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draft-ietf-dprive-xfr-over-tls-09.txt
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dprive W. Toorop
Internet-Draft NLnet Labs
Updates: 1995, 5936, 7766 (if approved) S. Dickinson
Intended status: Standards Track Sinodun IT
Expires: October 8, 2021 S. Sahib
P. Aras
A. Mankin
Salesforce
April 6, 2021
DNS Zone Transfer-over-TLS
draft-ietf-dprive-xfr-over-tls-09
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 the use of TLS, rather than clear text, to prevent
zone content collection via passive monitoring of zone transfers:
XFR-over-TLS (XoT). Additionally, this specification updates
RFC1995, RFC5936 and RFC7766.
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 October 8, 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
Toorop, et al. Expires October 8, 2021 [Page 1]
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(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
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Document work via GitHub . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Design Considerations for XoT . . . . . . . . . . . . . . . . 6
6. Connection and Data Flows in Existing XFR Mechanisms . . . . 7
6.1. AXFR Mechanism . . . . . . . . . . . . . . . . . . . . . 8
6.2. IXFR Mechanism . . . . . . . . . . . . . . . . . . . . . 9
6.3. Data Leakage of NOTIFY and SOA Message Exchanges . . . . 11
6.3.1. NOTIFY . . . . . . . . . . . . . . . . . . . . . . . 11
6.3.2. SOA . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Updates to existing specifications . . . . . . . . . . . . . 11
7.1. Update to RFC1995 for IXFR-over-TCP . . . . . . . . . . . 13
7.2. Update to RFC5936 for AXFR-over-TCP . . . . . . . . . . . 13
7.3. Updates to RFC1995 and RFC5936 for XFR-over-TCP . . . . . 13
7.3.1. Connection reuse . . . . . . . . . . . . . . . . . . 13
7.3.2. AXFRs and IXFRs on the same connection . . . . . . . 14
7.3.3. XFR limits . . . . . . . . . . . . . . . . . . . . . 14
7.3.4. The edns-tcp-keepalive EDNS0 Option . . . . . . . . . 15
7.3.5. Backwards compatibility . . . . . . . . . . . . . . . 15
7.4. Update to RFC7766 . . . . . . . . . . . . . . . . . . . . 15
8. XoT specification . . . . . . . . . . . . . . . . . . . . . . 17
8.1. TLS versions . . . . . . . . . . . . . . . . . . . . . . 17
8.2. Port selection . . . . . . . . . . . . . . . . . . . . . 17
8.3. High level XoT descriptions . . . . . . . . . . . . . . . 17
8.4. XoT transfers . . . . . . . . . . . . . . . . . . . . . . 19
8.5. XoT connections . . . . . . . . . . . . . . . . . . . . . 20
8.6. XoT vs ADoT . . . . . . . . . . . . . . . . . . . . . . . 20
8.7. Response RCODES . . . . . . . . . . . . . . . . . . . . . 21
8.8. AXoT specifics . . . . . . . . . . . . . . . . . . . . . 21
8.8.1. Padding AXoT responses . . . . . . . . . . . . . . . 21
8.9. IXoT specifics . . . . . . . . . . . . . . . . . . . . . 22
8.9.1. Condensation of responses . . . . . . . . . . . . . . 22
8.9.2. Fallback to AXFR . . . . . . . . . . . . . . . . . . 22
8.9.3. Padding of IXoT responses . . . . . . . . . . . . . . 23
8.10. Name compression and maximum payload sizes . . . . . . . 23
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9. Multi-primary Configurations . . . . . . . . . . . . . . . . 23
10. Authentication mechanisms . . . . . . . . . . . . . . . . . . 24
10.1. TSIG . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.2. SIG(0) . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.3. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.3.1. Opportunistic TLS . . . . . . . . . . . . . . . . . 25
10.3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . 26
10.3.3. Mutual TLS . . . . . . . . . . . . . . . . . . . . . 26
10.4. IP Based ACL on the Primary . . . . . . . . . . . . . . 26
10.5. ZONEMD . . . . . . . . . . . . . . . . . . . . . . . . . 27
11. XoT authentication . . . . . . . . . . . . . . . . . . . . . 27
12. Policies for Both AXoT and IXoT . . . . . . . . . . . . . . . 28
13. Implementation Considerations . . . . . . . . . . . . . . . . 29
14. Operational Considerations . . . . . . . . . . . . . . . . . 29
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
16. Implementation Status . . . . . . . . . . . . . . . . . . . . 29
