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

[Kyle Rose via Datatracker <[e-mail]>]

Reviewer: Kyle Rose
Review result: Ready with Issues

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>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.

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?

* 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. 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? 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?

* 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?