DNS Records

welcome to the i.t free training video on DNS records this video will look at some of the more common DNS records that are used with DNS DNS records contain the data that is requested by the client depending on the type of DNS record this will determine the kind of data returned to the client in what they can do with it there are a large number of DNS records that can be created in this video I will look at the following DNS records not a complete list but it does cover the more commonly used DNS records that you are likely to encounter and use in your organization to start with I will look at the most widely used DNS record the a record and quad a record and a record stores the IP address for a DNS name for example if a client on the network wanted to know the address of dc1 it would send the request to its DNS server the DNS server would look to see if it had an a record to match the name dc1 if it did it would return the IP address contained in that a record to the client in order to resolve computers and devices on your network and a record is required you can see why there are so many a records in a DNS server each device on the network requires one in order for other devices on the network to obtain an IP address for them a records are used for ipv4 addresses the quad-a records are used for ipv6 addresses this is the only difference between the two fundamentally they perform the same requirement of storing the IP address for a name except they do it for ipv6 addresses although an a or quad a record can contain all the IP addresses for all the devices on the network you may also use them with another type of record called the alias record or cname an alias record creates an alternative record or alias for another record this can also be referred to as a canonical name and thus that is where the name see name comes from to understand when you would use an alias consider on the network you have a server called web one it would be difficult for the users to remember the name web 1 so you create an alias record called WW now when a client asks for the IP address of WWE we'll be given the IP address of web 1 you may be thinking could you create another a record for www although this would work the problem with this approach is that you now have two records that need to be changed if the IP address of web 1 were to change having an alias record instead means that if the IP address of web 1 were to change the alias record would not need to be modified the next advantage of an alias record is that it can transparently be used to route a user to a server for example if you had a second server on the network called web 2 you could at any time change the cname record for the second server for example if the first server needs to be rebooted for maintenance you could change the wwa alias record to point to web - now all client requests 4ww will be given the IP address of web 2 of course an approach like this does not take into account a DNS record being stored in the client cache and using the old IP address until the cache expires the next record that I will look at is the mail exchange or MX record an MX record identifies a mail server for that DNS name for example if an MX record was created for I T free training com this would contain a mail server that could process emails for I T free training com when a mail server has an e mail that it needs to send it needs to locate a mail server that is able to accept mail for that DNS name it does this by using the MX record the mail server will send a query to the DNS server and for the MX record for the DNS name it is trying to send email to in this case the DNS server will respond with the MX record for IT free training com the MX record will contain the server mail 1.83 training comm the mail server now knows to send mail for IT free training com to the server mail 1 dot IT free training com this is how a mail server uses DNS to deliver emails to the Internet the MX record also has a priority associated with it there are multiple MX records for the same DNS name the MX record with the lowest priority value is tried first if there are two MX records with the same priority value the choice of MX record will be random the mail server that is used can be any mail server that knows how to deliver mail to the DNS name to consider an example this DNS server has two MX records for the DNS name IT free training com since the left MX record has a priority of five mail for IT free training com will be sent to the server in this MX record which is mail one dot e free training com notice that there is another MX record with a priority of ten what will happen is this if the mail server mail one I T free training com or to become unavailable an e mail server on the internet will still attempt to deliver email to this server when the delivery fails the MX record with the next highest priority will be tried in this case the mail server that is tried next is mail one highcosttraining com notice that the mail server does not need to have the same DNS name it only needs to know how to deliver mail to that domain what will happen is the mail server for highcosttraining will hold the email for I T free training until the I T free training mail server comes back online when this occurs the email will be transferred this setup is used by many companies for fault tolerance this helps prevent email being lost during extended outages a service record indicates the location of specific services for example 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 data about the service the main ones are the following service target port and priority in most cases service records are created automatically by the application that requires them assuming your DNS server allows dynamic updates for example in an Active Directory environment you could delete all the service records for a domain and they will be recreated automatically not something that I recommend you do in a production environment the next record type is the start of authority or SOA record for each zone created in DNS there is one SOA record associated with that zone and only one this record contains information on the primary name server for that zone the primary name server is the server that will 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 the DNS server that is considered the most up-to-date server to answer queries about DNS records for that zone the SOA record contains 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 SOA record also contains a serial number for that zone each time a change is made to the zone the serial number is incremented this serial number is used in order to keep duplicate copies of the zone data known as a secondary zone in sync if another copy of the zone has a different serial number the DNS server knows that it needs to update the copy of the zone data the last piece of data stored in an SOA record is the refresh time for that zone secondary zones use this information to determine when they should check for updates for that zone if your zone file changes often you may want to consider using a lower value so that your secondary zones will not be out of date for too long the next record type is name server or NS these records contain the authority DNS server for that DNS name this can be a DNS server that has a secondary zone even though the 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 for queries for that zone or in other words contain the source data or data copied from the source to put it another way name servers do not contain cached data for that zone when a DNS record is resolved by a DNS server the DNS server will contact one of these name servers and keep the results in its cache for a period of time if the data were to change the DNS server with a cached DNS record would not be aware of the change and thus would be giving out old and accurate information until the cached record expired in order to be assured that the latest data is used to resolve DNS requests the name server records can be used to identify a DNS server that holds this information the last record that I will look at in this video is the pointer or PTR record this record provides a mapping between an IP address and a name this essentially is the opposite of an a or quad a record this essentially means that given an IP address the name associated with that IP address can be looked up these records are used by reverse lookup tools pointer records are useful in cases where you have an IP address in a log file and you want to identify which computer or device that IP address came from pointer records are not required to run day-to-day activities for example they are not required in order to use Active Directory although not required reverse pointer records can be invaluable in troubleshooting thanks for watching this video from I T free training for more free videos please feel free to subscribe or look at our web page or youtube channel see you next time

Channel: itfreetraining

Views: 327,919

Keywords: DNS Records, AAAA, C Name, MX, ITFreeTraining, Domain Name System (Protocol), Software (Industry)

Id: 6uEwzkfViSM

Length: 11min 32sec (692 seconds)

Published: Sun Jun 16 2013