I’m planning to buy a router and modem and put OpenWrt on it for maximum control and privacy. While I could get a router with an integrated DSL modem, the previous tenants had cable internet so I’m not sure if the DSL connection even works and DSL internet is also more expensive (at least where I live). Fiber optic is not available. The problem is, there is apparently no open firmware for cable modems so I would have to buy a standalone router and a standalone cable modem. I would put OpenWrt on the router and use whatever proprietary firmware came with the modem.
So my question is:
Can a standalone modem that doesn’t do routing, spy on you?
If yes a rough explanation how would be appreciated.
It seems that modem and router are used interchangeably on the internet (probably because they are mostly combined) so it is really hard to find any information on modems. Here are both Wikipedia articles for reference:
https://en.wikipedia.org/wiki/Modem
https://en.wikipedia.org/wiki/Router_(computing)
Can it? Maybe. It’s not impossible; but it isn’t practical and most ISPs limit their shenanigans to grabbing your unencrypted DNS requests.
Will it? Probably no; aside from the previously mentioned DNS redirections; they’re not interested in most people’s packets, only in how many they deliver.
Should you care? I won’t tell you not to take precaution, but I do urge you to consider your threat model carefully and consider the tradeoffs. When Security & Privacy goes up, Convenience and Functionality WILL go down. Balance your needs. Don’t put yourself in a state of Privacy fatigue.
Are there easy fixes? Maybe. I think a VPN or using Tor would solve your concerns here anyways; it’s not required that your modem be running OSS that you can control. If you can achieve it; that’s still good for you; but it’s not something to be sweating if your modem isn’t capable and your invasive ISP is the only effective option.