Network Working Group Internet Draft Document: draft-gpaterno-wireless-pppoe-05.txt Giuseppe Paterno' Expires: May 2003 December 2002 Using PPP-over-Ethernet (PPPoE) to authenticate 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 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' Experimental [Page 1] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 Table of Contents Status of this memo............................................1 Conventions used in this document..............................1 Abstract.......................................................1 Table of contents..............................................2 1. Introduction................................................3 2. Current Wireless LAN scenario...............................3 2.1. Wireless standard IEEE 802.11 and security concerns.......3 2.2. Existing authentication methodologies.....................4 3 Proposed solution............................................5 3.1. A layered approach........................................5 3.2. The authentication layer: PPPoE...........................6 3.3. The encryption layer......................................8 4. An architecture example.....................................9 5. Conclusions................................................10 Acronyms......................................................11 References....................................................12 Copyright and disclaimer......................................14 Acknowledgments...............................................14 Author's Addresses............................................14 G. Paterno' Experimental [Page 2] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 1. Introduction Current wireless LANs technologies provide a feeble security architecture that can be broken by motivated malicious users. Moreover, these technologies are not able to uniquely identify the user that is accessing the network: as a result, corporations, ISPs, and mobile operators are unable to apply appropriate rights and/or services to the end-users. This document proposes the adoption of the Point-To-Point Protocol over Ethernet as an authentication methodology in wireless LANs and as an additional security component. Furthermore, it explores how consumers, corporations, ISPs, and mobile operators will benefit of the adoption of PPPoE as an alternative solution to IEEE 802.1X. 2. Current Wireless LANs scenario The need for mobility and network coverage in open spaces or places where cabling is difficult (such as airports, hospitals, warehouses or old buildings) has accelerated the development of Wireless alternatives. 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). 2.1. Wireless standard IEEE 802.11 and security concerns The most successful technology in wireless LANs is IEEE 802.11 [10] for its easy configuration, flexibility and performance with low costs. In particular, the extension named IEEE 802.11b [15] (also referred to as 802.11 High Rate or Wi-Fi), was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet. IEEE 802.11 focus mainly on Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, but it specifies also an optional security feature in the form of encryption, named the Wired Equivalent Privacy (WEP). WEP was initially developed to give to the end user the same protection as the wired network. Recent studies, such as [9] and [11], demonstrated that a malicious user might gain access to the network by breaking the WEP keys and without supplying any credential. WEP is based on the RC4 [17] encryption algorithm, a function that generates a pseudo-random infinitive streaming cypher by suppling two arguments: the actual WEP keys (referred as K), that might be 40 or 128 bytes long, and the Initial Vector (IV), that is 24 bytes long. Each IEEE 802.11 frame payload contains both the IV in clear and the cyphertext: the cyphertext is obtained by applying G. Paterno' Experimental [Page 3] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 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 of possible values (2^24), the number of IV would be repeated (i.e. 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 keys. The document [12] help understanding the RC4 algorithm weaknesses, and [11] explains how can be applied to break WEP. The Wired Equivalent Privacy gives therefore a false security feeling to the end-user: sensitive data that is not encrypted in the presentation layer, through SSL for example, would be easily eavesdropped. Using layer 3 network addresses over the wireless LAN raises also some concerns. For example the use of DHCP might represent a disadvantage for those service providers that are unable to 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 (DNS, WINS, routing, etc.). Many manufacturers of APs introduced another security feature by providing the ability to identify the MAC addresses of network cards permitted to use the AP, also known as MAC filtering. The use of MAC addresses introduces some issues, one of manageability: if a user changes the wireless adapter, for example to replace a broken one, he/she should contact the ISP and provide the new MAC address. Another issue is that MAC addresses can be changed easily and guessed by malicious users to gain access to the Wireless LAN. 2.2. Existing authentication methodologies The IEEE 802.1X standard [2], based on Extensive Authentication Protocol Over Lan (EAPOL), has been proposed to address the security concerns