sympl-firewall: iptables-persistent conflict
It looks like when iptables-persistent is installed with a reasonable standard config, it can prevent DNS lookups when there's no IPv4 resolvers, which leads to the sympl-firewall hook waiting indefinitely and eventually being killed, and therefore no IPv6 coming up (and therefore no DNS resolution) which leads to other oddities.
Likely fix: make sure the hook doesn't stall indefinitely and instead times out.
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