< draft-ietf-dnsop-dns-tcp-requirements-03.txt   draft-ietf-dnsop-dns-tcp-requirements-04.txt >
Domain Name System Operations J. Kristoff Domain Name System Operations J. Kristoff
Internet-Draft DePaul University Internet-Draft DePaul University
Updates: 1123 (if approved) D. Wessels Updates: 1123 (if approved) D. Wessels
Intended status: Best Current Practice Verisign Intended status: Best Current Practice Verisign
Expires: July 6, 2019 January 2, 2019 Expires: December 26, 2019 June 24, 2019
DNS Transport over TCP - Operational Requirements DNS Transport over TCP - Operational Requirements
draft-ietf-dnsop-dns-tcp-requirements-03 draft-ietf-dnsop-dns-tcp-requirements-04
Abstract Abstract
This document encourages the practice of permitting DNS messages to This document encourages the practice of permitting DNS messages to
be carried over TCP on the Internet. It also considers the be carried over TCP on the Internet. It also considers the
consequences with this form of DNS communication and the potential consequences with this form of DNS communication and the potential
operational issues that can arise when this best common practice is operational issues that can arise when this best common practice is
not upheld. not upheld.
Status of This Memo Status of This Memo
skipping to change at page 1, line 35 skipping to change at page 1, line 35
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 6, 2019. This Internet-Draft will expire on December 26, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Uneven Transport Usage and Preference . . . . . . . . . . 4 2.1. Uneven Transport Usage and Preference . . . . . . . . . . 4
2.2. Waiting for Large Messages and Reliability . . . . . . . 4 2.2. Waiting for Large Messages and Reliability . . . . . . . 5
2.3. EDNS0 . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3. EDNS0 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Fragmentation and Truncation . . . . . . . . . . . . . . 5 2.4. Fragmentation and Truncation . . . . . . . . . . . . . . 6
2.5. "Only Zone Transfers Use TCP" . . . . . . . . . . . . . . 6 2.5. "Only Zone Transfers Use TCP" . . . . . . . . . . . . . . 7
3. DNS over TCP Requirements . . . . . . . . . . . . . . . . . . 7 3. DNS over TCP Requirements . . . . . . . . . . . . . . . . . . 7
4. Network and System Considerations . . . . . . . . . . . . . . 8 4. Network and System Considerations . . . . . . . . . . . . . . 8
4.1. Connection Admission . . . . . . . . . . . . . . . . . . 8 4.1. Connection Admission . . . . . . . . . . . . . . . . . . 8
4.2. Connection Management . . . . . . . . . . . . . . . . . . 9 4.2. Connection Management . . . . . . . . . . . . . . . . . . 9
4.3. Connection Termination . . . . . . . . . . . . . . . . . 10 4.3. Connection Termination . . . . . . . . . . . . . . . . . 10
5. DNS over TCP Filtering Risks . . . . . . . . . . . . . . . . 10 5. DNS over TCP Filtering Risks . . . . . . . . . . . . . . . . 10
5.1. DNS Wedgie . . . . . . . . . . . . . . . . . . . . . . . 11 5.1. DNS Wedgie . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. DNS Root Zone KSK Rollover . . . . . . . . . . . . . . . 11 5.2. DNS Root Zone KSK Rollover . . . . . . . . . . . . . . . 11
5.3. DNS-over-TLS . . . . . . . . . . . . . . . . . . . . . . 11 5.3. DNS-over-TLS . . . . . . . . . . . . . . . . . . . . . . 11
6. Logging and Monitoring . . . . . . . . . . . . . . . . . . . 12 6. Logging and Monitoring . . . . . . . . . . . . . . . . . . . 12
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12 9. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13 10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 11. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.1. Normative References . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.2. Informative References . . . . . . . . . . . . . . . . . 13 12.1. Normative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Standards Related to DNS Transport over TCP . . . . 18 12.2. Informative References . . . . . . . . . . . . . . . . . 13
A.1. TODO - additional, relevant RFCs . . . . . . . . . . . . 18 Appendix A. Standards Related to DNS Transport over TCP . . . . 19
A.2. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR) . . . . 18 A.1. IETF RFC 1035 - DOMAIN NAMES - IMPLEMENTATION AND
A.3. IETF RFC 6304 - AS112 Nameserver Operations . . . . . . . 18 SPECIFICATION . . . . . . . . . . . . . . . . . . . . . . 19
A.4. IETF RFC 6762 - Multicast DNS . . . . . . . . . . . . . . 18 A.2. IETF RFC 1536 - Common DNS Implementation Errors and
A.5. IETF RFC 6950 - Architectural Considerations on Suggested Fixes . . . . . . . . . . . . . . . . . . . . . 19
Application Features in the DNS . . . . . . . . . . . . . 18 A.3. IETF RFC 1995 - Incremental Zone Transfer in DNS . . . . 19
A.6. IETF RFC 7477 - Child-to-Parent Synchronization in DNS . 18 A.4. IETF RFC 1996 - A Mechanism for Prompt Notification of
A.7. IETF RFC 7720 - DNS Root Name Service Protocol and Zone Changes (DNS NOTIFY) . . . . . . . . . . . . . . . . 20
Deployment Requirements . . . . . . . . . . . . . . . . . 19 A.5. IETF RFC 2181 - Clarifications to the DNS Specification . 20
A.8. IETF RFC 7766 - DNS Transport over TCP - Implementation A.6. IETF RFC 2694 - DNS extensions to Network Address
Requirements . . . . . . . . . . . . . . . . . . . . . . 19 Translators (DNS_ALG) . . . . . . . . . . . . . . . . . . 20
A.9. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option . . . 19 A.7. IETF RFC 3225 - Indicating Resolver Support of DNSSEC . . 20
A.10. IETF RFC 7858 - Specification for DNS over Transport A.8. IETF RFC 3326 - DNSSEC and IPv6 A6 aware server/resolver
Layer Security (TLS) . . . . . . . . . . . . . . . . . . 19 message size requirements . . . . . . . . . . . . . . . . 20
A.11. IETF RFC 7873 - Domain Name System (DNS) Cookies . . . . 19 A.9. IETF RFC 4472 - Operational Considerations and Issues
A.12. IETF RFC 7901 - CHAIN Query Requests in DNS . . . . . . . 20 with IPv6 DNS . . . . . . . . . . . . . . . . . . . . . . 21
A.13. IETF RFC 8027 - DNSSEC Roadblock Avoidance . . . . . . . 20 A.10. IETF RFC 5452 - Measures for Making DNS More Resilient
A.14. IETF RFC 8094 - DNS over Datagram Transport Layer against Forged Answers . . . . . . . . . . . . . . . . . 21
Security (DTLS) . . . . . . . . . . . . . . . . . . . . . 20 A.11. IETF RFC 5507 - Design Choices When Expanding the DNS . . 21
A.15. IETF RFC 8162 - Using Secure DNS to Associate A.12. IETF RFC 5625 - DNS Proxy Implementation Guidelines . . . 21
Certificates with Domain Names for S/MIME . . . . . . . . 20 A.13. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR) . . . . 21
A.16. IETF RFC 8324 - DNS Privacy, Authorization, Special Uses, A.14. IETF RFC 5966 - DNS Transport over TCP - Implementation
Requirements . . . . . . . . . . . . . . . . . . . . . . 22
A.15. IETF RFC 6304 - AS112 Nameserver Operations . . . . . . . 22
A.16. IETF RFC 6762 - Multicast DNS . . . . . . . . . . . . . . 22
A.17. IETF RFC 6891 - Extension Mechanisms for DNS (EDNS(0)) . 22
A.18. IETF RFC 6950 - Architectural Considerations on
Application Features in the DNS . . . . . . . . . . . . . 22
A.19. IETF RFC 7477 - Child-to-Parent Synchronization in DNS . 22
A.20. IETF RFC 7720 - DNS Root Name Service Protocol and
Deployment Requirements . . . . . . . . . . . . . . . . . 23
A.21. IETF RFC 7766 - DNS Transport over TCP - Implementation
Requirements . . . . . . . . . . . . . . . . . . . . . . 23
A.22. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option . . . 23
A.23. IETF RFC 7858 - Specification for DNS over Transport
Layer Security (TLS) . . . . . . . . . . . . . . . . . . 23
A.24. IETF RFC 7873 - Domain Name System (DNS) Cookies . . . . 23
A.25. IETF RFC 7901 - CHAIN Query Requests in DNS . . . . . . . 24
A.26. IETF RFC 8027 - DNSSEC Roadblock Avoidance . . . . . . . 24
A.27. IETF RFC 8094 - DNS over Datagram Transport Layer
Security (DTLS) . . . . . . . . . . . . . . . . . . . . . 24
A.28. IETF RFC 8162 - Using Secure DNS to Associate
Certificates with Domain Names for S/MIME . . . . . . . . 24
A.29. IETF RFC 8324 - DNS Privacy, Authorization, Special Uses,
Encoding, Characters, Matching, and Root Structure: Time Encoding, Characters, Matching, and Root Structure: Time