17. Security Considerations . . . . . . . . . . . . . . . . . . . 30
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
19. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 31
20. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 31
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
21.1. Normative References . . . . . . . . . . . . . . . . . . 33
21.2. Informative References . . . . . . . . . . . . . . . . . 35
21.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Appendix A. XoT server connection handling . . . . . . . . . . . 36
A.1. Only listen on TLS on a specific IP address . . . . . . . 37
A.2. Client specific TLS acceptance . . . . . . . . . . . . . 37
A.3. SNI based TLS acceptance . . . . . . . . . . . . . . . . 37
A.4. TLS specific response policies . . . . . . . . . . . . . 37
A.4.1. SNI based response policies . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
DNS has a number of privacy vulnerabilities, as discussed in detail
in [I-D.ietf-dprive-rfc7626-bis]. 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. However there is currently no RFC that specifically defines
recursive to authoritative DNS-over-TLS (ADoT).
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[I-D.ietf-dprive-rfc7626-bis] 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
o MX-for-Location.example.org
o Service-tenant-from-another-org.example.org
Additionally, the full zone contents expose all the IP addresses of
endpoints held in the DNS records which can make reconnaissance
trivial, particularly for IPv6 addresses. 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 [I-D.ietf-dprive-rfc7626-bis]
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.
Zone enumeration is trivially possible for DNSSEC zones which use
NSEC; i.e. queries for the authenticated denial of existences
records allow a client to walk through the entire zone contents.
[RFC5155] specifies NSEC3, a mechanism to provide measures against
zone enumeration for DNSSEC signed zones (a goal was to make it as
hard to enumerate an DNSSEC signed zone as an unsigned zone). Whilst
this is widely used, zone walking is now possible with NSEC3 due to
crypto-breaking advances. This has prompted further work on an
alternative mechanism for DNSSEC authenticated denial of existence -
NSEC5 [I-D.vcelak-nsec5] - however questions remain over the
practicality of this mechanism.
[RFC5155] does not address data obtained outside zone enumeration
(nor does [I-D.vcelak-nsec5]). Preventing eavesdropping of zone
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transfers (this draft) is orthogonal to preventing zone enumeration,
though they aim to protect the same information.
[RFC5936] specifies using TSIG [RFC8945] 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.
Section 8 of the NIST guide on 'Secure Domain Name System (DNS)
Deployment' [nist-guide] discusses restricting access for zone
transfers using ACLs and TSIG in more detail. It also discusses the
possibility that specific deployments might choose to use a lower
level network layer to protect zone transfers, e.g., IPSec.
It is noted that in all the common open source implementations such
ACLs are applied on a per query basis. Since requests typically
occur on TCP connections authoritatives must cater for accepting any
TCP connection and then handling the authentication of each XFR
request individually.
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. Document work via GitHub
[THIS SECTION TO BE REMOVED BEFORE PUBLICATION] The Github repository
for this document is at <https://github.com/hanzhang0116/hzpa-dprive-
xfr-over-tls>. Proposed text and editorial changes are very much
welcomed there, but any functional changes should always first be
discussed on the IETF DPRIVE WG (dns-privacy) mailing list.
3. 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].
DNS terminology is as described in [RFC8499]. Note that as in
[RFC8499], the terms 'primary' and 'secondary' are used for two
servers engaged in zone transfers.
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DoT: DNS-over-TLS as specified in [RFC7858]
XFR-over-TCP: Used to mean both IXFR-over-TCP [RFC1995] and AXFR-
over-TCP [RFC5936].
