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Bug#860809: installation-reports: ipv6 SLAAC autoconf not working with stretch's network install

Hi Frederic,

On 2017-04-24 14:09, Frederic Bonnard wrote:
You seem to indicate that you need to use IPv4 to access the nameserver?

Yes, I actually was given only static IPv4 parameters to connect the
network (and no DHCPv4 server is available on it).

Is the real failure you report that it did not configure IPv4 correctly?
I.e. didn't show a prompt and continued without it?

Well yes, at least I don't mind it tries to probe for DHCPv[46] (that
makes sense) but if it fails like it seems to do here, shouldn't it
propose a manual setup? In that case, I would have continue with a
static IPv4 configuration and the installation.

I managed to get the manual setup menus, by changing the priority of the installer, also undoing the IPv6 (if I remember correctly) and so on but
that's not straightforward.

Also when it asks for the nameserver in the ncurses gui, how am I
supposed to know that it's trying to do an IPv6 setup ? (tell me if I'm
wrong, but given the ip output, it seems it was in the middle of an IPv6
setup after having failed the IPv4 dhcp probing and thus expecting an
IPv6 nameserver)

the thing is that you received an IP allocation from the IPv6 router on your subnet. That it's 6to4 makes me suspicious that it's a misconfigured ancient Windows Server with ICS on the same subnet. Is that possible?

We're kind of in a bound here in terms of IPv6: We have received IP addressing and routing but we're not allowed to call home to our servers to check if actual connectivity exists. So we assume that given that one autoconfiguration mechanism exists and succeeded that we don't have to prompt for the other protocol. (The same would happen for IPv4 if there would be a malicious IPv4 DHCP server and static IPv6-only connectivity.)

I'd somewhat argue that you need to keep your network sane or resort to manual configuration as you did. But it's likely that the latter needs some better documentation, so if you can point at what's missing where, we can add it. :)

If so, shouldn't the installer gui let the user know for which kind of
network setup it asks a parameter (such as the nameserver) ?

It can be either if you're dual-stacked. If you don't receive an IPv6 announcement nor IPv4 DHCP you're prompted for static configuration and you're unlikely to use another protocol in the nameserver prompt. I suppose it said "IPv6 autoconfiguration succeeded" somewhere in the process, but only very briefly. :/

~ # ip -6 route
2002:903:15f:550::/64 dev enP5p1s0f1  metric 256  expires 0sec
default via fe80::4e96:14ff:fe59:d7f0 dev enP5p1s0f1 metric 1024 expires 0sec

So 4C:96:14:59:D7:F0 provides broken IPv6 connectivity on the same subnet. It provided you with an address out of's 6to4 space. If it'd route and you'd use a valid IPv6 nameserver, you'd be fine.

However, there's another catch - the one why Windows made so many Linux boxes sad in the past and why things like RA Guard matter: the expiry is set to 0 sec. So it actually says that you shouldn't really send traffic there. And then I suppose that Linux thinks "well, it's better than nothing". On the other hand the address it handed out has a lifetime of 5 months. -_-

~ # ip add
    inet6 2002:903:15f:550:72e2:84ff:fe14:24ee/64 scope global dynamic
       valid_lft 2591868sec preferred_lft 604668sec

Kind regards
Philipp Kern

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