How to fix localhost networking issues that affect several apps by resetting networking settings on macOS?

Question

I had started encountering crashes on startup in multiple apps that all had networking issues at the core:

• RStudio Desktop failed to start, giving error messages [rsession-ka37] ERROR system error 49 (Can't assign requested address); OCCURRED AT rstudio::core::Error rstudio::core::http::initTcpIpAcceptor(SocketAcceptorService<boost::asio::ip::tcp> &, const std::string &, const std::string &) src/cpp/session/http/SessionPosixHttpConnectionListener.cpp:103; LOGGED FROM: int main(int, char *const *) src/cpp/session/SessionMain.cpp:2088
• Adobe Premiere crashed on startup during "Initializing Application", before even opening a project.
• Audacity failed to start with "The Audacity IPC server failed to initialize. This is likely due to a resource shortage and a reboot may be required.".

The last one was my most helpful clue: something was up with my computer's networking settings. It may have been because I had followed these instructions and created a fake network adapter to use for connection sharing. I had deleted that adapter and no evidence remained of it, but I still suspected something wrong with networking. (As it turns out, probably deleting the interface was the cause of my problems.)

OS is Big Sur (macOS 11.6.2).

Answer 1

Try the easy way first: under Prefs:Sharing:Internet Sharing, enable some sort of Internet sharing, e.g., Wifi to Bluetooth PAN. Disable. See if things work. (Based on my postmortem, this probably would have fixed the specific problem that I encountered.)

I followed the basic instructions under the "Reset the Network Configuration Files" heading of this article, but with some variations because I found some other junk in the same folder. Here's my best reconstruction of what I did at the Terminal. Since this could potentially break things, don't do this unless you're experienced with editing your system files!

• Close everything else, save, make sure your backups are up to date, etc.
• Go to /Library/Preferences/SystemConfiguration.
• Make a backup directory, say, ~/prefs-sysconfig-backup
• Using sudo, move all files in SystemConfiguration to your backup directory. (This will fail for com.apple.Boot.plist but everything else should work fine.)
• Reboot. The files should be recreated automatically.

Looking at the diffs, I suspect that my problem was actually caused by com.apple.nat.plist referring to a PrimaryInterface that no longer existed. The suspicious entry looked like:

        <key>PrimaryInterface</key>
        <dict>
            <key>Device</key>
            <string>lo0</string>
            <key>Enabled</key>
            <integer>0</integer>
            <key>HardwareKey</key>
            <string></string>
            <key>PrimaryUserReadable</key>
            <string>FakeNet</string>
        </dict>