for Another Look? . . . . . . . . . . . . . . . . . . . . 20 for Another Look? . . . . . . . . . . . . . . . . . . . . 24
A.17. IETF RFC 8467 - Padding Policies for Extension Mechanisms A.30. IETF RFC 8467 - Padding Policies for Extension Mechanisms
for DNS (EDNS(0)) . . . . . . . . . . . . . . . . . . . . 21 for DNS (EDNS(0)) . . . . . . . . . . . . . . . . . . . . 25
A.18. IETF RFC 8483 - Yeti DNS Testbed . . . . . . . . . . . . 21 A.31. IETF RFC 8483 - Yeti DNS Testbed . . . . . . . . . . . . 25
A.19. IETF RFC 8484 - DNS Queries over HTTPS (DoH) . . . . . . 21 A.32. IETF RFC 8484 - DNS Queries over HTTPS (DoH) . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 A.33. IETF RFC 8490 - DNS Stateful Operations . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction 1. Introduction
DNS messages may be delivered using UDP or TCP communications. While DNS messages may be delivered using UDP or TCP communications. While
most DNS transactions are carried over UDP, some operators have been most DNS transactions are carried over UDP, some operators have been
led to believe that any DNS over TCP traffic is unwanted or led to believe that any DNS over TCP traffic is unwanted or
unnecessary for general DNS operation. As usage and features have unnecessary for general DNS operation. When DNS over TCP has been
evolved, TCP transport has become increasingly important for correct restricted, a variety of communication failures and debugging
and safe operation of the Internet DNS. Reflecting modern usage, the challenges often arise. As DNS and new naming system features have
DNS standards were recently updated to declare support for TCP is now evolved, TCP as a transport has become increasingly important for the
a required part of the DNS implementation specifications in correct and safe operation of an Internet DNS. Reflecting modern
[RFC7766]. This document is the formal requirements equivalent for usage, the DNS standards were recently updated to declare support for
the operational community, encouraging operators to ensure DNS over TCP is now a required part of the DNS implementation
TCP communications support is on par with DNS over UDP specifications.[RFC7766] This document is the formal requirements
equivalent for the operational community, encouraging system
administrators, network engineers, and security staff to ensure DNS
over TCP communications support is on par with DNS over UDP
communications. communications.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
2. Background 2. Background
The curious state of disagreement in operational best practices and The curious state of disagreement in operational best practices and
guidance for DNS transport protocols derives from conflicting guidance for DNS transport protocols derives from conflicting
messages operators have gotten from other operators, implementors, messages operators have gotten from other operators, implementors,
and even the IETF. Sometimes these mixed signals have been explicit, and even the IETF. Sometimes these mixed signals have been explicit,
on other occasions they have suspiciously implicit. Here we on other occasions they have suspiciously implicit. This section
summarize our interpretation of the storied and conflicting history presents an interpretation of the storied and conflicting history
that has brought us to this document. that led to this document.
2.1. Uneven Transport Usage and Preference 2.1. Uneven Transport Usage and Preference
In the original suite of DNS specifications, [RFC1034] and [RFC1035] In the original suite of DNS specifications, [RFC1034] and [RFC1035]
clearly specified that DNS messages could be carried in either UDP or clearly specified that DNS messages could be carried in either UDP or
TCP, but they also made clear a preference for UDP as the transport TCP, but they also stated a preference for UDP as the best transport
for queries in the general case. As stated in [RFC1035]: for queries in the general case. As stated in [RFC1035]:
"While virtual circuits can be used for any DNS activity, "While virtual circuits can be used for any DNS activity,
datagrams are preferred for queries due to their lower overhead datagrams are preferred for queries due to their lower overhead
and better performance." and better performance."
Another early, important, and influential document, [RFC1123], Another early, important, and influential document, [RFC1123], marked
detailed the preference for UDP more explicitly: the preference for a transport protocol more explicitly:
"DNS resolvers and recursive servers MUST support UDP, and SHOULD "DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries." support TCP, for sending (non-zone-transfer) queries."
and further stipulated: and further stipulated:
"A name server MAY limit the resources it devotes to TCP queries, "A name server MAY limit the resources it devotes to TCP queries,
but it SHOULD NOT refuse to service a TCP query just because it but it SHOULD NOT refuse to service a TCP query just because it
would have succeeded with UDP." would have succeeded with UDP."
Culminating in [RFC1536], DNS over TCP came to be associated Culminating in [RFC1536], DNS over TCP came to be associated
primarily with the zone transfer mechanism, while most DNS queries primarily with the zone transfer mechanism, while most DNS queries
and responses were seen as the dominion of UDP. and responses were seen as the dominion of UDP.
2.2. Waiting for Large Messages and Reliability 2.2. Waiting for Large Messages and Reliability
In the original specifications, the maximum DNS over UDP message size In the original specifications, the maximum DNS over UDP message size
was enshrined at 512 bytes. However, even while [RFC1123] made a was enshrined at 512 bytes. However, even while [RFC1123] preferred
clear preference for UDP, it foresaw DNS over TCP becoming more UDP for non-zone transfer queries, it foresaw DNS over TCP becoming
popular in the future to overcome this limitation: more popular in the future to overcome this limitation:
"[...] it is also clear that some new DNS record types defined in "[...] it is also clear that some new DNS record types defined in
the future will contain information exceeding the 512 byte limit the future will contain information exceeding the 512 byte limit
that applies to UDP, and hence will require TCP. that applies to UDP, and hence will require TCP.
At least two new, widely anticipated developments were set to elevate At least two new, widely anticipated developments were set to elevate
the need for DNS over TCP transactions. The first was dynamic the need for DNS over TCP transactions. The first was dynamic
updates defined in [RFC2136] and the second was the set of extensions updates defined in [RFC2136] and the second was the set of extensions
collectively known as DNSSEC originally specified in [RFC2541]. The collectively known as DNSSEC originally specified in [RFC2541]. The
former suggested "requestors who require an accurate response code former suggested "requestors who require an accurate response code
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At least two new, widely anticipated developments were set to elevate At least two new, widely anticipated developments were set to elevate
the need for DNS over TCP transactions. The first was dynamic the need for DNS over TCP transactions. The first was dynamic
updates defined in [RFC2136] and the second was the set of extensions updates defined in [RFC2136] and the second was the set of extensions
collectively known as DNSSEC originally specified in [RFC2541]. The collectively known as DNSSEC originally specified in [RFC2541]. The
former suggested "requestors who require an accurate response code former suggested "requestors who require an accurate response code
must use TCP", while the later warned "[...] larger keys increase the must use TCP", while the later warned "[...] larger keys increase the
size of KEY and SIG RRs. This increases the chance of DNS UDP packet size of KEY and SIG RRs. This increases the chance of DNS UDP packet
overflow and the possible necessity for using higher overhead TCP in overflow and the possible necessity for using higher overhead TCP in
responses." responses."
