In the end firewall is implemented as .Get and .Set rules, with policy
statically set to 'drop'. This way allow atomic firewall updates.
Since we already have appropriate firewall format handling in
qubes.firewall module - reuse it from there, but adjust the code to be
prepared for potentially malicious input. And also mark such variables
with untrusted_ prefix.
There is also third method: .Reload - which cause firewall reload
without making any change.
QubesOS/qubes-issues#2622FixesQubesOS/qubes-issues#2869
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