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Using DHCP for Passive OS Identification

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

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Using DHCP for Passive OS Identification

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  1. David LaPorteHarvard University Eric KollmannBoise State University Using DHCP for Passive OS Identification

  2. Who We Are • David LaPorte • Network Security ManagerHarvard 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. Linux Group 1 TTL 16 MS Windows 95 TTL 32 Linux Group 2 TTL 64 MS Windows >95 TTL 128 Mac OS X TTL 255 IP TTL on DHCP Packets • 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. MS Windows XP 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 • Number and order of requested parameters forms a fingerprint • eg., 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. Option 60, contd. • 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. 0 Intel x86PC 5 Intel Lean Client 1 NEC/PC98 6 EFI IA32 2 EFI Itanium 7 EFI BC 3 DEC Alpha 8 EFI Xscale 4 Arc x86 9 EFI x86-64 Option 93 – client system architecture • PXE boot • Determine the underlying hardware

  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 --------------- ---------------------- 1 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 http://www.fingerbank.org

  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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