In our previous article ” DNS Records” we have already discussed two DNS records like Host (A and AAAA) and CNAME. In this article we will be discussing the remaining DNS Records; so let’s start:-

The next DNS Record which we will be discussing now is Mail Exchange or MX Record:

v  Mail Exchanger Record : –

A mail exchanger record identifies mail server for that DNS name. For ex: – If a Mail Exchange record was created for hostdens.com this would contain a mail server that could process emails for hostdens.com. When mail server has an email that it would need to send then it needs to locate a mail server that is able to accept mails for that DNS name. It does this by using an MX record.

The mail server will send a query to a DNS server and ask it for the MX record for the DNS name which is trying to send emails to. In this case, the DNS server would respond with MX record for the hostdens.com. The MX record would contain the server “mail.hostdens.com”. This is how a mail server uses DNS to deliver emails through the internet. The MX record also has a priority associated with it. There are multiple MX records for the same DNS name.

Note: –The MX record with the lowest priority value is tried first. If there are two MX records with same priority value then the choice of MX record would be random.

The mail server that is used can be any mail server that knows how to deliver a mail to the DNS name. For Ex: – Let’s assume that there is a DNS server which has 2 MX records, for the DNS name hostdens.com. Since one MX record has priority of 5 and another MX record has a priority of 10.

The mail for the hostdens.com would be sent to the server in this MX record which is mail.hostdens.com. Notice that there is another MX record with a priority of 10. What would happen if this mail server mail.hostdens.com has become unavailable? An email server on the internet would still try to attempt to deliver the email to this server.

When the delivery fails, the MX record with the next highest priority would be tried. In this case, the mail server that tried next is the mail.sarpshostdens.com. Notice that the mail server does not need to have the same DNS name it only needs to know how to deliver the mail to that domain.

What would happen if the mail server for the sarpshostdens.com would hold the mail for the hostdens.com until the hostdens.com mail server comes back online? When this occurs, the email would be transferred. This set up is used by many companies for fault tolerance.

This helps to prevent an email from being lost during extended outrages.

The next record which we are going to discuss is Service Record or SRV Record.

v  Service Record or SRV Record: –

A Service Record or SRV indicates the location of specific services. For ex. The active directory uses DNS service records to allow clients on the network to find servers like domain controllers on the network. A service record contains the data about a service. The main ones are as follows:

v  Service

v  Target

v  Port

v  Priority

In most cases, service records are created automatically by the application that requires them. A server allows dynamic updates. For Ex:- In an active directory environment you could delete all the service records, for a domain and they would be recreated automatically. Not something recommended that you could do in a production environment.

The next record which we are going to discuss is the Start of Authority (SOA).

v  Start Of Authority(SOA):-

For each zone created in DNS, there is a Start of Authority record which is associated with Zone with one and only one per zone. This record contains an information on the primary name server for that zone. The primary name server is the server that would hold a read-write copy of the zone and is considered to be the DNS server with the most authority for that zone. It is also a DNS server that is considered to be the most up to date server to answer queries about DNS records for that zone.

The SOA records contain the email address for an administrator for that domain. for publicly available DNS zones this allows an individual to retrieve this information to contact the administrator for that DNS zone.

The record also contains a serial number for that zone. Each time changes are made to a zone the serial number is incremented. This serial number is used in order to keep the duplicate copies of the zoned data known as a secondary zone in the sink. If another copy of the zone has a different serial number then the DNS server knows that it needs to update the copy of the zone data.

The last piece of data stored in the SOA record is the refresh time for that zone. Secondary zones use this information to determine when they should check for that zone. If your zone file changes often you may want to consider using a lower value so that your secondary zones would not be out of dates for too long.

The next record type is Name Server (NS).

v    Name Server(NS): –

These records contain the authority DNS servers for that domain or domain name. This can be a DNS server that has a secondary zone. Even though a secondary zone is considered a copy it is still considered to be an authority for that zone.

Name Servers are considered to be the best source of answers to the queries for that zones or in another word contains the source data or data copied from the source. To put it in another words name servers do not contain cached data for that zone. When a DNS record is resolved by a DNS server the DNS server would contact one of these Name Servers and would keep the results in its cache for a period of time.

If the data would get changed the DNS server with a cached DNS record would not be aware of the change and does would be giving out all the inaccurate information until the cached record expired.

In order to be assured that the latest data is used to resolve DNS request the Name Server records can be used to identify a DNS server that holds this information.

 

v  The Pointer Record (PTR):-

This record provides a mapping between an IP address and a name. This essentially is the opposite of ‘A’ or ‘AAAA’ record. This means given an IP address, the name associated with that IP address can be looked up. These records are used by a reverse lookup tool.

Pointer records are useful in cases where you have an IP address and a log file and you want to identify which device or computer that IP address came from. They are not required to run a day to day activities for ex:  They are required in order to use active directory although not required reverse pointer directories can be invaluable in troubleshooting.

 

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