RE: Disable IPv4-mapped - enforce IPV6_V6ONLY

[David Laight]

> > A proper solution would be to either return false if net.ipv6.bindv6only is true and optval is false
> (which would break downward compatibility because it wouldn't just be a default and setsockopt might
> return an error) or to introduce a new sysctl variable like net.ipv6.bindv6only_enforced_silently.
> ("silently" because setsockopt() wouldn't return an error if net.ipv6.bindv6only is true and optval
> (v6only in the example above) is false.)
> >
> > I would volunteer to write a patch which introduces something like
> net.ipv6.bindv6only_enforced_silently if some maintainer would give me his ok.
> >
> > If so, the question remains if
> >
> > systemctl net.ipv6.bindv6only_enforced_silently = 1
> >
> > should set systemctl.net.ipv6.bindv6only too or if an error should be returned if
> net.ipv6.bindv6only is false.
> 
> I am not convinced why you need this, and I am not in favor of
> enfocing IPV6_V6ONLY, but... some points:
> 
> - We should allow system-admin to "enforce" IPV6_V6ONLY to 0 as well.
> - CAP_NET_ADMIN users should always be able to use both modes
>   (They can do sysctl anyway.)
> - setsockopt should fail w/ EPERM if user tries to override.

I can imagine that some programs will always try to clear IPV6_V6ONLY
(maybe for portability with other OS which default to setting it
for security reasons) and will error-exit if it fails.
So non-silent enforcing could be a PITA.

OTOH there might be programs/systems where silent failure is wrong.

You really don't want to (globally) stop an application setting
IPV6_V6ONLY, such a program may well be creating separate IPv4
and IPv6 sockets.

Some of this needs to be part of some application wide 'security'
framework - that probably doesn't exist!

Should there also be similar controls for the use of IPv4
mapped addresses in actual on-the-wire IPv6 packets - eg those
destined for a remote gateway on an IPv6 only system?

	David



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