DNS Zone Transfer-over-TLS
draft-ietf-dprive-xfr-over-tls-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 9103.
|
|
|---|---|---|---|
| Authors | Willem Toorop , Sara Dickinson , Shivan Kaul Sahib , Pallavi Aras , Allison Mankin | ||
| Last updated | 2020-07-13 (Latest revision 2020-05-20) | ||
| Replaces | draft-hzpa-dprive-xfr-over-tls | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 9103 (Proposed Standard) | |
| Consensus boilerplate | Yes | ||
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| Send notices to | (None) |
draft-ietf-dprive-xfr-over-tls-02
dprive W. Toorop
Internet-Draft NLnet Labs
Updates: 1995, 7766 (if approved) S. Dickinson
Intended status: Standards Track Sinodun IT
Expires: January 14, 2021 S. Sahib
P. Aras
A. Mankin
Salesforce
July 13, 2020
DNS Zone Transfer-over-TLS
draft-ietf-dprive-xfr-over-tls-02
Abstract
DNS zone transfers are transmitted in clear text, which gives
attackers the opportunity to collect the content of a zone by
eavesdropping on network connections. The DNS Transaction Signature
(TSIG) mechanism is specified to restrict direct zone transfer to
authorized clients only, but it does not add confidentiality. This
document specifies use of TLS, rather then clear text, to prevent
zone contents collection via passive monitoring of zone transfers.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 14, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Use Cases for XFR-over-TLS . . . . . . . . . . . . . . . . . 5
4. Connection and Data Flows in Existing XFR Mechanisms . . . . 5
4.1. AXFR Mechanism . . . . . . . . . . . . . . . . . . . . . 6
4.2. IXFR Mechanism . . . . . . . . . . . . . . . . . . . . . 7
4.3. Data Leakage of NOTIFY and SOA Message Exchanges . . . . 8
4.3.1. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 8
4.3.2. SOA . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Connections and Data Flows in XoT . . . . . . . . . . . . . . 8
5.1. TLS versions . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Connection usage . . . . . . . . . . . . . . . . . . . . 8
5.2.1. High level XoT descriptions . . . . . . . . . . . . . 9
5.2.2. Previous specifications . . . . . . . . . . . . . . . 9
5.3. Update to RFC7766 . . . . . . . . . . . . . . . . . . . . 10
5.4. Connection Establishment . . . . . . . . . . . . . . . . 10
5.4.1. Draft Version Identification . . . . . . . . . . . . 11
5.5. Port selection . . . . . . . . . . . . . . . . . . . . . 11
5.6. AXoT mechanism . . . . . . . . . . . . . . . . . . . . . 11
5.6.1. Coverage and relationship to RFC5936 . . . . . . . . 12
5.6.2. AXoT connection and message handling . . . . . . . . 12
5.6.3. Padding AXoT responses . . . . . . . . . . . . . . . 14
5.7. IXoT mechanism . . . . . . . . . . . . . . . . . . . . . 15
5.7.1. Coverage and relationship to RFC1995 . . . . . . . . 15
5.7.2. IXoT connection and message handling . . . . . . . . 15
5.7.3. Condensation of responses . . . . . . . . . . . . . . 16
5.7.4. Fallback to AXFR . . . . . . . . . . . . . . . . . . 16
5.7.5. Padding of IXoT responses . . . . . . . . . . . . . . 16
6. Multi-primary Configurations . . . . . . . . . . . . . . . . 16
7. Zone Transfer with DoT - Authentication . . . . . . . . . . . 17
7.1. TSIG . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2. SIG(0) . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.1. Opportunistic . . . . . . . . . . . . . . . . . . . . 18
7.3.2. Strict . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.3. Mutual . . . . . . . . . . . . . . . . . . . . . . . 18
7.4. IP Based ACL on the Primary . . . . . . . . . . . . . . . 18
7.5. ZONEMD . . . . . . . . . . . . . . . . . . . . . . . . . 19
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7.6. Comparison of Authentication Methods . . . . . . . . . . 19
8. Policies for Both AXFR and IXFR . . . . . . . . . . . . . . . 20
9. Implementation Considerations . . . . . . . . . . . . . . . . 21
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 21
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
11.1. Registration of XoT Identification String . . . . . . . 21
12. Security Considerations . . . . . . . . . . . . . . . . . . . 21
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 22
15. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 22
16. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
16.1. Normative References . . . . . . . . . . . . . . . . . . 23
16.2. Informative References . . . . . . . . . . . . . . . . . 24
16.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
DNS has a number of privacy vulnerabilities, as discussed in detail
in [RFC7626]. Stub client to recursive resolver query privacy has
received the most attention to date, with standards track documents
for both DNS-over-TLS (DoT) [RFC7858] and DNS-over-HTTPS (DoH)
[RFC8484], and a proposal for DNS-over-QUIC
[I-D.ietf-dprive-dnsoquic]. There is ongoing work on DNS privacy
requirements for exchanges between recursive resolvers and
authoritative servers [I-D.ietf-dprive-phase2-requirements] and some
suggestions for how signaling of DoT support by authoritatives might
work, e.g., [I-D.vandijk-dprive-ds-dot-signal-and-pin]. However
there is currently no RFC that specifically defines authoritative
support for DNS-over-TLS.
