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Using DHCP for Passive OS Identification David LaPorte Harvard University Eric Kollmann Boise State University.

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Presentation on theme: "Using DHCP for Passive OS Identification David LaPorte Harvard University Eric Kollmann Boise State University."— Presentation transcript:

1 Using DHCP for Passive OS Identification David LaPorte Harvard University Eric Kollmann Boise State University

2 Who We Are David LaPorte –Network Security Manager Harvard University Network and Server Systems –Co-developer of PacketFence, an open-source NAC solution Eric Kollmann –Systems Engineer, Boise State University –Developer of Satori, a Windows-based passive OS fingerprinting tool

3 Types of OS Fingerprinting Active –Port interrogation nmap Passive –traffic analysis P0f DHCP fingerprinting

4 Why DHCP is Unique Broadcast protocol –Totally passive collection Most networks come with a built-in probe –DHCP relay agents! Extremely accurate

5 DHCP Primer Dynamic Host Configuration Protocol Entirely client-driven (currently) Main types of packets –DHCP Discover –DHCP Offer –DHCP Request –DHCP Acknowledgement –DHCP Information –DHCP Release

6 DHCP Primer, contd. Relevant RFCs –RFC 1541 –RFC 2131 Added DHCPINFORM, extended vendor classes –RFC 2132 Vendor Extensions –RFC 4361 Option 61 updates –RFC 4578 PXE Boot Information

7 DHCP Primer, contd. Server Client Server (not selected) (selected) v v v | | | | Begins initialization | | | | | _____________/|\____________ | |/DHCPDISCOVER | DHCPDISCOVER \| | | | Determines | Determines configuration | configuration |\ | | | \ | ____________/| | \________ | /DHCPOFFER | | DHCPOFFER\ |/ | | \ | | | Collects replies | | \| | | Selects configuration | | | | | _____________/|\____________ | |/ DHCPREQUEST | DHCPREQUEST\ | | | | | | Commits configuration | | | | | _____________/| | |/ DHCPACK | | | | | Initialization complete | | | |... | | | | Graceful shutdown | | | | | |\ ____________ | | | DHCPRELEASE \| | | | | | Discards lease | | | v v v

8 Which ones are useful Discover, Request, Information –All will help you identify the client OS, some are more useful than others Offer –Useful in a SOHO environment Release –Seen on a graceful shutdown on some OS's

9 Fingerprinting the hard way When there is no DHCP Server responding –DHCP retransmission timing How long does each OS wait between DHCP Discover packets before it sends another one RFC's state they should wait 4, 8, 16, 32, up to 64, all +/- 1 second RFC's also state that the seconds field should not be set to a constant value

10 Fingerprinting the hard way, contd. Seconds Elapsed Field

11 Fingerprinting the hard way, contd. What it should look like –RFC's state they should wait 4, 8, 16, 32, up to 64, all +/- 1 second

12 Fingerprinting the hard way, contd. Problem 1 – Incorrect time difference Problem 2 – Incorrect use of 'secs' field –1 Second does not = 256

13 Fingerprinting the hard way, contd. Seconds Elapsed Field set to a constant –RFC's state that the seconds field should not be set to a constant value

14 Fingerprinting the hard way, contd. Two overlapping attempts at the same time

15 IP TTL on DHCP Packets TTL 255Mac OS X TTL 128MS Windows >95 TTL 64Linux Group 2 TTL 32MS Windows 95 TTL 16Linux Group 1 Provides a rough guide to OS

16 More with TTL and DHCP Typically, no guessing required

17 Issues with TTL with DHCP DHCP Relay –Some Cisco devices will change the TTL to 255 –Some HP devices will leave the TTL field alone

18 Fingerprinting the easy way Using DHCP Options –All of the options –Option 55 (requested parameter list)‏ –Option 60 (vendor id)‏ –Option 61 (client id)‏ –Option 77 (user class information)‏ –Option 82 (relay agent information)‏ –Option 93 (client system architecture)‏

19 All of the Options Of limited use, but may get us to the “family” of the OS. –53, 61, 50, 54, 12, 55, 43

20 All of the Options, contd. Still can't be ruled out – Some systems will not provide you with other options that you want Windows 95 Discover –Note that hostname below is what we put in, the OS isn't nice enough to tell us this!

