Re: [Dots] Tsvart last call review of draft-ietf-dots-server-discovery-11

[[e-mail]]

Hi Kyle, 

Thank you for the review.

Please see inline. 

Cheers,
Med

> -----Message d'origine-----
> De : Kyle Rose via Datatracker [mailto:[e-mail]]
> Envoyé : lundi 12 octobre 2020 02:10
> À : [e-mail]
> Cc : [e-mail]; [e-mail]; draft-ietf-dots-server-
> [e-mail]
> Objet : Tsvart last call review of draft-ietf-dots-server-discovery-
> 11
> 
> Reviewer: Kyle Rose
> Review result: Ready with Issues
> 
> This document has been reviewed as part of the transport area review
> team's ongoing effort to review key IETF documents. These comments
> were written primarily for the transport area directors, but are
> copied to the document's authors and WG to allow them to address any
> issues raised and also to the IETF discussion list for information.
> 
> When done at the time of IETF Last Call, the authors should consider
> this review as part of the last-call comments they receive. Please
> always CC [e-mail] if you reply to or forward this review.
> 
> >From the perspective of the transport area, this document is Ready:
> the
> specification of new SRV application protocol names appears to
> comply with current TSVART guidance.
> 

[Med] Thanks. 

> That said, I have some questions and comments that may or may not be
> of interest to the transport ADs:
> 
> * Section 4 says:
> 
>    The discovery method is reiterated by a DOTS agent upon the
> following
>    events:
> 
>    o  Expiry of a a validity timer (e.g., DHCP lease, DNS TTL)
>       associated with a discovered DOTS agent.
> 
> >From my reading, rendezvous information for the DOTS server is
> "pushed"
> >to the
> client via DHCP, so it's not clear the above is actionable. Is it
> simply the case that a client should use the DOTS server assigned in
> the most recent lease?
> 

[Med] The intent is to avoid using "stale" servers. So yes, this is about using the server that was most recently discovered but also about not using a server beyond a validity time associated with the discovered server. 

> * Section 5.1 describes a trust mechanism that can charitably be
> described as "hopeful". Fundamentally, the security of TLS relies on
> a three-legged stool:
> 
>     (1) The integrity of the X.509 server certificate issuance
> procedures
>     (2) The cryptographic trust anchors configured in a client
>     (3) Pre-established trust in the server's identity, which must
> match a
>     ServerAltName in the signed server certificate
> 
> The security considerations section briefly alludes to the problems
> with using DHCP via reference to security considerations sections in
> DHCP RFCs, but I don't think that quite does justice to the problem
> here. TLS is cripped by a failure in any of the above:
> 
>     * A trusted CA wrongly issues a certificate to a third party
> with the
>     ability to hijack rendezvous (e.g., through BGP or DNS attacks)
> * A user's
>     certificate store is augmented by a CA under the control of an
> adversary *
>     Via a mistaken or injected hostname, a user establishes a
> connection to the
>     wrong endpoint that nonetheless has a legitimately-issued
> certificate
> 
> Using DHCP, an inherently insecure protocol, to inform DOTS clients
> of the hostname of DOTS server knocks this third leg out, calling
> into question the entire mechanism.

[Med] We didn't include such discussion as this is a variation of agent impersonation (covered by the pointer to RFC8811) and rogue servers in RFC8415. Also, we have the following:

   This document assumes that security credentials to authenticate DOTS
   server(s) are provisioned to a DOTS client using a mechanism such as
   (but not limited to) those discussed in [RFC8572] ...

We can add some text to elaborate on this further. 

 Measures can be taken to
> mitigate this risk, such as configuring clients with a domain
> whitelist (e.g., accept DOTS names only within a particular domain),
> configuring clients with a set of private CAs as trust anchors,
> configuring border routers with network-layer packet filtering for
> DOTS traffic, etc., but ultimately without some preconfiguration,
> clients are at the mercy of rogue DHCP.
> 
> * Section 5.2.1 has:
> 
>    The DHCPv4 [RFC2132] DOTS Reference Identifier option is used to
>    configure a name of the peer DOTS agent.  The format of this
> option
>    is illustrated in Figure 6.
> 
>             Code  Length   Peer DOTS agent name
>            +-----+-----+-----+-----+-----+-----+-----+--
>            |TBA3 |  n  |  s1 |  s2 |  s3 |  s4 | s5  |  ...
>            +-----+-----+-----+-----+-----+-----+-----+--
> 
>      The values s1, s2, s3, etc. represent the domain name labels in
> the
>      domain name encoding.
> 
> Is the final label zero-length?

[Med] Yes, as indicated in 5.2.1: 

   o  Peer DOTS agent name: The domain name of the peer DOTS agent.
      This field is formatted as specified in Section 10 of [RFC8415]. 

 Other parts of the DHCPv4 protocol
> terminate domain names with a zero-length label. What if a zero-
> length label appears in some s_i other than the final one?
> 

[Med] Names are validated as per the rules in Section 10 of RFC8415:  

   If the DHCP client receives OPTION_V4_DOTS_RI only, but
   OPTION_V4_DOTS_RI option contains more than one name, as
   distinguished by the presence of multiple root labels, the DHCP
   client MUST use only the first name.  Once the name is validated
   (Section 10 of [RFC8415]), the name is passed to a name resolution
   library.

> * Section 7 says:
> 
>    This document does not define any keys; the TXT record of a DNS-
> SD
>    service is thus empty (Section 6 of [RFC6763]).
> 
> RFC 6763 says that an empty TXT record MUST be included alongside
> the SRV record where DNS-SD is concerned. Should normative language
> be used here, or should we rely on the normative reference in case
> that guidance changes in the future?

[Med] I don't think that the use of normative language is needed here. RFC6763 is sufficient by its own.

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