Yet defying some expectations, DNS over TCP remained little used in Yet defying some expectations, DNS over TCP remained little used in
real traffic across the Internet. Dynamic updates saw little real traffic across the Internet. Dynamic updates saw little
deployment between autonomous networks. Around the time DNSSEC was deployment between autonomous networks. Around the time DNSSEC was
first defined, another new feature helped solidify UDP's transport first defined, another new feature helped solidify UDP transport
dominance for message transactions. dominance for message transactions.
2.3. EDNS0 2.3. EDNS0
In 1999 the IETF published the Extension Mechanisms for DNS (EDNS0) In 1999 the IETF published the Extension Mechanisms for DNS (EDNS0)
in [RFC2671] (superseded in 2013 by an update in [RFC6891]). This in [RFC2671] (superseded in 2013 by an update in [RFC6891]). This
document standardized a way for communicating DNS nodes to perform document standardized a way for communicating DNS nodes to perform
rudimentary capabilities negotiation. One such capability written rudimentary capabilities negotiation. One such capability written
into the base specification and present in every ENDS0 compatible into the base specification and present in every ENDS0 compatible
message is the value of the maximum UDP payload size the sender can message is the value of the maximum UDP payload size the sender can
support. This unsigned 16-bit field specifies in bytes the maximum support. This unsigned 16-bit field specifies in bytes the maximum
(possibly fragmented) DNS message size a node is capable of (possibly fragmented) DNS message size a node is capable of
receiving. In practice, typical values are a subset of the 512 to receiving. In practice, typical values are a subset of the 512 to
4096 byte range. EDNS0 became widely deployed over the next several 4096 byte range. EDNS0 became widely deployed over the next several
years and numerous surveys have shown many systems currently support years and numerous surveys have shown many systems currently support
larger UDP MTUs [CASTRO2010], [NETALYZR] with EDNS0. larger UDP MTUs [CASTRO2010], [NETALYZR] with EDNS0.
The natural effect of EDNS0 deployment meant DNS messages larger than The natural effect of EDNS0 deployment meant DNS messages larger than
512 bytes would be less reliant on TCP than they might otherwise have 512 bytes would be less reliant on TCP than they might otherwise have
been. While a non-negligible population of DNS systems lack EDNS0 or been. While a non-negligible population of DNS systems lacked EDNS0
may still fall back to TCP for some transactions, DNS over TCP or fall back to TCP when necessary, DNS over TCP transactions
transactions remain a very small fraction of overall DNS traffic remained a very small fraction of overall DNS traffic [VERISIGN].
[VERISIGN].
2.4. Fragmentation and Truncation 2.4. Fragmentation and Truncation
Although EDNS0 provides a way for endpoints to signal support for DNS Although EDNS0 provides a way for endpoints to signal support for DNS
messages exceeding 512 bytes, the realities of a diverse and messages exceeding 512 bytes, the realities of a diverse and
inconsistently deployed Internet may result in some large messages inconsistently deployed Internet may result in some large messages
being unable to reach their destination. Any IP datagram whose size being unable to reach their destination. Any IP datagram whose size
exceeds the MTU of a link it transits will be fragmented and then exceeds the MTU of a link it transits will be fragmented and then
reassembled by the receiving host. Unfortunately, it is not uncommon reassembled by the receiving host. Unfortunately, it is not uncommon
for middleboxes and firewalls to block IP fragments. If one or more for middleboxes and firewalls to block IP fragments. If one or more
fragments do not arrive, the application does not receive the message fragments do not arrive, the application does not receive the message
and the request times out. and the request times out.
For IPv4-connected hosts, the de-facto MTU is often the Ethernet For IPv4-connected hosts, the de-facto MTU is often the Ethernet
payload size of 1500 bytes. This means that the largest unfragmented payload size of 1500 bytes. This means that the largest unfragmented
UDP DNS message that can be sent over IPv4 is likely 1472 bytes. For UDP DNS message that can be sent over IPv4 is likely 1472 bytes. For
IPv6, the situation is a little more complicated. First, IPv6 IPv6, the situation is a little more complicated. First, IPv6
headers are 40 bytes (versus 20 without option in IPv4). Second, it headers are 40 bytes (versus 20 without options in IPv4). Second, it
seems as though some people have mis-interpreted IPv6's required seems as though some people have mis-interpreted IPv6's required
minimum MTU of 1280 as a required maximum. Third, fragmentation in minimum MTU of 1280 as a required maximum. Third, fragmentation in
IPv6 can only be done by the host originating the datagram. The need IPv6 can only be done by the host originating the datagram. The need
to fragment is conveyed in an ICMPv6 "packet too big" message. The to fragment is conveyed in an ICMPv6 "packet too big" message. The
originating host indicates a fragmented datagram with IPv6 extension originating host indicates a fragmented datagram with IPv6 extension
headers. Unfortunately, it is quite common for both ICMPv6 and IPv6 headers. Unfortunately, it is quite common for both ICMPv6 and IPv6
extension headers to be blocked by middleboxes. According to extension headers to be blocked by middleboxes. According to
[HUSTON] some 35% of IPv6-capable recursive resolvers are unable to [HUSTON] some 35% of IPv6-capable recursive resolvers were unable to
receive a fragmented IPv6 packet. receive a fragmented IPv6 packet.
The practical consequence of all this is that DNS requestors must be The practical consequence of all this is that DNS requestors must be
prepared to retry queries with different EDNS0 maximum message size prepared to retry queries with different EDNS0 maximum message size
values. Administrators of BIND are likely to be familiar with seeing values. Administrators of BIND are likely to be familiar with seeing
"success resolving ... after reducing the advertised EDNS0 UDP packet "success resolving ... after reducing the advertised EDNS0 UDP packet
size to 512 octets" messages in their system logs. size to 512 octets" messages in their system logs.
Often, reducing the EDNS0 UDP packet size leads to a successful Often, reducing the EDNS0 UDP packet size leads to a successful
response. That is, the necessary data fits within the smaller response. That is, the necessary data fits within the smaller
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easily in 512 bytes. The KSK Rollover design team [DESIGNTEAM] spent easily in 512 bytes. The KSK Rollover design team [DESIGNTEAM] spent
a lot of time thinking and worrying about response sizes. There is a lot of time thinking and worrying about response sizes. There is
growing sentiment in the DNSSEC community that RSA key sizes beyond growing sentiment in the DNSSEC community that RSA key sizes beyond
2048-bits are impractical and that critical infrastructure zones 2048-bits are impractical and that critical infrastructure zones
should transition to elliptic curve algorithms to keep response sizes should transition to elliptic curve algorithms to keep response sizes
manageable. manageable.
2.5. "Only Zone Transfers Use TCP" 2.5. "Only Zone Transfers Use TCP"
Today, the majority of the DNS community expects, or at least has a Today, the majority of the DNS community expects, or at least has a
desire, to see DNS over TCP transactions to occur without desire, to see DNS over TCP transactions occur without interference.
interference. However there has also been a long held belief by some However there has also been a long held belief by some operators,
operators, particularly for security-related reasons, that DNS over particularly for security-related reasons, that DNS over TCP services
TCP services should be purposely limited or not provided at all should be purposely limited or not provided at all [CHES94],
[CHES94], [DJBDNS]. A popular meme has also held the imagination of [DJBDNS]. A popular meme has also held the imagination of some that
some that DNS over TCP is only ever used for zone transfers and is DNS over TCP is only ever used for zone transfers and is generally
generally unnecessary otherwise, with filtering all DNS over TCP unnecessary otherwise, with filtering all DNS over TCP traffic even
traffic even described as a best practice. described as a best practice.
