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How do I whitelist a dynamically changing application sync server IP ? (programming.dev)

submitted 1 year ago* (last edited 1 year ago) by mathiuscov@programming.dev to c/linux@programming.dev

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cross-posted from: https://programming.dev/post/2768533

I have a vm for which I have s specific whitelist only firewall. It is supposed to only allow connections to the IPs an app connects to when syncing.

I first got the sync server IP's listening to tcpdump, then when I had the IP's I activated the whitelist.

This worked perfectly for some time, but now it appears that the IP's have changed. I could do the same thing again but repeating the process regularly is annoying and defeats the whole purpose of only ever allowing network connections to specific whitelisted serves.

Alternatively, I could set up a process to only allow network traffic from that app somewhat.

Using debian-11 btw.

Any help is appreceated !!!

EDIT: I don't own the sync servers, my app simply connects them, so I can get the updated state from my other devices

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[–] Nath@aussie.zone 3 points 1 year ago

There's no tidy way to do it. You can configure a dynamicDNS from the dynamic IP, then set up a cron from the server to check that record every hour or so. Update the whitelist accordingly.

The other way to do it is to have an intermediate jump point and whitelist that.

Both have their drawbacks. I'd personally go the dyndns route. But I'd be sure a had a static IP I could ssh from as a fallback in case I had problems. You don't want to lock yourself out.