| Domain Name System Operations | J. Kristoff |
| Internet-Draft | DePaul University |
| Intended status: Best Current Practice | March 13, 2017 |
| Expires: September 14, 2017 |
DNS Transport over TCP - Operational Requirements
draft-kristoff-dnsop-dns-tcp-requirements-02
This document encourages the practice of permitting DNS messages to be carried over TCP on the Internet. It also describes some of the consequences of this behavior and the potential operational issues that can arise when this best common practice is not upheld.
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DNS messages may be delivered using UDP or TCP communications. While most DNS transactions are carried over UDP, some operators have been led to believe that any DNS over TCP traffic is unwanted or unnecessary for general DNS operation. As usage and features have evolved, TCP transport has become increasingly important for correct and safe operation of the Internet DNS. Reflecting modern usage, the DNS standards were recently updated to declare support for TCP is now a required part of the DNS implementation specifications in [RFC7766]. This document is the formal requirements equivalent for the operational community, encouraging operators to ensure DNS over TCP communications support is on par with DNS over UDP communications.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
In the original suite of DNS specifications, [RFC1034] and [RFC1035] 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 for queries in the general case. As stated in [RFC1035]:
Another early, important, and influential document, [RFC1123], detailed the preference for UDP more explicitly:
and further stipulated:
Culminating in [RFC1536], DNS over TCP came to be associated primarily with the zone transfer mechanism, while most DNS queries and responses were seen as the dominion of UDP.
As stipulated in the original specifications, DNS messages over UDP were restricted to a 512-byte message size. However, even while [RFC1123] made a clear preference for UDP, it foresaw DNS over TCP becoming more popular in the future: [RFC2136] and the second was the set of extensions collectively known as DNSSEC originally specified in [RFC2541]. The former suggested "requestors who require an accurate response code 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 overflow and the possible necessity for using higher overhead TCP in responses."
At least two new, widely anticipated developments were set to elevate the need for DNS over TCP transactions. The first was dynamic updates defined in
Yet defying some expectations, DNS over TCP remained little used in real traffic across the Internet. Dynamic updates saw little deployment between autonomous networks. Around the time DNSSEC was first defined, another new feature affecting DNS over UDP helped solidify its dominance for message transactions.
In 1999 the IETF published the Extension Mechanisms for DNS (EDNS0) in [RFC2671]. This document standardized a way for communicating DNS nodes to perform rudimentary capabilities negotiation. One such capability written into the base specification and present in every ENDS0 compatible message is the value of the maximum UDP payload size the sender can support. This unsigned 16-bit field specifies in bytes the maximum DNS MTU. In practice, typical values are a subset of the 512 to 4096 byte range. EDNS0 was rapidly and widely deployed over the next several years and numerous surveys have shown many systems currently support larger UDP MTUs [CASTRO2010], [NETALYZR] with EDNS0.
The natural effect of EDNS0 deployment meant large DNS messages would be less reliant on TCP than they might otherwise have been. While a nonneglible population of DNS systems lack EDNS0 or may still fall back to TCP for some transactions, DNS over TCP transactions remain a very small fraction of overall DNS traffic [VERISIGN]. Nevertheless, some average increase in DNS message size, the continued development of new DNS features and a denial of service mitigation technique (see Section 9) have suggested that DNS over TCP transactions are as important to the correct and safe operation of the Internet DNS as ever, if not more so. Furthermore, there has been serious research that has suggested connection-oriented DNS transactions may provide security and privacy advantages over UDP transport [TDNS]. In fact, [RFC7858], a Standards Track document is just this sort of specification. Therefore, it might be desirable for network operators to avoid artificially inhibiting the potential utility and advances in the DNS such as these.
Even while many in the DNS community expect DNS over TCP transactions to occur without interference, in practice there has been a long held belief by some operators, particularly for security-related reasons, to the contrary [CHES94], [DJBDNS]. A popular meme has also held the imagination of some that DNS over TCP is only ever used for zone transfers and is generally unnecessary otherwise, with filtering all DNS over TCP traffic even described as a best practice. Arguably any exposed Internet service poses some risk, but this reasoning is often invalid.
Section 6.1.3.2 in [RFC1123] is updated: All general-purpose DNS servers MUST be able to service both UDP and TCP queries. [RFC1123] also says:
Regarding the choice of limiting the resources a server devotes to queries, Section 6.1.3.2 in
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 query just because it would have succeeded with another transport protocol.
