DNS server split principle

DNS split is required when you need to provide some resources both internal (intranet) and external (internet) using the same name.

Simple DNS zone

Let's start with a simple reminder.

A standard DNS zone is only internal = no one access any internal resources from the outside. If you want to do so, you have to use a VPN connection.

Simple DNS

The Internet connection is just OUT.

Limitations of simple DNS zone

Shared name

Sometimes you can use the same name on 2 different locations. Therefore, some resources are only accessible when you are inside or outside.

DNS 01 shared name issue

In that example the web server "www" is not reachable from the private zone because the authoritative DNS for the domain doesn't know it!

  • The desktop is only using the DNS in the private area (blue zone)
  • The DNS server in that area doesn't know any "www" server... So the request will be rejected.

Shared internal resources

Another problem might occurred when you want to make an internal resource available from both the inside AND the outside.

DNS 02 split horizon issue

Problem description

From the outside:

  • Smartcards.vehco.com == web-server ... Thanks to the port forwarding TCP 80.

From the inside:

  • Smartcards.vehco.com == domain root !! By default the domain root = DNS server @, not www

As you can see, from the inside you cannot reach "smartcards.vehco.com" website! You need to use "http://www.smartcards.vehco.com" or "http://www"


Think of a mobile user...

  • When using WIFI he'll be in the blue area ; when using 4G he'll be in the Internet.
  • But he only has 1 bookmark to reach the website! And that's smartcards.vehco.com


Shared name fix

That issue is very simple to fix: you just need to add a new entry (type A or AAA) for the remote server into the private's zone DNS.

DNS 03 shared name fix

Now, "www" will be redirect to the remote IP @.

Share resource fix

That is a tricky issue!

To resolve that situation you must use a "DNS split" = "Split horizon" = "brain split".

DNS 04 split horizon fix

Solution principle

Depending on where the request comes from (using source IP @ filter) the DNS will use the "internal" or "external" zone description.

  • Local network + VPN network + loopback => INTERNAL zone
  • Everything else => EXTERNAL zone

>> To access "smartcards.vehco.com" from the INTERNAL zone, you must create a link (CNAME) that point to the web server.

Technical note: the link is using the FQDN "smartcards.vehco.com" while the domain is using the "@" notation.

>> The EXTERNAL zone has only access to a limited set of resources.