Re: [secdir] review of draft-ietf-dhc-secure-dhcpv6-07

Sheng Jiang <jiangsheng@huawei.com> Tue, 26 February 2013 03:31 UTC

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From: Sheng Jiang <jiangsheng@huawei.com>
To: Stephen Kent <kent@bbn.com>, secdir <secdir@ietf.org>, =?utf-8?B?U2VhbiBTaGVuIOayiOeDgQ==?= <shenshuo@cnnic.cn>, "john_brzozowski@cable.comcast.com" <john_brzozowski@cable.comcast.com>, "ted.lemon@nominum.com" <ted.lemon@nominum.com>, "volz@cisco.com" <volz@cisco.com>, "rdroms.ietf@gmail.com" <rdroms.ietf@gmail.com>, "brian@innovationslab.net" <brian@innovationslab.net>
Thread-Topic: review of draft-ietf-dhc-secure-dhcpv6-07
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Date: Tue, 26 Feb 2013 03:31:40 +0000
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Subject: Re: [secdir] review of draft-ietf-dhc-secure-dhcpv6-07
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Hi, Stephen,

Thanks so much for your detailed review. Your comments are important for us to improve the draft. We will work on that in the next update. Maybe some private communication will be for your further advices.

Many thanks and best regards,

Sheng

From: Stephen Kent [mailto:kent@bbn.com]
Sent: Monday, February 25, 2013 8:42 PM
To: secdir; Sheng Jiang; Sean Shen 沈烁; john_brzozowski@cable.comcast.com; ted.lemon@nominum.com; volz@cisco.com; rdroms.ietf@gmail.com; brian@innovationslab.net
Subject: review of draft-ietf-dhc-secure-dhcpv6-07

SECDIR review of draft-ietf-dhc-secure-dhcpv6-07

I reviewed this document as part of the security directorate's ongoing effort to review all IETF documents being processed by the IESG.  These comments were written primarily for the benefit of the security area directors.  Document editors and WG chairs should treat these comments just like any other last call comments.

This document proposes use of CGAs to secure DHCPv6 traffic, principally to provide data origin authentication and connectionless integrity for messages transferred between a DHCPv6 server (or relay agent) and a client. These services are derived from the use of a CGA by the communicating peers.

The document does not provide a thorough system-level description of how the security mechanisms are to be used, and how clients, servers, and relay agents might need to be configured accordingly. For example, if the primary focus is thwarting fake DHCPv6 server attacks, then a client might not need to signature a query directed to a server. On the other hand, if the goal is to enable servers to selectively provide service to clients, e.g., based on cached CGA values, then a client would need to sign a query. The document needs to provide additional background in this regard, to enable readers (and implementers) to understand what features need to be present in system components making use of these security mechanisms. A description of local configuration variables that can be used to achieve the desired system-level behavior is needed.

Section 3 provides a very brief discussion of the vulnerabilities associated with unsecured DHCPv6, and then reviews the security mechanisms offered in RFC 3315. It notes that the symmetric key management approach offered in 3315 is difficult to manage, a conclusion with which I concur. However, the authors overstate the simplicity of their proposed approach, deferring to the Security Considerations section a discussion of public key management.

This section also notes that 3315 suggests use of IPsec to secure communications between servers and relay agents (or between relay agents), but dismisses that approach due to complexity. I am less sympathetic to this statement. IPsec is already implemented in all major operating systems, so an argument about its complexity, relevant to a set of proposed new mechanisms, is not especially relevant. Perhaps the authors intend to argue that management of IPsec for this application is more complex than their proposed solution. If so, I suggest that the text in bullet “c” of Section 3 be revised.


Section 4 describes the proposed mechanism. The section states the assumptions that underlie use of CGAs in this context, but it does so in a confusing manner. The use of CGAs provides intrinsic authentication of the sender of a signed message, in terms of the IPv6 address of the sender. For DHCPv6 clients, this may be all that is required, but the text does not make that argument. For DHCPv6 servers, clients (and relay agents) simple address authentication does not suffice; a client (or a relay agent) needs to know that the sender of a message is authorized to act as a server (or relay agent). The text is not at all clear on this point, i.e., it fails to state for which entities it is necessary to pre-configure CGA parameters, to enable meaningful authentication (and authorization).



The text here states an exception to the need for pre-configuration saying  that an entity could have “ … recorded [the parameters] from previous communications.” This leap of faith (LoF) key management approach is discussed again only in Section 7. The discussion there is superficial. More text is needed to explain when LoF may be viewed as appropriate, and to address issues such as how a client that acquired a server’s CGA would transition to a new server CGA, securely.





