Feature request for parallel queries for name servers from different protocol families (IPv4 vs IPv6)

Brad House brad at brad-house.com
Tue Jan 18 23:56:10 CET 2022

We would certainly take patches to accomplish something like this. I'm 
not sure how easy it would be to work the internals of c-ares to 
understand parallel nameservers for the same query, it would probably be 
a substantial change though.

Also, I'm also not sure how "friendly" it is to actually perform the 
requests in parallel at all times.

Infact, happyeyeballs itself doesn't always do parallel connection 
attempts, its an implementation-defined delay before also attempting the 
next address in the list. It also has you maintain a feedback loop in 
order to sort known bad addresses to be least prefered.  It would be 
much easier to stay closer to happy eyeballs and just sort the dns 
server list using prior result success/fail (even upfront sorting using 
some algorithm to interleave ipv6/ipv4 in a pattern would help, maybe 
with using logic such as from RFC6724 sec 2.1 like we do in 
ares_getaddrinfo for returned addresses, but instead of the nameservers 

Obviously, if you're using something that isn't maintaining state, the 
feedback loop won't help, but maybe the pre-sorting suggestion would. 
Assuming you're using libcurl, and not a stateless command line curl, I 
think it would ultimately accomplish your goal.


On 1/18/22 4:23 PM, Dmitry Karpov via c-ares wrote:
> Hello,
> Using libcurl with c-ares for dual-stack scenarios, I observed that 
> c-ares doesn’t distinguish between IPv4 and IPv6 nameservers in the 
> name server list (i.e. listed in resolv.conf on Linux systems)  and 
> iterates through them sequentially.
> Such approach creates problems for dual-stack systems, when one stack 
> is either not fully functional or have not responding/reachable name 
> servers put on top of the list.
> In such scenarios, problems with one stack (i.e. IPv6) may create name 
> resolution delays and timeouts for the other fully functional stack 
> (i.e. IPv4) if name servers from the not functioning stack go before 
> the good stack like:
> [IPv6 - BAD]
> 2001:4860:4860::8888
> 2001:4860:4860::8844
> [IPv4 - GOOD]
> In this scenario, it will take two resolution timeouts for bad IPv6 
> name servers before good IPv4 name servers are reached, and this 
> negatively impacts Happy Eyeball implementations in client 
> applications (i.e. libcurl) which have to wait for too long before 
> they can start dual-stack connections.
> So here is the feature request which should help dual-stack client 
> applications to work more efficiently when name servers from not 
> functioning stack are listed before name servers from a good stack:
>   * Split the flat name server list for both stacks into two lists for
>     each stack.
>   * Execute parallel DNS queries for each stack list, iterating each
>     stack list sequentially as it is currently done for the whole
>     dual-stack list.
>   * Return the result whichever comes first.
> This feature request can be also considered in a broader scope:
>   * Run parallel queries for each name server in the dual-stack list
>     regardless of whether it is IPv4 or IPv6 address.
>   * Return the result whichever comes first.
> Such broader scope will also allow to skip over bad name servers and 
> get host resolution results much more quickly even for the same stack 
> (i.e. when name server list for a single stack contains some not 
> responding name servers at the top).
> But in this case, it should be probably controlled by some new c-ares 
> option, so the current sequential approach may be enforced if needed.
> Thanks,
> Dmitry Karpov
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