Randomized and Changing MAC Address State of Affairs

Privacy is becoming a huge concern, as more and more devices are getting directly (e.g., via Wi-Fi) or indirectly (e.g., via a smartphone using Bluetooth) connected to the Internet. This ubiquitous connectivity, together with the lack of proper education about privacy make it very easy to track/monitor the location of users and/or eavesdrop their physical and online activities. This is due to many factors, such as the vast digital footprint that users leave on the Internet with or without their consent, for instance sharing information on social networks, cookies used by browsers and servers for various reasons, connectivity logs that allow tracking of a user's Layer-2 (L2/MAC) or Layer-3 (L3) address, web trackers, etc.; and/or the weak (or even null in some cases) authentication and encryption mechanisms used to secure communications. This privacy concern affects all layers of the protocol stack, from the lower layers involved in the access to the network (e.g., the MAC/Layer-2 and Layer-3 addresses can be used to obtain the location of a user) to higher layer protocol identifiers and user applications . In particular, IEEE 802 MAC addresses have historically been an easy target for tracking users . There have been several initiatives at the IETF and the IEEE 802 standards committees to overcome some of these privacy issues. This document provides an overview of these activities to help coordinating standardization activities within these bodies.

Most mobile devices used today are WLAN enabled (i.e., they are equipped with an IEEE 802.11 wireless local area network interface). Wi-Fi interfaces, as any other kind of IEEE 802-based network interface, like Ethernet (i.e., IEEE 802.3) have a Layer-2 address also referred to as MAC address, which can be seen by anybody who can receive the radio signal transmitted by the network interface. The format of these addresses (for 48-bit MAC addresses) is shown in .

MAC addresses can either be universally administered or locally administered. Universally administered and locally administered addresses are distinguished by setting the second-least-significant bit of the most significant byte of the address (the U/L bit). A universally administered address is uniquely assigned to a device by its manufacturer. Most physical devices are provided with a universally administered address, which is composed of two parts: (i) the Organizationally Unique Identifier (OUI), which are the first three octets in transmission order and identify the organization that issued the identifier, and (ii) Network Interface Controller (NIC) Specific, which are the following three octets, assigned by the organization that manufactured the NIC, in such a way that the resulting MAC address is globally unique. Locally administered addresses override the burned-in address, and they can either be set-up by the network administrator, or by the Operating System (OS) of the device to which the address pertains. However, as explained in further sections of this document, there are new initiatives at the IEEE 802 and other organizations to specify ways in which these locally administered addresses should be assigned, depending on the use case.

Since universally administered MAC addresses are by definition globally unique, when a device uses this MAC address over a shared medium to transmit data -especially over the air- it is relatively easy to track this device by simple medium observation. Since a device is usually directly associated to an individual, this poses a privacy concern . MAC addresses can be easily observed by a third party, such as a passive device listening to communications in the same layer-2 network. In an 802.11 network, a station exposes its MAC address in two different situations: While actively scanning for available networks, the MAC address is used in the Probe Request frames sent by the device (a.k.a. IEEE 802.11 STA). Once associated to a given Access Point (AP), the MAC address is used in frame transmission and reception, as one of the addresses used in the unicast address fields of an IEEE 802.11 frame. One way to overcome this privacy concern is by using randomly generated MAC addresses. The IEEE 802 addressing includes one bit to specify if the hardware address is locally or globally administered. This allows generating local addresses without the need of any global coordination mechanism to ensure that the generated address is still unique within the local network. This feature can be used to generate random addresses, which decouple the globally unique identifier from the device and therefore make it more difficult to track a user device from its MAC/L2 address . Note that there are reports of some mobile Operating Systems (OSes) reporting persistently (every 20 minutes or so) on MAC addresses (among other information), which would defeat MAC address randomization. While these practices might have changed by now, it is important to highlight that privacy preserving techniques should be conducted considering all layers of the protocol stack.