Section 4 ends by noting that the “authentication options” (presumably the signature option) can be used to counter replay attacks. This is not quite accurate, i.e., it is the integrity aspect of the signature that provides a basis for anti-replay mechanisms, vs. the authentication  aspect. More worrisome is that fact that there is no later mention of anti-reply in the document. The authors need to add text discussing anti-replay in this context.



Section 4.2 discusses algorithm agility, but does so only for hash algorithms. This section needs to be expanded to discuss signature algorithm agility as well. Also, the text here states that “mainly unilateral notification” is the means by which an algorithm change is made known to a peer communicant, but that not all senders in a network need to transition to a new algorithm at the same time. This section needs to describe how an orderly transition to a new algorithm can be effected in a network. For example, one might add an option that a sender could include in a message, indicating a preferred list of algorithms (signature and has) that it supports. This would enable a server to know whether clients are prepared to transition to a new algorithm.



Much of the text in 4.2 should be moved to the Security Considerations section, as it is motivational material not describing the working of the protocol.



Section 5 describes the enhancements to DHCPv6 to support the proposed security features. Section 5.1 defines an option to transfer a CGA parameters (as pre RFC 3972). This is a simple option and I didn’t see any problems with the description provided.



Section 5.2 defines a signature option. There is a timestamp here, which is present to help “reduce the danger of replay attacks.” However, the processing rules for a receiver (Section 6.2) make no mention of this timestamp. This mismatch needs to be fixed. The description of what data is protected by the signature is a bit complex to follow. A diagram is needed. A padding field is defined, but there is no mention of what padding bits are to be used.



Section 5.3 defines a signature option for relayed replies. It is used to enable encapsulation of a reply that passes through one or more relay agents, so that there are separate signatures for each agent and for the target client. The description here is not clear to me, especially if there is more than one relay agent



Section 5.4 describes an option that carries the IPv6 address of a server, preserving it for authentication when a reply is relayed through an agent. This is a simple option, and its description seems fairly clear.



Sections 6.1 and 6.2 provides processing rules for senders and receivers, respectively. This is a very good structure, but, as noted above, some details are missing, e.g., there is no mention of timestamp use. (If timestamp processing rules are defined elsewhere, e.g., 3515, then this text should include the relevant cite. The description of when the CGA, Signature and DUID options MUST and MUST NOT be used is sufficiently complex that a table needs be included. There is a reference to an Authentication Option near the bottom of page 11, but none is defined in this document.



The opening discussion for Section  6.2 is confusing when it discusses how a Secure DHCPv6 “enabled” receiver might, or might not, discard messages that omit the CGA and Signature options. The authors may need to distinguish between a receiver that is Secure DHCPv6 “capable” vs. “enabled” to clarify what they mean. Maybe the purported source address of the sender plays a roll here as well. Discarding a packet for which the required options are absent is a SHOULD, here. But, near the bottom of page 12, there is text that says a packet that does not pass all of the checks MUST be discarded. These two statement need to be reconciled. There is no discussion of how a receiver verifies that a message is from an authorized server or relay agent, e.g., by reference to pre-configured CGA data.  There is no discussion of when a receiver should cache CGA data for future use, despite an allusion to this possibility in Section 4. These topics must be addressed if the processing rules are to be considered complete. The “minbits” description is  bit confusing, as its name specifies a minimum key size, but the description discusses both min and max key sizes. Also, this variable needs to be augmented with an algorithm ID, so that it is clear to which algorithm the key size applies.



Section 6.3 describes special processing performed by relay agents, above what is described for them as senders and receivers, in the preceding two sections. Because relay agent processing has already been discussed in 6.1 and 6.2, the additional text here seems confusing to me. The closing paragraph is especially confusing to me, but maybe DHCPv6 experts will find it understandable.



The security considerations section states that “… clients should be pre-configured with the DHCPv6 server CGA.” This seems important enough to be “SHOULD” vs. “should.” Similar language is used to describe the need to pre-configure CGAs for relays and servers, and it too needs to be stated more firmly. In both cases the text states that how secure pre-configuration of CGAs is achieved is out of scope. Later in this section the authors suggest that a leaf of faith (LoF) approach to acquisition of these CGAs by clients may be a reasonable alternative to secure distribution of server CGA values. This suggestion ignores the issue of how a key change for a server will be accommodated, or how the addition of a new server would work. Absent a discussion of these issues, the LoF suggestion seems questionable.



This section contains a brief discussion of collision attacks against hash functions, and why the current levels of attacks are not a serious concern in this context. Hash functions, per se, do not have a “non-repudiation feature” so I think the text here should be improved. Discussion of the hash function security in the SeND context seems relevant. As noted earlier, the test in 4.2 that talks about why hash functions are adequately secure for this context should move here, and the redundancies should be removed.