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cloudflare-ddns
Discontinued A script to update your Cloudflare DNS records at a glance. (by adrienbrignon)
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
If one of them has a static IP (or you get a DigitalOcean droplet for $5/month), then you can run nebula (https://github.com/slackhq/nebula). It only relies on one server having a static IP and the rest can connect, then it creates a network overlay (kind of like a VPN) that works very well.
No special router or infrastructure necessary. As other people in this thread have mentioned, sign up with cloudflare and then use a cron at the site to change your A records. This is exactly DDNS. Here's a premade script: https://github.com/adrienbrignon/cloudflare-ddns