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[Freedombox-discuss] Thoughts on MAC Addresses

It is worthwhile considering the privacy implications of MAC
addresses.  Here's some info.

  *  MAC addresses on Ethernet gear come in two parts: the company ID
     number, and the manufacturer-assigned extension number.  The
     company ID reveals who built the Ethernet gear; the extension
     number is similar to a serial number.  So, a database of MAC
     addresses can be used to find equipment made by particular
     manufacturers, for example if you know a vulnerability in that
     product as shipped and want to exploit it; or if you are a
     malevolent government intending to confiscate all hardware
     that's likely to be running FreedomBox software.

  *  Apple iPhones record the MAC addresses that are nearby, report
     these to Apple, and Apple uses them to return a physical position
     fix.  This is used to more rapidly cause the GPS algorithm to
     converge on a position, and also used when GPS isn't working.
     The phones often report their GPS position and any nearby MAC
     addresses back to Apple servers.  (Apple formerly used startup
     Skyhook Wireless for this service, but ended up disintermediating
     them by covertly using their customers' iPhones to collect an
     equivalent database.)  See:
     It's easy for hackers to query that database of MAC addresses
     and locations, by pretending to be an iPhone seeking its location.

  *  Google Street View vehicles recorded the MAC addresses of all
     accessible WiFi access points that they passed.  Google then used
     this database to guess at the physical location of Android mobile
     phones who can also hear beacons from the same MAC addreses.
     Android phones may now be doing what iPhones do, reporting nearby
     MAC addresses plus the phone's GPS location to a Google server.

  *  By default, every IPv6 interface's MAC address is in the low order
     bits of its IPv6 address.  See RFC 4291 (IPv6 Addressing
     Architecture) Section 2.5.1 and Appendix A; RFC 2464
     (Transmission of IPv6 Packets over Ethernet Networks); RFC 4862
     (IPv6 Stateless Address Autoconfiguration); RFC 4941 (Privacy
     Extensions for Stateless Address Autoconfiguration in IPv6).  So,
     anyone who ever communicates with a machine via IPv6 will
     generally learn the MAC address of one of its interfaces, unless
     that machine specifically uses the RFC 4941 privacy extensions to
     generate a temporary random address and change it periodically.
     (In Linux, the default is to not use such temporary addresses;
     you can change that default by writing 1 to
     /proc/sys/net/ipv6/conf/default/use_tempaddr.  You can change it
     for all current devices by writing to
     /proc/sys/net/ipv6/conf/all/use_tempaddr.  If you want privacy,
     it's probably good to write a 1 to both.)

  *  If someone who is in radio range of a WiFi access point can send
     it packets that cause it to communicate over the Internet, that
     someone can figure out the correlation between the access point's
     MAC address and the IP address (v4 or v6) it uses over the
     Internet.  For example, connecting to the access point and then
     sending a DNS query for your own domain, will cause a DNS query
     packet to be forwarded to your own domain server, from the global
     IP address of the access point.  Your domain server can then log
     that packet and correlate it with the access point's MAC address
     seen by the wireless device that generated it.  Many closed
     access points handle DNS packets even before authentication,
     since they rely on Web page spoofing to force people to "log in"
     or "check a box to agree to terms".  It's even simpler if the
     access point is open.  If a police car crusing past a FreedomBox
     can cause it to access a police Internet DNS site, they can map
     the MAC address to the IP address of that FreedomBox.

So, there are two aspects of MAC addresses that are problematic: that
they are long-term identifiers, and that they actually reveal things
about that device.

If upon installation the FreedomBox software merely changed each
machine's MAC address to a random value, we'd solve the second problem
(avoid revealing who made the device).  However it's trickier than
that, since we may not have a good source of randomness at
installation time (making our addresses too predictable), and also,
devices that have a random MAC address (instead of one assigned to a
company on the IEEE-maintained registry of Ethernet manufacturers)
might make it too obvious that the manufacturer's MAC address had been
overwritten, which would lead totalitarians to pay more attention.

We could carefully pick a random number and then package it into a MAC
address that looks like it comes from a popular manufacturer.  For
example, we could have a table of a hundred big manufacturers, and for
each, the known range of "serial number" bits that they shipped.  We'd
use part of our random number to pick a manufacturer, and another part
to pick a valid-looking serial number within that manufacturer's
products.  To avoid drawing scrutiny, we might have to be more
selective, e.g. avoid putting a MAC address from a 10-megabit 1990s
3Com Ethernet card onto a 2010s WiFi link.

Changing the MAC address to a packaged random value *periodically* --
perhaps daily or weekly -- would solve the first problem of its being
a long-term identifier.  IPv6 can cope with that without trouble; it
encourages interfaces to have multiple IPv6 addresses, deprecating old
ones while allowing existing communications to work.  IPv4 can also
cope with changed MAC addresses; within seconds, any neighbor on the
Ethernet or radio who is communicating with the node will know the new
MAC address that matches the same old IPv4 address.

We would have to test any new candidate MAC address, before using it,
by trying to communicate with it and seeing if anything responds.  See
the "Duplicate Address Detection" algorithm in RFC 4862.

> *If* preventing people from being identified by MAC addresses should be
> a goal, how do we accomplish that?  The MAC address can, but shouldn't
> be set in the firmware, we can't change it or set it, from the running
> system, there.  That's good, because it means the bootloader can't be
> changed on a running system.

MAC addresses in all modern networking chips can be set from software.

I think what you mean is that the *default* MAC address is stored in
flash, near where the boot firmware is stored, and that some boards
running Linux can't rewrite that flash memory.  "Can't" is probably
too strong a word -- many can, but how to do so is often merely
undocumented, providing a little security-by-obscurity.

> So, where can we set the MAC address?  The obvious solution is to put it
> into /etc/networking/interfaces, but that'll harm folks who want some
> form of anonymity.  We can use 0:0:0:0:0:0 as a default MAC address in
> /etc/networking/interfaces and folks who want to set a static one can
> edit the file to set it there, while folks who want to use a MAC changer
> can run a service hooked through Plinth.

"ifconfig DEV hwaddr xx:xx:xx:xx:xx:xx" or "ip link set DEV
xx:xx:xx:xx:xx:xx" lets you set the MAC address of DEV at any time
from a shell, as root.  There is also an equivalent low level

We can't set 0:0:0:0:0:0 as every interface's MAC address.  There is
good reason to have unique addresses on Ethernet interfaces.  On a
given Ethernet, or in a given WiFi radio range, communication will
fail if multiple interfaces have the same address (unless those
interfaces specifically coordinate with each other, e.g. are plugged
into the same node and use custom software to pretend to be a single
interface).  If you and your neighbor both have a FreedomBox with WiFi
address 0:0:0:0:0:0, there will be no way to send a packet to YOUR
FreedomBox; your neighbor's box will also receive the packet and is
just as likely to respond to it -- which will confuse the communication
when BOTH boxes respond to it.


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