[RFC7626] established that stub client DNS query transactions are not
public and needed protection, but on zone transfer [RFC1995]
[RFC5936] it says only:
"Privacy risks for the holder of a zone (the risk that someone
gets the data) are discussed in [RFC5936] and [RFC5155]."
In what way is exposing the full contents of a zone a privacy risk?
The contents of the zone could include information such as names of
persons used in names of hosts. Best practice is not to use personal
information for domain names, but many such domain names exist. The
contents of the zone could also include references to locations that
allow inference about location information of the individuals
associated with the zone's organization. It could also include
references to other organizations. Examples of this could be:
o Person-laptop.example.org
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o MX-for-Location.example.org
o Service-tenant-from-another-org.example.org
There may also be regulatory, policy or other reasons why the zone
contents in full must be treated as private.
Neither of the RFCs mentioned in [RFC7626] contemplates the risk that
someone gets the data through eavesdropping on network connections,
only via enumeration or unauthorized transfer as described in the
following paragraphs.
[RFC5155] specifies NSEC3 to prevent zone enumeration, which is when
queries for the authenticated denial of existences records of DNSSEC
allow a client to walk through the entire zone. Note that the need
for this protection also motivates NSEC5 [I-D.vcelak-nsec5]; zone
walking is now possible with NSEC3 due to crypto-breaking advances,
and NSEC5 is a response to this problem.
[RFC5155] does not address data obtained outside zone enumeration
(nor does [I-D.vcelak-nsec5]). Preventing eavesdropping of zone
transfers (this draft) is orthogonal to preventing zone enumeration,
though they aim to protect the same information.
[RFC5936] specifies using TSIG [RFC2845] for authorization of the
clients of a zone transfer and for data integrity, but does not
express any need for confidentiality, and TSIG does not offer
encryption. Some operators use SSH tunneling or IPSec to encrypt the
transfer data.
Because both AXFR and IXFR zone transfers are typically carried out
over TCP from authoritative DNS protocol implementations, encrypting
zone transfers using TLS, based closely on DoT [RFC7858], seems like
a simple step forward. This document specifies how to use TLS as a
transport to prevent zone collection from zone transfers.
2. Terminology
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 BCP
14 [RFC2119] and [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Privacy terminology is as described in Section 3 of [RFC6973].
Note that in this document we choose to use the terms 'primary' and
'secondary' for two servers engaged in zone transfers.
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DNS terminology is as described in [RFC8499].
DoT: DNS-over-TLS as specified in [RFC7858]
XoT: Generic XFR-over-TLS mechanisms as specified in this document
AXoT: AXFR-over-TLS
IXoT: IXFR over-TLS
3. Use Cases for XFR-over-TLS
o Confidentiality. Clearly using an encrypted transport for zone
transfers will defeat zone content leakage that can occur via
passive surveillance.
o Authentication. Use of single or mutual TLS authentication (in
combination with ACLs) can complement and potentially be an
alternative to TSIG.
o Performance. Existing AXFR and IXFR mechanisms have the burden of
backwards compatibility with older implementations based on the
original specifications in [RFC1034] and [RFC1035]. For example,
some older AXFR servers don't support using a TCP connection for
multiple AXFR sessions or XFRs of different zones because they
have not been updated to follow the guidance in [RFC5936]. Any
implementation of XFR-over-TLS (XoT) would obviously be required
to implement optimized and interoperable transfers as described in
[RFC5936], e.g., transfer of multiple zones over one connection.
o Performance. Current usage of TCP for IXFR is sub-optimal in some
cases i.e. connections are frequently closed after a single IXFR.
4. Connection and Data Flows in Existing XFR Mechanisms
The original specification for zone transfers in [RFC1034] and
[RFC1035] was based on a polling mechanism: a secondary performed a
periodic SOA query (based on the refresh timer) to determine if an
AXFR was required.
[RFC1995] and [RFC1996] introduced the concepts of IXFR and NOTIFY
respectively, to provide for prompt propagation of zone updates.
This has largely replaced AXFR where possible, particularly for
dynamically updated zones.
[RFC5936] subsequently redefined the specification of AXFR to improve
performance and interoperability.
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In this document we use the phrase "XFR mechanism" to describe the
entire set of message exchanges between a secondary and a primary
that concludes in a successful AXFR or IXFR request/response. This
set may or may not include
o NOTIFY messages
o SOA queries
o Fallback from IXFR to AXFR
o Fallback from IXFR-over-UDP to IXFR-over-TCP
The term is used to encompasses the range of permutations that are
possible and is useful to distinguish the 'XFR mechanism' from a
single XFR request/response exchange.
4.1. AXFR Mechanism
The figure below provides an outline of an AXFR mechanism including
NOTIFYs.
Figure 1. AXFR Mechanism [1]
1. An AXFR is often (but not always) preceded by a NOTIFY (over UDP)
from the primary to the secondary. A secondary may also initiate
an AXFR based on a refresh timer or scheduled/triggered zone
maintenance.