The position on restricting DNS over TCP had some justification given The position on restricting DNS over TCP had some justification given
that historic implementations of DNS nameservers provided very little that historic implementations of DNS nameservers provided very little
in the way of TCP connection management (for example see in the way of TCP connection management (for example see
Section 6.1.2 of [RFC7766] for more details). However modern Section 6.1.2 of [RFC7766] for more details). However modern
standards and implementations are moving to align with the more standards and implementations are nearing parity with the more
sophisticated TCP management techniques employed by, for example, sophisticated TCP management techniques employed by, for example,
HTTP(S) servers and load balancers. HTTP(S) servers and load balancers.
3. DNS over TCP Requirements 3. DNS over TCP Requirements
An average increase in DNS message size, the continued development of An average increase in DNS message size, the continued development of
new DNS features and a denial of service mitigation technique (see new DNS features [Appendix A], and a denial of service mitigation
Section 9) have suggested that DNS over TCP transactions are as technique [Section 9] have suggested that DNS over TCP transactions
important to the correct and safe operation of the Internet DNS as are as important to the correct and safe operation of the Internet
ever, if not more so. Furthermore, there has been serious research DNS as ever, if not more so. Furthermore, there has been serious
that has suggested connection-oriented DNS transactions may provide research that argues connection-oriented DNS transactions may provide
security and privacy advantages over UDP transport [TDNS]. In fact, security and privacy advantages over UDP transport. [TDNS] In fact
[RFC7858], a Standards Track document is just this sort of [RFC7858], a Standards Track document, is just this sort of
specification. Therefore, we now believe it is undesirable for specification. Therefore, this document makes explicit that it is
network operators to artificially inhibit the potential utility and undesirable for network operators to artificially inhibit DNS over
advances in the DNS such as these. TCP transport.
TODO: I think the text below needs some work/discussion because 7766
already updated 1123 in a very similar way except that 7766 speaks of
"implement" and this one speaks of "service". 1123 speaks of
"support" and doesn't distinguish between implement/service.
Section 6.1.3.2 in [RFC1123] is updated: All general-purpose DNS Section 6.1.3.2 in [RFC1123] is updated: All DNS resolvers and
servers MUST be able to service both UDP and TCP queries. servers MUST support and service both UDP and TCP queries.
o Authoritative servers MUST service TCP queries so that they do not o Authoritative servers MUST support and service all TCP queries so
limit the size of responses to what fits in a single UDP packet. that they do not limit the size of responses to what fits in a
single UDP packet.
o Recursive servers (or forwarders) MUST service TCP queries so that o Recursive servers (or forwarders) MUST support and service all TCP
they do not prevent large responses from a TCP-capable server from queries so that they do not prevent large responses from a TCP-
reaching its TCP-capable clients. capable server from reaching its TCP-capable clients.
Regarding the choice of limiting the resources a server devotes to Regarding the choice of limiting the resources a server devotes to
queries, Section 6.1.3.2 in [RFC1123] also says: queries, Section 6.1.3.2 in [RFC1123] also says:
"A name server MAY limit the resources it devotes to TCP queries, "A name server MAY limit the resources it devotes to TCP queries,
but it SHOULD NOT refuse to service a TCP query just because it but it SHOULD NOT refuse to service a TCP query just because it
would have succeeded with UDP." would have succeeded with UDP."
This requirement is hereby updated: A name server MAY limit the the This requirement is hereby updated: A name server MAY limit the the
resources it devotes to queries, but it MUST NOT refuse to service a resources it devotes to queries, but it MUST NOT refuse to service a
query just because it would have succeeded with another transport query just because it would have succeeded with another transport
protocol. protocol.
Filtering of DNS over TCP is considered harmful in the general case. Filtering of DNS over TCP is considered harmful in the general case.
DNS resolver and server operators MUST provide DNS service over both DNS resolver and server operators MUST suport and provide DNS service
UDP and TCP transports. Likewise, network operators MUST allow DNS over both UDP and TCP transports. Likewise, network operators MUST
service over both UDP and TCP transports. It must be acknowledged allow DNS service over both UDP and TCP transports. It is
that DNS over TCP service can pose operational challenges that are acknowledged that DNS over TCP service can pose operational
not present when running DNS over UDP alone, and vice-versa. challenges that are not present when running DNS over UDP alone, and
However, it is the aim of this document to argue that the potential vice-versa. However, it is the aim of this document to argue that
damage incurred by prohibiting DNS over TCP service is more the potential damage incurred by prohibiting DNS over TCP service is
detrimental to the continued utility and success of the DNS than when more detrimental to the continued utility and success of the DNS than
its usage is allowed. when its usage is allowed.
4. Network and System Considerations 4. Network and System Considerations
This section describes measures that systems and applications can This section describes measures that systems and applications can
take to optimize performance over TCP and to protect themselves from take to optimize performance over TCP and to protect themselves from
TCP-based resource exhaustion and attacks. TCP-based resource exhaustion and attacks.
4.1. Connection Admission 4.1. Connection Admission
The SYN flooding attack is a denial-of-service method affecting hosts The SYN flooding attack is a denial-of-service method affecting hosts
skipping to change at page 9, line 41 skipping to change at page 10, line 12
used to ensure that a single or small set of users can not consume used to ensure that a single or small set of users can not consume
all TCP resources and deny service to other users. Operators SHOULD all TCP resources and deny service to other users. Operators SHOULD
ensure this limit is configured appropriately, based on their number ensure this limit is configured appropriately, based on their number
of diversity of users. of diversity of users.
DNS server software SHOULD provide a configurable timeout for idle DNS server software SHOULD provide a configurable timeout for idle
TCP connections. For very busy name servers this might be set to a TCP connections. For very busy name servers this might be set to a
low value, such as a few seconds. For less busy servers it might be low value, such as a few seconds. For less busy servers it might be
set to a higher value, such as tens of seconds. DNS clients and set to a higher value, such as tens of seconds. DNS clients and
servers SHOULD signal their timeout values using the edns-tcp- servers SHOULD signal their timeout values using the edns-tcp-
keepalive option [RFC7828]. keepalive option.[RFC7828]
DNS server software MAY provide a configurable limit on the number of DNS server software MAY provide a configurable limit on the number of
transactions per TCP connection. This document does not offer advice transactions per TCP connection. This document does not offer advice
on particular values for such a limit. on particular values for such a limit.
Similarly, DNS server software MAY provide a configurable limit on Similarly, DNS server software MAY provide a configurable limit on
the total duration of a TCP connection. This document does not offer the total duration of a TCP connection. This document does not offer
advice on particular values for such a limit. advice on particular values for such a limit.
Since clients may not be aware of server-imposed limits, clients Since clients may not be aware of server-imposed limits, clients
skipping to change at page 10, line 39 skipping to change at page 11, line 8
Networks that filter DNS over TCP risk losing access to significant Networks that filter DNS over TCP risk losing access to significant
or important pieces of the DNS name space. For a variety of reasons or important pieces of the DNS name space. For a variety of reasons
a DNS answer may require a DNS over TCP query. This may include a DNS answer may require a DNS over TCP query. This may include
large message sizes, lack of EDNS0 support, DDoS mitigation large message sizes, lack of EDNS0 support, DDoS mitigation
techniques, or perhaps some future capability that is as yet techniques, or perhaps some future capability that is as yet
unforeseen will also demand TCP transport. unforeseen will also demand TCP transport.
