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Clear saved HSTS pinnings in Safari and macOS

So today I was trying to get SSL working on my machine with localhost. In the process I set the HSTS headers, telling the browser to never access the non-secure http:// version of localhost. Thus going to http://localhost:4000 would always fail as HSTS tells the browser to only access https://.

Here is how you can reset the cache Safari has for HSTS pinnings so you can access your local http://localhost again.

Open a terminal and follow those steps:

sudo killall nsurlstoraged

# or edit file, search for "localhost", remove it and then save
rm -f ~/Library/Cookies/HSTS.plist

launchctl start /System/Library/LaunchAgents/

You may have to restart Safari afterwards.

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