In some versions of macOS (10.14 and 10.15), Apple’s support for SMB network shares may send packets over a VPN tunnel interface that can trigger this warning. These packets have the same source and destination address which is what triggers the LAND attack warning.
These messages can be ignored, or, if you have the option, you can disable the NetBIOS daemon that is sending out these packets on the Mac using the following command:
launchctl unload -w /System/Library/LaunchDaemons/com.apple.netbiosd.plist
Please note that disabling the NetBIOS daemon may affect your network file shares.