Firefox private mode should not allow entries to remain in the DNS cache

[AnotherOldGuy]

Presently, when browsing in a Firefox private session, entries are added to the machine's DNS cache, listing the web site names visited during the session. These entries remain in the machine's DNS cache after the private session ends, thereby leaving browsing traces for anyone with access to the machine's DNS cache, for example, by using Command Prompt. So, for the private session to actually not leave any traces behind, no entries into the DNS cache should be allowed, or the DNS cache should be flushed when the session ends.

[Jon]

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[luis123456789]

The problem with this is that, in a correctly-functioning machine, the DNS cache not under Firefox's control. There are also multiple DNS caches (bind9, dnscrypt, dnsmasq, Windows's default one to name a few) and not all of them provide the sort of API to "bypass" resolution because that would defeat the point of them serving the DNS feature in the first place (or because they are machines "managed by the enterprise").

The only way towards making sure that Private Browsing DNS queries do not persist in the system after, is for Private Browsing mode to use its own external channel for DNS queries (be it something like a separate DoH target, or maybe by Mozilla providing its own DNS system dæmon).