2. The secondary will normally (but not always) make a SOA query to
the primary to obtain the serial number of the zone held by the
primary.
3. If the primary serial is higher than the secondaries serial
(using Serial Number Arithmetic [RFC1982]), the secondary makes
an AXFR request (over TCP) to the primary after which the AXFR
data flows in one or more AXFR responses on the TCP connection.
[RFC5936] specifies that AXFR must use TCP as the transport protocol
but details that there is no restriction in the protocol that a
single TCP connection must be used only for a single AXFR exchange,
or even solely for XFRs. For example, it outlines that the SOA query
can also happen on this connection. However, this can cause
interoperability problems with older implementations that support
only the trivial case of one AXFR per connection.
Further details of the limitations in existing AXFR implementations
are outlined in [RFC5936].
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4.2. IXFR Mechanism
The figure below provides an outline of the IXFR mechanism including
NOTIFYs.
Figure 1. IXFR Mechanism [2]
1. An IXFR is normally (but not always) preceded by a NOTIFY (over
UDP) from the primary to the secondary. A secondary may also
initiate an IXFR based on a refresh timer or scheduled/triggered
zone maintenance.
2. The secondary will normally (but not always) make a SOA query to
the primary to obtain the serial number of the zone held by the
primary.
3. If the primary serial is higher than the secondaries serial
(using Serial Number Arithmetic [RFC1982]), the secondary makes
an IXFR request to the primary after the primary sends an IXFR
response.
[RFC1995] specifies that Incremental Transfer may use UDP if the
entire IXFR response can be contained in a single DNS packet,
otherwise, TCP is used. In fact is says in non-normative language:
"Thus, a client should first make an IXFR query using UDP."
So there may be a forth step above where the client falls back to
IXFR-over-TCP. There may also be a forth step where the secondary
must fall back to AXFR because, e.g., the primary does not support
IXFR.
However it is noted that at least two widely used open source
authoritative nameserver implementations (BIND [3] and NSD [4]) do
IXFR using TCP by default in their latest releases. For BIND TCP
connections are sometimes used for SOA queries but in general they
are not used persistently and close after an IXFR is completed.
It is noted that the specification for IXFR was published well before
TCP was considered a first class transport for DNS. This document
therefore updates [RFC1995] to state that DNS implementations that
support IXFR-over-TCP MUST use [RFC7766] to optimize the use of TCP
connections and SHOULD use [RFC7858] to manage persistent
connections.
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4.3. Data Leakage of NOTIFY and SOA Message Exchanges
This section attempts to presents a rationale for also encrypting the
other messages in the XFR mechanism.
Since the SOA of the published zone can be trivially discovered by
simply querying the publicly available authoritative servers leakage
of this RR is not discussed in the following sections.
4.3.1. NOTIFY
Unencrypted NOTIFY messages identify configured secondaries on the
primary.
[RFC1996] also states:
"If ANCOUNT>0, then the answer section represents an
unsecure hint at the new RRset for this (QNAME,QCLASS,QTYPE).
But since the only supported QTYPE for NOTIFY is SOA, this does not
pose a potential leak.
4.3.2. SOA
For hidden primaries or secondaries the SOA response leaks the degree
of lag of any downstream secondary.
5. Connections and Data Flows in XoT
5.1. TLS versions
For improved security all implementations of this specification MUST
use only TLS 1.3 [RFC8446] or later.
5.2. Connection usage
It is useful to note that in these mechanisms it is the secondary
that initiates the TLS connection to the primary for a XFR request,
so that in terms of connectivity the secondary is the TLS client and
the primary the TLS server.
The details in [RFC7766], [RFC7858] and [RFC8310] about, e.g.,
persistent connection and message handling are fully applicable to
XoT as well. However any behavior specified here takes precedence
for XoT.
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5.2.1. High level XoT descriptions
The figure below provides an outline of the AXoT mechanism including
NOTIFYs.
Figure 3: AXoT mechanism [5]
The figure below provides an outline of the IXoT mechanism including
NOTIFYs.
Figure 4: IXoT mechanism [6]
5.2.2. Previous specifications
We note that whilst [RFC5936] already recommends re-using open TCP
connections, it does state:
"Non-AXFR session traffic can also use an open TCP connection."
when discussing AXFR-over-TCP. It defines an AXFR session as an AXFR
query message and the sequence of AXFR response messages returned for
it. Note that this excludes any SOA queries issued as part of the
overall AXFR mechanism. This requirement needs to be re-evaluated
when considering applying the same model to XoT since
o There is no guarantee that a XoT server (which is very likely, but
not necessarily, a purely authoritative server) will also support
DoT for regular queries. Requiring a purely authoritative server
to also respond to any query over a TLS connection would be
equivalent to defining a form of authoritative DoT. We consider
this to be out of scope for this document, which is focussed
purely on zone transfers.
o It would, however, be optimal for XoT to include the capability to
send SOA queries over an already open TLS connection.
