Filter A records in an IPv6 only environment

Mark Andrews marka at
Tue May 1 23:43:54 UTC 2018

If you have a IPv6 only network then there should be no default IPv4 route so
the connect(), sendto() etc. should all fail immediately with network unreachable.
If they don’t then complain to your OS developer. If the application does not
detect these failures complain to the application developer.

Applications should fail over immediately if there is not a covering route.

If you have a local default IPv4 route, the router should be returning network
unreachable and the OS should be processing them. This should cause network
errors to be delivered to the application.

The application should also be implementing happy eyeballs where you fail
over to other addresses with sub second delays when there are multiple
addresses to try.  There is nothing wrong with having multiple connection
attempts happening in parallel.

Mucking with DNS responses is only covering up application defects.  Filtering
A records also breaks DNS64 for applications that are using DNSSEC as they
need to see the A records as well as the AAAA responses.

Notify the application vendor they they have a denial-of-service bug.


> On 2 May 2018, at 9:00 am, Kim Ebert <kim at> wrote:
> Hi All,
> I recently came across an issue where the user program preferred IPv4 over IPv6. This is a problem because I've created a network that is IPv6 only. I'm currently using NAT64 to provide access to services that don't have any IPv6 access.
> Now this particular program could work if the DNS records didn't provide A records. I would rather do everything I can as an admin to make the network just work instead of waiting for all of the end user programs to be fixed, so I started working on filter-a functionality to filter out A responses. Currently implementation status is that it sort of works, which is further than I expected to get in 2 days.
> I wonder if anyone in the community is interested in me sharing the work, and if I should go to the effort to publish the effort?
> If anyone is curious, the offending program is nodejs and npm.
> Thanks,
> Kim
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Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742              INTERNET: marka at

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