1. DHCP Lease Query DHC Working Group Kim Kinnear Cisco Systems
December 11, 2000
Phone example
Laptop at friend’s house
Copyright 1998 American Internet Corporation, Company Confidential

2. DHCP Lease Query What’s the Problem?
Access concentrators can “glean” DHCP information to build internal tables relating IP, MAC, and circuit, but this information is lost after reboot.
This information is used to increase security in public networks which use DHCP.

3. DHCP Lease Query What’s the Solution?
The access concentrator can ask the DHCP server about the IP addresses that it encounters, and rebuild its internal tables in real time.
It asks the DHCP server because the DHCP server has the most up to date information.

4. What is DHCP Lease Query?
A lightweight method for relay agents to get “location information” from the DHCP server(s)
A message designed for the needs of broadband access concentrators (e.g. DOCSIS CMTS, DSL AC)
A DHCP message that does not modify server lease state (like DHCPINFORM)

5. Location Information Often includes the following information:
Device hardware (MAC) address
Port/virtual circuit that leads to the device*
Hardware address of the intervening subscriber modem*
* contained in relay-agent-info option
Can be used for both downstream transmission, and upstream verification

6. Location Information in a Cable Access Network
Computers
Subscriber Modems
Access Concentrator
DHCP Server
Access Concentrators use location info. for:
Choosing specific broadband access network
Encrypting traffic for specific subscriber modem
Forwarding traffic to specific subscriber modem

7. Why Propose a New DHCP Message: Alternatives
Use broadcast ARP
Chatty on public network
Vulnerable to subscriber spoofing
Capture information from relayed DHCP messages (gleaning)
Process starts from scratch with reboot or replacement of relay agent
Relay agent misses unicast DHCP messages (e.g. Renewals)

8. Why Propose a New DHCP Message: More Alternatives
Leverage DHCP Server MIB
Access concentrators act as SNMP agents, but not as SNMP managers
Leverage DHCP LDAP Schema
Access concentrators don’t act as LDAP clients, LDAP information may not be up to date.

9. DHCP Lease Query Exchange
DHCP Lease Query message
Ciaddr refers to IP address lease to query
Giaddr refers to requestor (i.e. access concentrator)
Parameter request list includes IP Address Lease Time option (51) and Relay Agent Information option (82)
DHCP Lease Query response - DHCPACK or DHCPNAK

10. DHCP Lease Query Example - DOCSIS CMTS
CMTS receives packet to forward downstream across cable
CMTS has no local location information
CMTS sends DHCP Lease Query, gets DHCPACK
Chaddr contains the PC MAC address
Option 82 contains subscriber modem info
CMTS transmits packet using BPI

11. Interactions with Lease Query
Relay Agent Gleaning
Gleaning state replaces Lease Query state
Lease Query with Failover
Access concentrator sends Lease Query messages to multiple DHCP servers
Failover BNDUPD messages need to include option 82 relay-agent-info
Lease Query uses DHCP Authentication

12. DHCP Lease Query Status
DHCP Lease Query internet draft updated with comments and submitted under DHC working group
DHCP Lease Query variant implemented in Cisco uBR, Cisco Network Registrar

13. DHCP Lease Query Recent Updates
Restructured draft for clarity
Specified detailed client and server behavior
Added information about static (reserved) addresses
Fleshed out NAK semantics

14. DHCP Lease Query Plans
Gather additional technical comments (some received already)
Update draft prior to Minneapolis IETF in March.
When is last call? After Minneapolis review?