I like the idea of adding additional bit to CURLOPT_IPRESOLVE, but actually I
think about the "Auto" or "Dual" mode, which should probably provide the
current "both" IPv4 and IPv6 instead of the "Also" mode enforcing certain name
resolution (i.e. IPv4),.
And probably the "auto" mode should not be used by default, because when I
enabled IPv6 in libcurl, I stepped on name resolution regressions and even
timeouts, which didn't occur when IPv6 wasn't used.
I used explicit CURL_IPRESOLVE_V4 settings to avoid any dual-stack issues, but
still stepped on them in a quite unexpected way.
So, I think that results delivered by explicit CURL_IPRESOLVE_V4 should be
agnostic by default to whether IPv6 is enabled in libcurl or not.
It is just doesn't seem natural to specify CURL_IPRESOLVE_V4 to indicate that
only IPv4 addresses should be used and step on problems with not relevant IPv6
queries.
That's why I think that by default the behavior should be probably like what my
patch provides:
CURL_IPRESOLVE_V4 - only IPv4 DNS resolution is used
CURL_IPRESOLVE_WHATEVER - both IPv4 and IPv6 DNS resolutions
CURL_IPRESOLVE_V6 - both IPv4 and IPv6 DNS resolutions
The reason why I think that CURL_IPRESOLVE_V6 should provide both IPv4 and IPv6
name resolutions is because IPv4 is still much better supported than IPv6, and
it is more likely to have DNS problems with IPv6 than with IPv4 (at least
that's my experience with a quite large set of ISPs and routers).
This means that if some client wants manually (not via the
CURL_IPRESOLVE_WHATEVER dual-stack option) to try IPv6 first and switch to IPv4
in case of failure, then successful IPv4 name resolution will be very handy.
> Your patch looks like it might work. I'm still mostly concerned that this
> takes us back in behavior a little bit and that it will make someone else not
> like that and come here in a few months with an issue about it.
Sure, I understand the concern. But with the IPv6 DNS issues that I observed
(slow and missing AAAA responses) the use case described in 84d2839740ca7804
may result into timeouts or slow name resolution regressions.
With my patch (or some other similar solution), it will be able to avoid such
regressions with a price of additional AAAA query when the client decides to
switch to IPv6 resolve mode after IPv4.
This switch to IPv6 will fail under such circumstances, but because IPv4 host
name resolution is already in the cache, the client can quickly switch back to
IPv4 and proceed the transfer.
And with the "Auto" mode bit turned on, some special clients can enforce "dual"
queries even for IPv4 resolve mode, but this seems like a very special use case
to me to be enabled by default.
Thanks,
Dmitry Karpov
-----Original Message-----
From: Daniel Stenberg <dan...@haxx.se>
Sent: Saturday, July 9, 2022 3:14 PM
To: Dmitry Karpov via curl-library <curl-library@lists.haxx.se>
Cc: Dmitry Karpov <dkar...@roku.com>
Subject: RE: IPv6 resolution problems for IPv4 resolve mode
On Thu, 7 Jul 2022, Dmitry Karpov via curl-library wrote:
> So, the intent described in the statement "It is important that *all*
> addresses versions are resolved, even if not used..." from
> 84d2839740ca7804 will cause transfer timeouts for IPv4-only resolution
> mode in case of bad/missing AAAA responses.
I would rather say that it *may* cause timouts, as it certainly is not how
things *should* work.
Your patch looks like it might work. I'm still mostly concerned that this takes
us back in behavior a little bit and that it will make someone else not like
that and come here in a few montths with an issue about it.
Wouldn't it be better if we add a bit to the CURLOPT_IPRESOLVE option that says
something like "enforce this in name resolve too" ? Like:
#define CURL_IPRESOLVE_ALSO_RESOLVE 0x10 /* name to be debated */
to be used like this:
curl_easy_setopt(c, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4 |
CURL_IPRESOLVE_ALSO_RESOLVE );
... which then makes the name resolve for this transfer also just ask for IPv4
and the connect only be done using IPv4?
Maybe even make the DNS cache use family preference as a key for lookups so
that we have separate IPv6-only and IPv4-only results in memory for the same
host?
--
/ daniel.haxx.se
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