Re: [gentoo-user] Wayland! Beware of!

[jay@jvf.cc]

On 9/25/24 7:26 AM, Eli Schwartz <eschwartz@gentoo.org> wrote:
> On 9/25/24 6:00 AM, Walter Dnes wrote:
> >    My system is actually very stable.  In the shitstorm that erupted on
> > this list at "ipv6" enabling I did not see any mention of sysctl.  In my
> > /etc/default/grub file I have...
> >
> > GRUB_CMDLINE_LINUX_DEFAULT="noexec=on net.ifnames=0 ipv6.disable=1"
> >
> > With this setting is it guaranteed that a program compiled with "ipv6"
> > flag will not try IPV6 first and timeout before dropping down to IPV4?
> 
> 
> (Note that the sysctl dynamically disables ipv6 support so that you can
> manually toggle it after boot, e.g. for testing. The kernel command line
> option hard-disables it at boot time. Your choice which to use, I guess.)
> 
> 
> If the kernel has disabled ipv6 there is no timeout because no attempt
> is made.
> 
> If the kernel has enabled ipv6 then an attempt will be made and it may:
> 
> - succeed, if your network has functioning ipv6 connectivity
> 
> - fail instantly, if your network is correctly configured (you may not
>    be in control of the network you use)
> 
> - fail after a lengthy timeout after your network "valiantly" attempts
>    to send your connection attempt into a black hole of doom
> 

This was actually a pretty common failure mode around 10-15 years ago. An
early Apple Airport Express had a bug where it would issue global-scope v6
addresses and send RAs even if it didn't have global connectivity.

Those issues (and similar ones on less ubiquitus routers), fed a lot of the
paranoia around desires to disable IPv6. It still can happen today, but IME
it's more often in the form of an ISP with inferior IPv6 connectivity.

--
Jay Faulkner

> As Arsen mentioned, RFC 8305 defines the "Happy Eyeballs" mechanism for
> trying both ipv4 and ipv6 at the same time, incurring the cost of
> slightly more traffic for the benefit of avoiding timeouts (since ipv4
> will still succeed just as fast regardless of whether a parallel ipv6 is
> timing out, and as soon as ipv4 succeeds, the ipv6 timeout is ignored
> and made redundant).
> 
> Not all software uses Happy Eyeballs. In particular, emerge --sync does
> not, because the python library that portage uses to check for updated
> PGP keys used when validating manifests, does not. This pained me
> tremendously since "emerge --sync" would literally hang forever, until I
> disabled ipv6 via the kernel. Note that since Aug 31, 2021, Gentoo's
> package for python has not supported USE=ipv6, but the sysctl works
> quite well.
> 
> 
> >    How OS-specific is this?  I "asked Mr. Google" and the NordVPN web
> > page recommended for Redhat based distros...
> 
> 
> It is specific to the linux kernel, that is all. You may replace "all"
> with the name of a machine-specific interface (as listed by "ip addr")
> to express settings that are specific to a given interface. Most people
> do not need that flexibility and simply want all interfaces to look the
> same.
> 
> 
>