DNS Configuration - CompTIA A+ 220-1101 - 2.6
Summary
TLDRThis script delves into the intricate workings of the Domain Name System (DNS), highlighting its role as a distributed database translating domain names into IP addresses. It outlines the DNS hierarchy, including root servers, top-level domains, and the importance of redundancy for server addresses. The script also covers various DNS record types, such as A, AAAA, MX, TXT, SPF, DKIM, and DMARC, explaining their purposes in email routing, security, and verification. The necessity of a secure and well-maintained DNS configuration is emphasized, with examples of how to manage DNS records through text files or web interfaces.
Takeaways
- 🌐 DNS is a distributed database that translates fully qualified domain names into IP addresses used by networks.
- 📚 There is a hierarchical structure to DNS with multiple servers that include 13 root server clusters and over 1,000 different servers.
- 🌐 Top-level domains (TLDs) are divided into generic TLDs like .com, .org, and .net, and country-code TLDs like .us, .ca, and .uk.
- 🏢 DNS records are organized hierarchically, with a root domain, subdomains for services like mail, and potentially regional subdomains for large networks.
- 🔍 Users can use the 'dig' command or 'nslookup' to query DNS records and see the IP addresses associated with a domain.
- 🔁 DNS provides redundancy by having multiple IP addresses for a single domain to ensure availability even if one IP fails.
- 📝 DNS servers store resource records, which contain various types of data, including IP addresses, mail exchangers, and text information.
- 🛠️ A DNS server's configuration can be edited using a simple text file or through a web-based interface, depending on the service.
- 📑 Common DNS record types include A records for IPv4 addresses, AAAA records for IPv6, MX records for mail exchangers, and TXT records for text information.
- ⏱️ Time to Live (TTL) in DNS specifies how long information is cached before a device needs to request it again from the DNS server.
- 🔒 DNS plays a critical role in email security with records like SPF, DKIM, and DMARC that help verify the origin and integrity of emails.
Q & A
What is DNS and what does it do?
-DNS, or the Domain Name System, is the service that translates fully qualified domain names entered into web browsers into IP addresses that networks can understand and use for communication.
Is DNS a standalone server or part of a larger system?
-DNS is not a standalone server; it is a distributed database with multiple servers across the internet that work on a hierarchical system to provide translations for domain names.
How many root server clusters are there in the DNS system?
-There are 13 root server clusters in the DNS system, which actually consists of over 1,000 different servers.
What are the different types of top-level domains mentioned in the script?
-The script mentions generic top-level domains like .com, .org, .net, and country-level top-level domains such as .us, .ca, and .uk.
Can you explain the hierarchy of a fully qualified domain name using the example from the script?
-The hierarchy starts with a period at the top, followed by the top-level domain (e.g., .com), then the second-level domain (e.g., .professormesser), and finally the subdomains like www or mail.
What is the purpose of the 'dig' command in the context of DNS?
-The 'dig' command is used to perform DNS lookups from the command line, showing a summary of the requested information and the IP addresses associated with a domain name.
What is the significance of having multiple IP addresses for a web server?
-Multiple IP addresses for a web server provide redundancy, ensuring that if one IP address becomes unavailable, devices can use any of the other IP addresses to communicate with the server.
What are resource records in the context of DNS?
-Resource records are the data entries in a DNS server that contain information such as fully qualified domain names, IP addresses, and other details necessary for the translation between domain names and IP addresses.
Why is it important to have backups when making changes to a DNS server configuration?
-Backups are crucial because if a DNS server becomes unavailable or misconfigured, it can prevent the translation between domain names and IP addresses, making websites and services inaccessible.
What are some common types of resource records found on a DNS server?
-Common types of resource records include A records (for IPv4 addresses), AAAA records (for IPv6 addresses), MX records (for mail exchangers), and TXT records (for storing text information).
Can you describe the purpose and function of an MX record in DNS?
-An MX record, or mail exchanger record, specifies the mail server responsible for accepting email messages on behalf of a domain, ensuring that emails are delivered to the correct server.
What is the role of a TXT record in DNS?
-A TXT record in DNS is used to store text information that can be queried by others. It is often used for verification purposes, email security, and providing information for SPF, DKIM, and DMARC configurations.
What is SPF and how does it relate to TXT records?
-SPF, or Sender Policy Framework, is a mechanism that uses TXT records to specify which mail servers are authorized to send emails on behalf of a domain, helping to prevent email spoofing.
Can you explain what DKIM and DMARC are and their roles in email security?
-DKIM, or Domain Keys Identified Mail, is a method of associating a digital signature with outgoing mail using a public key in a TXT record to validate the authenticity of an email. DMARC, or Domain-based Message Authentication, Reporting, and Conformance, extends SPF and DKIM by specifying how to handle emails that fail validation and generating reports on email validation results.
Outlines
🌐 Understanding DNS Structure and Hierarchy
The paragraph introduces the Domain Name System (DNS) as a critical service that translates human-readable domain names into IP addresses for network use. It explains that DNS is a hierarchical and distributed database with multiple servers worldwide, including 13 root server clusters and over 1,000 servers, as well as top-level domains (TLDs) like .com, .org, and country-code TLDs like .us and .uk. The script uses professormesser.com to illustrate the DNS hierarchy, showing how subdomains like www and mail fit into the structure. It also discusses the importance of redundancy in DNS with multiple IP addresses for a server and introduces command-line tools like 'dig' and 'nslookup' for querying DNS records. The paragraph concludes with an overview of the types of resource records stored in DNS, emphasizing the system's complexity and the need for careful configuration and backup.
📝 DNS Record Types and Email Security
This paragraph delves into the specifics of DNS records, starting with address records (A and AAAA) for IPv4 and IPv6, respectively, and explaining how they associate domain names with IP addresses. It discusses the time to live (TTL) setting, which dictates how long information is cached before being refreshed. The paragraph then covers mail exchanger (MX) records, which direct email delivery, and text (TXT) records, which serve various purposes, including email security. It introduces SPF (Sender Policy Framework) records that list authorized email servers to prevent spoofing, and DKIM (DomainKeys Identified Mail) records that provide a digital signature for emails. The paragraph also touches on DMARC (Domain-based Message Authentication, Reporting, and Conformance) records, which dictate actions for unverified emails and provide reporting on email validation. Examples of configuring and querying these records are given, highlighting the importance of DNS in securing and managing email communication.
🔒 Advanced Email Security with DNS Records
The final paragraph focuses on advanced email security measures implemented through DNS records. It explains the process of creating a text-based DNS record for SPF to authorize email servers and prevent unauthorized use of the domain name in emails. The paragraph also describes the configuration of DKIM records, which involve adding a public key to a DNS TXT record to digitally sign outgoing emails, ensuring their authenticity. DMARC records are introduced as an extension of SPF and DKIM, providing a policy for handling emails that fail validation checks. The script outlines how to create and add DMARC records to a DNS server, emphasizing the importance of defining the disposition of unverified emails and the process of receiving reports on email validation status. The paragraph concludes by illustrating how these DNS-based security measures work together to protect and verify email communications.
Mindmap
Keywords
💡DNS
💡Fully Qualified Domain Name (FQDN)
💡IP Address
💡Root Server Clusters
💡Top-Level Domain (TLD)
💡dig command
💡nslookup
💡Resource Records
💡A Record
💡MX Record
💡TXT Record
💡SPF Record
💡DKIM Record
💡DMARC Record
Highlights
DNS is a distributed database that translates domain names into IP addresses.
There are 13 root server clusters with over 1,000 different servers in the DNS hierarchy.
DNS includes generic and country-level top-level domains such as .com, .org, .net, .us, .ca, and .uk.
The DNS hierarchy is visualized starting from the top-level domain down to the fully qualified domain name.
DNS records can be configured for specific structures like subdomains and organizational domains.
The 'dig' command can be used to see the translation process from domain name to IP address.
Redundancy in DNS is achieved by having multiple IP addresses associated with a single domain.
The 'nslookup' command can be used to query the local DNS server for domain IP addresses.
DNS servers contain a large database of resource records for domain name and IP address translations.
DNS configurations can be managed through a simple text file or a web-based interface.
A and AAAA records in DNS are used for mapping domain names to IPv4 and IPv6 addresses, respectively.
The Time to Live (TTL) in DNS specifies how long a record is cached before it is refreshed.
MX records in DNS determine where emails should be delivered by pointing to mail servers.
TXT records in DNS are used for storing text information, often for verification or security purposes.
SPF records in DNS specify authorized email servers to prevent spoofing of domain names in emails.
DKIM records provide a digital signature for emails to verify their origin and authenticity.
DMARC extends SPF and DKIM by determining the disposition of emails that fail validation.
DNS is a critical resource; changes should be made with backups and a clear understanding of configurations.
Transcripts
DNS is the domain name system, and we often
refer to this as the service that
converts the fully qualified domain names that we might
type into our browser to something our networks might
use, like an IP address.
But what you may not realize about DNS
is that this is not simply a standalone server that
provides this resource.
There are multiple servers across the internet that
provide these translations and they
work on a hierarchy across all of the different fully
qualified domain names.
This is also a very distributed database,
because we have many different DNS servers on the internet.
There are 13 root server clusters.
In actuality, this consists of over 1,000 different servers.
There are hundreds of generic top-level domains.
These are the domains that are the .com, the .org, the .net,
and others.
And then there are also country-level top-level
domains, like .us, .ca for Canada,
or .uk for the United Kingdom.
Here's a very simple visual representation
of this hierarchy from the perspective
of professormesser.com.
We'll start at the top with a period.
This designates the end of the fully qualified domain name.
And working backwards, we would have a .com, a .net, a .edu,
and others.
Obviously, professormesser.com would be the next layer
in this hierarchy.
So you can see underneath .com, we have .professormesser.
And of course, there may be multiple servers
at professormesser.com.
If you go to my website, then you're
visiting www.professormesser.com,
but I might have a mail server, which
is mail.professormesser.com.
And in very large networks, you may have organizational domain
names-- for example, east.professormesser.com
and west.professormesser.com.
And in the east, there may be certain servers.
In the west, there might be other servers.
Having this hierarchy allows us to configure a very specific
structure, and this works across every fully qualified domain
name on the internet.
If you'd like to see visually how this translation operates
and your system supports the dig command,
you can run at the command line dig www.professormessor.com.
The results of this command show us
a summary of what we requested.
It shows the information that was sent asking specifically
for an address associated with www.professormesser.com.
And then you can see in the ANSWER SECTION,
there are actually three different IP addresses
associated with my web server, and you
can see those IP addresses are listed here.
The reason there are three different addresses for my web
server is for redundancy.
If one of those IP addresses is no longer available,
your device can use any of these other IP addresses
to communicate back to www.professormesser.com.
If you're not on a system that supports the dig command,
you can use nslookup professormesser.com.
This will go out to your locally configured DNS server
and provide answers for the professormesser.com IP
addresses, and you can see the results
of this query show exactly the same three IP addresses.
Behind the scenes, the DNS server
has a large database that contains fully qualified domain
names, IP addresses, and other details that
can help your systems perform this translation between fully
qualified domain name and IP address.
We refer to these as resource records, and in this video,
we'll look at a number of different types of resource
records that are used on a DNS server.
There are over 30 different record types.
We won't go through all 30 of those in this video.
Those record types might be IP addresses, certificates,
host names, and other details.
As you can imagine, a DNS server is a critical resource.
If a DNS server isn't available, you
can make the request to visit www.professormesser.com,
but there's nothing behind the scenes
to make the translation between the fully qualified domain
name and the IP address.
This is why we tell people, if you're making a change to DNS,
make sure you have very good backups
of the previous configuration and that you
know exactly what you're changing in that DNS server.
Many DNS servers have a very simple configuration file
that's written in text, and this is an example of one
of those DNS configurations.
The section at the top is the Start of Authority Record.
This has some mail exchanger records inside of it,
a list of IP addresses, and fully qualified domain names,
and you've got some canonical or alias
names that you've also assigned inside of this DNS server.
This makes it relatively easy if you
need to make changes to a DNS server's configuration
because you can use any text editor to modify or update
this configuration file.
You might also find that the DNS service that you're using
can provide you with a web-based front end to the configuration,
so instead of understanding all of those different records
and understanding where they go in the configuration file,
you can put them all into a web-based front end
and make your changes from there.
The first record will look at and perhaps one
of the most common records you see in a DNS server
is an address record.
We often refer to these as in A record or a AAAA record.
The A records are address records for IP version 4.
So this a record will have a fully qualified domain
name and the associated IP version 4 address.
The quad A records are for IPv6.
The same thing applies.
We'd have a fully qualified domain name,
and we would associate the IPv6 associated with that domain
name.
Here's an example of an A record that's
on the professormesser.com DNS server.
You can see that I am specifying that www.professormesser.com
is an internet address using the A record,
and the IP address associated with that fully qualified
domain name is 162.159.246.164.
If you're configuring this in a DNS text file,
then you also have the option to add remarks or other comments
on that record line.
If the front end to your DNS server
is in a web-based configuration, it's the same information,
but we've separated out the A record, the hostname, the IP
address, and the time to live for this IP address.
The time to live in a DNS server is
specifying how long an end station will remember
this match between fully qualified domain name and IP
address.
This 15 minute time to live means
that a device will make the request to a DNS server
and store or cache that information for 15 minutes.
After 15 minutes, that information
is removed from the cache, and if this device
needs to communicate back to the www server,
it will need to request, again, the IP address
for that particular record.
Here's the same thing, but from the perspective
of IPv6, where we're configuring a quad A record,
and we have a hostname, an IPv6 address, and, again, a time
to live.
Another important record in a DNS is where all of your emails
should be delivered.
This is a mail exchanger record or an MX record.
To make this work, you would need two separate records
inside of your DNS server.
The first would be the MX record.
You can see the mail exchange record in this server points
to mail.mydomain.name.
To be able to obtain the IP address for mail.mydomain.name,
we would need to look at an A record,
and you can see there is an A record
for the mail.mydomain.name, which is 123.12.41.41,
and it is a Linux server.
DNS servers have many different functions they can provide.
One of those is to stored text information that can then
be used for other purposes.
We would store the text information
in a text record or TXT record.
This is usually public information.
Since people are able to query these text
records on your server, these text records
were originally designed for very informal purposes.
But today, we have very specific uses for a TXT or text
record in our DNS server.
For example, we might use this for verification purposes.
We might be making a configuration change
to our domain, and that domain change
requires that you add something very specific to a text
record in your DNS server.
This is because the DNS server configuration is usually
very secure, and only authorized individuals
would have access to make changes to a DNS server's
configuration.
We also use this text record extensively for email security,
and you'll see in a moment how we're able to add information
to a text record that can help verify
the origination of a particular email.
If you want to see some example of text records,
you can look at the text records that
are on the professormesser.com DNS server.
If you're running dig, you can use dig professormesser.com
and then txt.
And then it will show all of the text records
that I have currently configured on my DNS server.
You can see that I have two currently configured,
one for a stripe verification, and another one that
is used for mailgun.org, which is used to send out my email
messages.
If dig isn't available, you can also
view these using an nslookup.
You would use nslookup-type=txt, and then the domain name such
as google.com or professormesser.com.
You can see when you perform a google.com lookup that Google
has a number of text records on their device, which include
things like a Facebook domain verification, a Google site
verification, and a DocuSign text record.
A common text record you might find
is an SPF record, or a sender policy framework.
This is a list of all of the email servers that
are authorized to send messages using your fully
qualified domain name.
This was created to help prevent others
from spoofing your fully qualified domain name
and sending email as if you would send it yourself.
A mail server receiving an email that
says it was from professormesser.com
will query the professormesser.com DNS server,
retrieve this SPF record in the DNS server,
and be able to determine is this something that really
came from an authorized host?
Here's the same process for creating a text-based DNS
record, and you can see, you just
paste in the text that is associated with the record
that you'd like to add.
In this example, you can see that I'm adding the SPF
record into my DNS server, and, again,
I have a TTL of 15 minutes.
We can even take this email security one step further
and provide a digital signature that we can
associate with outgoing mail.
We do this through the use of a DKIM text record,
or Domain Keys Identified Mail.
This is going to be validated by the mail servers
as that message is traversing the network,
and the public key associated with this digital signature
is added to a text record in your DNS server.
Here's the same configuration for that DKIM record,
except we're making the configuration
change in this web-based front end, and you can see,
it's a large bit of text that is the public key for all
of the digital signatures that have been sent from my domain.
Now that we have a way to verify messages that have been sent
and to digitally sign messages that are being sent,
we need some way to determine what we do with those messages
if the verification fails.
We would use DMARC for that purpose.
This is the Domain-based Message Authentication Reporting
and Conformance.
This is an extension of the SPF and DKIM processes
that we've already seen, except DMARC takes the extra step
to determine the disposition that
should be used when someone receives a message that
can't be validated.
You might create a DMARC record on your DNS server that
says, if a message is not validated,
simply accept it, or maybe send it to a spam folder,
or simply reject the email entirely.
The mail servers behind the scenes
keep track of how many mail messages have been validated
and how many have failed the validation,
and then you can receive a report that shows exactly how
many messages were able to get through based on the SPF
or DKIM configuration.
And here's an example of adding one of those DMARC text records
to your DNS.
You can see that the content specifies
what to do with the email messages
and where to send the report so that you can examine how
your mail has been distributed.
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