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)
In an absolute sense, yes a modem can spy on you by hijacking requests and redirecting them to controlled locations. We use TLS to prevent this, even stronger with technologies like HSTS.
Does this happen in real life though? More then you think, but less of an impact then you think.
Some US providers will hijack DNS requests and redirect them to their own DNS servers, but this can be solved with DOH or DOT.
TLS interception is a thing but it requires the device you are using (phone, PC, tablet, laptop) to have a root certificate installed that the ISP also controls. Almost all browsers will only install root certificates from root certificate providers with good standing and have no quarm in untrusing the root certificate if things go badly.