Moreover, it is worth noting that [RFC7766] made general
implementation recommendations with regard to TCP/TLS connection
handling:
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"To mitigate the risk of unintentional server overload, DNS
clients MUST take care to minimize the number of concurrent TCP
connections made to any individual server. It is RECOMMENDED
that for any given client/server interaction there SHOULD be no
more than one connection for regular queries, one for zone
transfers, and one for each protocol that is being used on top
of TCP (for example, if the resolver was using TLS). However,
it is noted that certain primary/ secondary configurations with
many busy zones might need to use more than one TCP connection
for zone transfers for operational reasons (for example, to
support concurrent transfers of multiple zones)."
Whilst this recommends a particular behavior for the clients using
TCP, it does not relax the requirement for servers to handle 'mixed'
traffic (regular queries and zone transfers) on any open TCP/TLS
connection. It also overlooks the potential that other transports
might want to take the same approach with regard to using separate
connections for different purposes.
5.3. Update to RFC7766
This specification for XoT updates the guidance in [RFC7766] to
provide the same separation of connection purpose (regular queries
and zone transfers) for all transports being used on top of TCP.
Therefore, it is RECOMMENDED that for each protocol used on top of
TCP in any given client/server interaction there SHOULD be no more
than one connection for regular queries and one for zone transfers.
We provide specific details in the following sections of reasons
where more than one connection might be required for zone transfers.
5.4. Connection Establishment
This specification additionally limits the scope of XoT as defined
here to be the use of dedicated TLS connections (XoT connections) to
exchange only traffic specific to enabling zone transfers. The set
of transactions supported on such connections is limited to:
o AXFR
o IXFR
o SOA
and is collectively referred to hereafter as 'XoT traffic'.
Such connections MUST use an ALPN token of 'xot' during the TLS
handshake (see Section 11).
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In the absence of DNS specific capability signaling mechanisms this
greatly simplifies the implementation of XoT such that a XoT exchange
can occur between any primary and secondary regardless of the role of
each (e.g. purely authoritative, recursive resolver also
authoritatively hosting zones, stub) or of other DNS transport
capability each may have. It also clearly makes XoT support
orthogonal to any set of zone transfer authentication mechanisms
chosen by the two parties.
XoT clients MUST only send XoT traffic on XoT connections. If a XoT
server receives traffic other than XoT traffic on a XoT connection it
MUST respond with the extended DNS error code 21 - Not Supported
[I-D.ietf-dnsop-extended-error]. It SHOULD treat this as protocol
error and close the connection.
With the update to [RFC7766] guidance above, clients are free to open
separate connections to the server to make any other queries they may
need over either TLS, TCP or UDP. A specification for connections
that support both XoT traffic and non-XoT traffic may be the subject
of a future work.
5.4.1. Draft Version Identification
_RFC Editor's Note:_ Please remove this section prior to publication
of a final version of this document.
Only implementations of the final, published RFC can identify
themselves as "xot". Until such an RFC exists, implementations MUST
NOT identify themselves using this string.
Implementations of draft versions of the protocol MUST add the string
"-" and the corresponding draft number to the identifier. For
example, draft-ietf-dprive-xfr-over-tls-02 is identified using the
string "xot-02".
5.5. Port selection
The connection for XoT SHOULD be established using port 853, as
specified in [RFC7858], unless there is mutual agreement between the
secondary and primary to use a port other than port 853 for XoT.
There MAY be agreement to use different ports for AXoT and IXoT.
5.6. AXoT mechanism
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5.6.1. Coverage and relationship to RFC5936
[RFC5936] re-specified AXFR providing additional guidance beyond that
provided in [RFC1034] and [RFC1035]. For example, sections 4.1,
4.1.1 and 4.1.2 of [RFC5936] provide improved guidance for AXFR
clients and servers with regard to re-use of connections for multiple
AXFRs and AXFRs of different zones. However [RFC5936] was
constrained by having to be backwards compatible with some very early
basic implementations of AXFR.
Here we specify some optimized behaviors for AXoT, based closely on
those in [RFC5936], but without the constraint of backwards
compatibility since it is expected that all implementations of AXoT
fully implement the behavior described here.
Where any behavior is not explicitly described here, the behavior
specified in [RFC5936] MUST be followed. Any behavior specified here
takes precedence for AXoT implementations over that in [RFC5936].
5.6.2. AXoT connection and message handling
The first paragraph of Section 4.1.1 of [RFC5936] says that clients
SHOULD close the connection when there is no 'apparent need' to use
the connection for some time period.
For AXoT this requirement is updated: AXoT clients and servers SHOULD
use EDNS0 Keepalive [RFC7828] to establish the connection timeouts to
be used. The client SHOULD send the EDNS0 Keepalive option on every
AXoT request sent so that the server has every opportunity to update
the Keepalive timeout. The AXoT server may use the frequency of
recent AXFRs to calculate an average update rate as input to the
decision of what EDNS0 Keepalive timeout to use. If the server does
not support EDNS0 Keepalive the client MAY keep the connection open
for a few seconds ([RFC7766] recommends that servers use timeouts of
at least a few seconds).
Whilst the specification for EDNS0 [RFC6891] does not specifically
mention AXFRs, it does say
"If an OPT record is present in a received request, compliant
responders MUST include an OPT record in their respective
responses."