of Wireless LANs. The protocol has been designed to provide user authentication for both wireless and wireline LANs, giving to the corporations the opportunity to provide their users with personalised services such as grouping in specific Virtual LANs. While the enhancements proposed by both IEEE 802.1X and the work-in- progress IEEE 802.11i could improve the security of 802.11 networks, these solutions come out of the market too late, causing impatient vendors to implement proprietary solutions. These vendors may not be willing to replace these proprietary fixes with 802.11i as it becomes available. G. Paterno' Experimental [Page 4] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 Many of the high-end AP manufacturers anyway embraced the IEEE 802.1X standard, sometimes with proprietary extensions: although 802.1X provides flexibility and extended LAN support, purchasing compliant hardware is still an expensive solution for small businesses and consumers. In fact, as of today, many of the Wireless Access Points and hub/switches do not support EAPOL. Furthermore, many 802.1X compliant hardware do not implement the dynamic WEP-key exchange feature [2] (EAPOL-Key), adding potential security issues. Most consumers, small ISPs and small corporations will not be able to afford such equipment, but are nevertheless in need of security and of being able to identify users that are accessing their resources: in fact, some malicious users today are gaining access to home users' equipment through WLANs in order to attack remote sites and preserve their anonymity. It is also to be considered that many of ISPs and mobile operators are not interested in implementing encryption to their customer, for example for public Internet access, nevertheless they are in need to identify the user for billing purposes. The ideal solution for ISPs and mobile operators would be able to integrate with the existing dial-up infrastructure (modem, GPRS, etc) with little effort, and should bring the same subscrbed services (fixed IP address, Quality of Service, etc) to the end user. 3. Proposed solution 3.1. A layered approach As suggested by the OSI specifications, a good solution might be the adoption of a layered approach, focusing on specific aspects of a given layer. By analysing physical/data link, authentication, and encryption separately the advantage is that the resulting framework would allow changes in one layer to occur without affecting the other layers. As Wireless LANs, including IEEE 802.11, will evolve and new standards become available, authentication and encryption will remain unchanged or vice versa. The schema below summaries the proposed authentication layer and the resulting framework: G. Paterno' Experimental [Page 5] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 +-------+ +-------+ +----------------+ | HTTPS | | IMAPS | | Other secure | Application Layer | | | | | protocol (TLS) | +-------+ +-------+ +----------------+ +-------+ +------------+ +-----------+ | IPSec | | Other | | MPPE | | | | encryption | | PPP | Encryption Layer +-------+ +------------+ | extension | (optional) | | +------------------------+ | | Point-To-Point Protocol | Authentication Layer +------------------------------------+ +--------+ +----------+ +------------+ | 802.11 | | HyperLAN | | Other WLAN | Physical/Data Link Layer +--------+ +----------+ +------------+ 3.2. The authentication layer: PPPoE With the introduction of cable and ADSL technologies, ISPs have adopted a methodology for resolving the authentication layer problem for the broadband world. In standard configuration, these technologies are able to emulate an ethernet network. 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. The 802.11 technology, in a similar way to the previous broadband technologies, is able to emulate the ethernet network. The idea then is to apply PPPoE technology to Wireless LANs. The advantage is clear: consumers, corporations, Internet Service Providers, and mobile operators can perform authentication, authorisation, and accounting easily on the wireless users without adding new components and, more importantly, with little effort. A practical aspect of this technology might be to provide, for example, fixed IP addresses to a roaming wireless user: wherever the user is located, he/she can have his/her IP address and subscribed class of services. G. Paterno' Experimental [Page 6] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 Furthermore, the use of PPP will introduce another obstacle to malicious users, that would have to break both the WEP and the PPP layer to gain access to the IP-based network. It is envisaged that passwords must not be exchanged through the Password Authentication Protocol (PAP), since PAP transmit passwords in cleartext: a stronger protocol such as MS-CHAPv2 [4], EAP-TLS [5] or better should be used instead. From a traditional ISP and corporations perspective, there is no additional benefit in using PPPoE technology, if compared to IEEE 802.1X: the concerns of using PPPoE are the PPP frame overhead and the MTU size. However, one aspect must be considered when deploying IEEE 802.1X: current implementations are based on EAP-TLS [5]. Although EAP-TLS is the perfect choice for corporations that already deployed X.509 certificates, it is not for ISPs, mobile operators and corporations that do not own or plan to have an X.509 infrastructure. Creating and maintaining a PKI infrastructure is expensive, expecially