Internet Engineering Task Force (IETF) B. Volz Request for Comments: 8213 Y. Pal Category: Standards Track Cisco Systems ISSN: 2070-1721 August 2017
Security of Messages Exchanged between Servers and Relay Agents
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) has no guidance for how to secure messages exchanged between servers and relay agents. The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) states that IPsec should be used to secure messages exchanged between servers and relay agents but does not require encryption. With recent concerns about pervasive monitoring and other attacks, it is appropriate to require securing relay-to-relay and relay-to-server communication for DHCPv6 and relay-to-server communication for DHCPv4.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.
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RFC 8213 DHCP Relay/Server Security August 2017
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
Table of Contents
1. Introduction ....................................................2 2. Requirements Language and Terminology ...........................3 3. Security of Messages Exchanged between Servers and Relay Agents ..........................................................3 4. Security Considerations .........................................5 5. IANA Considerations .............................................5 6. References ......................................................6 6.1. Normative References .......................................6 6.2. Informative References .....................................6 Acknowledgments ....................................................8 Authors' Addresses .................................................8
The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) [RFC2131] and the Bootstrap Protocol [RFC1542] have no guidance for how to secure messages exchanged between servers and relay agents. The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315] states that IPsec should be used to secure messages exchanged between servers and relay agents but does not recommend encryption. With recent concerns about pervasive monitoring [RFC7258], it is appropriate to require use of IPsec with encryption for relay-to- server communication for DHCPv4 and require use of IPsec with encryption for relay-to-relay and relay-to-server communication for DHCPv6.
This document specifies the optional requirements for relay agent and server implementations to support IPsec authentication and encryption and recommends that operators enable this IPsec support.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
3. Security of Messages Exchanged between Servers and Relay Agents
For DHCPv6 [RFC3315], this specification REQUIRES relay and server implementations to support IPsec encryption of relay-to-relay and relay-to-server communication as documented below. The remainder of this section replaces the text in Section 21.1 of [RFC3315] when this specification is followed.
For DHCPv4 [RFC2131], this specification REQUIRES relay and server implementations to support IPsec encryption of relay-to-server communication as documented below.
This specification RECOMMENDS that operators enable IPsec for this communication.
By using IPsec with encryption for this communication, potentially sensitive client message and relay included information, such as the DHCPv4 Relay Agent Information option (82) [RFC3046], vendor-specific information (for example, the options defined in [CableLabs-DHCP]), and Access-Network-Identifier option(s) [RFC7839], are protected from pervasive monitoring and other attacks.
Relay agents and servers MUST be able to exchange messages using the IPsec mechanisms described in [RFC4301] with the conditions below. If a client message is relayed through multiple relay agents (relay chain), each of the relay agents MUST have established independent, pairwise trust relationships. That is, if messages from client C will be relayed by relay agent A to relay agent B and then to the server, relay agents A and B MUST be configured to use IPsec for the messages they exchange, and relay agent B and the server MUST be configured to use IPsec for the messages they exchange.
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RFC 8213 DHCP Relay/Server Security August 2017
Relay agents and servers use IPsec with the following conditions:
Selectors Relay agents are manually configured with the addresses of the relay agent or server to which DHCP messages are to be forwarded. Each relay agent and server that will be using IPsec for securing DHCP messages MUST also be configured with a list of the relay agents to which messages will be returned. The selectors for the relay agents and servers will be the pairs of addresses defining relay agents and servers and the direction of DHCP message exchange on DHCPv4 UDP port 67 or DHCPv6 UDP port 547.
Mode Relay agents and servers MUST use IPsec in transport mode and use Encapsulating Security Payload (ESP).
Encryption and authentication algorithms This document REQUIRES combined mode algorithms for ESP authenticated encryption, ESP encryption algorithms, and ESP authentication algorithms as per Sections 2.1, 2.2, and 2.3 of [RFC7321], respectively. Encryption is required as relay agents may forward unencrypted client messages as well as include additional sensitive information, such as vendor-specific information (for example, the options defined in [CableLabs-DHCP]) and the Access-Network-Identifier Option defined in [RFC7839].
Key management Because both relay agents and servers tend to be managed by a single organizational entity, public key schemes MAY be optional. Manually configured key management MAY suffice but does not provide defense against replayed messages. Accordingly, Internet Key Exchange Protocol Version 2 (IKEv2) [RFC7296] with pre-shared secrets SHOULD be supported. IKEv2 with public keys MAY be supported. Additional information on manual vs. automated key management and when one should be used over the other can be found in [RFC4107].
Security policy DHCP messages between relay agents and servers MUST only be accepted from DHCP peers as identified in the local configuration.
Authentication Shared keys, indexed to the source IP address of the received DHCP message, are adequate in this application.
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RFC 8213 DHCP Relay/Server Security August 2017
Note: As using IPsec with multicast has additional complexities (see [RFC5374]), relay agents SHOULD be configured to forward DHCP messages to unicast addresses.
The security model specified in this document is hop by hop. For DHCPv6, there could be multiple relay agents between a client and server, and each of these hops needs to be secured. For DHCPv4, there is no support for multiple relays.
As this document only mandates securing messages exchanged between relay agents and servers, the message exchanges between clients and the first-hop relay agent or server are not secured. Clients may follow the recommendations in [RFC7844] to minimize what information they expose or make use of secure DHCPv6 [SEC-DHCPv6] to secure communication between the client and server.
As mentioned in Section 14 of [RFC4552], the following are known limitations of the usage of manual keys:
o As the sequence numbers cannot be negotiated, replay protection cannot be provided. This leaves DHCP insecure against all the attacks that can be performed by replaying DHCP packets.
o Manual keys are usually long lived (changing them often is a tedious task). This gives an attacker enough time to discover the keys.
It should be noted that if the requirements in this document are followed, while the DHCP traffic on the wire between relays and servers is encrypted, the unencrypted data may still be available through other attacks on the DHCP servers, relays, and related systems. Securing these systems and the data in databases and logs also needs to be considered on both the systems themselves and when transferred over a network (i.e., to network attached storage for backups or to operational support systems).
Use of IPsec as described herein is also applicable to Lightweight DHCPv6 Relay Agents [RFC6221], as they have a link-local address that can be used to secure communication with their next-hop relay(s).