We clarify here that if an OPT record is present in a received AXoT
request, compliant responders MUST include an OPT record in each of
the subsequent AXoT responses. Note that this requirement, combined
with the use of EDNS0 Keepalive, enables AXoT servers to signal the
desire to close a connection due to low resources by sending an EDNS0
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Keepalive option with a timeout of 0 on any AXoT response (in the
absence of another way to signal the abort of a AXoT transfer).
An AXoT server MUST be able to handle multiple AXFR requests on a
single XoT connection (for the same and different zones).
[RFC5936] says:
"An AXFR client MAY use an already opened TCP connection to
start an AXFR session. Using an existing open connection is
RECOMMENDED over opening a new connection. (Non-AXFR session
traffic can also use an open connection.)"
For AXoT this requirement is updated: AXoT clients SHOULD re-use an
existing open XoT connection when starting any new AXoT session to
the same primary, and for issuing SOA queries, instead of opening a
new connection. The number of XoT connections between a secondary
and primary SHOULD be minimized.
Valid reasons for not re-using existing connections might include:
o reaching a configured limit for the number of outstanding queries
allowed on a single XoT connection
o the message ID pool has already been exhausted on an open
connection
o a large number of timeouts or slow responses have occurred on an
open connection
o an EDNS0 Keepalive option with a timeout of 0 has been received
from the server and the client is in the process of closing the
connection
If no XoT connections are currently open, AXoT clients MAY send SOA
queries over UDP, TCP or TLS.
[RFC5936] says:
"Some old AXFR clients expect each response message to contain
only a single RR. To interoperate with such clients, the server
MAY restrict response messages to a single RR."
This is opposed to the normal behavior of containing a sufficient
number of RRs to reasonably amortize the per-message overhead. We
clarify here that AXoT clients MUST be able to handle responses that
include multiple RRs, up to the largest number that will fit within a
DNS message (taking the required content of the other sections into
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account, as described here and in [RFC5936]). This removes any
burden on AXoT servers of having to accommodate a configuration
option or support for restricting responses to containing only a
single RR.
An AXoT client SHOULD pipeline AXFR requests for different zones on a
single XoT connection. An AXoT server SHOULD respond to those
requests as soon as the response is available i.e. potentially out of
order.
5.6.3. Padding AXoT responses
The goal of padding AXoT responses would be two fold:
o to obfuscate the actual size of the transferred zone to minimize
information leakage about the entire contents of the zone.
o to obfuscate the incremental changes to the zone between SOA
updates to minimize information leakage about zone update activity
and growth.
Note that the re-use of XoT connections for transfers of multiple
different zones complicates any attempt to analyze the traffic size
and timing to extract information.
We note here that any requirement to obfuscate the total zone size is
likely to require a server to create 'empty' AXoT responses. That
is, AXoT responses that contain no RR's apart from an OPT RR
containing the EDNS(0) option for padding. However, as with existing
AXFR, the last AXoT response message sent MUST contain the same SOA
that was in the first message of the AXoT response series in order to
signal the conclusion of the zone transfer.
[RFC5936] says:
"Each AXFR response message SHOULD contain a sufficient number
of RRs to reasonably amortize the per-message overhead, up to
the largest number that will fit within a DNS message (taking
the required content of the other sections into account, as
described below)."
'Empty' AXoT responses generated in order to meet a padding
requirement will be exceptions to the above statement. In order to
guarantee support for future padding policies, we state here that
secondary implementations MUST be resilient to receiving padded AXoT
responses, including 'empty' AXoT responses that contain only an OPT
RR containing the EDNS(0) option for padding.
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Recommendation of specific policies for padding AXoT responses are
out of scope for this specification. Detailed considerations of such
policies and the trade-offs involved are expected to be the subject
of future work.
5.7. IXoT mechanism
5.7.1. Coverage and relationship to RFC1995
[RFC1995] says nothing with respect to optimizing IXFRs over TCP or
re-using already open TCP connections to perform IXFRs or other
queries. Therefore, there arguably is an implicit assumption
(probably unintentional) that a TCP connection is used for one and
only one IXFR request. Indeed, several open source implementations
currently take this approach.
We provide new guidance here specific to IXoT that aligns with the
guidance in [RFC5936] for AXFR, that in section Section 5.6 for AXoT,
and with that for performant TCP/TLS usage in [RFC7766] and
[RFC7858].
Where any behavior is not explicitly described here, the behavior
specified in [RFC1995] MUST be followed. Any behavior specified here
takes precedence for IXoT implementations over that in [RFC1995].
5.7.2. IXoT connection and message handling
In a manner entirely analogous to that described in paragraph 2 of
Section 5.6.2 IXoT clients and servers SHOULD use EDNS0 Keepalive
[RFC7828] to establish the connection timeouts to be used.
An IXoT server MUST be able to handle multiple IXoT requests on a
single XoT connection (for the same and different zones).
IXoT clients SHOULD re-use an existing open XoT connection when
making any new IXoT request to the same primary, and for issuing SOA
queries, instead of opening a new connection. The number of XoT
connections between a secondary and primary SHOULD be minimized.
