DNS on the BIG-IP: IPv6 to IPv4 Translation

I've been writing some DNS articles over the past couple of months, and I wanted to keep the momentum going with a discussion on IPv6 translation.  As a reminder, my first four articles are:

  1. Let's Talk DNS on DevCentral
  2. DNS The F5 Way: A Paradigm Shift
  3. DNS Express and Zone Transfers
  4. The BIG-IP GTM: Configuring DNSSEC


The Address Space Problem

I'm pretty sure all of you have heard about the problem of IPv4 address depletion, so I won't go too crazy on that.  But, I did want to share one quick analogy of how the IPv4 address space relates to the IPv6 space.  There are ~4 billion possible IPv4 addresses and ~3.4 x 1038 IPv6 addresses.  Sometimes when I see a comparison of large numbers like these, it's hard for me to grasp the magnitude of the difference.  Here's the analogy that helped put this in perspective: if the entire IPv4 address space was a single drop of water, the IPv6 address space would be the equivalent of 68 times the entire volume of the world's oceans!  I can't imagine ever needing more IP address space than that, but I guess we will see.

As IPv4 address space is used up and new IP-enabled devices continue to hit the market, companies need to support and manage existing IPv4 devices and content while transitioning to IPv6.  Just last week, ICANN announced that IPv4 addresses are nearing total exhaustion.  Leo Vegoda, operational excellence manager at ICANN, said "Redistributing increasingly small blocks of IPv4 address space is not a sustainable way to grow the Internet. IPv6 deployment is a requirement for any network that needs to survive."  As companies transition to IPv6, they still face a real issue of handling IPv4 traffic.  Despite the need to move to IPv6, the fact is most Internet traffic today is still IPv4.  Google has a really cool graph that tracks IPv6 adoption, and they currently report that only 3.5% of all Internet traffic is IPv6. 

You would think that the people who developed IPv6 would have made it backward compatible with IPv4 thus making the transition fairly easy and straightforward...but that's not true.  This leaves companies in a tough spot.  They need a services fabric that is flexible enough to handle both IPv4 and IPv6 at the same time.  The good news is that the BIG-IP is the best in the business at doing just that. 


BIG-IP Configuration

Let's say you built an IPv6 network and things are running smoothly within your own network...IPv6 talking to IPv6 and all is well.  But remember that statistic I mentioned about most of the Internet traffic running IPv4?  That creates a big need for your network to translate from IPv6 to IPv4 and back again.  The BIG-IP can do this by configuring a DNS profile and assigning it to a virtual server.  You can create this DNS profile by navigating to Local Traffic >> Profiles >> Services >> DNS and create/modify a DNS profile.  There are several options to configure in the DNS profile, but for this article, we are just going to look at the DNS IPv6 to IPv4 translation part.  Notice the three DNS IPv6 to IPv4 settings in the screenshot below:  DNS IPv6 to IPv4, IPv6 to IPv4 Prefix, and IPv6 to IPv4 Additional Section Rewrite.



The DNS IPv6 to IPv4 setting has four options.  This setting specifies whether you want the BIG-IP to convert IPv6-formatted IP addresses to IPv4-formatted IP addresses.  The options for DNS IPv6 to IPv4 are:

  • Disabled: The BIG-IP does not map IPv4 addresses to IPv6 addresses.  This is the default setting.

  • Secondary: The BIG-IP receives an AAAA (IPv6) query and forwards the query to a DNS server. Only if the server fails to return a response does the BIG-IP system send an A (IPv4) query. If the BIG-IP system receives an A response, it prepends a 96-bit user-configured prefix to the record and forwards it to the client.

  • Immediate: The BIG-IP system receives an AAAA query and forwards the query to a DNS server. The BIG-IP then forwards the first good response from the DNS server to the client. If the system receives an A response first, it prepends a 96-bit prefix to the record and forwards it to the client. If the system receives an AAAA response first, it simply forwards the response to the client. The system disregards the subsequent response from the DNS server.

  • v4 Only: The BIG-IP receives an AAAA query, but forwards an A query to a DNS server. After receiving an A response from the server, the BIG-IP system prepends a 96-bit user-configured prefix to the record and forwards it to the client. Only select the v4 Only option if you know that all DNS servers are IPv4-only servers.


When you select one of the options listed above (except the "Disabled" option), you must also provide a prefix in the IPv6 to IPv4 Prefix field and make a selection from the IPv6 to IPv4 Additional Section Rewrite list. 

The IPv6 to IPv4 Prefix specifies the prefix to use for the IPv6-formatted IP addresses that the BIG-IP converts to IPv4-formatted IP addresses. The default is 0:0:0:0:0:0:0:0.

The IPv6 to IPv4 Additional Section Rewrite allows improved network efficiency for both Unicast and Multicast DNS-SD responses. This setting has 4 options:

  • Disabled: The BIG-IP does not perform additional rewrite.  This is the default setting.

  • V4 Only: The BIG-IP accepts only A records. The system prepends the 96-bit user-configured prefix (mentioned previously) to a record and returns an IPv6 response to the client.

  • V6 Only: The BIG-IP accepts only AAAA records and returns an IPv6 response to the client.

  • Any: The BIG-IP accepts and returns both A and AAAA records. If the DNS server returns an A record in the Additional section of a DNS message, the BIG-IP prepends the 96-bit user-configured prefix to the record and returns an IPv6 response to the client.


Like any configuration change, I would recommend initial testing in a lab to see how your network performs with these settings.  This one is pretty straightforward, though.  Hopefully this helps with any hesitation you may have with transitioning to an IPv6 network.  Go ahead and take advantage of that vast IPv6 space, and let the BIG-IP take care of all the translation work!  Stay tuned for more DNS articles, and let me know if you have any specific topics you'd like to see.


One final and related note: check out the F5 CGNAT products page to learn more about seamless migration to IPv6.


Published May 29, 2014
Version 1.0

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  • Hi,


    I have a question.


    Is it compulsory to enable option "DNS IPv6 to IPv4" if we host IPv6 listener on BIG-IP DNS (GTM)?


    We are experiencing strange issue. User belong to one of the Europe region not able to connect application when they connect from their home Wi-Fi which have IPv6 addresses enabled.


    On GTM we have IPv6 listener and IPv4 listener which shares same DNS profile which enabled with option "DNS IPv6 to IPv4" (Secondary). Because of this end user receives two records in IPv6 addresses in format of "::xxx.xxx.xxx.xxx".


    Do you think this could be a reason for issue we are currently experiencing?


    When we ask client to change their IP schema from IPv6 to IPv4 it works perfectly fine.