XoT: Generic XFR-over-TLS mechanisms as specified in this document
AXoT: AXFR-over-TLS
IXoT: IXFR over-TLS
4. Threat Model
The threat model considered here is one where the current contents
and size of the zone are considered sensitive and should be protected
during transfer.
The threat model does not, however, consider the existence of a zone,
the act of zone transfer between two entities, nor the identities of
the nameservers hosting a zone (including both those acting as hidden
primaries/secondaries or directly serving the zone) as sensitive
information. The proposed mechanisms does not attempt to obscure
such information. The reasons for this include:
o much of this information can be obtained by various methods
including active scanning of the DNS
o an attacker who can monitor network traffic can relatively easily
infer relations between nameservers simply from traffic patterns,
even when some or all of the traffic is encrypted
It is noted that simply using XoT will indicate a desire by the zone
owner that the contents of the zone remain confidential and so could
be subject to blocking (e.g., via blocking of port 853) if an
attacker had such capabilities. However this threat is likely true
of any such mechanism that attempts to encrypt data passed between
nameservers, e.g., IPsec.
5. Design Considerations for XoT
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 (mTLS) authentication
(in combination with ACLs) can complement and potentially be an
alternative to TSIG.
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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 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.
6. 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.
In this document we use the term "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.
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6.1. AXFR Mechanism
The figure below provides an outline of an AXFR mechanism including
NOTIFYs.
Secondary Primary
| NOTIFY |
| <-------------------------------- | UDP
| --------------------------------> |
| NOTIFY Response |
| |
| |
| SOA Request |
| --------------------------------> | UDP (or part of
| <-------------------------------- | a TCP session)
| SOA Response |
| |
| |
| |
| AXFR Request | ---
| --------------------------------> | |
| <-------------------------------- | |
| AXFR Response 1 | |
| (Zone data) | |
| | |
| <-------------------------------- | | TCP
| AXFR Response 2 | | Session
| (Zone data) | |
| | |
| <-------------------------------- | |
| AXFR Response 3 | |
| (Zone data) | ---
| |
Figure 1. AXFR Mechanism
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.
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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] defines this specific step as an 'AXFR session' i.e. as
an AXFR query message and the sequence of AXFR response messages
returned for it.
[RFC5936] re-specified AXFR providing additional guidance beyond that
provided in [RFC1034] and [RFC1035] and importantly specified that
AXFR must use TCP as the transport protocol.
Additionally, 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 TCP 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. 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.
6.2. IXFR Mechanism
The figure below provides an outline of the IXFR mechanism including
NOTIFYs.
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Secondary Primary
| NOTIFY |
| <-------------------------------- | UDP
| --------------------------------> |
| NOTIFY Response |
| |
| |
| SOA Request |
| --------------------------------> | UDP or TCP
| <-------------------------------- |
| SOA Response |
| |
| |
| |
| IXFR Request |
| --------------------------------> | UDP or TCP
| <-------------------------------- |
| IXFR Response |
| (Zone data) |
| |
| | ---
| IXFR Request | |
| --------------------------------> | | Retry over
| <-------------------------------- | | TCP if
| IXFR Response | | required
| (Zone data) | ---
Figure 2. IXFR Mechanism
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 which 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 it says:
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"Thus, a client should first make an IXFR query using UDP."
So there may be a fourth step above where the client falls back to
IXFR-over-TCP. There may also be a fourth step where the secondary
must fall back to AXFR because, e.g., the primary does not support
IXFR.
However it is noted that most widely used open source authoritative
nameserver implementations (including both [BIND] and [NSD]) 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.
6.3. Data Leakage of NOTIFY and SOA Message Exchanges
This section attempts to presents a rationale for considering
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.
6.3.1. 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.
6.3.2. SOA
For hidden primaries or secondaries the SOA response leaks only the
degree of SOA serial number lag of any downstream secondary.
7. Updates to existing specifications
For convenience, the term 'XFR-over-TCP' is used in this document to
mean both IXFR-over-TCP and AXFR-over-TCP and therefore statements
that use that term update both [RFC1995] and [RFC5936], and
implicitly also apply to XoT. Differences in behavior specific to
XoT are discussed in Section 8.
