Stretch generates SLAAC IPv6 address even with /etc/network/interfaces set to manual static address

[Eike Lantzsch <zp6cge@gmx.net>]

Hi to all!

I'm trying to teach myself to work with IPv6.

On a Stretch client I'd like to have manually set static IPv6 addresses.
But what I get are SLAAC addresses with $prefix + MAC-derived according to the 
IEEE-Tutorial EUI-64 .
I can cope with a SLAAC address AND an additional manual static address on the 
same interface - no problem - but what is going on here? Why is the manual 
address in the file "/etc/network/interfaces" ignored?
What is it that I don't understand?
I don't have such problem with OpenBSD-clients - only with Debian Stretch.
Is it a bug, a misconfiguration or is it me misunderstanding  IPv6 address 
schemes?

excerpt of my /etc/network/interfaces:

# The primary network interface
auto enp3s0
allow-hotplug enp3s0
iface enp3s0 inet static
	address 192.168.12.21
	netmask 255.255.255.0
	network 192.168.12.0
	gateway 192.168.12.1
# dns-* options are implemented by the resolvconf package, if installed
	dns-nameservers nnn.nnn.nnn.nnn nnn.nnn.nnn.nnn 2001:470:20::2 74.82.42.42
	dns-search MYDOMAIN.co.py

# /48 network obtained from HE which allows subnets
iface enp3s0 inet6 static
	address 2001:470:7075:e2::21
	netmask 64
	gateway 2001:470:7075:e2::

Anyway I get ip addr:

2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP 
group default qlen 1000
    link/ether 78:24:af:37:6d:60 brd ff:ff:ff:ff:ff:ff
    inet 192.168.12.21/24 brd 192.168.12.255 scope global enp3s0
       valid_lft forever preferred_lft forever
    inet6 2001:470:7075:e2:7a24:afff:fe37:6d60/64 scope global mngtmpaddr \ 
dynamic
       valid_lft 2591999sec preferred_lft 604799sec
    inet6 fe80::7a24:afff:fe37:6d60/64 scope link
       valid_lft forever preferred_lft forever

Ideally I'd like static private addresses to route through to the IN but 
static manual addresses used only in my internal network. But that will be an 
exercise for later.

What about "autoconf 0" in /etc/network/interfaces - would that help?
But according to man interfaces that is the default - or isn't it?

Or would
iface enp3s0 inet6 static
	address 2001:470:7075:e2::21/64
	gateway 2001:470:7075:e2::
be any better?

Thanks for your consideration.
Eike

-- 
Eike Lantzsch ZP6CGE
Zuviel Zucker ist ungesund. Daher:
Tragt den Zuckerberg ab!