Implementation review

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 follow the requirements for radio spectrum use. For non-technical details on our position on regulatory support on Linux see our Linux wireless regulatory support statement. 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 a set of RSA digital signatures or can read a set of them from a preconfigured directory. 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.

crda.png

Status

The new regulatory infrastructure went in as of 2.6.28, so CRDA can be used in kernels >= 2.6.28. It is required for 802.11d operation in 2.6.29.

Code releases

Kernel integration

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

CRDA is our userspace agent which uploads regulatory domains into the kernel, it acts as a udev helper.

wireless-regdb

wireless-regdb is our regulatory database used by CRDA.

Old regulatory implementation

This section exists to explain how we used to do things and to also explain what CONFIG_WIRELESS_OLD_REGULATORY was exactly. Prior to our new regulatory implementation explained throughout this page we had 3 static regulatory domains built-in to the Linux kernel for all cfg802111 drivers (therefore all mac80211 drivers). Apart from the 3 static regulatory domains in the old implementation we also gave users the option to set the regulatory domain via the ieee80211_regdom module parameter. We cover these details below.

Old static regulatory domains

The 3 old static regulatory domains we had implemented in-kernel were for:

  • US
  • EU
  • JP

By static regulatory domains we mean that they were defined in kernel-space and the only way to make changes due to regulatory updates by different countries was to send a patch for submission for inclusion into the Linux kernel. There are several downsides to this approach. We review them briefly below.

  • Country regulatory changes would need to accounted for completely in kernel space, requiring regulatory updates to be backported to older kernel releases.
  • Each country can have their own regulatory rules requiring an entry for each country or some conglomeration of countries into custom groups. This can lead to huge debates on implementation and efficiency – each vendor has their own set of custom regulatory domains to group regulatory information into groups, taking one vendor approach would imply preferring one implementation over another

  • Only accounted for countries in each kernel release would get proper regulatory consideration

Our initial implementation approach for our new regulatory infrastructure was to populate a regulatory domain in-kernel for each country. It was decided that it is a lot easier to deal with this in userspace and so that was one of the design changes for new regulatory implementation.

The ieee80211_regdom module parameter

Another old option for users from the old regulatory implementation was to set the regulatory domain using a module parameter for the cfg80211 module. The module parameter name is ieee80211_regdom. This module parameter only exists in 2.6.27, 2.6.28 when the CONFIG_WIRELESS_OLD_REGULATORY option is enabled. The ieee80211_regdom module parameter has become available as of recent kernel to users without the CONFIG_WIRELESS_OLD_REGULATORY enabled, in those kernels it is treated as a userspace regulatory hint request but the compromise was that when using the "EU" regulatory domain the user will world roam as "EU" is not an ISO / IEC 3166 country code. Users of "EU" are encouraged to be more specific and supply their country ISO3166-alpha2 instead when not using CONFIG_WIRELESS_OLD_REGULATORY.

The ieee80211_regdom module parameter is inherited from our old regulatory implementation. We now have a userspace API which allows userspace to inform the kernel what country you are in through nl80211. Currently two userspace applications exists that supports this, iw and wpa_supplicant. Using the ieee80211_regdom module parameter on modern kernels is treated as a userspace regulatory hint as if it came through nl80211 through utilities like iw and wpa_supplicant.

Although modern kernels do support the ieee80211_regdom module parameter distributions are encouraged to use userspace utilties to supply country hints instead since in the future the Linux desktop may be providing userspace regulatory hints by default through things like geoclue (more on this below).

Feature removal

CONFIG_WIRELESS_OLD_REGULATORY has been replaced completely as of the 2.6.34 Linux kernel release and disabled by default as of 2.6.30. The alternative for those seeking in-kernel regulatory databases is to now build the entire regulatory database into the kernel itself, therefore not requiring a userspace agent. This is achieved with CFG80211_INTERNAL_REGDB. Users of CFG80211_INTERNAL_REGDB should be aware though that new regulatory updates would not be possible when this mechanism is used unless a new kernel is provided for each new regulatory update, for more information see the documentation on CFG80211_INTERNAL_REGDB.

Automatic country discovery

The Linux desktop is expected to advance to be able to discover what country it is in at any point in time and to pass this off to the kernel to enhance regulatory compliance. To aid with these efforts we had started a Google Summer of Code (GSoC) project for 2009 to help integrate GeoClue to the GNOME desktop. This project did not coplete but for details please see the GeoClue regulatory integration GSoC project.

Custom regulatory information

The Linux regulatory infrastructure was designed to allow compliance but to also address flexibility where a manufacturer customizes hardware or wants to sell hardware that works on a licensed band or a customized regulatory environment not covered by the usual world wide regulatory agencies. Customizations are also likely to happen in research environments where local regulatory laws may not apply depending on jurisdiction.

The regulatory infrastructure supports both authorship and file integrity, and allows third parties to distribute binary-only regulatory databases even with custom licenses as the software for it is licensed under a permissive license, the ISC license. Below we cover how to achieve all this.

Editing the regulatory database

You can edit the regulatory database by modifying db.txt as you see fit.

