Even an old pallet works. Use BIOS passwords. I'm not sure if any BIOSes on 486 boards have universal passwords so be careful. Core Kernel Security Vulnerabilities between the Linux kernel and router routers differ. DoS attacks are easy to do and hard to defeat.
Port scanning can come from inside or outside.
Scanning, though not an 'attack' per se, could easily be construed in a court of law as network reconnaissance, or even a denial of service, depending on the intensity. To my knowledge we have yet to see precedents. We solve these two problems in the kernel with security patches and nmap. If you see a vulnerability, rest assured it's in the latest Oxygen. ICMP redirects and ICMP echo requests.