21 Option 55 - requested parameter list The easiest and most accurate way to identify a machine

22 Option 55, contd. Number and order of requested parameters forms a fingerprint –eg., 1,15,3,6,44,46,47,31,33,249,43 1,15,3,6,44,46,47,31,33,249,43,252 1,15,3,6,44,46,47,31,33,249,43,252,12 15,3,6,44,46,47,31,33,249,43 15,3,6,44,46,47,31,33,249,43,252 15,3,6,44,46,47,31,33,249,43,252,12 28,2,3,15,6,12,44,47 MS Windows XP Apple iPhone 1,3,6,15,119,78,79,95,252 1,3,6,15,119,95,252,44,46,47

23 Option 60 - vendor id Vendor ID –May be quite specific or very generic –May even be misleading

24 Option 60, contd.

25 Cisco VOIP devices –Generic Cisco Systems, Inc. IP Phone –Specific Cisco Systems, Inc. IP Phone 7905 Cisco Systems, Inc. IP Phone 7912 Cisco Systems, Inc. IP Phone CP-7960G

26 Option 60 (contd.)‏ Some Linux distributions make it easy!

27 Option 61 - client id Client Identifier –In most cases this will just be the MAC of the device, but, if you want to identify a MS RRAS server

28 Option 77 - user class information User Class Information –Be careful with this one, it is user-defined! –If you need to identify MS RRAS…

29 Option 93 – client system architecture PXE boot Determine the underlying hardware 9 EFI x Arc x86 8 EFI Xscale3 DEC Alpha 7 EFI BC2 EFI Itanium 6 EFI IA321 NEC/PC98 5 Intel Lean Client0 Intel x86PC

30 Option 82 - relay agent information RFC 3046, DHCP Relay Agent Information Option –Compatible devices “tag” DHCP packet with additional information What is included is varies by vendor Exposes information about client or switch –eg. Cisco provides port, vlan, and switch data. Data format is model-dependent Code Len Agent Information Field | 82 | N | i1 | i2 | i3 | i4 | | iN | SubOpt Len Sub-option Value | 1 | N | s1 | s2 | s3 | s4 | | sN | DHCP Agent Sub-Option Description Sub-option Code Agent Circuit ID Sub-option 2 Agent Remote ID Sub-option

31 Use Cases Targeted identification or enumeration System Inventory NAC integration to enforce OS-based policy –PacketFence –Cisco NAC Appliance

32 Mitigation Strategies Modify default DHCP client Keep IP segments as small as is reasonable –/24 segment = 254 hosts –/20 segment = 4094 hosts

33 Repository Submit, search, and export DHCP fingerprints –169+ fingerprints collected –eg., gaming consoles, DVRs, VoIP phones

34 Additional Links Satori & DHCP Fingerprinting Whitepaper –http://myweb.cableone.net/xnih PacketFence (and WRT54G tool) –http://www.packetfence.org Next Generation DHCP (SysAdmin, 02/2005) –http://insipid.com/NGDHCP.pdf

35 Related Publications 'New scheme for passive OS fingerprinting using DHCP message’ –Joho Shori Gakkai Kenkyu Hokoku, 02/2003 'Next Generation DHCP Deployments’ –SysAdmin Magazine, 02/2005

36 Other Implementations RINGS project RogueScanner (Network Chemistry)‏ DHCPListener Dhcprint Beacon (Great Bay)‏

37 Summary DHCP is an accurate and overlooked source of fingerprinting data Multiple methods available –Option 55, most reliable –Option 60, easiest (when accurate)‏ Many potential applications –NAC –Asset inventory

38 Demo

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