Generating your own private and public key

You typically do not have to build the wireless-regdb, unless you want to attach a customized RSA signature based on your public key. You can generate your own public and private keys by building wireless-regdb. Below is an example of building wireless-regdb:

mcgrof@tux ~/devel/wireless-regdb (git::master)$ make
Generating private key for mcgrof...
openssl genrsa -out ~/.wireless-regdb-mcgrof.key.priv.pem 2048
Generating RSA private key, 2048 bit long modulus
..........................+++
.....................................................................................................+++
e is 65537 (0x10001)
Generating public key for mcgrof...
openssl rsa -in ~/.wireless-regdb-mcgrof.key.priv.pem -out mcgrof.key.pub.pem -pubout -outform PEM
writing RSA key
Generating regulatory.bin digitally signed by mcgrof...
./db2bin.py regulatory.bin db.txt ~/.wireless-regdb-mcgrof.key.priv.pem

On this example the build produced three files:

  • ~mcgrof/.wireless-regdb-mcgrof.key.priv.pem - the RSA private key
  • mcgrof.key.pub.pem - the RSA public key
  • regulatory.bin - digitally signed binary regulatory database

The private key is built into your home directory by default. The public key is built into the wireless-regdb directory. The binary wireless regulatory database is then built and then digitally sign it using your private key. When you run make again only a binary regulatory database file will be built as the public and private keys would have been built already.

Importing your public key into CRDA

CRDA has a directory, pubkeys of all trusted public keys it can use to embed onto the binary for RSA signature verification against any particular binary regulatory database. This is used to allow CRDA to trust different authors for regulatory information. By default John Linville's key is always present on the pubkeys directory. You can remove it if for your particular application you cannot trust the upstream community regulatory database information.

CRDA can be built with gcrypt or openssl support. If using openssl (USE_OPENSSL=1) you can enable dynamic loading of trusted public keys and stuff custom public keys at any time into the /etc/wireless-regdb/pubkeys directory (by default).

You can also import your public key to be built into the CRDA binary though. This is required for gcrypt support as gcrypt support lacks a PEM parser. To import your public key to be built into CRDA all you have to do is copy it into the pubkeys directory of crda source code prior to building CRDA:

mcgrof@tux ~/devel/crda (git::master)$ cp ../wireless-regdb/mcgrof.key.pub.pem pubkeys/

Building using extra public keys

To build CRDA with extra public keys built-in to the final binary CRDA just run make with the list of public keys you trust in the pubkeys directory. For example to build wireless-regdb with a custom mcgrof.key.pub.pem stuffed into the pubkeys directory you would do:

mcgrof@tux ~/devel/crda (git::master)$ make
  GEN  keys-gcrypt.c
  Trusted pubkeys: pubkeys/linville.key.pub.pem pubkeys/mcgrof.key.pub.pem
  CC   reglib.o
  CC   crda.o
  LD   crda
  CC   intersect.o
  CC   print-regdom.o
  LD   intersect
  CC   regdbdump.o
  LD   regdbdump
  CHK  /usr/lib/crda/regulatory.bin

Redistribution licenses

Since both wireless-regdb and CRDA are licensed under a permissive license, the ISC license, you can choose to modify wireless-regdb, create your own keys and redistribute only the binary regulatory.bin without providing the source code or keys.

The license is important. You are free to redistribute your binary and public key under a new license, even a proprietary one, but you must still keep the original copyright notice from wireless-regdb somewhere on your new license. A custom license would enable third parties to enable on the Linux kernel custom 802.11 devices which may operate, for example, on actual licensed bands the end users have licenses for. Another example would be if a manufacturer has customized some 802.11 hardware and has verified the integrity of the modified hardware to operate on different frequencies and has taken the time to ensure regulatory compliance for usage of those cards. And yet another example would be the use of 802.11 hardware in research settings where regulatory compliance, depending on your jurisdiction, may allow you to use higher EIRP or custom frequencies for research purposes.

What is needed for end users

If you are customizing a regulatory database you need to redistribute three things:

  • Your custom regulatory.bin
  • Your public key
  • A license for the above two

With the above an end user should be able to either build CRDA with gcrypt support to trust your regulatory.bin files or to just stuff it into the /etc/wireless-regdb/ directory if openssl support was enabled which allows dynamic reading of trusted public keys.

CONFIG_CFG80211_CERTIFICATION_ONUS

The CONFIG_CFG80211_CERTIFICATION_ONUS is available for features which require additional regulatory compliance testing and validation by the system integrator. This allows us to define 802.11 specific kernel features under a flag that is intended by design to be disabled by standard Linux distributions, and only enabled by system integrators or distributions that have done work to ensure regulatory certification on the system with the enabled features. Regulatory verification may at times only be possible until you have the final system in place. Examples of features which depend on this option are DFS, cellular base station regulatory hints, custom 802.11 research features, and OEM / ODM chip verification features useful for testing / validation.

This option should only be enabled by system integrators or distributions that have done work necessary to ensure regulatory certification on the system with the enabled features. Alternatively you can enable this option if you are a wireless researcher and are working in a controlled and approved environment by your local regulatory agency.

Processing rules

If you would like to become familiar with the cfg80211 algorithm used to process regulatory rules you can review this on the cfg80211 regulatory processing rules section.