We take regulatory considerations seriously as its one of the major key components to getting proper vendor support on drivers due to fear uncertainty and doubt that Linux drivers cannot not follow the requirements for radio spectrum use. Despite the fact that drivers and hardware can have their own regulatory solutions we provide this framework as a safety net for regulatory considerations to account for changes and updates on regulatory rules world wide and to provide an API to allow drivers to export their own regulatory restrictions. Our regulatory infrastructure consists of three major components:
We embrace proper regulatory compliance in the Linux kernel by making it part of cfg80211, used by new wireless drivers. We maintain a thorough and flexible regulatory database in userspace and provide a Central Regulatory Domain Agent (CRDA), a userspace agent, which can be triggered to update the kernel wireless core's definition of the regulatory permissions for a specific country. Keeping the database in userspace allows distributions to provide updates without kernel upgrades. The database is shipped in binary form using a binary file format designed for size efficiency that also includes an RSA digital signature. When a regulatory domain change is detected (for example by observing an AP with country information), the kernel will request, from CRDA, the regulatory permissions for the new domain to enforce those on drivers.
For some hardware, regulatory permissions are programmed into the EEPROM, these can be observed as well, depending on the driver. Some drivers rely on EEPROM values for enforcement or calibration and drivers can continue to rely on these values by filtering the CRDA data according to the EEPROM settings. For these type of drivers, CRDA provides an extra layer of regulatory compliance, for instance when the card is in a laptop that roams between countries.
The diagram below illustrates best the current design of CRDA and its interaction kernel and the regulatory database.
The new regulatory infrastructure went in as of 2.6.28, so CRDA can be used in kernels kernels >= 2.6.28. It is required for 802.11d operation in 2.6.29.
We have factored common regulatory driver code as part of the wireless stack and provided a way for a userspace agent to update the currently set regulatory domain. All new drivers registered with cfg80211 can reap benefits from this through cfg80211's regulatory support. mac80211 also uses this regulatory infrastructure to support 802.11d. An important component to Linux' own kernel integration is to allow drivers themselves to hint to the wireless core an alpha2 and have a callback to review the data passed by crda based on its own driver or EEPROM data. This allows vendors to use their own regulatory information to help enhance regulatory compliance even further. For more details on the Linux kernel integration see how you can set the regulatory domain.
CRDA is our userspace agent which uploads regulatory domains into the kernel, it acts as a udev helper.
The regulatory database
CRDA requires a regulatory database (Web view or gitweb) to be build and maintained. Our hope is that this database can be used by other platforms (open or proprietary), not just Linux. John Linville maintains this database through the wireless-regdb git tree:
The regulatory.bin file there is signed with his RSA private key. We keep the RSA public key embedded as part of CRDA which allows us to verify the authorship and integrity of the regulatory database.
Releases of wireless-regdb
You can find official wireless-regdb releases here: http://wireless.kernel.org/download/wireless-regdb/
ASCII file format
Below is an example of a country entry for the db.txt regulatory file for EC (Ecuador)
country EC: (2402 - 2482 @ 40), (N/A, 20) (5170 - 5250 @ 20), (6, 17) (5250 - 5330 @ 20), (6, 23), DFS (5735 - 5835 @ 20), (6, 30)
Note that the frequency range (e.g. 2402-2482) covers the complete used bandwidth, so this definition allows using the 2 GHz channels 1 through 13 as 40 MHz channels. 5 GHz channels of a bandwidth of 20 MHz can be used if the frequencies used by the channel fit into the specified frequency ranges.
Binary file format
We define a new custom binary file format for use with CRDA, to have the data available quickly and as compact as possible as well as allowing to distribute the data along with the digital signature (see below) as easily as possible. The file format is defined in the regdb.h header file.
RSA Digital Signature
Integrity of the binary regulatory file is ensured by digitally signing the regulatory data using a private key and embedding the signature into the binary file. When the file is loaded by the regulatory daemon the signature is checked against a list of public keys built into the regulatory daemon binary. This ensures regulatory.bin file authorship and integrity.
Note that both CRDA and wireless-regdb allows you to build it without RSA key signature checking, if this is something you find useless then do not use them, but we advise against it. The reason RSA digital signature checks are an option and is what is recommend is that regulatory bodies are highly sensitive towards compliance and the current infrastructure we have gives us best effort on our part of doing the best we can to ensure integrity of the files and also gives us a mechanism to use files from trusted parties on-the-fly. Distribution packaging tends to guarantee file integrity upon installation time and from a specific source but it does not give you on-the-fly file integrity checks. Integrity checks are possible through alternate means such as simple CRC checks but you'd then need a list of all allowed CRCs, by using RSA digital signatures you get both file integrity checks for _any_ binary built with the private key by checking for the signature – and while at it you also can get file authorship protection – all of this while the file is being read for usage in memory. Distributions do not protect against file corruption after the files are in place, for example.
John Linville is the default trusted party in CRDA if you enable RSA digital signature checks because he is the maintainer of the Linux wireless subsystem and wireless-regdb. But note that CRDA lets you enable multiple trusted parties by letting you throw in alternative public keys into the pubkeys directory. If your distribution requires you to build your own regulatory.bin then simply add your own pubkey into the pubkeys. CRDA will then run using a regulatory.bin built by John Linville or your distribution's wirelss-regdb package maintainer's key. It would allows users to upgrade using your distribution's built regulatory.bin, or simply upgrade to using John's.
Sending updates to the regulatory database
Patches sent to the wireless-regdb git tree should be addressed as follows:
To: firstname.lastname@example.org Cc: email@example.com Subject: wireless-regdb: Update regulatory rules for France (FR) on 5GHz
Changing the database file format
To change the file format you will need to send patches to both crda (start off with regdb.h) and wireless-regdb/dbparse.py. You should send your patch as an RFC on the linux-wireless mailing list and CC both the wireless-regdb and crda maintainers.