Whenever we access a domain name services server in order
to gather information about the name or the IP
address of a resource, we're really
looking through a local database that
has all of these associations between a fully qualified
domain name and an IP address.
We called these records on a DNS server the resource records,
and it's really a text file that happens
to contain all of the information we need to be
able to perform these lookups.
We'll look at a number of different record types
in this video, but there are over 30 different types
that you may find inside of a DNS server.
The database of a DNS server is simply a text file.
This is where you would configure the DNS server
and this is where all the lookups will take place.
You can see information in this sample lookup
file that has information about the DNS server itself
and information associated with the caching.
You also see mail exchange records.
There's records that are specifying the name and the IP
address of other devices and this look up also contains
aliases.
This allows us to use different names to associate
to a single fully qualified domain name.
One of the most important roles of a DNS
is the one where we are associating an IP
address with a fully qualified domain name.
And if we look at this section of the lookup file,
you'll see there are some machine names
and you'll see IP addresses associated with those machine
names.
Let's break out these individual address resource records.
These address records are called A records or quad A records.
And they're used to associate the name with an IP address.
We use the A record to associate an IP version 4 address
with the name of a device.
And we use a quad A record to associate an IP version 6
address with the name of a device.
Here's an example of an record.
I have the name www.professormesser.com.
There's also a specification of the class of this record.
And for most of us, the class of this record
will be the internet class.
This is an A record, so this will
be an IP version 4 address.
And you can see the IP version 4 address is also
included as part of this record that associates this IP address
with www.professrmesser.com.
You can also add a semi-colon at the end of the record
to designate a comment, so that you can add other contexts
to this particular record.
There may be times when we may want
to associate a number of different names
with an existing device.
In those situations, you may want to use a canonical name
record or see CNAME record.
This is where you would add the alias to something
that already exists.
For example, if you had a device called mail.mydomain.name,
you could also call this the ftp server for ftp.mydomain.name,
www.mydomain.name.
Your DNS server will recognize that this is a canonical name.
It will associate it with the primary name of this device
and then perform the proper lookup.
There are also DNS records that can
help you find particular services on your network.
For example, you could create a service record or SRV
record that would allow Windows devices to find Windows domain
controllers on the network or you might add a service record
so that voiceover IP devices can find the centralized voiceover
IP controller on your network.
For example, here's the service record
that you use for Windows devices to find an Active Directory
controller.
You can see the service is ldap, the protocol name
is tcp.domain.com in this example.
The time to live is 300 seconds.
This is the internet class.
It is an SRV or service record.
There's priority and weight values, a port number, and then
the target that allows you to associate
this particular service with a particular device.
If you're in charge of the mail servers on your network,
then you know the MX record or the mail exchange record
is an extremely important record that
allows third parties to be able to find your local mail
servers.
In your DNS configuration file, the MX record
starts with the IN class for internet.
It is an MX record.
And then you put the name of the mail server.
Later in the configuration, you'll
find the name of that device is listed
with an A record that will specify the IP
address of that mail server.
Many DNS servers also contain configuration details
for additional name servers on the domain.
These are NS records or name server records.
The format of a name server record
would specify the class of this record, which is internet.
This is the name server record, or NS.
And then you'll list out the names associated
with those name servers.
Later on in the configuration, you
can find A, or address records, that
will associate the name of the name server
to a specific IP address.
We commonly use DNS servers to associate a name with an IP
address, but we can also do the reverse
where we provide the name server with an IP address
and it will provide us with the name.
It's able to do this by using pointer records,
or PTR records, which are effectively the reverse of an A
or a quad A record.
In the configuration of our DNS, we
would list out the IP addresses associated with these devices.
We would specify that these are PTR or pointer records,
and then we would provide the fully qualified domain
name of these devices.
Another common DNS resource record you'll find
is a text record, or a TXT record.
This record usually contains information
that will be valuable to other third party services that
are accessing your DNS.
For example, a text record might contain
SPF protocol information-- this stands for Sender Policy
Framework.
And it's a type of text message that
helps prevent someone else from spoofing your domain name
as emails are sent out across the internet.
Other mail servers can check this text record
and determine if this particular email address really
came from your email server.
We also use text records to add DKIM, or Domain Keys Identified
Mail.
This allows your mail servers to digitally sign outgoing mail.
And when the external mail server receives this,
they can confirm that this was, indeed, digitally signed
by your mail server.
You're effectively adding your public key
into this particular DNS text record.
Here's an example of these two text records in my DNS.
This first record is an SPF TXT record.
It contains information that tells other mail servers what
the valid mail servers will be for professormesser.com.
So I list out my domain name, professormesser.com.
I've got a timeout value of 300 seconds.
This is a text record, and there's the SPF information.
Third parties will be able to look at this text
record on my DNS server, and see that if mail is coming
from mailgun.org, than it is authorized as a mail
server for professormesser.com.
I also have a domain key text record listed here,
which shows the domain key information
for professormesser.com.
This is a text value.
And you can see the entire public key
is listed under this DKIM1 value.
This allows third-party mail servers
that have received a digitally signed email from my mail
server to get the public key and validate that signature.