Re: [musl] Behavior change in getaddrbyname() with AF_UNSPEC

[Rich Felker <dalias@libc.org>]

On Sat, Jan 14, 2023 at 10:56:28PM +0000, Barry Bond wrote:
> This is related to this change:  https://git.musl-libc.org/cgit/musl/commit/?id=5cf1ac2443ad0dba263559a3fe043d929e0e5c4c made back in 2020.
> 
> In the repro case, getaddrbyname() with AF_UNSPEC sends out two
> requests, but only gets back a single response, with the ipv4
> address. There is no ipv6 on the network.
> 
> name_from_dns() contains the relevant code. After __res_msend_rc()
> returns, 'nq' is 2, and 'alens' is [96, 0], indicating that there
> was an ipv4 response of 96 bytes, but no response for ipv6. Then the
> validation code runs:
> 
>                 for (i=0; i<nq; i++) {
>                                 if (alens[i] < 4 || (abuf[i][3] & 15) == 2) return EAI_AGAIN;
>                                 if ((abuf[i][3] & 15) == 3) return 0;
>                                 if ((abuf[i][3] & 15) != 0) return EAI_FAIL;
>                 }
> 
> and the result is EAI_AGAIN, because alens[1]==0.
> 
> Before this patch, the code would have parsed the ipv4 response via
> __dns_parse(), failed to parse the empty second response because
> alens[1]<12, and the function would return with ctx.cnt==1.

That was the wrong behavior that this patch fixed. Previously, the
query was timing out, but because there was an answer to the other
query, we were erroneously hiding the failure from the application and
presenting a timing-/network-congestion-dependent incorrect result
(wrongly claiming only A or only AAAA exist when in fact we didn't get
enough information to make that determination).

"There is no ipv6 on the network" is not cause for the AAAA query to
timeout. The ability to lookup a particular RR type has nothing to do
with what network protocols are supported on your network. Can you
describe the environment this is happening in and why it might be
happening?

Rich