For example, [RFC7901] describes a latency-avoiding technique that For example, [RFC7901] describes a latency-avoiding technique that
sends extra data in DNS responses. This makes responses larger and sends extra data in DNS responses. This makes responses larger and
potentially increases the risk of DDoS reflection attacks. The potentially increases the risk of DDoS reflection attacks. The
specification mandates the use of TCP or DNS Cookies ([RFC7873]). specification mandates the use of TCP or DNS Cookies.[RFC7873]
Even if any or all particular answers have consistently been returned Even if any or all particular answers have consistently been returned
successfully with UDP in the past, this continued behavior cannot be successfully with UDP in the past, this continued behavior cannot be
guaranteed when DNS messages are exchanged between autonomous guaranteed when DNS messages are exchanged between autonomous
systems. Therefore, filtering of DNS over TCP is considered harmful systems. Therefore, filtering of DNS over TCP is considered harmful
and contrary to the safe and successful operation of the Internet. and contrary to the safe and successful operation of the Internet.
This section enumerates some of the known risks we know about at the This section enumerates some of the known risks known at the time of
time of this writing when networks filter DNS over TCP. this writing when networks filter DNS over TCP.
5.1. DNS Wedgie 5.1. DNS Wedgie
Networks that filter DNS over TCP may inadvertently cause problems Networks that filter DNS over TCP may inadvertently cause problems
for third party resolvers as experienced by [TOYAMA]. If for for third party resolvers as experienced by [TOYAMA]. If for
instance a resolver receives a truncated answer from a server, but instance a resolver receives a truncated answer from a server, but
when the resolver resends the query using TCP and the TCP response when the resolver resends the query using TCP and the TCP response
never arrives, not only will full answer be unavailable, but the never arrives, not only will a complete answer be unavailable, but
resolver will incur the full extent of TCP retransmissions and time the resolver will incur the full extent of TCP retransmissions and
outs. This situation might place extreme strain on resolver time outs. This situation might place extreme strain on resolver
resources. If the number and frequency of these truncated answers resources. If the number and frequency of these truncated answers
are sufficiently high, we refer to the steady-state of lost resources are sufficiently high, the steady-state of lost resources as a result
as a result a "DNS" wedgie". A DNS wedgie is often not easily or is a "DNS" wedgie". A DNS wedgie is generally not easily or
completely mitigated by the affected DNS resolver operator. completely mitigated by the affected DNS resolver operator.
5.2. DNS Root Zone KSK Rollover 5.2. DNS Root Zone KSK Rollover
Recent plans for a new root zone DNSSEC KSK have highlighted a Recent plans for a new root zone DNSSEC KSK have highlighted a
potential problem in retrieving the keys [LEWIS]. Some packets in potential problem in retrieving the keys.[LEWIS] Some packets in the
the KSK rollover process will be larger than 1280 bytes, the IPv6 KSK rollover process will be larger than 1280 bytes, the IPv6 minimum
minimum MTU for links carrying IPv6 traffic.[RFC2460] While studies MTU for links carrying IPv6 traffic.[RFC2460] While studies have
have shown that problems due to fragment filtering or an inability to shown that problems due to fragment filtering or an inability to
generate and receive these larger messages are negligible, any DNS generate and receive these larger messages are negligible, any DNS
server that is unable to receive large DNS over UDP messages or server that is unable to receive large DNS over UDP messages or
perform DNS over TCP may experience severe disruption of DNS service perform DNS over TCP may experience severe disruption of DNS service
if performing DNSSEC validation. if performing DNSSEC validation.
TODO: Is this "overcome by events" now? We've had 1414 byte DNSKEY TODO: Is this "overcome by events" now? We've had 1414 byte DNSKEY
responses at the three ZSK rollover periods since KSK-2017 became responses at the three ZSK rollover periods since KSK-2017 became
published in the root zone. published in the root zone.
5.3. DNS-over-TLS 5.3. DNS-over-TLS
skipping to change at page 12, line 38 skipping to change at page 12, line 48
This document was initially motivated by feedback from students who This document was initially motivated by feedback from students who
pointed out that they were hearing contradictory information about pointed out that they were hearing contradictory information about
filtering DNS over TCP messages. Thanks in particular to a teaching filtering DNS over TCP messages. Thanks in particular to a teaching
colleague, JPL, who perhaps unknowingly encouraged the initial colleague, JPL, who perhaps unknowingly encouraged the initial
research into the differences of what the community has historically research into the differences of what the community has historically
said and did. Thanks to all the NANOG 63 attendees who provided said and did. Thanks to all the NANOG 63 attendees who provided
feedback to an early talk on this subject. feedback to an early talk on this subject.
The following individuals provided an array of feedback to help The following individuals provided an array of feedback to help
improve this document: Sara Dickinson, Bob Harold, Tatuya Jinmei, and improve this document: Sara Dickinson, Bob Harold, Tatuya Jinmei, and
Paul Hoffman. The authors are indebted to their contributions. Any Paul Hoffman. The authors are also indebted to the contributions
remaining errors or imperfections are the sole responsibility of the stemming from discussion in the tcpm working group meeting at IETF
document authors. 104. Any remaining errors or imperfections are the sole
responsibility of the document authors.
8. IANA Considerations 8. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
9. Security Considerations 9. Security Considerations
Ironically, returning truncated DNS over UDP answers in order to Ironically, returning truncated DNS over UDP answers in order to
induce a client query to switch to DNS over TCP has become a common induce a client query to switch to DNS over TCP has become a common
response to source address spoofed, DNS denial-of-service attacks response to source address spoofed, DNS denial-of-service attacks
skipping to change at page 13, line 18 skipping to change at page 13, line 30
DNS over TCP is not unlike many other Internet TCP services. TCP DNS over TCP is not unlike many other Internet TCP services. TCP
threats and many mitigation strategies have been well documented in a threats and many mitigation strategies have been well documented in a
series of documents such as [RFC4953], [RFC4987], [RFC5927], and series of documents such as [RFC4953], [RFC4987], [RFC5927], and
[RFC5961]. [RFC5961].
10. Privacy Considerations 10. Privacy Considerations
TODO: Does this document warrant privacy considerations? TODO: Does this document warrant privacy considerations?
11. References 11. Examples
11.1. Normative References Suggestion from IETF104 to include example config snippets ala 7706.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
11.2. Informative References 12.2. Informative References
[CASTRO2010] [CASTRO2010]
Castro, S., Zhang, M., John, W., Wessels, D., and k. Castro, S., Zhang, M., John, W., Wessels, D., and k.
claffy, "Understanding and preparing for DNS evolution", claffy, "Understanding and preparing for DNS evolution",
2010. 2010.
[CHES94] Cheswick, W. and S. Bellovin, "Firewalls and Internet [CHES94] Cheswick, W. and S. Bellovin, "Firewalls and Internet
Security: Repelling the Wily Hacker", 1994. Security: Repelling the Wily Hacker", 1994.
[CLOUDFLARE] [CLOUDFLARE]
skipping to change at page 14, line 35 skipping to change at page 15, line 5
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989, DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>. <https://www.rfc-editor.org/info/rfc1123>.
[RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. [RFC1536] Kumar, A., Postel, J., Neuman, C., Danzig, P., and S.
Miller, "Common DNS Implementation Errors and Suggested Miller, "Common DNS Implementation Errors and Suggested
Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993, Fixes", RFC 1536, DOI 10.17487/RFC1536, October 1993,
<https://www.rfc-editor.org/info/rfc1536>. <https://www.rfc-editor.org/info/rfc1536>.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
DOI 10.17487/RFC1995, August 1996,
<https://www.rfc-editor.org/info/rfc1995>.