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Both [RFC1995] and [RFC5936] were published sometime before TCP was
considered a first class transport for DNS. [RFC1995], in fact, 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 that a TCP connection is
used for one and only one IXFR request. Indeed, many major open
source implementations currently take this approach. And whilst
[RFC5936] gives guidance on connection re-use for AXFR, it pre-dates
more recent specifications describing persistent TCP connections,
e.g., [RFC7766], [RFC7828] and AXFR implementations again often make
less than optimal use of open connections.
Given this, new implementations of XoT will clearly benefit from
specific guidance on TCP/TLS connection usage for XFR because this
will:
o result in more consistent XoT implementations with better
interoperability
o remove any need for XoT implementations to support legacy behavior
that XFR-over-TCP implementations have historically often
supported
Therefore this document updates both the previous specifications for
XFR-over-TCP to clarify that
o Implementations MUST use [RFC7766] (DNS Transport over TCP -
Implementation Requirements) to optimize the use of TCP
connections.
o Whilst RFC7766 states that 'DNS clients SHOULD pipeline their
queries' on TCP connections, it did not distinguish between XFRs
and other queries for this behavior. It is now recognized that
XFRs are not as latency sensitive as other queries, and can be
significantly more complex for clients to handle both because of
the large amount of state that must be kept and because there may
be multiple messages in the responses. For these reasons it is
clarified here that a valid reason for not pipelining queries is
when they are all XFR queries i.e. clients sending multiple XFRs
MAY choose not to pipeline those queries. Clients that do not
pipeline XFR queries, therefore, have no additional requirements
to handle out-of-order or intermingled responses (as described
later) since they will never receive them.
o Implementations SHOULD use [RFC7828] (The edns-tcp-keepalive EDNS0
Option) to manage persistent connections.
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The following sections include detailed clarifications on the updates
to XFR behavior implied in [RFC7766] and how the use of [RFC7828]
applies specifically to XFR exchanges. It also discusses how IXFR
and AXFR can reuse the same TCP connection.
For completeness, we also mention here the recent specification of
extended DNS error (EDE) codes [RFC8914]. For zone transfers, when
returning REFUSED to a zone transfer request from an 'unauthorized'
client (e.g., where the client is not listed in an ACL for zone
transfers or does not sign the request with the correct TSIG key),
the extended DNS error code 18 (Prohibited) can also be sent.
7.1. Update to RFC1995 for IXFR-over-TCP
For clarity - an IXFR-over-TCP server compliant with this
specification MUST be able to handle multiple concurrent IXoT
requests on a single TCP connection (for the same and different
zones) and SHOULD send the responses as soon as they are available,
which might be out-of-order compared to the requests.
7.2. Update to RFC5936 for AXFR-over-TCP
For clarity - an AXFR-over-TCP server compliant with this
specification MUST be able to handle multiple concurrent AXoT
sessions on a single TCP connection (for the same and different
zones). The response streams for concurrent AXFRs MAY be
intermingled and AXFR-over-TCP clients compliant with this
specification which pipeline AXFR requests MUST be able to handle
this.
7.3. Updates to RFC1995 and RFC5936 for XFR-over-TCP
7.3.1. Connection reuse
As specified, XFR-over-TCP clients SHOULD re-use any existing open
TCP connection when starting any new XFR request to the same primary,
and for issuing SOA queries, instead of opening a new connection.
The number of TCP connections between a secondary and primary SHOULD
be minimized (also see Section 7.4).
Valid reasons for not re-using existing connections might include:
o as already noted in [RFC7766], separate connections for different
zones might be preferred for operational reasons. In this case
the number of concurrent connections for zone transfers SHOULD be
limited to the total number of zones transferred between the
client and server.
