kCFErrorDomainCFNetwork error 310
Hello,
I have a HTTP proxy and Safari shows this error on MacOS Big Sur. It appears to work for other pages via the proxy but some do not work.
Mac mini 2018 or later
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Found the problem, my HTTP proxy was configured to only pass HTTP port (80) and HTTPS port (443). Most proxies are configured that way for security reasons. The solution was to allow port I was trying to connect to (9090 in my case). You can only do this if you are the proxy administrator.
You can tell if the site is using nonstandard port by looking at the URL in the browser: http://www.example.com:9090 uses a different port (9090) which must be allowed in proxy configuration.
I hope this was helpful.