Valid reasons for not re-using existing connections are the same as
those described in Section 5.6.2
If no XoT connections are currently open, IXoT clients MAY send SOA
queries over UDP, TCP or TLS.
An IXoT client SHOULD pipeline IXFR requests for different zones on a
single XoT connection. An IXoT server SHOULD respond to those
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requests as soon as the response is available i.e. potentially out of
order.
5.7.3. Condensation of responses
[RFC1995] says condensation of responses is optional and MAY be done.
Whilst it does add complexity to generating responses it can
significantly reduce the size of responses. However any such
reduction might be offset by increased message size due to padding.
This specification does not update the optionality of condensation.
5.7.4. Fallback to AXFR
Fallback to AXFR can happen, for example, if the server is not able
to provide an IXFR for the requested SOA. Implementations differ in
how long they store zone deltas and how many may be stored at any one
time.
After a failed IXFR a IXoT client SHOULD request the AXFR on the
already open XoT connection.
5.7.5. Padding of IXoT responses
The goal of padding IXoT responses would be to obfuscate the
incremental changes to the zone between SOA updates to minimize
information leakage about zone update activity and growth. Both the
size and timing of the IXoT responses could reveal information.
IXFR responses can vary in size greatly from the order of 100 bytes
for one or two record updates, to tens of thousands of bytes for
large dynamic DNSSEC signed zones. The frequency of IXFR responses
can also depend greatly on if and how the zone is DNSSEC signed.
In order to guarantee support for future padding policies, we state
here that secondary implementations MUST be resilient to receiving
padded IXoT responses.
Recommendation of specific policies for padding IXoT responses are
out of scope for this specification. Detailed considerations of such
policies and the trade-offs involved are expected to be the subject
of future work.
6. Multi-primary Configurations
Also known as multi-master configurations this model can provide
flexibility and redundancy particularly for IXFR. A secondary will
receive one or more NOTIFY messages and can send an SOA to all of the
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configured primaries. It can then choose to send an XFR request to
the primary with the highest SOA (or other criteria, e.g., RTT).
When using persistent connections the secondary may have a XoT
connection already open to one or more primaries. Should a secondary
preferentially request an XFR from a primary to which it already has
an open XoT connection or the one with the highest SOA (assuming it
doesn't have a connection open to it already)?
Two extremes can be envisaged here. The first one can be considered
a 'preferred primary connection' model. In this case the secondary
continues to use one persistent connection to a single primary until
it has reason not to. Reasons not to might include the primary
repeatedly closing the connection, long RTTs on transfers or the SOA
of the primary being an unacceptable lag behind the SOA of an
alternative primary.
The other extreme can be considered a 'parallel primary connection'
model. Here a secondary could keep multiple persistent connections
open to all available primaries and only request XFRs from the
primary with the highest serial number. Since normally the number of
secondaries and primaries in direct contact in a transfer group is
reasonably low this might be feasible if latency is the most
significant concern.
Recommendation of a particular scheme is out of scope of this
document but implementations are encouraged to provide configuration
options that allow operators to make choices about this behavior.
7. Zone Transfer with DoT - Authentication
7.1. TSIG
TSIG [RFC2845] provides a mechanism for two or more parties to use
shared secret keys which can then be used to create a message digest
to protect individual DNS messages. This allows each party to
authenticate that a request or response (and the data in it) came
from the other party, even if it was transmitted over an unsecured
channel or via a proxy. It provides party-to-party data
authentication, but not hop-to-hop channel authentication or
confidentiality.
7.2. SIG(0)
SIG(0) [RFC2535] similarly also provides a mechanism to digitally
sign a DNS message but uses public key authentication, where the
public keys are stored in DNS as KEY RRs and a private key is stored
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at the signer. It also provides party-to-party data authentication,
but not hop-to-hop channel authentication or confidentiality.
7.3. TLS
7.3.1. Opportunistic
Opportunistic TLS [RFC8310] provides a defense against passive
surveillance, providing on-the-wire confidentiality.
7.3.2. Strict
Strict TLS [RFC8310] requires that a client is configured with an
authentication domain name (and/or SPKI pinset) that should be used
to authenticate the TLS handshake with the server. This additionally
provides a defense for the client against active surveillance,
providing client-to-server authentication and end-to-end channel
confidentiality.
7.3.3. Mutual
This is an extension to Strict TLS [RFC8310] which requires that a
client is configured with an authentication domain name (and/or SPKI
pinset) and a client certificate. The client offers the certificate
for authentication by the server and the client can authentic the
server the same way as in Strict TLS. This provides a defense for
both parties against active surveillance, providing bi-directional
authentication and end-to-end channel confidentiality.
7.4. IP Based ACL on the Primary
Most DNS server implementations offer an option to configure an IP
based Access Control List (ACL), which is often used in combination
with TSIG based ACLs to restrict access to zone transfers on primary
servers.
This is also possible with XoT but it must be noted that as with TCP
the implementation of such an ACL cannot be enforced on the primary
until a XFR request is received on an established connection.