As an outcome to the STRINT W3C/IAB Workshop , a tutorial on "Pervasive Surveillance of the Internet - Designing Privacy into Internet Protocols" was given at the IEEE 802 Plenary meeting in San Diego in July of 2014. The tutorial provided an update on the recent developments regarding Internet privacy, the actions undertaken by other SDOs such as IETF, and guidelines that were being followed when developing new Internet protocol specifications (e.g., ). The tutorial highlighted some privacy concerns applicable specifically to link-layer technologies and provided suggestions on how IEEE 802 could help addressing them. Following the discussions and interest within the IEEE 802 community, on 18 July 2014 the IEEE 802 Executive Committee (EC) created an IEEE 802 EC Privacy Recommendation Study Group (SG) . The work and discussions from the group have generated multiple outcomes, such as: 802E PAR (Project Authorization Request, this is the means by which standards projects are started within the IEEE. PARs define the scope, purpose, and contact points for a new project): Recommended Practice for Privacy Considerations for IEEE 802 Technologies , and the 802c PAR: Standard for Local and Metropolitan Area Networks - Overview and Architecture Amendment - Local Medium Access Control (MAC) Address Usage . In order to test the effects of MAC address randomization, trials were conducted at the IETF and IEEE 802 meetings between November 2014 and March 2015 - IETF91, IETF92 and IEEE 802 Plenary in Berlin. The purpose of the trials was to evaluate the use of MAC address randomization from two different perspectives: (i) the effect on the connectivity experience of the end-user, also checking if applications and OSes were affected; and (ii) the potential impact on the network infrastructure itself. Some of the findings were published in . During the trials it was observed that the probability of address duplication in a network is negligible. The trials also revealed that other protocol identifiers (e.g., DHCP client identifier) can be correlated and therefore be used to still track an individual. Hence, effective privacy tools should not work in isolation at a single layer, but they should be coordinated with other privacy features at higher layers. Since then, MAC randomization has further been implemented by mobile OSes to provide better privacy for mobile phone users when connecting to public wireless networks , , .

Practical experiences of Randomized and Changing MAC addresses (RCM) in devices (some of them are explained in Section ) helped researchers fine-tune their understanding of attacks against randomization mechanisms . At the IEEE 802.11 group these research experiences eventually formed the basis for a specified mechanism introduced in the IEEE 802.11aq in 2018 which randomize MAC addresses . More recent developments include turning on MAC randomization in mobile OSes by default, which has an impact on the ability of network operators to customize services . Therefore, follow-on work in the IEEE 802.11 mapped effects of potentially large uptake of randomized MAC identifiers on a number of commonly offered operator services in 2019. In the summer of 2020 this work emanated in two new standards projects with the purpose of developing mechanisms that do not decrease user privacy but enable an optimal user experience when the MAC address of a device in an Extended Service Set (a group of interconnected IEEE 802.11 wireless access points and stations that form a single logical network) is randomized or changes and user privacy solutions applicable to IEEE Std 802.11 . IEEE Std 802 , as of the amendment IEEE 802c-2017 , specifies a local MAC address space structure known as the Structured Local Address Plan (SLAP) . The SLAP designates a range of Extended Local Identifiers for subassignment within a block of addresses assigned by the IEEE Registration Authority via a Company ID. A range of local MAC addresses is designated for Standard Assigned Identifiers to be specified by IEEE 802 standards. Another range of local MAC addresses is designated for Administratively Assigned Identifiers subject to assignment by a network administrator. "IEEE Std 802E-2020: Recommended Practice for Privacy Considerations for IEEE 802 Technologies" recommends the use of temporary and transient identifiers if there are no compelling reasons for a newly introduced identifier to be permanent. This recommendation is part of the basis for the review of user privacy solutions for IEEE Std 802.11 (a.k.a. Wi-Fi) devices as part of the RCM efforts. Annex T of IEEE Std 802.1AEdk-2023: MAC Privacy Protection discusses privacy considerations in bridged networks. As per 2024, two task groups in IEEE 802.11 are dealing with issues related to RCM: The IEEE 802.11bh task group, looking at mitigating the repercussions that RCM creates on 802.11 networks and related services, and The IEEE 802.11bi task group, which is chartered to define modifications to the IEEE Std 802.11 medium access control (MAC) specification to specify new mechanisms that address and improve user privacy.

At the Wireless Broadband Alliance (WBA), the Testing and Interoperability Work Group has been looking at the issues related to MAC address randomization and has identified a list of potential impacts of these changes to existing systems and solutions, mainly related to Wi-Fi identification. As part of this work, WBA has documented a set of use cases that a Wi-Fi Identification Standard should address in order to scale and achieve longer term sustainability of deployed services. A first version of this document has been liaised with the IETF as part of the MAC Address Device Identification for Network and Application Services (MADINAS) activities through the "Wi-Fi Identification In a post MAC Randomization Era v1.0" paper .