[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>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)", "Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997, RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>. <https://www.rfc-editor.org/info/rfc2136>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>. December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[RFC2541] Eastlake 3rd, D., "DNS Security Operational [RFC2541] Eastlake 3rd, D., "DNS Security Operational
Considerations", RFC 2541, DOI 10.17487/RFC2541, March Considerations", RFC 2541, DOI 10.17487/RFC2541, March
1999, <https://www.rfc-editor.org/info/rfc2541>. 1999, <https://www.rfc-editor.org/info/rfc2541>.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, DOI 10.17487/RFC2671, August 1999, RFC 2671, DOI 10.17487/RFC2671, August 1999,
<https://www.rfc-editor.org/info/rfc2671>. <https://www.rfc-editor.org/info/rfc2671>.
[RFC2694] Srisuresh, P., Tsirtsis, G., Akkiraju, P., and A.
Heffernan, "DNS extensions to Network Address Translators
(DNS_ALG)", RFC 2694, DOI 10.17487/RFC2694, September
1999, <https://www.rfc-editor.org/info/rfc2694>.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, DOI 10.17487/RFC3225, December 2001,
<https://www.rfc-editor.org/info/rfc3225>.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226,
DOI 10.17487/RFC3226, December 2001,
<https://www.rfc-editor.org/info/rfc3226>.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472,
DOI 10.17487/RFC4472, April 2006,
<https://www.rfc-editor.org/info/rfc4472>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007, RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>. <https://www.rfc-editor.org/info/rfc4953>.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common [RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007, Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
<https://www.rfc-editor.org/info/rfc4987>. <https://www.rfc-editor.org/info/rfc4987>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without "Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077, Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>. January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC5452, January 2009,
<https://www.rfc-editor.org/info/rfc5452>.
[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
Ed., "Design Choices When Expanding the DNS", RFC 5507,
DOI 10.17487/RFC5507, April 2009,
<https://www.rfc-editor.org/info/rfc5507>.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<https://www.rfc-editor.org/info/rfc5625>.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
DOI 10.17487/RFC5927, July 2010, DOI 10.17487/RFC5927, July 2010,
<https://www.rfc-editor.org/info/rfc5927>. <https://www.rfc-editor.org/info/rfc5927>.
[RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol
(AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010,
<https://www.rfc-editor.org/info/rfc5936>. <https://www.rfc-editor.org/info/rfc5936>.
[RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's [RFC5961] Ramaiah, A., Stewart, R., and M. Dalal, "Improving TCP's
Robustness to Blind In-Window Attacks", RFC 5961, Robustness to Blind In-Window Attacks", RFC 5961,
DOI 10.17487/RFC5961, August 2010, DOI 10.17487/RFC5961, August 2010,
<https://www.rfc-editor.org/info/rfc5961>. <https://www.rfc-editor.org/info/rfc5961>.
[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, DOI 10.17487/RFC5966, August
2010, <https://www.rfc-editor.org/info/rfc5966>.
[RFC6304] Abley, J. and W. Maton, "AS112 Nameserver Operations", [RFC6304] Abley, J. and W. Maton, "AS112 Nameserver Operations",
RFC 6304, DOI 10.17487/RFC6304, July 2011, RFC 6304, DOI 10.17487/RFC6304, July 2011,
<https://www.rfc-editor.org/info/rfc6304>. <https://www.rfc-editor.org/info/rfc6304>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013, DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>. <https://www.rfc-editor.org/info/rfc6762>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, for DNS (EDNS(0))", STD 75, RFC 6891,
skipping to change at page 17, line 27 skipping to change at page 18, line 46
October 2018, <https://www.rfc-editor.org/info/rfc8467>. October 2018, <https://www.rfc-editor.org/info/rfc8467>.
[RFC8483] Song, L., Ed., Liu, D., Vixie, P., Kato, A., and S. Kerr, [RFC8483] Song, L., Ed., Liu, D., Vixie, P., Kato, A., and S. Kerr,
"Yeti DNS Testbed", RFC 8483, DOI 10.17487/RFC8483, "Yeti DNS Testbed", RFC 8483, DOI 10.17487/RFC8483,
October 2018, <https://www.rfc-editor.org/info/rfc8483>. October 2018, <https://www.rfc-editor.org/info/rfc8483>.
[RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS [RFC8484] Hoffman, P. and P. McManus, "DNS Queries over HTTPS
(DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018, (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
<https://www.rfc-editor.org/info/rfc8484>. <https://www.rfc-editor.org/info/rfc8484>.
[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.
[RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting [RRL] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1 Draft1, April 2012. (DNS RRL)", ISC-TN 2012-1 Draft1, April 2012.
[Stevens] Stevens, W., Fenner, B., and A. Rudoff, "UNIX Network [Stevens] Stevens, W., Fenner, B., and A. Rudoff, "UNIX Network
Programming Volume 1, Third Edition: The Sockets Programming Volume 1, Third Edition: The Sockets
Networking API", November 2003. Networking API", November 2003.
[TDNS] Zhu, L., Heidemann, J., Wessels, D., Mankin, A., and N. [TDNS] Zhu, L., Heidemann, J., Wessels, D., Mankin, A., and N.
Somaiya, "Connection-oriented DNS to Improve Privacy and Somaiya, "Connection-oriented DNS to Improve Privacy and
Security", 2015. Security", 2015.
skipping to change at page 18, line 13 skipping to change at page 19, line 34
<https://en.wikipedia.org/wiki/TCP_Fast_Open>. <https://en.wikipedia.org/wiki/TCP_Fast_Open>.
Appendix A. Standards Related to DNS Transport over TCP Appendix A. Standards Related to DNS Transport over TCP
This section enumerates all known IETF RFC documents that are This section enumerates all known IETF RFC documents that are
currently of status standard, informational, best common practice or currently of status standard, informational, best common practice or
experimental and either implicitly or explicitly make assumptions or experimental and either implicitly or explicitly make assumptions or
statements about the use of TCP as a transport for the DNS germane to statements about the use of TCP as a transport for the DNS germane to
this document. this document.
A.1. TODO - additional, relevant RFCs A.1. IETF RFC 1035 - DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION
A.2. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR) The internet standard [RFC1035] is the base DNS specification that
explicitly defines support for DNS over TCP.
A.2. IETF RFC 1536 - Common DNS Implementation Errors and Suggested
Fixes
The informational document [RFC1536] states UDP is the "chosen
protocol for communication though TCP is used for zone transfers."
That statement should now be considered in its historical context and
is no longer a proper reflection of modern expectations.
A.3. IETF RFC 1995 - Incremental Zone Transfer in DNS
The [RFC1995] standards track document documents the use of TCP as
the fallback transport when IXFR responses do not fit into a single
UDP response. As with AXFR, IXFR messages are typically delivered
over TCP by default in practice. XXX: is this an accurate statement?
A.4. IETF RFC 1996 - A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)
The [RFC1996] standards track document suggests a zone master may
decide to issue NOTIFY messages over TCP. In practice NOTIFY
messages are generally sent over UDP, but this specification leaves
open the possibility that the choice of transport protocol is up to
the master, and therefore a slave ought to be able to operate over
both UDP and TCP.
A.5. IETF RFC 2181 - Clarifications to the DNS Specification
The [RFC2181] standards track document includes clarifying text on
how a client should react to the TC flag set on responses. It is
advised the the response should be discarded and the query resent
using TCP.
A.6. IETF RFC 2694 - DNS extensions to Network Address Translators
(DNS_ALG)
The informational document [RFC2694] enumerates considerations for
network address translation (NAT) middle boxes to properly handle DNS
traffic. This document is noteworthy in its suggestion that DNS over
TCP is "[t]ypically" used for zone transfer requests, further
evidence that helps explain why DNS over TCP may often have been
treated very differently than DNS over UDP in operational networks.