if a public Certification Authority is used, and requires expert human resources dedicated to the PKI. Moreover, if the ISP or corporate already owns non 802.1X compliant Access Points, such hardware should be replaced. Another advantage of embracing PPPoE for Network Access Provider (NAP) or Network Service Provider (NSP) is that they can provide secure access to a corporate gateway, by using Layer 3 routing, Layer 2 Tunneling Protocol (L2TP), and/or IPSec tunnels in a similar way of existing dial-up scenarios (modem, ISDN, ADSL, etc..). This makes the business model of selling wholesale services and Virtual Private Dial-up Networks (VPDNs) scalable. For consumers, small businesses, and local ISPs the PPPoE MTU limitation is not an issue, if compared to the cost of deploying both hardware and EAPOL compliant software to the client. The advantage is that, by preserving the existing access points and with a simple additional component (the PPPoE server), they are able to protect their LANs by identifying uniquely the user. As a result, adding a PPPoE server is easier than deploying EAPOL with EAP-TLS, that requires a more complex infrastructure. Moreover, most of today's operating systems include a PPPoE client, resulting in a low cost deployment for this technology. Finally, Access Point manufacturers can easily embed a PPPoE server in their products, that might be distributed as a firmware update, and provide an easy user configuration to the consumer, for example through a web interface. It has been mentioned in the former paragraphs that the use of PPPoE has an Maximum Transmission Unit (MTU) issue: as specified in [1], G. Paterno' Experimental [Page 7] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 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 to 1460 should generally suffice. 3.3. The encryption layer A Wireless LAN, being over the air, might be considered a public switched network, in a similar way of the telephone network. For example, in the traditional Plain Old Telephone Service (POTS), a malicious user intercept PPP packets by tapping the phone wire. The Wireless LAN can be managed therefore as a dial-up connection and encryption and/or access policies should be applied, such as protecting the access through a firewall or a proxy, allowing only specific applications. It is recommended that users that need privacy should add an encryption layer on top of their connection, be a wireless LAN or a standard PPP over modem. There can be different approaches for this layer: a simple, but efficient solution for companies, ISPs and mobile operators can be the Microsoft Point-To-Point Encryption Protocol [6] PPP extension. MPPE is an optional PPP extension that is negotiated within option 18 [16] in the Compression Control Protocol, and uses the Rivest-Shamir- Adleman (RSA) RC4 [17] algorithm to provide data confidentiality. MPPE can use 40-bit, 56-bit, or 128-bit encryption keys: the 40-bit key provides backward compatibility with old clients. 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. For environments that requires stronger privacy, it is recommended to use other encryption methodologies to access the corporate LAN, such as for example IPSec [7], the de-facto standard Point-to-Point Tunneling Protocol (PPTP) [8], or future encryption technologies. G. Paterno' Experimental [Page 8] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 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) +-------------------------+ | Firewall |--------| External Proxy/DNS/Mail | +----------+ +-------------------------+ | (DMZ2) | +---------------------------+ +--------------| Remote Access/VPDN server | | +---------------------------+ | | +--------------------------+ +--------------| Wireless Access Point(s) | | +--------------------------+ | +----------+ (DMZ3) +------------------+ | Firewall |--------| VPN concentrator | +----------+ | +------------------+ | | | | +------------------+ +---------------+ | +----| Internal Proxy |--| Radius Server | | +------------------+ +---------------+ +----------+ | Intranet | +----------+ We mentioned that remote access systems, such as modems, are subject to "wardialing", i.e. the attempt of a malicious user of guessing the modem telephone number and accessing the corporate network. Today, most of the corporate IT security policies do not allow to connect a modem and an analogue phone line to internally connected computers. In a security infrastructure, dial-up users are usually subject to an IP-based inspection (using a firewall or access lists for example) to limit access to corporate resources. While creating a security policy, dial-up user are usually considered more "trusted" than global Internet users, since appropriate credentials should be required. G. Paterno' Experimental [Page 9] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 In the example above, a border firewall separates global Internet access from both externally visible services (DNS, Mail, Proxy, etc..) and remote access users, creating two demilitarised zones, DMZ1 and DMZ2 respectively. DMZ2 should be more secure than the external services, that can be compromised by a malicious user: this zone is suitable for dial-up (be a RAS server or outsourcing through a Virtual Private Dial-up Network) and Wireless LANs user, that should supply credential to gain access to IP-based network. Once a dial-up/wireless user has obtained access, a second firewall connects the DMZ2 to a DMZ3 and the corporate Intranet. DMZ3 hosts a RADIUS server to authenticate users, an internal proxy and a VPN concentrator, if not included with the firewall. The VPN concentrator implements the encryption layer, offering a secure connection to the Intranet. An optional data flow, if encrypted, can be established from DMZ2 to the Intranet, for example IMAPS or HTTPS, so that VPN will be required only for specific unencrypted applications, such as TN3270E mainframe access. 