Resolver Cache)

Ipconfig /registerdns (this command refreshes all DHCP leases and re-registers DNS names)

Ipconfig /all (to verify).



-On internet, the most important things are: IP, DNS, routers and optical fiber.


-Remark:
few years ago, WINS was also used in networks: WINS is Windows Internet Name Service. WINS resolved NetBIOS name to IP address.
The following DOS command gave the NetBIOS name of a machine: nbtstat -n
WINS in a LAN was like DNS on internet.
If the network administrator forgot to configure the WINS in a computer of the LAN, then the other computers were not able to see it.


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