Re: Anybody Using IPv6?

Carlos E. R. <robin_listas@es.invalid> wrote:

On 2025-05-21 16:52, Marc Haber wrote:

"Carlos E. R." <robin_listas@es.invalid> wrote:

On 2025-05-21 13:16, Marc Haber wrote:

I think that the local resolver should? also refrain from asking for
AAAA records if the local system doesn't have IPv6, but I don't know
whether this special-case handling is implemented at all. And I'm too
lazy to look that up.
>
But all this needs to be taken into account before someone can comment
about speed of one IP protocol compared to the other on a level that
is beyond passing myths.

>
It just is a perceived fact. On some machines, if an application gets
back from the system a list of addresses to try, and tries IPv6 first
when there is no actual IPv6 internet connectivity, there is a small
delay waiting for the request to fail, and then try the next address in
the list.

 
pcap/strace or it didn't happen.

 
Asking for a trace _now_ is ridiculous. It certainly did happen, and to
to several people. Years ago.

Looks like it was only last year when I encountered package list
downloads failing in Aptitude on an IPv4-only VPS due to it trying
to connect on IPv6.

Disabling IPv6 on there made perfect sense - it's intended to be a
stable system, not a testing ground for applications. I knew IPv6
wasn't available, and I'd seen such behaviour before in an
unimportant program on another system, so really it's my fault for
leaving the door open to such bugs by pointlessly leaving the
kernel's IPv6 support enabled.

Please note that _broken_ IPv6, for example when the router announces
an IPv6 but the network doesn't return a host unreachable ICMPv6
message from the place where connectivity is missing, will cause an
IPv6-enabled application to wait for the time out. But that is an
error in the _network_ setup, and should not happen in the case where
the end system (the one with the application running) has v6 enabled
on a non-v6-enabled network.

But if that network error has been made, you'll avoid trouble if
IPv6 is disabled. So if you know IPv6 isn't available anyway,
there's a clear advantage to disabling it in the kernel and dodging
these potential sources of failure even if they "should not
happen".

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