curl / Mailing Lists / curl-library / Single Mail
Buy commercial curl support from WolfSSL. We help you work out your issues, debug your libcurl applications, use the API, port to new platforms, add new features and more. With a team lead by the curl founder himself.

Re: Feature request: provide ability to set a global callback function telling libcurl if IPv6 works on the system

From: Daniel F via curl-library <>
Date: Tue, 20 Sep 2022 09:00:23 +0200

W dniu 2022-09-20 08:22, Daniel Stenberg via curl-library napisaƂ(a):
> On Mon, 19 Sep 2022, Dmitry Karpov via curl-library wrote:
>> Working with IPv6-enabled dual-stack libcurl, I noticed that for some
>> Linux kernel configurations, it takes ~15-30ms more for dual-stack
>> libcurl to establish IPv4 connections than for IPv4 single-stack
>> libcurl. After looking into this issue, it turned out that the
>> regression was caused by Curl_ipv6works() function enabled in IPv6-on
>> builds.
> The origin for that IPv6 check was that back in the 90s it happened
> that configure detected IPv6 support but then at runtime curl couldn't
> actually use it.
> I dare to suggest that we don't see that kind of system setups and
> behavior anymore. I think maybe the time has come to make instead
> rather assume that it works when built IPv6 enabled. At least I think
> it is worth exploring that route first before we consider adding a new
> API function for it.

This is not true. Application can be built using curl with IPv6 enabled
(e.g. using RedHat 8 base docker image), then executed on another
system/machine with IPv6 disabled. If I remember correctly, attempt to
create IPv6 socket failed with error.

> --
>  /
>  | Commercial curl support up to 24x7 is available!
>  | Private help, bug fixes, support, ports, new features
>  |
Received on 2022-09-20