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DNS – Domain Name Servers

DNS is nothing more than a server that, through port 53, responds to text requests, and returns IP addresses. In other words ...
When you browse the internet, when you type the famous “www.google.com” into your browser, this typed text, which is the URL, goes to your internet provider's DNS server or some other particular chosen. You can do this through the MS-DOS Prompt, typing CMD in “Start> Run”, and in the command line, the program “NSLOOKUP”.

The DNS server responds to “www.google.com” the values: 64.233.161.99, 64.233.161.104, 64.233.161.147.
This is because the Google server, there are several spread over several places, usually when searching for addresses using the command "NSLOOKUP" we find different addresses for Google.
But, in general, this is not common for other sites, where they have a single, fixed address, which does not change, and which, if changed, creates a big problem.

Configuring a DNS serverIn rare cases, a company needs to have a private DNS server to answer the responses from the internet, and it is widely used for Intranet queries, since no other server in the world could respond to local addresses other than the local server, since the pages are local.
To define an address on the server, it depends a lot on the server used, but many have a friendly interface, and others not much, but for you to understand the logic, it goes like this:
Addresses are records as if they were in a table. There are several types of records.
Type A records are records that the IP address has authority over the domain. That is, the IP address is correctly configured for the domain, but if it is changed, it is pointed to another server.

CNAME type records are records that point to a specific destination. One of the big mistakes of some sites on the internet, is to define the A record with "www" in front, this causes that if the user types the address without the "www", he ends up entering another site, or not being able to enter. The best thing to do is to define “example.com” as an address in the registry as A for an IP address, and “www.example.com” as a CNAME with the value “@”.
If the DNS server you are using does not accept the value "@", you can enter the IP address normally, as in the "A" record.

Well, this is the basics for the address to be located on the internet, or on the intranet. Remember that for intranet, the address range must be those that own local networks, that is, those beginning with “10. *. *. *” Or “192.168. *. *” Or “172.168. *. * ”.

Other than that, there are reverse registrations, which in reality are registered so that if the user searches for the IP address, the name of the domain owner is returned.

That is, if you type http://10.159.0.5/ in your internet browser, some browsers look for the reverse address to check the reliability of the website. Typically, servers that receive text messages from emails, always check the reverse address to verify the reliability of the message. If it is not recognized, it is automatically marked as SPAM, or it is automatically deleted, as with the Terra server, they delete these messages with wrong or false reverse records automatically.

It is necessary to configure the reverse address so that a computer or server can send emails, because many servers mark messages as SPAM when receiving messages with the reverse missing or with errors.

To register Reverso, it goes something like this:
IP: 10.159.219.54
Reverse: 54.219.159.10.domain.com

You can see your reverse IP address from your internet, accessing some sites like  www.meuip.com.br .

In addition, there are also records of type TXT, SPF, AAAA and others.
SPF is a registry with information of authenticity of the domain, who is the owner, size of the server, among others, this serves to release e-mail messages on various sites that check the veracity of servers that send messages. Without configuring SPF, it is very unlikely that messages will be delivered.
AAAA are records of IP addresses of the IPv6 type, this, in this case, is for you to already allow your server to be seen through the ipv6 network. The google site, launched an address to verify that you are connected correctly over IPv6, is  http://ipv6.google.com/  . You can log in to test if your connection is correct and functional for the new mode for the sixth generation of IP addresses.
NS records
These are the names of the servers where the DNS settings are hosted. It is mandatory to have two DNS servers to answer the requests for each website, usually a website hosting company, or reseller of domains like  www.godaddy.com, offer this service for free, and it comes practically pre-configured, without needing to change many things. It is also possible to assign local IP addresses for these servers, however, it must exist on your local network, the server with a given IP address, otherwise the page will not be displayed. For example, suppose you register a domain for your company, such as "yourcompany.com", and you want the subdomain "intranet.sycompany.com" to work only within your company, it happens that the DNS server is Godaddy's, so , just create the subdomain on servers with local IP addresses on your local network. Of course, on the internet, IP addresses will not work, no one will have access to your local network, but at most, you could know what IP address you have assigned for a particular record, but as the network is local,it cannot be accessed externally by normal means.
Another way to make a subdomain work in your company, without working on the internet, would be like the example of the NS record, you create a subdomain in Godaddy, and only point out the new NS servers, which are located in your company. Every time a user or employee accesses the intranet, they will access the Godaddy server, which will inform the DNS server (NS Record) responsible for the domain, (it must be two servers, or 1 with two fixed IP addresses registered). On the local server, they must contain type “A” or “AAAA” records for your network and services to work correctly.

MX records

Main functionality, is to receive emails. That is, without MX records, your company does not receive emails, because it does not know where to send them. Example:
Someone sends an email to [email protected] This person's messaging server, "SMTP", will search the DNS server records for your domain "yourcompany.com" for which computer or server is responsible for receiving messages. The SMTP server makes the query in your domain and comes the answer: Priority: 10, mail exchanger = smtp1.yourcompany.com. Then SMTP will know that the domain smtp1.yourcompany.com is responsible for receiving emails. But it's not over yet. The SMTP server will look for the “A” or “CNAME” type record to obtain the IP address.

So let's assume that when you find it, you receive the record “CNAME, 64.39.211.33 ″ (it can be any other server, it can even be servers like Google or Hotmail, they already work to receive emails from different domains, so that it works perfectly in your company, as an efficient alternative to corporate email).
Well, we learned that there are some basic records, and their main needs. I believe that with this, you will already be able to have a logic of how to configure a server. There are servers that work through the command line, others through the browser interface and others through programs and applications, and others that are spread over the internet, which offer the service for free.
Of course, for many services, it is necessary to have a fixed IP address, which can be contracted by the internet provider, so that your computer or server is connected directly to the internet.
Note: Home connections are usually dynamic IP addresses, that is, it changes with each access. The records of the servers have a TTL (Time to Left), which is a time to be expired, that is, every time you make a change to the DNS server, this goes into effect, immediately, however, the browsers that accessed it your site, received that the domain has an IP address with a long lifetime, and the browser will not bother to look for your IP again, occurring that, therefore, it is impossible to define a website on the internet with dynamic IP, due to this.

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