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o reaching a configured limit for the number of outstanding queries
or XFR requests allowed on a single TCP 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 edns-tcp-keepalive EDNS0 option with a timeout of 0 has been
received from the server and the client is in the process of
closing the connection (see Section 7.3.4)
If no TCP connections are currently open, XFR clients MAY send SOA
queries over UDP or a new TCP connection.
7.3.2. AXFRs and IXFRs on the same connection
Neither [RFC1995] nor [RFC5936] explicitly discuss the use of a
single TCP connection for both IXFR and AXFR requests. [RFC5936]
does make the general statement:
"Non-AXFR session traffic can also use an open TCP connection."
We clarify here that implementations capable of both AXFR and IXFR
and compliant with this specification SHOULD
o use the same TCP connection for both AXFR and IXFR requests to the
same primary
o pipeline such requests (if they pipeline XFR requests in general)
and MAY intermingle them
o send the response(s) for each request as soon as they are
available i.e. responses MAY be sent intermingled
7.3.3. XFR limits
The server MAY limit the number of concurrent IXFRs, AXFRs or total
XFR transfers in progress, or from a given secondary, to protect
server resources. Servers SHOULD return SERVFAIL if this limit is
hit, since it is a transient error and a retry at a later time might
succeed.
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7.3.4. The edns-tcp-keepalive EDNS0 Option
XFR clients that send the edns-tcp-keepalive EDNS0 option on every
XFR request provide the server with maximum opportunity to update the
edns-tcp-keepalive timeout. The XFR server may use the frequency of
recent XFRs to calculate an average update rate as input to the
decision of what edns-tcp-keepalive timeout to use. If the server
does not support edns-tcp-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 AXFR
request, compliant responders MUST include an OPT record in each of
the subsequent AXFR responses. Note that this requirement, combined
with the use of edns-tcp-keepalive, enables AXFR servers to signal
the desire to close a connection (when existing transactions have
competed) due to low resources by sending an edns-tcp-keepalive EDNS0
option with a timeout of 0 on any AXFR response. This does not
signal that the AXFR is aborted, just that the server wishes to close
the connection as soon as possible.
7.3.5. Backwards compatibility
Certain legacy behaviors were noted in [RFC5936], with provisions
that implementations may want to offer options to fallback to legacy
behavior when interoperating with servers known not to support
[RFC5936]. For purposes of interoperability, IXFR and AXFR
implementations may want to continue offering such configuration
options, as well as supporting some behaviors that were
underspecified prior to this work (e.g., performing IXFR and AXFRs on
separate connections). However, XoT implementations should have no
need to do so.
7.4. Update to RFC7766
[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.
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.
As an illustration, it could be imagined that in future such an
interaction could hypothetically include one or all of the following:
o one TCP connection for regular queries
o one TCP connection for zone transfers
o one TLS connection for regular queries
o one TLS connection for zone transfers
o one DoH connection for regular queries
o one DoH connection for zone transfers
Section 7.3.1 has provided specific details of reasons where more
than one connection for a given transport might be required for zone
transfers from a particular client.
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8. XoT specification
8.1. TLS versions
For improved security all implementations of this specification MUST
use only TLS 1.3 [RFC8446] or later.
8.2. 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, or
for different zones.
8.3. High level XoT descriptions
It is useful to note that in XoT 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 figure below provides an outline of the AXoT mechanism including
NOTIFYs.
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Secondary Primary
| NOTIFY |
| <-------------------------------- | UDP
| --------------------------------> |
| NOTIFY Response |
| |
| |
| SOA Request |
| --------------------------------> | UDP (or part of
| <-------------------------------- | a TCP/TLS session)
| SOA Response |
| |
| |
| |
| AXFR Request | ---
| --------------------------------> | |
| <-------------------------------- | |
| AXFR Response 1 | |
| (Zone data) | |
| | |
| <-------------------------------- | | TLS
| AXFR Response 2 | | Session
| (Zone data) | |
| | |
| <-------------------------------- | |
| AXFR Response 3 | |
| (Zone data) | ---
| |
Figure 3. AXoT Mechanism
The figure below provides an outline of the IXoT mechanism including
NOTIFYs.