specifies Stateless Address Autoconfiguration (SLAAC) for IPv6, which typically results in hosts configuring one or more "stable" addresses composed of a network prefix advertised by a local router, and an Interface Identifier (IID). formally updated the original IPv6 IID selection mechanism to avoid generating the IID from the MAC address of the interface (via EUI64), as this potentially allowed for tracking of a device at L3. Additionally, the prefix part of an IP address provides meaningful insights of the physical location of the device in general, which together with the MAC address-based IID, made it easier to perform global device tracking. identifies and describes the privacy issues associated with embedding MAC stable addressing information into the IPv6 addresses (as part of the IID). It describes an extension to IPv6 SLAAC that causes hosts to generate temporary addresses with randomized interface identifiers for each prefix advertised with autoconfiguration enabled. Changing addresses over time limits the window of time during which eavesdroppers and other information collectors may trivially perform address-based network-activity correlation when the same address is employed for multiple transactions by the same host. Additionally, it reduces the window of exposure of a host as being accessible via an address that becomes revealed as a result of active communication. These temporary addresses are meant to be used for a short period of time (hours to days) and would then be deprecated. Deprecated addresses can continue to be used for already established connections, but are not used to initiate new connections. New temporary addresses are generated periodically to replace temporary addresses that expire. In order to do so, a node produces a sequence of temporary global scope addresses from a sequence of interface identifiers that appear to be random in the sense that it is difficult for an outside observer to predict a future address (or identifier) based on a current one, and it is difficult to determine previous addresses (or identifiers) knowing only the present one. Temporary addresses should not be used by applications that listen for incoming connections (as these are supposed to be waiting on permanent/well-known identifiers). If a node changes network and comes back to a previously visited one, the temporary addresses that the node would use will be different, and this might be an issue in certain networks where addresses are used for operational purposes (e.g., filtering or authentication). , summarized next, partially addresses the problems aforementioned. describes a method to generate Interface Identifiers that are stable for each network interface within each subnet, but that change as a host moves from one network to another. This method enables keeping the "stability" properties of the Interface Identifiers specified in , while still mitigating address-scanning attacks and preventing correlation of the activities of a host as it moves from one network to another. The method defined to generate the IPv6 IID is based on computing a hash function which takes as input information that is stable and associated to the interface (e.g., a local interface identifier), stable information associated to the visited network (e.g., IEEE 802.11 SSID), the IPv6 prefix, and a secret key, plus some other additional information. This basically ensures that a different IID is generated when any of the input fields changes (such as the network or the prefix), but that the IID is the same within each subnet. Currently, recommends nodes to implement as the default scheme for generating stable IPv6 addresses with SLAAC, to mitigate the privacy threats posed by the use of MAC-derived IIDs. In addition to the former documents, proposes "an extension to DHCPv6 that allows a scalable approach to link-layer address assignments where preassigned link-layer address assignments (such as by a manufacturer) are not possible or unnecessary". proposes "extensions to DHCPv6 protocols to enable a DHCPv6 client or a DHCPv6 relay to indicate a preferred SLAP quadrant to the server, so that the server may allocate MAC addresses in the quadrant requested by the relay or client". Not only MAC and IP addresses can be used for tracking purposes. Some DHCP options carry unique identifiers. These identifiers can enable device tracking even if the device administrator takes care of randomizing other potential identifications like link-layer addresses or IPv6 addresses. introduces anonymity profiles, "designed for clients that wish to remain anonymous to the visited network. The profiles provide guidelines on the composition of DHCP or DHCPv6 messages, designed to minimize disclosure of identifying information". also indicates that the link-layer address, IP address, and DHCP identifier shall evolve in synchrony.

This section documents different policies for MAC address selection. Some OSes might use combination of multiple of these policies. Note about the used naming convention: the "M" in MAC is included in the acronym, but not the "A" from address. This allows one to talk about a PVOM Address, or PNGM Address.

This form of MAC address selection is the historical default. The vendor obtains an Organizationally Unique Identifier (OUI) from the IEEE. This has been a 24-bit prefix (including two upper bits which are set specifically) that is assigned to the vendor. The vendor generates a unique 24-bit value for the lower 24-bits, forming the 48-bit MAC address. It has not been unusual for the 24-bit value to be taken as an incrementing counter, assigned at the factory, and burnt into non-volatile storage. Note that 802.15.4 use 64-bit MAC addresses, and the IEEE assigns 32-bit prefixes. The IEEE has indicated that there may be a future Ethernet specification using 64-bit MAC addresses.

This form of MAC address is randomly generated by the device, usually upon first boot. The resulting MAC address is stored in non-volatile storage and is used for the rest of the device lifetime.

This form of MAC address is randomly generated by the device, each time the device is booted. The resulting MAC address is *not* stored in non-volatile storage. It does not persist across power cycles. This case may sometimes be a PDGM where the non-volatile storage is no longer functional (or has failed).

This form of MAC address is generated each time a new network attachment is created. This is typically used with Wi-Fi (802.11) networks where the network is identified by an SSID Name. The generated address is stored on non-volatile storage, indexed by the SSID. Each time the device returns to a network with the same SSID, the device uses the saved MAC address. It is possible to use PNGM for wired Ethernet connections through some passive observation of network traffic, such as STP , LLDP , DHCP or Router Advertisements to determine which network has been attached.

