Ask HN: What are current best security practices for custom servers?

It’s been a while since I’ve done any low-level network programming, and I was thinking about doing a hobby project to keep my hand in. What safety/security precautions should I take before punching a firewall hole from the Internet at large to a custom-written TCP server? Is the obscurity of an unadvertised server running an undicumented protocol enough protection, or is there something else that should be done?