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Note that despite the comment on the second line (see interfaces(5)), not everything is explained there. You have to look at resolvconf(8) as well: all the options that used to be available to resolv.conf, can be put in interfaces, if you prefix them with "dns-". Take care that instead of configuring multiple nameserver-lines, they are now on one dns-nameservers line.
There is a bug that effectively overrides the privext value with what's in /etc/sysctl.d/10-ipv6-privacy.conf. You have to work around it by commenting out the two lines there.
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