[dns-operations] IPv6 Connectivity on Firefox

Shane Kerr shane at time-travellers.org
Wed Nov 25 12:44:46 UTC 2015


I'm not sure that this is the best list to ask this question. Possibly
a Firefox development list would be a better place.

Still, lets have a look at your questions...

At 2015-11-25 02:06:41 -0800
Fatemah Alharbi <falha008 at ucr.edu> wrote:

> I am a PhD student at UC Riverside, CA, USA. I am conducting
> experiments using Firefox to test Ipv6 connectivity and I would
> appreciate your cooperation and help to answer the following
> questions. 
> Experiment 1:
> By default, Firefox has Ipv6 enabled. When I visit a particular
> website, Firefox always sends 2 DNS queries: A query(For IPv4
> address) and AAAA query (for IPv6 address). It always prefers the
> IPv4 address, why?

Firefox wants to know the IP addresses of the web server you are
connecting to. These can be either IPv4 or IPv6, so two separate
queries are needed.

As for why Firefox prefers IPv4... Firefox uses what we call "happy


You can read their ticket about the details of the implementation here:


Most likely in your case it is always preferring IPv4 because it is
faster (or at least lower-latency).

> Experiment 2:
> Please see the attached file: block_A_record_google.pcapng. I blocked
> the IPv4 response packets (for the A queries) of the domain name
> clients1.google.com to see how Firefox would behave if it gets only
> the AAAA response. Even if Firefox gets a positive response for the
> AAAA query, it ignores the response and keeps asking for the Ipv4
> address! Why?
> Does that have anything to do with my ISP Ipv6 connectivity? My ISP,
> Charter, doesn’t support Ipv6 connectivity to residential customers
> yet. If so, how Firefox does know that my ISP doesn’t support Ipv6
> connectivity? How does Firefox know that the host triggering the DNS
> requests is not within an Ipv6 network?
> This is the result of the Ipv6 connectivity test on Firefox (Please
> see the attached file: Test Your Ipv6 Connectivity-Firefox.pdf):

Since you don't have IPv6 connectivity, disabling IPv4 by blocking DNS
lookup for A records just means that you can't connect to the Internet.
It looks like the browser is continuing to try to connect to the web
page by retrying the queries. I'm not sure what you think the browser
should do?



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