[dhcwg] questions about DHCPv6 authentication

[JINMEI Tatuya / 神明達哉 <[e-mail]>]

(Trying to resend it from a different mail address...sorry if this is
a duplicate.)

Hello,

I have a couple of questions on DHCPv6 authentication described in
RFC3315 (Section 21).

1. the RFC says in 21.4.4.4 as follows:

    21.4.4.4. Sending Information-request Messages

    If the server has selected a key for the client in a previous message
    exchange (see section 21.4.5.1), the client MUST use the same key to
    generate the authentication information throughout the session.

  I don't really understand this part...does this assume a separate
  Solicit-Advertise (or Reply if rapid-commit option is used?)
  exchanges to negotiate the key?  If so, what if the server and/or
  the client only implement the "stateless" subset described in
  RFC3736?  Note that RFC3736 says:

    Authenticated DHCP, as described in
    sections 21 and 22.11 of the DHCP specification [1], can be used to
    avoid attacks mounted through the stateless DHCP service.

  I read this to mean that Authenticated DHCP can be used for
  implementations only support RFC3736.  But how can such
  implementations select the key without the "previous message
  exchange"?

2. RFC3315 says in Section 21.4.2 (Message Validation) that

     If the MAC
     computed by the receiver does not match the MAC contained in the
     authentication option, the receiver MUST discard the DHCP message.

   It seems to me this part of the specification some other parts of
   Section 21.4:

   2-1) in Section 21.4.4.2, the RFC allows the client to respond to
        an Advertise even if it fails on authentication:

          Client behavior, if no Advertise messages include authentication
	  information or pass the validation test, is controlled by local
	  policy on the client.  According to client policy, the client MAY
	  choose to respond to an Advertise message that has not been
	  authenticated.

        Doesn't this contradict with Section 21.4.2?

   2-2) this can be a more serious issue.  The RFC says in Section
        21.4.4.4 that:

          If the Reply
          fails to pass the validation test, the client MUST restart the DHCP
          configuration process by sending a Solicit message.

        Doesn't this contradict with Section 21.4.2?  Moreover, it
        seems to me that restarting the configuration process in this
        case can open up a possibility of DoS attack.  What is the
        background of this specification?

Any hints or answers would be highly appreciated.  Thanks in advance,

					JINMEI, Tatuya
					Communication Platform Lab.
					Corporate R&D Center, Toshiba Corp.
					[e-mail]

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