Network Working Group Internet Draft Document: draft-gpaterno-wireless-pppoe-12.txt Giuseppe Paterno' Expires: April 2004 October 2003 Using PPP-over-Ethernet (PPPoE) in Wireless LANs Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT","SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC-2119]. Abstract This document explores the use of Point-To-Point Protocol over Ethernet (PPPoE) to provide wireless access to the Internet and suggests how the infrastructure can be deployed. The document targets consumers, corporations, Internet Service Providers, and mobile phone operators that provide user access through Wireless LANs technologies such as IEEE 802.11. G. Paterno' Informational [Page 1] Internet-Draft Using PPPoE in Wireless LANs October 2003 Table of Contents Status of this memo............................................1 Conventions used in this document..............................1 Abstract.......................................................1 Table of contents..............................................2 1. Introduction................................................3 2. Wireless Standard IEEE 802.11 and Security Concerns.........3 3. Proposed Solution: PPPoE....................................5 3.1. The Authentication Layer..................................5 3.2. The Encryption Layer......................................7 3.3. An Architecture Example...................................8 Abbreviations.................................................10 Normative references..........................................11 Informative references........................................13 Copyright and disclaimer......................................14 Acknowledgments...............................................14 Author's Addresses............................................14 G. Paterno' Informational [Page 2] Internet-Draft Using PPPoE in Wireless LANs October 2003 1. Introduction Increased personal mobility and need for network coverage in open spaces or in places where cabling is difficult (such as airports, hospitals, warehouses or old buildings) has accelerated the development of several wireless solutions. Different technologies exist for transmitting data "over-the-air", for example GSM Packet Radio Service (GPRS), Bluetooth, and IEEE 801.11, also known as Wireless Ethernet or Wireless Fidelity (Wi-Fi). It has been demonstrated that the security architecture within Wi-Fi is feeble and can be broken by motivated malicious users. Moreover, IEEE 802.1b is not able to uniquely identify natively the user that is accessing the network. Several workarounds and new protocols has been developed -such as IEEE 802.1x and the adoption of VPN technologies (IPSec, PPTP, etc.)- to overcome the issues. This document proposes the adoption of the Point-To-Point Protocol over Ethernet (PPPoE) [1] as an authentication methodology in wireless LAN. PPPoE expecially targets Small Office Home Office (SOHO) users and ISP. 2. Wireless Standard IEEE 802.11b and Security Concerns Currently the most successful technology in wireless LANs is IEEE 802.11 [8], due to its easy configuration, flexibility, and cost/performance ratio. In particular, the extension named IEEE 802.11b [9] (also known as 802.11 High Rate or Wi-Fi), one of the 1999 ratifications of the original 802.11 standard, provides wireless functionality comparable to Ethernet. IEEE 802.11 focuses mainly on Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, but it defines also an optional security feature in the form of encryption, named the Wired Equivalent Privacy (WEP). WEP was initially developed to give end users a protection comparable with that available on wired networks. Recent studies, such as [24] and [25], demonstrated, however, that a malicious user might gain access to the network by breaking the WEP keys and without ever needing to supply additional credentials after that. WEP is based on the RC4 [11] encryption algorithm, a function that generates a pseudo-random infinitive streaming cypher by suppling two arguments: the actual WEP key (referred to as K), that may be 40 or 104 bits long, and the Initial Vector (referred to as IV), that is 24 bits long. Each IEEE 802.11 frame payload contains the IV and cyphertext data: cyphertext data is obtained by applying XOR between the RC4 (IV, K) resulting stream and the clear text. Moreover, when a frame is transmitted a new IV is generated. As the IV has 16777216 possible values (2^24), the number of IVs would be G. Paterno' Informational [Page 3] Internet-Draft Using PPPoE in Wireless LANs October 2003 exhausted (i.e. it would collide) every 5 hours by constantly transmitting at 11 Mb/s. With about 1500 IV collisions, and with a probabilistic attack to the RC4 algorithm, it is possible to decrypt the transmission and derive the original WEP key. Document [26] describes in detail the weaknesses of the RC4 algorithm, while document [25] explains how such weaknesses can be used to break WEP. The Wired Equivalent Privacy gives therefore a false security feeling to end-users, as sensitive data, not encrypted in the presentation layer (e.g. through SSL), can be easily eavesdropped by motivated malicious users. Using layer 3 network addresses over the wireless LANs raises also some concerns. For example, the use of DHCP might be a disadvantage for those service providers unable to uniquely identify a specific user, typically for authorisation and accounting purposes. We must also consider that, once a malicious user gains access to the WEP keys, DHCP immediately gives an IP address and network information to the intruder (e.g. DNS, WINS, routing, etc.). Many manufacturers of Access Points (AP) introduced an additinal security feature known as MAC filtering. APs were enabled to identify MAC addresses of the network cards allowed to access the AP itself. The use of MAC addresses presents some issues. First one of manageability: if a user changes his/her wireless adapter, for example to replace a broken one, he/she must contact the ISP and communicate his/her new MAC address. Secondly, network cards MAC addresses can be easily changed by malicious users to find out the ones with access permissions and so gain access to the Wireless LAN. G. Paterno' Informational [Page 4] Internet-Draft Using PPPoE in Wireless LANs October 2003 3. Proposed Solution: PPPoE 3.1. The Authentication Layer Several workarounds have been adopted to overcome the security concerns described in paragraph 2, for example, the use of VPN technologies (such as IPSec) and "hardening" the AP (disabling ESSID advertisement, limiting coverage area, etc...). IEEE has developed a new technology to authenticate wired and wireless LAN users, that is the IEEE 802.1x protcol. The IEEE 802.1x standard [2], based on Extensive Authentication Protocol Over Lan (EAPOL), aimed at addressing the security issues of Wireless LANs. This protocol provides user authentication for both wireless and wireline LANs: it allows unique user identification and therefore makes it possible to provide personalised services such as grouping of users in specific Virtual LANs. When used over an IEEE 802.11 media, EAPOL is capable creating and distributing per-session WEP keys through the EAPOL-Key frame. More and more AP manufacturers are embracing the IEEE 802.1X standard, sometimes with proprietary extensions. Although IEEE 802.1x is the ideal choice for authenticating Wireless LANs users, the author wish to propose another solution based on PPPoE. With the introduction of cable and ADSL technologies, ISPs have adopted a methodology for resolving the authentication layer problem for the broadband world. ADSL and cable technologies are able to emulate an ethernet network in their standard configuration. Although DHCP is easy to deploy for a Service Provider, and to configure from an user perspective, it does not provide a way to authenticate the user, and therefore cannot be used for accounting or authorization. This need was solved with the introduction of the Point-To-Point over Ethernet protocol (PPPoE), described in [1]. Through the adoption of this protocol, access control, billing, and several type of services can be performed on a per-user, rather than a per-site or cell basis. Like the previous broadband technologies, 802.11 is able to emulate an ethernet network. The idea then is to apply PPPoE technology to Wireless LANs. The schema below summaries the proposed authentication layer and the resulting framework: G. Paterno' Informational [Page 5] Internet-Draft Using PPPoE in Wireless LANs October 2003 +-------+ +-------+ +----------------+ | HTTPS | | IMAPS | | Other secure | Application-Layer | | | | | protocol (TLS) | Security +-------+ +-------+ +----------------+ +-------+ +------------+ +-----------+ | IPSec | | Other | | MPPE/ECP | | | | encryption | | (3DESE) | Encryption +-------+ +------------+ | PPP | (optional) | extension | +------------------------+ | | Point-To-Point Protocol | Authentication | over Ethernet | +------------------------------------+ +--------+ +----------+ +------------+ | 802.11 | | HyperLAN | | Other WLAN | Physical/Data Link +--------+ +----------+ +------------+ Protocols In author's opinion, for SOHO users and for whom own APs that not support IEEE 802.1x, PPPoE can be an easier solution to authenticate users: PPPoE is widely adopted in ADSL and other broadband technologies, and the software is embedded in most of the current OSes or can be easily available. ISP might benefit of the adoption of PPPoE because fixed IP addresses (or other traditional PPP services) can be provided and because it makes simpler selling wholesale services and Virtual Private Dial-up Networks (VPDNs). By reusing proven technologies, e.g. Layer 2 Tunneling Protocol (L2TP) [19] or IPSec tunnels, Wireless LAN can extend existing ISPs dial-up offerings. Two aspects must be considered while deploying PPPoE, i.e. password exchange and the Maximum Transmission Unit (MTU) size. As specified in [1], the Maximum Receive Unit (MRU) option must not be negotiated to a size larger than 1492; Ethernet has a maximum payload size of 1500 octets. The PPPoE header is 6 octets and the PPP protocol ID is 2 octets, so that the PPP MTU must not be greater than 1492. However, based on the author's experience, some misbehaved VPN software packages add their own overhead to the MTU reported by the PPPoE interface, making the network packets too large to pass through a PPP over Ethernet connection: reducing the MTU by 32 bytes, i.e. reducing it to 1460, should generally suffice. Passwords must not be exchanged through the Password Authentication Protocol (PAP) between the user and the PPPoE concentrator, because PAP transmits passwords in cleartext: a stronger protocol such as MS-CHAPv2 [4], EAP-TLS [5] or better should be used instead. G. Paterno' Informational [Page 6] Internet-Draft Using PPPoE in Wireless LANs October 2003 3.2. The Encryption Layer Although network access can be secured through the adoption of PPPoE, data is still being sent in clear "over the air", with several concerns on users' privacy. Data can be easily decrypted even when WEP is enabled on the Wireless LAN (please refer Section 2.1). Those who intend to adopt PPPoE as their authentication methodology can easily encrypt users' data by enabling ECP or MPPE on the PPPoE link. The Encryption Control Protocol (ECP) [12] is able to configure and to enable data encryption on both ends of the point-to-point link. ECP uses the same packet exchange mechanism as the Link Control Protocol (LCP) and, more generally, provides a framework for negotiating and applying parameters associated with encryption, e.g. choosing the algorithm. At present, two algorithms has been defined, i.e. DESE-bis [13] and 3DESE [14]. Although ECP would be a better choice than MPPE to encrypt data, the author is not aware of any implementation of ECP, nor any associated option, and therefore ECP cannot be immediatly used to secure a Wireless LAN. The second option is to negotiate the Microsoft Point-To-Point Encryption Protocol (MPPE) [6] on the point-to-point link. MPPE is a a PPP Compression Control Protocol (CCP) [15] option that implements encryption: Microsoft, in fact, created MPPE to secure its Point-to- Point Tunneling Protocol (PPTP) [23,21]. MPPE is an optional PPP extension that is negotiated within option 18 [10] in the Compression Control Protocol, and uses the Rivest-Shamir-Adleman (RSA) RC4 [11] algorithm to provide data confidentiality. It was originally designed for encryption across a point-to-point link where packets arrive in the same order in which they were sent with little packet loss. MPPE can use 40-bit, 56-bit, or 128-bit encryption keys: the 40-bit key provides backward compatibility with old clients. The RFC [6] specifies also a stateless mode for MPPE, which changes the encryption keys on every packet: most implementations are capable of negotiating such an option and it is envisaged to use the stateless mode, when available, to overcome the RC4 algorithm issue on a long- lived PPPoE session. The MPPE protocol itself is already available and it has been implemented in several devices and dial-in products. Therefore MPPE can be a simple, but efficient solution for companies, ISPs, and mobile operators who wish to implement secure wireless access. Whenever corporations need more data confidentiality, the author suggests to use an application-layer encryption, for example an SSL/TLS secured applications (HTTPS, IMAPS, etc...), on