If control were to be any more fine-grained than this then a
separate, dedicated port would need to be agreed between primary and
secondary for XoT such that implementations would be able to refuse
connections on that port to all clients except those configured as
secondaries.
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7.5. ZONEMD
Message Digest for DNS Zones (ZONEMD)
[I-D.ietf-dnsop-dns-zone-digest] digest is a mechanism that can be
used to verify the content of a standalone zone. It is designed to
be independent of the transmission channel or mechanism, allowing a
general consumer of a zone to do origin authentication of the entire
zone contents. Note that the current version of
[I-D.ietf-dnsop-dns-zone-digest] states:
"As specified at this time, ZONEMD is not designed for use in large,
dynamic zones due to the time and resources required for digest
calculation. The ZONEMD record described in this document includes
fields reserved for future work to support large, dynamic zones."
It is complementary the above mechanisms and can be used in
conjunction with XoT but is not considered further.
7.6. Comparison of Authentication Methods
The Table below compares the properties of a selection of the above
methods in terms of what protection they provide to the secondary and
primary servers during XoT in terms of:
o 'Data Auth': Authentication that the DNS message data is signed by
the party with whom credentials were shared (the signing party may
or may not be party operating the far end of a TCP/TLS connection
in a 'proxy' scenario). For the primary the TSIG on the XFR
request confirms that the requesting party is authorized to
request zone data, for the secondary it authenticates the zone
data that is received.
o 'Channel Conf': Confidentiality of the communication channel
between the client and server (i.e. the two end points of a TCP/
TLS connection).
o Channel Auth: Authentication of the identity of party to whom a
TCP/TLS connection is made (this might not be a direct connection
between the primary and secondary in a proxy scenario).
It is noted that zone transfer scenarios can vary from a simple
single primary/secondary relationship where both servers are under
the control of a single operator to a complex hierarchical structure
which includes proxies and multiple operators. Each deployment
scenario will require specific analysis to determine which
authentication methods are best suited to the deployment model in
question.
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Table 1: Properties of Authentication methods for XoT [7]
Based on this analysis it can be seen that:
o A combination of Opportunistic TLS and TSIG provides both data
authentication and channel confidentiality for both parties.
However this does not stop a MitM attack on the channel which
could be used to gather zone data.
o Using just mutual TLS can be considered a standalone solution if
the secondary has reason to place equivalent trust in channel
authentication as data authentication, e.g., the same operator
runs both the primary and secondary.
o Using TSIG, Strict TLS and an ACL on the primary provides all 3
properties for both parties with probably the lowest operational
overhead.
8. Policies for Both AXFR and IXFR
We call the entire group of servers involved in XFR (all the
primaries and all the secondaries) the 'transfer group'.
Within any transfer group both AXFRs and IXFRs for a zone SHOULD all
use the same policy, e.g., if AXFRs use AXoT all IXFRs SHOULD use
IXoT.
In order to assure the confidentiality of the zone information, the
entire transfer group MUST have a consistent policy of requiring
confidentiality. If any do not, this is a weak link for attackers to
exploit.
A XoT policy should specify
o If TSIG or SIG(0) is required
o What kind of TLS is required (Opportunistic, Strict or mTLS)
o If IP based ACLs should also be used.
Since this may require configuration of a number of servers who may
be under the control of different operators the desired consistency
could be hard to enforce and audit in practice.
Certain aspects of the Policies can be relatively easily tested
independently, e.g., by requesting zone transfers without TSIG, from
unauthorized IP addresses or over cleartext DNS. Other aspects such
as if a secondary will accept data without a TSIG digest or if
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secondaries are using Strict as opposed to Opportunistic TLS are more
challenging.
The mechanics of co-ordinating or enforcing such policies are out of
the scope of this document but may be the subject of future
operational guidance.
9. Implementation Considerations
TBD
10. Implementation Status
The 1.9.2 version of Unbound [8] includes an option to perform AXoT
(instead of AXFR-over-TCP). This requires the client (secondary) to
authenticate the server (primary) using a configured authentication
domain name.
It is noted that use of a TLS proxy in front of the primary server is
a simple deployment solution that can enable server side XoT.
11. IANA Considerations
11.1. Registration of XoT Identification String
This document creates a new registration for the identification of
XoT in the "Application Layer Protocol Negotiation (ALPN) Protocol
IDs" registry [RFC7301].
The "xot" string identifies XoT:
Protocol: XoT
Identification Sequence: 0x64 0x6F 0x72 ("xot")
Specification: This document
12. Security Considerations
This document specifies a security measure against a DNS risk: the
risk that an attacker collects entire DNS zones through eavesdropping
on clear text DNS zone transfers.
This does not mitigate:
o the risk that some level of zone activity might be inferred by
observing zone transfer sizes and timing on encrypted connections
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(even with padding applied), in combination with obtaining SOA
records by directly querying authoritative servers.
o the risk that hidden primaries might be inferred or identified via
observation of encrypted connections.
o the risk of zone contents being obtained via zone enumeration
techniques.
Security concerns of DoT are outlined in [RFC7858] and [RFC8310].