DNS over TCP filtering is considered harmful in the general case. DNS resolver and server operators MUST provide DNS service over both UDP and TCP transports. Likewise, network operators MUST allow DNS service over both UDP and TCP transports. It must be acknowledged that DNS over TCP service can pose operational challenges that are not present when running DNS over UDP alone. However, it is the aim of this document to argue that the potential damage incurred by prohibiting DNS over TCP service is more detrimental to the continued utility and success of the DNS than when its usage is allowed.
TODO: refer to IETF RFC 7766 connection handling discussion, various TCP hardening documents, network operator protocol and traffic best practices, etc.
Networks that filter DNS over TCP may inadvertently cause problems for third party resolvers as experienced by [TOYAMA]. If for instance a resolver receives a truncated answer from a server, but if when the resolver resends the query using TCP and the TCP response never arrives, the resolver will incur the full extent of TCP retransmissions and time outs.
Networks that filter DNS over TCP risk losing access to significant 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 large message sizes, lack of EDNS0 support, DDoS mitigation techniques, or perhaps some future capability that is as yet unforeseen will also demand TCP transport.
Even if any or all particular answers have consistently been returned successfully with UDP in the past, this continued behavior cannot be guaranteed when DNS messages are exchanged between autonomous systems. Therefore, filtering of DNS over TCP is considered harmful and contrary to the safe and successful operation of the Internet.
This section enumerates all known IETF RFC documents that are currently of status standard, informational, best common practice or experimental and either implicitly or explicitly make assumptions or statements about the use of TCP as a transport for the DNS germane to this document.
This standards track document [RFC7477] specifies a RRType and protocol to signal and synchronize NS, A, and AAAA resource record changes from a child to parent zone. Since this protocol may require multiple requests and responses, it recommends utilizing DNS over TCP to ensure the conversation takes place between a consistent pair of end nodes.
The standards track document [RFC7766] is might be considered the direct ancestor of this operational requirements document. The implementation requirements document codifies mandatory support for DNS over TCP in compliant DNS software.
This standards track document [RFC7828] defines an EDNS0 option to negotiate an idle timeout value for long-lived DNS over TCP connections. Consequently, this document is only applicable and relevant to DNS over TCP sessions and between implementations that support this option.
This standards track document [RFC7873] describes an EDNS0 option to provide additional protection against query and answer forgery. This specification mentions DNS over TCP as a reasonable fallback mechanism when DNS Cookies are not available. The specification does 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 server should consider whether the request arrived over TCP and if so, it should consider accept TCP as sufficient to authenticate the request and respond accordingly. In another, when a client receives a BADCOOKIE reply using a fresh server cookie, the client should retry using TCP as the transport.
This experimental specification [RFC7901] describes an EDNS0 option that can be used by a security-aware validating resolver to request 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 verified transport mechanism such as EDNS-COOKIE.[RFC7873]
This document [RFC8027] details observed problems with DNSSEC deployment and mitigation techniques. Network traffic blocking and restrictions, including DNS over TCP messages, are highlighted as one reason for DNSSEC deployment issues. While this document suggests 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 mitigation techniques to avoid so-called DNSSEC roadblocks.
This document was initially motivated by feedback from students who pointed out that they were hearing contradictory information about filtering DNS over TCP messages. Thanks in particular to my teaching colleague, JPL, who perhaps unknowingly encouraged the initial research into the differences of what the IETF community has historically said and did. Thanks to all the NANOG 63 attendees who provided feedback to an early talk on this subject.
The following individuals provided an array of feedback to help improve this document: Bob Harold, Paul Hoffman, and Sara Dickinson. The author is indebted to their contributions. Any remaining errors or imperfections are the sole responsbility of the document author.
This memo includes no request to IANA.
Ironically, returning truncated DNS over UDP answers in order to induce a client query to switch to DNS over TCP has become a common response to source address spoofed, DNS denial-of-service attacks [RRL]. Historically, operators have been wary of TCP-based attacks, but in recent years, UDP-based flooding attacks have proven to be the most common protocol attack on the DNS. Nevertheless, a high rate of short-lived DNS transactions over TCP may pose challenges. While many operators have provided DNS over TCP service for many years without duress, past experience is no guarantee of future success.
DNS over TCP is not unlike many other Internet TCP services. TCP threats and many mitigation strategies have been well documented in a series of documents such as [RFC4953], [RFC4987], [RFC5927], and [RFC5961].
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |