- [Voiceover] DNS servers can live on the internet of course
but they can also exist within private networks
hosting information about the resources that are available
within that private network.
DNS servers can be placed safely almost anywhere.
You can start up a DNS server on a network
without fear of disrupting that network
because no client will request information
from your DNS server unless it has been configured
to use your DNS server to look up information.
That's called a recursive query.
They're only used by client systems
that are configured to use them.
This is dramatically different from a DHCP server
which can be dangerous on a new network
if it conflicts with another DHCP server.
This is because DHCP responds to broadcasted queries
that are sent to an entire subnet by a device
when that device joins the network.
On the other hand, DNS servers will sit quietly
until a client registers a request specifically
with that server directly.
You can, of course, and in most cases should
configure a DHCP server to automatically
configure client computers and devices
to use a specific DNS server that you have decided
will serve your network operation best.
This gives client systems a common place to look
for the answers to their domain name resolution queries.
Having your domain listed in both internet-based
and private DNS servers
is sometimes called split horizon DNS.
This split system has your local private DNS server
for your domain directing traffic
to the private IPs for those resources
and the public DNS servers directing traffic
to the public IP addresses for those same resources.
Your DNS administrator will do this when they want
to host their servers inside the same network
with the client systems.
The local private DNS server will tell clients
on the local network to send their traffic
to the server that is on the local network
using internal private addressing.
This avoids the unnecessary traffic that would have
to go out to the internet, only to come back in the same way
that would be used by outside clients.
That same admin will have internet traffic
come to an externally available IP address
and be routed through a firewall
to the specific server inside the network
that matches that host name or usually a specific service
using the standard port to which that service is assigned.
By deploying a private DNS server, that DNS administrator
also has the ability to provide names
and IP address mappings and service listings
for services that may only exist inside the network.
This kind of private DNS name resolution is very common
and allows for easy identification of resources
like printers and servers that have static
or unchanging IP addresses
so they can always be found in the same place on the network
by all client systems.
Now many printers and client and server systems today
are also using something called Bonjour! or mDNS.
Bonjour! also performs this mapping of names
to IP addresses, but without using a DNS server.
Bonjour! will be outside of the scope of this class
because it doesn't use DNS.
But I wanted to introduce the concept here
because Bonjour! is a very popular way to access resources
on a local network via a device's name
that does not require static addressing
or the entry of data manually into a table or database.
Of course it is only available on a local subnet
so if you try to use Bonjour! across subnets
or on the internet
you'll have limited to no success depending upon
which you're trying.
It is handled completely automatically
by operating systems that support it, however,
and is increasing in popularity.
So DNS is a system for mapping names to numbers
and numbers to names.
It is used when you send mail addresses to a given domain.
It is even used in recent incarnations
for listing which services reside on which systems
for a given domain.
DNS is even used today to assist with special instructions
for mail security and spam prevention.
DNS is safe to play with on a non-production server
as it will only affect clients configured to use it.
So if you feel like learning DNS, as I'm sure you do,
spool up a small test network
fire up a copy of MAC OS 10 Server
or a router that can handle Bind or Unbound
like the pfSense router we will show in this course,
or maybe a copy of Windows Server 2012
with its DNS manager.
Any of the above will be perfect platforms on which
to learn about DNS.
And you should choose the one that is most likely to be
in use on the network you use either at home or at work.
As we proceed with this course
you will see the interfaces for each of the systems
I mentioned so you have a broad exposure to interfaces.
Principles of DNS are the same on all of them by necessity.
DNS is an internet standard and as such,
all of the players are best served
when they all obey the same rules.