5. Conclusions At the time of writing, it is extremely easy from a malicious users perspective to gain access to wireless networks, even if encrypted. Many Wireless LANs are unencrypted and their access points are configured to release dynamic IP address through the Dynamic Host Configuration Protocol. In such a configuration, it is even easier for an intruder to gain access to the network. Moreover, this raises some legal concerns: in some countries it is not illegal to gain access to a network that is not protected in any way or limited through a warning statement, for example through the usual "restricted area" banner, because the user is not accessing the Wireless LAN by "forcing" the system. Public services, such as mobile operators, ISPs, and free wireless networks, will not take advantage of any evolution of the WEP protocol. Today the encryption keys are unique for the whole Wireless LAN segment, which means that keys should be made publically available, in turn making the WEP protection mechanism ineffective. For consumers and corporations, using WEP or future protocols to encrypt "over-the-air" transmission is still an advantage: although easy to decrypt, the intruder should be very motivated to enter the network because an observation of thousands of interesting packets is needed to gain access to the encryption keys. Through this paper the author analyses the advantages of using Point- To-Point over Ethernet protocol as a solution for a Wireless LAN authentication layer: it has been demonstrated that, through the G. Paterno' Experimental [Page 10] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 reuse of existing elements of the network and without changing the existing infrastructure, consumers, corporations and Internet Service Providers can take advantage of PPPoE, resulting in a more secure environment with no or little additional cost. After a draft of this document was released to the public, some implementations of PPPoE authentication were deployed, demonstrating the willingness to implement this methodology: in fact, some premier universities, government institutions and private users, including free access city wireless networks, implemented the PPPoE solutions for their Wireless LANs. Acronyms ADSL ............. Asymmetric Digital Subscriber Line AP ............... Access Point DMZ .............. Demilitarised Zone EAPOL ............ Extensive Authentication Protocol over Ethernet GPRS ............. GSM Packet Radio Service GSM .............. Global System for Mobile Communications IEEE ............. Institute of Electrical and Electronics Engineers ISP .............. Internet Service Provider MPPE ............. Microsoft Point-to-Point Encryption Protocol MRU .............. Maximum Receive Unit MTU .............. Maximum Transmission Unit NAP .............. Network Access Provider NSP .............. Network Service Provider POTS ............. Plain Old Telephone Service PPPoE ............ Point-To-Point Protocol over Ethernet PPTP ............. Point-To-Point Tunneling Protocol SSL .............. Secure Sockets Layer TLS .............. Transport Layer Security VLAN ............. Virtual LAN WEP .............. Wired Equivalent Privacy Wi-Fi ............ Wireless Fidelity WLAN ............. Wireless LAN G. Paterno' Experimental [Page 11] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 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] Microsoft Corporation, "Understanding Point-to-Point Tunneling Protocol (PPTP)", WhitePaper, September 1999 [9] M. Sutton, iDEFENSE Labs, "Hacking the Invisible Network. Insecurities in 802.11x", WhitePaper, July 2002 [10] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE Standard 802.11, 1999 Edition [11] Stubblefield, Ioannidis, and Rubin, "Using the Fluhrer, Mantin, and Shamir Attack to Break WEP", AT&T Labs Technical Report TD-4ZCPZZ, August 2001 [12] Fluhrer, Mantin, and Shamir, G. Paterno' Experimental [Page 12] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 "Weaknesses in the Key Scheduling Algorithm of RC4", WhitePaper [13] Cisco Systems, "Troubleshooting MTU Size in PPPoE Dialin Connectivity", WhitePaper [14] Cisco Systems, "PPPoE Baseline Architecture for the Cisco 6400 UAC", WhitePaper [15] 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 [16] Pall, "Microsoft Point-to-Point Compression (MPPC) Protocol", RFC 2118, March 1997 [17] 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 G. Paterno' Experimental [Page 13] Internet-Draft Using PPPoE to authenticate Wireless LANs December 2002 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 of this document wishes to thank Luca Sciortino for his precious moral support and his contribution to this document, Silvio Danesi and Daniele Todde for providing the technical infrastructure. Many thanks go to Maria Di Biccari, Alberto Paterno' and Elisa Stella for their patience and loveliness. Author's addresses Giuseppe Paterno' Email: gpaterno@gpaterno.com G. Paterno' Experimental [Page 14]