1. The Future of DHCP
Dr. Ralph Droms
Bucknell University

2. Futures Draft Standard status New options DHCP and DNS
Inter-server protocol
Authentication
DHCPv6

3. DHCP to “Draft Standard”
DHCP has been accepted as a “Draft Standard”
Rules for progression in STD 1 (currently, RFC 1920)
Multiple, independent, interoperable implementations
Sufficient time for review as ”Proposed Standard”
Will be submitted for full “Standard” status
New options will progress through process independently

4. New Options Acceptance
New options must have non-overlapping option codes
Numbers handed out by Internet Assigned Numbers Authority (IANA)
New mechanism approves each new option as a separate RFC (like TELNET)

5. User Class Identifier
Encodes category or type of user or applications - for example, ACCOUNTING or SALES
Classes locally defined by DHCP administrator
Client may specify more than one class
Server interpretation is implementation dependent; policy is determined by DHCP administrator
Server returns class in response so client knows if it was accepted
See “The User Class Option for DHCP, “ by Stump and Droms (draft-ietf-dhc-userclass-01.txt)

6. FQDN Modifier
Options that identify servers only allow for 32-bit IP addresses
If servers change IP addresses, clients may not be informed
FQDN (Fully Qualified Domain Name) modifier allow options to replace IP addresses with FQDNs
See “An Option for FQDNs in DHCP Options,” by Rekhter and Droms (draft-ietf-dhc-fqdn-opt-03.txt)

7. FQDN Modifier Example

8. Option Extension
BOOTP established as globally defined, as locally defined options
Currently have defined roughly 80 options
Option extension defines option 127 as a tag for more suboptions
See “An Extension to the DHCP Option Codes,” by Droms (draft-ietf-dhc-options-opt txt)

9. Server Selection Option
Client may receive multiple OFFERs and must choose one server
OFFERs may not include enough information for client to make “good” choice
Server selection option will include server identification on which client can base selection
See “The Server Selection Option for DHCP,” by Stump and Gupta (draft-ietf-dhc-sso-00.txt)

10. Allocation From Network With Multiple Subnets
Relay agent must pick one address to put in ‘giaddr’
If there are multiple subnets on one physical network, which address should relay agent choose?
Always picking one limits flexibility in allocation
Specifying rules requires configuration of each relay agent

11. Allocation From Network With Multiple Subnets (con’t)
Server must have knowledge of network architecture and set of policies about allocating addresses based on ‘giaddr’
Different relay agents may insert different values for ‘giaddr’
Must be described as set of subnets that appear on same physical subnet
DHCP administrator can use classing options to define allocation policy
See “Subnet Selection Option for DHCP,” by Townsley (draft-ietf-dhc-subsel-00.txt)

12. Other New Options Options for Service Location Protocol
IEEE POSIX timezone specification
Relay agent information
Multicast address allocation
Netware/IP and NDS
Subnet selection
Domain search
See for pointers

13. Dynamic DNS
When client is allocated a new address, DNS records need to be updated
A record: Name to IP address
PTR record: IP address to name
Newly defined extensions to DNS for dynamic updates allow updates through the network
DHCP extended to allow coordination between client and server
Which does updates
Error conditions

14. Inter-Server Communication
Becomes a distributed database problem
Windows when information managed by servers is inconsistent
Newly allocated addresses
Extended leases
Released addresses
Solution - look at those windows carefully
Determine which are really a problem
Fix just those problem windows
This slide used to be titled “problems with multiple servers”

15. Windows Newly allocated address, not yet propagated to other servers
Other servers can’t provide redundancy
Won’t return incorrect information
Extended lease, not yet propagated to other servers
Client can choose longest outstanding lease
Early expiration simply implies server discards lease from database
Released lease, not yet propagated to other servers
Client may reboot and send DISCOVER
May get back old lease from servers that haven’t been notified

16. Reusing Addresses
When reusing an address, must be very careful to ensure that it is not in use
All servers must be polled to make sure there are no outstanding leases
In response to a RELEASE, server informs all other servers to terminate that lease
Server can’t reuse until all other servers have been polled
If any servers have extended the lease after the RELEASE message was received, address can’t be reused

17. Inter-Server Protocol
“An Inter-server Protocol for DHCP,” by Kinnear, Cole and Droms (draft-ietf-interserver-02.txt) addresses the “windows”
Based on the Server Cache Synchronization Protocol (draft-ietf-ion-scsp-01.txt)
From IP-over-ATM (ION) WG
Used for, e.g., ATMARP
Currently under discussion in WG

18. Security / Authentication
Unauthorized - either intentional or accidental - server can cause denial of service problems
Server authentication allows clients to discard messages from bogus servers
Some sites may want to limit IP address allocation to authorized client
Client authentication allows servers to disregard requests from unauthorized clients

19. Security / Authentication (con’t)
Authentication based on shared private key, an authentication ticket and a message digest
Assures source of message is valid and message hasn’t been tampered with en route
‘giaddr’ causes problems with end-to-end IP security

20. Security / Authentication (con’t)
Alternative 1: simple cleartext identifier in message
Alternative 2: shared secret between server and client (Schiller/Huitema)
Alternative 3: public key exchange (Gudmundsson)

21. Change Management Many new options have been proposed
Some make fundamental changes to DHCP as interpreted by client and server
Others involve new data types (e.g., FQDN)
Each change requires development and deployment of new software

22. DHCPv2 ?
“Freeze” DHCP at present state and study process for developing new options
Define and deploy new option set
Accommodate data typing for options that may carry multiple types
Use new “cookie” value to identify new syntax

23. IPv6
IP Version 6 (aka IPv6 or IPng) is a new internet protocol to replace IP
Includes new features for host configuration:
Router advertisement
Autoconfiguration
Link-local addresses
To accommodate sites that want centralized management of addresses, DHCP for IPv6 (DHCPv6) is being developed by the DHC WG.

24. DHCPv6
DHCPv6 client uses link-local address to find relay agent and server
Client puts relay agent and server addresses in request
avoids relay agent modifying message
client must perform PMTU fragmentation
Client can request multiple addresses from server
May use DHCPv6 more than once to obtain addresses from server
Client can request server drop all current addresses at initialization

25. Reconfiguring IPv6 Clients
IPv6 accommodates dynamic renumbering
Server may need to force clients to reconfirm current addresses
RECONFIGURE message tells client to contact server for new configuration information

26. Summary
DHCP works today as a tool for automatic configuration of TCP/IP hosts
It is an open Internet standard and interoperable client implementations are widely available
Ongoing work will extend DHCP with authentication, DHCP-DNS interaction and inter-server communication