A.7. IETF RFC 3225 - Indicating Resolver Support of DNSSEC
The [RFC3225] standards track document makes statements indicating
DNS over TCP is "detrimental" as a result of increased traffic,
latency, and server load. This document is a companion to the next
document in the RFC series expressing the requirement for EDNS0
support for DNSSEC.
A.8. IETF RFC 3326 - DNSSEC and IPv6 A6 aware server/resolver message
size requirements
The [RFC3226] standards track document, although updated by later
DNSSEC strongly argued in favor of UDP messages over TCP largely for
performance reasons. The document declares EDNS0 a requirement for
DNSSEC servers and advocated packet fragmentation may be preferable
to TCP in certain situations
A.9. IETF RFC 4472 - Operational Considerations and Issues with IPv6
DNS
This informational document [RFC4472] notes that IPv6 data may
increase DNS responses beyond what would fit in a UDP message.
Particularly noteworthy, perhaps less common today then when this
document was written, refers to implementations that truncate data
without setting the TC bit to encourge the client to resend the query
using TCP.
A.10. IETF RFC 5452 - Measures for Making DNS More Resilient against
Forged Answers
This informational document [RFC5452] arose as public DNS systems
began to experience widespread abuse from spoofed queries, resulting
in amplification and reflection attacks against unwitting victims.
One of the leading justifications for supporting DNS over TCP to
thwart these attacks is briefly described in this document's 9.3
Spoof Detection and Countermeasure section.
A.11. IETF RFC 5507 - Design Choices When Expanding the DNS
This informational document [RFC5507] was largely an attempt to
dissuade new DNS data types from overloading the TXT resource record
type. In so doing it summarizes the conventional wisdom of DNS
design and implementation practices. The authors suggest TCP
overhead and stateful properties pose challenges compared to UDP, and
imply that UDP is generally preferred for performance and robustness.
A.12. IETF RFC 5625 - DNS Proxy Implementation Guidelines
This best current practice document [RFC5625] provides DNS proxy
implementation guidance including the mandate that a proxy "MUST
[...] be prepared to receive and forward queries over TCP" even
though it suggests historically TCP transport has not been strictly
mandatory in stub resolvers or recursive servers.
A.13. IETF RFC 5936 - DNS Zone Transfer Protocol (AXFR)
The [RFC5936] standards track document provides a detailed The [RFC5936] standards track document provides a detailed
specification for the zone transfer protocol, as originally outlined specification for the zone transfer protocol, as originally outlined
in the early DNS standards. AXFR operation is limited to TCP and not in the early DNS standards. AXFR operation is limited to TCP and not
specified for UDP. This document discusses TCP usage at length. specified for UDP. This document discusses TCP usage at length.
A.3. IETF RFC 6304 - AS112 Nameserver Operations A.14. IETF RFC 5966 - DNS Transport over TCP - Implementation
Requirements
This standards track document [RFC5966] instructs DNS implementers to
provide support for carrying DNS over TCP messages in their software.
The authors explicitly make no recommendations to operators, which we
seek to address here.
A.15. IETF RFC 6304 - AS112 Nameserver Operations
[RFC6304] is an informational document enumerating the requirements [RFC6304] is an informational document enumerating the requirements
for operation of AS112 project DNS servers. New AS112 nodes are for operation of AS112 project DNS servers. New AS112 nodes are
tested for their ability to provide service on both UDP and TCP tested for their ability to provide service on both UDP and TCP
transports, with the implication that TCP service is an expected part transports, with the implication that TCP service is an expected part
of normal operations. of normal operations.
A.4. IETF RFC 6762 - Multicast DNS A.16. IETF RFC 6762 - Multicast DNS
This standards track document [RFC6762] the TC bit is deemed to have In this standards track document [RFC6762] the TC bit is deemed to
essentially the same meaning as described in the original DNS have essentially the same meaning as described in the original DNS
specifications. That is, if a response with the TCP bit set is specifications. That is, if a response with the TCP bit set is
receiver "[...] the querier SHOULD reissue its query using TCP in receiver "[...] the querier SHOULD reissue its query using TCP in
order to receive the larger response." order to receive the larger response."
A.5. IETF RFC 6950 - Architectural Considerations on Application A.17. IETF RFC 6891 - Extension Mechanisms for DNS (EDNS(0))
Features in the DNS
This standards track document [RFC6891] helped slow the use and need
for DNS over TCP messages. This document highlights concerns over
server load and scalability in widespread use of DNS over TCP.
A.18. IETF RFC 6950 - Architectural Considerations on Application
Features in the DNS
An informational document [RFC6950] that draws attention to large An informational document [RFC6950] that draws attention to large
data in the DNS. TCP is referenced in the context as a common data in the DNS. TCP is referenced in the context as a common
fallback mechnanism and counter to some spoofing attacks. fallback mechnanism and counter to some spoofing attacks.
A.6. IETF RFC 7477 - Child-to-Parent Synchronization in DNS A.19. IETF RFC 7477 - Child-to-Parent Synchronization in DNS
This standards track document [RFC7477] specifies a RRType and This standards track document [RFC7477] specifies a RRType and
protocol to signal and synchronize NS, A, and AAAA resource record protocol to signal and synchronize NS, A, and AAAA resource record
changes from a child to parent zone. Since this protocol may require changes from a child to parent zone. Since this protocol may require
multiple requests and responses, it recommends utilizing DNS over TCP multiple requests and responses, it recommends utilizing DNS over TCP
to ensure the conversation takes place between a consistent pair of to ensure the conversation takes place between a consistent pair of
end nodes. end nodes.
A.7. IETF RFC 7720 - DNS Root Name Service Protocol and Deployment A.20. IETF RFC 7720 - DNS Root Name Service Protocol and Deployment
Requirements Requirements
This best current practice[RFC7720] declares root name service "MUST This best current practice[RFC7720] declares root name service "MUST
support UDP [RFC768] and TCP [RFC793] transport of DNS queries and support UDP [RFC768] and TCP [RFC793] transport of DNS queries and
responses." responses."
A.8. IETF RFC 7766 - DNS Transport over TCP - Implementation A.21. IETF RFC 7766 - DNS Transport over TCP - Implementation
Requirements Requirements
The standards track document [RFC7766] might be considered the direct The standards track document [RFC7766] might be considered the direct
ancestor of this operational requirements document. The ancestor of this operational requirements document. The
implementation requirements document codifies mandatory support for implementation requirements document codifies mandatory support for
DNS over TCP in compliant DNS software. DNS over TCP in compliant DNS software.
A.9. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option A.22. IETF RFC 7828 - The edns-tcp-keepalive EDNS0 Option
This standards track document [RFC7828] defines an EDNS0 option to This standards track document [RFC7828] defines an EDNS0 option to
negotiate an idle timeout value for long-lived DNS over TCP negotiate an idle timeout value for long-lived DNS over TCP
connections. Consequently, this document is only applicable and connections. Consequently, this document is only applicable and
relevant to DNS over TCP sessions and between implementations that relevant to DNS over TCP sessions and between implementations that
support this option. support this option.
A.10. IETF RFC 7858 - Specification for DNS over Transport Layer A.23. IETF RFC 7858 - Specification for DNS over Transport Layer
Security (TLS) Security (TLS)
This standards track document [RFC7858] defines a method for putting This standards track document [RFC7858] defines a method for putting
DNS messages into a TCP-based encrypted channel using TLS. This DNS messages into a TCP-based encrypted channel using TLS. This
specification is noteworthy for explicitly targetting the stub-to- specification is noteworthy for explicitly targetting the stub-to-
recursive traffic, but does not preclude its application from recursive traffic, but does not preclude its application from
recursive-to-authoritative traffic. recursive-to-authoritative traffic.