This form of MAC address is generated periodically. Typical numbers are around every twelve hours. Like PNGM, it is used primarily with Wi-Fi. When the MAC address changes, the station disconnects from the current session and reconnects using the new MAC address. This will involve a new WPA/802.1x session: new EAP, TLS, etc. negotiations. A new DHCP, SLAAC will be done. If DHCP is used, then a new DUID is generated so as to not link to the previous connection, and the result is usually new IP addresses allocated.

This form of MAC address is generated on a per session basis. How a session is defined is implementation-dependant, for example, a session might be defined by logging in a portal, VPN, etc. Like PNGM, it is used primarily with Wi-Fi. Since the address changes only when a new session is established, there is no disconnection/reconnection involved.

Most modern OSes (especially mobile ones) do implement by default some MAC address randomization policy. Since the mechanism and policies OSes implement can evolve with time, the content is now hosted at . For completeness, a snapshot of the content at the time of publication of this document is included below. Note that the extensive testing reported in this document was conducted in 2021, but no significant changes have been detected at the time of publication of this document. summarizes current practices for Android and iOS, as the time of writing this document (original source posted at: https://www.fing.com/news/private-mac-address-on-ios-14, latest wayback machine's snapshot available here: https://web.archive.org/web/20230905111429/https://www.fing.com/news/private-mac-address-on-ios-14, updated based on findings from the authors). Android 10+ iOS 14+ The randomized MAC address is bound to the SSID The randomized MAC address is bound to the Basic SSID The randomized MAC address is stable across reconnections for the same network The randomized MAC address is stable across reconnections for the same network The randomized MAC address does not get re-randomized when the device forgets a WiFI network The randomized MAC address is reset when the device forgets a WiFI network MAC address randomization is enabled by default for all the new Wi-Fi networks. But if the device previously connected to a Wi-Fi network identifying itself with the real MAC address, no randomized MAC address will be used (unless manually enabled) MAC address randomization is enabled by default for all the new Wi-Fi networks In September 2021, we have performed some additional tests to evaluate how most widely used OSes behave regarding MAC address randomization. summarizes our findings, where show on different rows whether the OS performs address randomization per network (PNGM according to the taxonomy introduced in ), per new connection (PSGM), daily (PPGM with a period of 24h), supports configuration per SSID, supports address randomization for scanning, and whether it does that by default. OS Linux (Debian "bookworm") Android 10 Windows 10 iOS 14+ Random per net. (PNGM)YYYY Random per connec. (PSGM)YNNN Random daily (PPGM)NNYN SSID config.YNNN Random. for scanYYYY Random. for scan by defaultNYNY According to , starting in Android 12, Android uses non-persistent randomization in the following situations: (i) a network suggestion app specifies that non-persistant randomization be used for the network (through an API); or (ii) the network is an open network that hasn't encountered a captive portal and an internal config option is set to do so (by default it is not).

This document has no IANA actions.

Privacy considerations regarding tracking the location of a user through the MAC address of this device are discussed throughout this document. Given the informational nature of this document, no protocols/solutions are specified, but current state of affairs is documented. Any future specification in this area would have to look into security and privacy aspects, such as, but not limited to: i) mitigating the problem of location privacy while minimizing the impact on upper layers of the protocol stack; ii) providing means to network operators to authenticate devices and authorize network access despite the MAC addresses changing following some pattern; and, iii) provide means for the device not to use MAC addresses it is not authorized to use or that are currently in use. A major conclusion of the work in IEEE Std 802E concerned the difficulty of defending privacy against adversaries of any sophistication. Individuals can be successfully tracked by fingerprinting using aspects of their communication other than MAC Addresses or other permanent identifiers.

Authors would like to thank Guillermo Sanchez Illan for the extensive tests performed on different OSes to analyze their behavior regarding address randomization. Authors would like to thank Jerome Henry, Hai Shalom, Stephen Farrel, Alan DeKok, Mathieu Cunche, Johanna Ansohn McDougall, Peter Yee, Bob Hinden, Behcet Sarikaya, David Farmer, Mohamed Boucadair, Éric Vyncke, Christian Amsüss, Roma Danyliw, Murray Kucherawy and Paul Wouters for their reviews and comments on previous versions of this document. Authors would also like to thank Michael Richardson for his contributions on the taxonomy section. Finally, authors would also like to thank the IEEE 802.1 Working Group for its review and comments, performed as part of the Liaison statement on Randomized and Changing MAC Address (https://datatracker.ietf.org/liaison/1884/).