13. Acknowledgements
The authors thank Benno Overeinder, Shumon Huque and Tim Wicinski for
review and discussions.
14. Contributors
Significant contributions to the document were made by:
Han Zhang
Salesforce
San Francisco, CA
United States
Email: hzhang@salesforce.com
15. Changelog
draft-ietf-dprive-xfr-over-tls-02
o Significantly update descriptions for both AXoT and IXoT for
message and connection handling taking into account previous
specifications in more detail
o Add use of APLN and limitations on traffic on XoT connections.
o Add new discussions of padding for both AXoT and IXoT
o Add text on SIG(0)
o Update security considerations
o Move multi-primary considerations to earlier as they are related
to connection handling
draft-ietf-dprive-xfr-over-tls-01
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o Minor editorial updates
o Add requirement for TLS 1.3. or later
draft-ietf-dprive-xfr-over-tls-00
o Rename after adoption and reference update.
o Add placeholder for SIG(0) discussion
o Update section on ZONEMD
draft-hzpa-dprive-xfr-over-tls-02
o Substantial re-work of the document.
draft-hzpa-dprive-xfr-over-tls-01
o Editorial changes, updates to references.
draft-hzpa-dprive-xfr-over-tls-00
o Initial commit
16. References
16.1. Normative References
[I-D.vcelak-nsec5]
Vcelak, J., Goldberg, S., Papadopoulos, D., Huque, S., and
D. Lawrence, "NSEC5, DNSSEC Authenticated Denial of
Existence", draft-vcelak-nsec5-08 (work in progress),
December 2018.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996, <https://www.rfc-
editor.org/info/rfc1995>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
editor.org/info/rfc2119>.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<https://www.rfc-editor.org/info/rfc2845>.
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[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<https://www.rfc-editor.org/info/rfc5155>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013, <https://www.rfc-
editor.org/info/rfc6973>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015, <https://www.rfc-
editor.org/info/rfc7626>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018, <https://www.rfc-
editor.org/info/rfc8310>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
16.2. Informative References
[I-D.ietf-dnsop-dns-zone-digest]
Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
Hardaker, "Message Digest for DNS Zones", draft-ietf-
dnsop-dns-zone-digest-08 (work in progress), June 2020.
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[I-D.ietf-dnsop-extended-error]
Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
Lawrence, "Extended DNS Errors", draft-ietf-dnsop-
extended-error-16 (work in progress), May 2020.
[I-D.ietf-dprive-dnsoquic]
Huitema, C., Mankin, A., and S. Dickinson, "Specification
of DNS over Dedicated QUIC Connections", draft-ietf-
dprive-dnsoquic-00 (work in progress), April 2020.
[I-D.ietf-dprive-phase2-requirements]
Livingood, J., Mayrhofer, A., and B. Overeinder, "DNS
Privacy Requirements for Exchanges between Recursive
Resolvers and Authoritative Servers", draft-ietf-dprive-
phase2-requirements-01 (work in progress), June 2020.
[I-D.vandijk-dprive-ds-dot-signal-and-pin]
Dijk, P., Geuze, R., and E. Bretelle, "Signalling
Authoritative DoT support in DS records, with key
pinning", draft-vandijk-dprive-ds-dot-signal-and-pin-00
(work in progress), May 2020.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
DOI 10.17487/RFC1982, August 1996, <https://www.rfc-
editor.org/info/rfc1982>.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <https://www.rfc-editor.org/info/rfc1996>.
[RFC2535] Eastlake 3rd, D., "Domain Name System Security
Extensions", RFC 2535, DOI 10.17487/RFC2535, March 1999,
<https://www.rfc-editor.org/info/rfc2535>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013, <https://www.rfc-
editor.org/info/rfc6891>.
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[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
16.3. URIs
[1] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
master/02-draft-dprive-svg/AXFR_mechanism.svg
[2] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
master/02-draft-dprive-svg/IXFR_mechanism.svg
[3] https://www.isc.org/bind/
[4] https://www.nlnetlabs.nl/projects/nsd/about/
[5] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
master/02-draft-dprive-svg/AXoT_mechanism.svg
[6] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/blob/
master/02-draft-dprive-svg/IXoT_mechanism.svg
[7] https://github.com/hanzhang0116/hzpa-dprive-xfr-over-tls/
blob/02_updates/02-draft-svg/
Properties_of_Authentication_methods_for_XoT.svg
[8] https://github.com/NLnetLabs/unbound/blob/release-1.9.2/doc/
Changelog
Authors' Addresses
Willem Toorop
NLnet Labs
Science Park 400
Amsterdam 1098 XH
The Netherlands
Email: willem@nlnetlabs.nl
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Sara Dickinson
Sinodun IT
Magdalen Centre
Oxford Science Park
Oxford OX4 4GA
United Kingdom
Email: sara@sinodun.com
Shivan Sahib
Salesforce
Vancouver, BC
Canada
Email: ssahib@salesforce.com
Pallavi Aras
Salesforce
Herndon, VA
United States
Email: paras@salesforce.com
Allison Mankin
Salesforce
Herndon, VA
United States
Email: allison.mankin@gmail.com
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