Security of Messages Exchanged Between Servers and Relay Agents

The Dynamic Host Configuration Protocol for IPv4 (DHCPv4) and 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 recommend encryption. And, with recent concerns about pervasive monitoring , it is appropriate to provide recommendations for DHCPv4 and also improve the recommendations for DHCPv6.

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 . This document uses terminology from , , and .

The following text replaces the text in RFC3315 section 21.1 and also applies to DHCPv4 (RFC1542). This revised text essentially adds encryption as relay agents may forward unencrypted client messages as well as include additional sensitive information, such as vendor-specific information (for example, ) and . While IPsec is not mandated for relay to relay, relay to server, and server to relay communication, it is highly recommended unless some other security mechanisms are already in place (such as VPN tunnels) that protect this potentially sensitive traffic from pervasive monitoring. Relay agents and servers that exchange messages securely use the IPsec mechanisms for IPv6 . If a client message is relayed through multiple relay agents, 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. 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. Relay agents and servers MUST use IPsec in transport mode and Encapsulating Security Payload (ESP). This document recommends combined mode algorithms for ESP authenticated encryption, ESP encryption algorithms, and ESP authentication algorithms as per section 2.1, 2.2, and 2.3 of respectively. Encryption is recommended as relay agents may forward unencrypted client messages as well as include additional sensitive information, such as vendor-specific information (for example, ) and . Because the relay agents and servers are used within an organization, public key schemes are not necessary. Because the relay agents and servers must be manually configured, manually configured key management may suffice, but does not provide defense against replayed messages. Accordingly, IKE with preshared secrets SHOULD be supported. IKE with public keys MAY be supported. DHCP messages between relay agents and servers should only be accepted from DHCP peers as identified in the local configuration. Shared keys, indexed to the source IP address of the received DHCP message, are adequate in this application. Appropriate IPsec implementations are likely to be available for servers and for relay agents in more full featured devices used in enterprise and core ISP networks. IPsec is less likely to be available for relay agents in low end devices primarily used in the home or small office markets.

This entire document is about security considerations and thus there is little else to add in this particular section. As this document addresses 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 to minimize what information they expose or make use of to secure communication between the client and server. As mentioned in section 14, the following are known limitations of the usage of manual keys: 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. 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 if the recommendations 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 the systems themselves and if 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 , as they have a link-local address which can be used to secure communication with their next hop relay(s).

This document has no requests of the fantastic IANA team.

The motivation for this document was several IESG discusses on recent DHCP relay agent options. Thanks to Kim Kinnear and Jinmei Tatuya for reviewing drafts and helping to improve the document. And, thanks to the authors of for the original Section 21.1 text.