top of MPPE or ECP. G. Paterno' Informational [Page 7] Internet-Draft Using PPPoE in Wireless LANs October 2003 4. An Architecture Example In the previous chapter, the Wireless LAN has been compared to a dial-up infrastructure from a security perspective. Using this similarity, a typical corporate scenario can be analysed as an example. +----------+ | Internet | +----------+ | +-------------+ (DMZ1) +-------------------------+ | FE Firewall |--------| External Proxy/DNS/Mail | +-------------+ +-------------------------+ | (DMZ2) | +-----------------------+ +--------------| Network Access Server | | +-----------------------+ | | +---------------------------+ +--------------| PPPoE Access Concentrator | | +------------+--------------+ | | | +------------+-------------+ | | Wireless Access Point(s) | | +--------------------------+ | +-------------+ (DMZ3) +------------------+ | BE Firewall |---+----| VPN concentrator | +-------------+ | +------------------+ | | | | +------------------+ +---------------+ | +----| Internal Proxy |--| Radius Server | | +------------------+ +---------------+ +----------+ | Intranet | +----------+ It has been mentioned that remote access systems, such as modems, are subject to "war-dialing", i.e. the attempt of a malicious user to guess the modem telephone number and access the corporate network. This is the reason why most of IT security policies do not allow connections using a modem and an analogue phone line to internally connected computers. Although, in a corporate security policy, dial-up users are usually G. Paterno' Informational [Page 8] Internet-Draft Using PPPoE in Wireless LANs October 2003 considered more "trusted" than global Internet users, they are however subject to an IP-based inspection (e.g. using firewalls or access lists) to limit access to corporate resources. In the example above, dial-up users are placed between two firewalls, in a zone named DMZ2. The front-end (FE) border firewall separates global Internet access from externally visible services (DMZ1), which contains for example DNS and Mail, and remote access users (DMZ2). DMZ2 is considered more secure than DMZ1: this zone is suitable for dial-up and Wireless LAN user: both access require the user to supply credentials to gain access to IP-based network. In particular, wireless users are subject to PPPoE access verification and encryption (MPPE) through the PPPoE Access Concentrator (i.e. the PPPoE server) Once a dial-up/wireless user has obtained access, a back-end (BE) firewall connects the DMZ2 to another security zone (DMZ3) and the corporate Intranet. DMZ3 hosts a RADIUS server to authenticate users, an internal proxy and a VPN concentrator, which implements encrypted tunnels for users connecting from the Internet In a highly-secure environment, the security policies might only allow an encrypted data flow, e.g. through SSL, from wireless users in DMZ2 to the Intranet (HTTPS, IMAPS, etc...). For specific unencrypted applications, e.g. TN3270E mainframe access, the policies might require a VPN secure tunnel to be established prior to accessing the service. G. Paterno' Informational [Page 9] Internet-Draft Using PPPoE in Wireless LANs October 2003 Abbreviations ADSL ............. Asymmetric Digital Subscriber Line AP ............... Access Point CLID ............. Caller ID DMZ .............. Demilitarised Zone EAPOL ............ Extensive Authentication Protocol over Ethernet ECP .............. Encryption Control Protocol GPRS ............. GSM Packet Radio Service GSM .............. Global System for Mobile Communications IEEE ............. Institute of Electrical and Electronics Engineers ISP .............. Internet Service Provider L2TP ............. Layer 2 Tunneling Protocol LAC .............. L2TP Access Concentrator LNS .............. L2TP Network Server MN ............... Mobile Node MPPE ............. Microsoft Point-to-Point Encryption Protocol MRU .............. Maximum Receive Unit MTU .............. Maximum Transmission Unit NAP .............. Network Access Provider NAS .............. Network Access Server NSP .............. Network Service Provider PAC .............. PPPoE Access Concentrator PARA ............. PPPoE Advanced Relay Agent POTS ............. Plain Old Telephone Service PPPoE ............ Point-To-Point Protocol over Ethernet PPTP ............. Point-To-Point Tunneling Protocol SSID ............. Service Set Identifier SSL .............. Secure Sockets Layer TLS .............. Transport Layer Security UMTS ............. Universal Mobile Telecommunications System VMAC ............. Virtual MAC VLAN ............. Virtual LAN WEP .............. Wired Equivalent Privacy WASS ............. Wireless Access SubSystem Wi-Fi ............ Wireless Fidelity WiRAN ............ Wireless Roaming Area Network WLAN ............. Wireless LAN G. Paterno' Informational [Page 10] Internet-Draft Using PPPoE in Wireless LANs October 2003 Normative references [1] Mamakos, et. al., "A Method for Transmitting PPP Over Ethernet (PPPoE)", RFC 2516, February 1999 [2] Institute of Electrical and Electronics Engineers, "Local and metropolitan area networks Port-Based Network Access Control", ANSI/IEEE Standard 802.1X, October 2001 [3] Simpson, "PPP Challenge Handshake Authentication Protocol (CHAP)", RFC 1994, August 1996 [4] Zorn, "Microsoft PPP CHAP Extensions, Version 2", RFC 2759, January 2000 [5] Aboba & Simon, "PPP EAP TLS Authentication Protocol", RFC 2716, October 1999 [6] Pall & Zorn, "Microsoft Point-To-Point Encryption (MPPE) Protocol" RFC 3078, March 2001 [7] Kent & Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998 [8] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE Standard 802.11, 1999 Edition [9] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band", IEEE Standard 802.11b, September 1999 [10] Pall, "Microsoft Point-to-Point Compression (MPPC) Protocol", RFC 2118, March 1997 G. Paterno' Informational [Page 11] Internet-Draft Using PPPoE in Wireless LANs October 2003 [11] RC4 is a proprietary encryption algorithm available under license from RSA Data Security Inc. For licensing information, contact: RSA Data Security, Inc. 100 Marine Parkway Redwood City, CA 94065-1031 [12] G. Meyer, "The PPP Encryption Control Protocol (ECP)", RFC 1968, June 1996 [13] K. Sklower, G. Meyer, "The PPP DES Encryption Protocol, Version 2 (DESE-bis)", RFC 2419, September 1998 [14] H. Kummert, "The PPP Triple-DES Encryption Protocol (3DESE)", RFC 2420, September 1998 [15] D. Rand, "The PPP Compression Control Protocol (CCP)", RFC 1962, June 1996 [16] C. Perkins, "IP Mobility Support for IPv4", RFC 3344, August 2002 [17] J. Solomon, S. Glass, "Mobile-IPv4 Configuration Option for PPP IPCP", RFC 2290, February 1998 [18] W. Simpson, "The Point-to-Point Protocol (PPP)" RFC 1661 / STD 51, July 1994 [19] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter, "Layer Two Tunneling Protocol (L2TP)", RFC 2661, August 1999 [20] D. Farinacci, T. Li, S.Hanks, D. Meyer, P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000 [21] K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, G. Zorn "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637, July 1999 G. Paterno' Informational [Page 12] Internet-Draft Using PPPoE in Wireless LANs October 2003 [22] J. Reynolds, J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. See also: http://www.iana.org/numbers.html Informative references [23] Microsoft Corporation, "Understanding Point-to-Point Tunneling Protocol (PPTP)", WhitePaper, September 1999 [24] M. Sutton, iDEFENSE Labs, "Hacking the Invisible Network. Insecurities in 802.11x", WhitePaper, July 2002 [25] Stubblefield, Ioannidis, and Rubin, "Using the Fluhrer, Mantin, and Shamir Attack to Break WEP", AT&T Labs Technical Report TD-4ZCPZZ, August 2001 [26] Fluhrer, Mantin, and Shamir, "Weaknesses in the Key Scheduling Algorithm of RC4", WhitePaper [27] Cisco Systems, "Troubleshooting MTU Size in PPPoE Dialin Connectivity", WhitePaper [28] Cisco Systems, "PPPoE Baseline Architecture for the Cisco 6400 UAC", WhitePaper [29] Uyless Black, "Internet Security Protocols: Protecting IP Traffic", Published by Prentice Hall / Pearson Education in year 2000 G. Paterno' Informational [Page 13] Internet-Draft Using PPPoE in Wireless LANs October 2003 Copyright and disclaimer Copyright (C) Giuseppe Paterno' (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the author of this document or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the author or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and Giuseppe Paterno' DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgments The author wishes to thank his good friend Luca Sciortino for his precious moral support and his contribution to this document. Many thanks go to Maria Di Biccari, Alberto Paterno' and Elisa Stella for their patience and loveliness. Author's addresses Giuseppe Paterno' Casella Postale 133 20090 Trezzano S/N (MI) Italy Email: gpaterno@gpaterno.com G. Paterno' Informational [Page 14]