In USA, is it illegal to modify your WiFi router or to use a WiFi router with custom firmware?

Question

Open source projects such as DD-WRT and Tomato provide custom firmware for wireless routers.

However some information on the web seems to imply that FCC rules no longer allow the modification of the firmware on software based radios.

Looking at the FCC web site, this question is linked to two PDFs:

1. 594280 D01 Configuration Control v02r01
2. 594280 D02 U-NII Device Security v01r02

The second PDF lists the requirements that a software radio operating in 5GHz range (a frequency provided by many WiFi routers) must meet in order to become certified by FCC. On p.2 this document says the following under "Third-Party Access Control":

What prevents third parties from loading non-US versions of the software / firmware on the device?

Describe in detail how the device is protected from "flashing" and the installation of third-party firmware such as DD-WRT.

However all of this documentation is for manufacturers, about their responsibility to block customizations like this. The questions I wanted to ask are from the perspective of the end user:

1. is it illegal to install custom firmware on a WiFi router?
2. if I have a router with custom firmware from another country, is it illegal to operate it in USA?

Answer 1

The FCC has clarified their stance on this:

... were we mandating wholesale blocking of Open Source firmware modifications?

We were not, but we agree that the guidance we provide to manufacturers must be crystal-clear to avoid confusion. So, today we released a revision to that guidance to clarify that our instructions were narrowly-focused on modifications that would take a device out of compliance. The revised guidance now more accurately reflects our intent in both the U-NII rules as well as our current rulemaking, and we hope it serves as a guidepost for the rules as we move from proposal to adoption.

Essentially, Open Source firmware modifications are allowed, the modifications are disallowed are things like excessively increasing signal strength.