I am using a more complicated version of the scenario you are describing and it works well for me. I have multiple USB interfaces which gets complicacated, but if you are using only one USB interface, then you should be fine. I would use the USB to connect to your ISP and the built-in to connect to your LAN. That way, if something goes wrong with the USB, you still have access to the RPi.
You absolutely want heat sinks at a minimum. They are cheap. If you are going to be using it in a case, then you want a fan too. One thing in particular is that the USB controller gets very hot. The metal in the connectors attached to the USB can be painful to touch when removing them.
As for adapters, it really depends on the chipset of the adapter and OpenBSD's usb support for Ethernet adapters. usb(4) has a list. If you look closely, you'll notice that only axe(4), axen(4), mue(4) and ure(4) specifically say that they support GigE, which you will need. Then try to find listings for adapters online that describe the chipset. I had one adapter lie about it's chipset on Amazon and had to return it. The adapters I'm using most are using the axen(4) driver and have worked fine for me.
Power consumption is another consideration. Unless you are using a lot of other USB devices with the RPi, then you should be fine powering directly from the RPi. I'm using a USB 3 hub with external power since I'm using multpile adapters. If you notice strange behavior, like the adapter suddenly disappearing, you might try an externally powered hub.
If you are going to using PF, then you might want some sort of semi-permanent serial connection or make sure you have a keyboard and monitor connected just in case you create a rule that locks you out. Because I do a lot with microcontrollers, I had a dozen or more ttl serial to usb adapters (not cables) in a drawer. All I need in case I lock myself out is a USB cable to connect to the adapter. That save me from having to remember pinouts and hope I got it right when I'm already angry at myself for locking myself out of because I was stupid with pf. I also connected an I2C real-time clock since the RPi has no onboard RTC.
On Mon, 16 Nov 2020, Mihai Popescu wrote:
Hello, Do you think is practical to run RPi 4 as an OpenBSD router, one interface from the board and one from USB Ethernet adapter? I try to replace my ISP router, the connection is 300/150 Mbps down/up. It will be a router with dhcp only, pf active. If anyone is running this scenario, what usb to ethernet adapter do you use? Do I need some special measures like extra cooling, etc? Basically I need a two interface board able to route that amount of traffic. Do you have another suggestion in this price range, please? APU is out of the question, I can't afford the price. Thank you.