A.11. IETF RFC 7873 - Domain Name System (DNS) Cookies A.24. IETF RFC 7873 - Domain Name System (DNS) Cookies
This standards track document [RFC7873] describes an EDNS0 option to This standards track document [RFC7873] describes an EDNS0 option to
provide additional protection against query and answer forgery. This provide additional protection against query and answer forgery. This
specification mentions DNS over TCP as a reasonable fallback specification mentions DNS over TCP as a reasonable fallback
mechanism when DNS Cookies are not available. The specification does mechanism when DNS Cookies are not available. The specification does
make mention of DNS over TCP processing in two specific situations. make mention of DNS over TCP processing in two specific situations.
In one, when a server receives only a client cookie in a request, the In one, when a server receives only a client cookie in a request, the
server should consider whether the request arrived over TCP and if server should consider whether the request arrived over TCP and if
so, it should consider accepting TCP as sufficient to authenticate so, it should consider accepting TCP as sufficient to authenticate
the request and respond accordingly. In another, when a client the request and respond accordingly. In another, when a client
receives a BADCOOKIE reply using a fresh server cookie, the client receives a BADCOOKIE reply using a fresh server cookie, the client
should retry using TCP as the transport. should retry using TCP as the transport.
A.12. IETF RFC 7901 - CHAIN Query Requests in DNS A.25. IETF RFC 7901 - CHAIN Query Requests in DNS
This experimental specification [RFC7901] describes an EDNS0 option This experimental specification [RFC7901] describes an EDNS0 option
that can be used by a security-aware validating resolver to request that can be used by a security-aware validating resolver to request
and obtain a complete DNSSEC validation path for any single query. and obtain a complete DNSSEC validation path for any single query.
This document requires the use of DNS over TCP or a source IP address This document requires the use of DNS over TCP or a source IP address
verified transport mechanism such as EDNS-COOKIE.[RFC7873] verified transport mechanism such as EDNS-COOKIE.[RFC7873]
A.13. IETF RFC 8027 - DNSSEC Roadblock Avoidance A.26. IETF RFC 8027 - DNSSEC Roadblock Avoidance
This document [RFC8027] details observed problems with DNSSEC This document [RFC8027] details observed problems with DNSSEC
deployment and mitigation techniques. Network traffic blocking and deployment and mitigation techniques. Network traffic blocking and
restrictions, including DNS over TCP messages, are highlighted as one restrictions, including DNS over TCP messages, are highlighted as one
reason for DNSSEC deployment issues. While this document suggests reason for DNSSEC deployment issues. While this document suggests
these sorts of problems are due to "non-compliant infrastructure" and these sorts of problems are due to "non-compliant infrastructure" and
is of type BCP, the scope of the document is limited to detection and is of type BCP, the scope of the document is limited to detection and
mitigation techniques to avoid so-called DNSSEC roadblocks. mitigation techniques to avoid so-called DNSSEC roadblocks.
A.14. IETF RFC 8094 - DNS over Datagram Transport Layer Security (DTLS) A.27. IETF RFC 8094 - DNS over Datagram Transport Layer Security (DTLS)
This experimental specification [RFC8094] details a protocol that This experimental specification [RFC8094] details a protocol that
uses a datagram transport (UDP), but stipulates that "DNS clients and uses a datagram transport (UDP), but stipulates that "DNS clients and
servers that implement DNS over DTLS MUST also implement DNS over TLS servers that implement DNS over DTLS MUST also implement DNS over TLS
in order to provide privacy for clients that desire Strict Privacy in order to provide privacy for clients that desire Strict Privacy
[...]". This requirement implies DNS over TCP must be supported in [...]". This requirement implies DNS over TCP must be supported in
case the message size is larger than the path MTU. case the message size is larger than the path MTU.
A.15. IETF RFC 8162 - Using Secure DNS to Associate Certificates with A.28. IETF RFC 8162 - Using Secure DNS to Associate Certificates with
Domain Names for S/MIME Domain Names for S/MIME
This experimental specification [RFC8162] describes a technique to This experimental specification [RFC8162] describes a technique to
authenticate user X.509 certificates in an S/MIME system via the DNS. authenticate user X.509 certificates in an S/MIME system via the DNS.
The document points out that the new experimental resource record The document points out that the new experimental resource record
types are expected to carry large payloads, resulting in the types are expected to carry large payloads, resulting in the
suggestion that "applications SHOULD use TCP -- not UDP -- to perform suggestion that "applications SHOULD use TCP -- not UDP -- to perform
queries for the SMIMEA resource record." queries for the SMIMEA resource record."
A.16. IETF RFC 8324 - DNS Privacy, Authorization, Special Uses, A.29. IETF RFC 8324 - DNS Privacy, Authorization, Special Uses,
Encoding, Characters, Matching, and Root Structure: Time for Encoding, Characters, Matching, and Root Structure: Time for
Another Look? Another Look?
An informational document [RFC8324] that briefly discusses the common An informational document [RFC8324] that briefly discusses the common
role and challenges of DNS over TCP throughout the history of DNS. role and challenges of DNS over TCP throughout the history of DNS.
A.17. IETF RFC 8467 - Padding Policies for Extension Mechanisms for DNS A.30. IETF RFC 8467 - Padding Policies for Extension Mechanisms for DNS
(EDNS(0)) (EDNS(0))
An experimental document [RFC8467] reminds implementers to consider An experimental document [RFC8467] reminds implementers to consider
the underlying transport protocol (e.g. TCP) when calculating the the underlying transport protocol (e.g. TCP) when calculating the
padding length when artificially increasing the DNS message size with padding length when artificially increasing the DNS message size with
an EDNS(0) padding option. an EDNS(0) padding option.
A.18. IETF RFC 8483 - Yeti DNS Testbed A.31. IETF RFC 8483 - Yeti DNS Testbed
This informational document [RFC8483] describes a testbed environment This informational document [RFC8483] describes a testbed environment
that highlights some DNS over TCP behaviors, including issues that highlights some DNS over TCP behaviors, including issues
involving packet fragmentation and operational requirements for TCP involving packet fragmentation and operational requirements for TCP
stream assembly in order to conduct DNS measurement and analysis. stream assembly in order to conduct DNS measurement and analysis.
A.19. IETF RFC 8484 - DNS Queries over HTTPS (DoH) A.32. IETF RFC 8484 - DNS Queries over HTTPS (DoH)
This standards track document [RFC8484] defines a protocol for This standards track document [RFC8484] defines a protocol for
sending DNS queries and responses over HTTPS. This specification sending DNS queries and responses over HTTPS. This specification
assumes TLS and TCP for the underlying security and transport layers assumes TLS and TCP for the underlying security and transport layers
respectively. Self-described as a a technique that more closely respectively. Self-described as a a technique that more closely
resembles a tunneling mechanism, DoH nevertheless likely implies DNS resembles a tunneling mechanism, DoH nevertheless likely implies DNS
over TCP in some sense if not directly. over TCP in some sense if not directly.
A.33. IETF RFC 8490 - DNS Stateful Operations
This standards track document [RFC8490] updates the base protocol
specification with a new OPCODE to help manage stateful operations in
persistent sessions such as those that might be used by DNS over TCP.
Authors' Addresses Authors' Addresses
John Kristoff John Kristoff
DePaul University DePaul University
Chicago, IL 60604 Chicago, IL 60604
US US
Phone: +1 312 493 0305 Phone: +1 312 493 0305
Email: jtk@depaul.edu Email: jtk@depaul.edu
URI: https://aharp.iorc.depaul.edu URI: https://aharp.iorc.depaul.edu
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