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operations Internet-Draft EDNS0 enables a DNS server to send large responses using UDP and is widely deployed. Large DNS/UDP responses are fragmented, and IP fragmentation has exposed weaknesses in application protocols. It is possible to avoid IP fragmentation in DNS by limiting response size where possible, and signaling the need to upgrade from UDP to TCP transport where necessary. This document proposes techniques to avoid IP fragmentation in DNS.

Introduction DNS has an EDNS0 mechanism. It enables a DNS server to send large responses using UDP. EDNS0 is now widely deployed, and DNS over UDP relies on IP fragmentation when the EDNS buffer size is set to a value larger than the path MTU. Fragmented DNS UDP responses have systemic weaknesses, which expose the requestor to DNS cache poisoning from off-path attackers. (See for references and details.) summarized that IP fragmentation introduces fragility to Internet communication. The transport of DNS messages over UDP should take account of the observations stated in that document. TCP avoids fragmentation using its Maximum Segment Size (MSS) parameter, but each transmitted segment is header-size aware such that the size of the IP and TCP headers is known, as well as the far end's MSS parameter and the interface or path MTU, so that the segment size can be chosen so as to keep the each IP datagram below a target size. This takes advantage of the elasticity of TCP's packetizing process as to how much queued data will fit into the next segment. In contrast, DNS over UDP has little datagram size elasticity and lacks insight into IP header and option size, and so must make more conservative estimates about available UDP payload space. This document proposes that implementations set the "Don't Fragment (DF) bit" on IPv4 and not use the "Fragment header" on IPv6 in DNS/UDP messages in order to avoid IP fragmentation. It also describes how to avoid packet losses due to DF bit and small MTU links. A path MTU different from the recommended value could be obtained from static configuration, or server routing hints, or some future discovery protocol; that would be the subject of a future specification and is beyond our scope here.

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 BCP14 when, and only when, they appear in all capitals, as shown here. "Requestor" refers to the side that sends a request. "Responder" refers to an authoritative server, recursive resolver or other DNS component that responds to questions. (Quoted from EDNS0 ) "Path MTU" is the minimum link MTU of all the links in a path between a source node and a destination node. (Quoted from ) In this document, the term "Path MTU discovery" includes both Classical Path MTU discovery , , and Packetization Layer Path MTU discovery . Many of the specialized terms used in this document are defined in DNS Terminology .

Proposal to avoid IP fragmentation in DNS These recommendations are intended for nodes with global IP addresses on the Internet. Private networks or local networks are out of the scope of this document. The methods to avoid IP fragmentation in DNS are described below:

Recommendations for UDP responders R1. UDP responders SHOULD send DNS responses without "Fragment header" on IPv6. R2. UDP responders MAY set IP "Don't Fragment flag (DF) bit" on IPv4. R3. UDP responders SHOULD compose response packets that fit in the offered requestor's maximum UDP payload size , the interface MTU, and the RECOMMENDED maximum DNS/UDP payload size 1400. R4. If the UDP responder detects an immediate error indicating that the UDP packet cannot be sent beyond the path MTU size (EMSGSIZE), the UDP responder MAY recreate response packets fit in path MTU size, or with the TC bit set. R5. UDP responders SHOULD limit the response size when UDP responders are located on small MTU (<1500) networks. The cause and effect of the TC bit are unchanged from EDNS0 .

Recommendations for UDP requestors R6. UDP requestors SHOULD limit the requestor's maximum UDP payload size to the RECOMMENDED size of 1400 or a smaller size. R7. UDP requestors MAY drop fragmented DNS/UDP responses without IP reassembly to avoid cache poisoning attacks. R8. DNS responses may be dropped by IP fragmentation. Upon a timeout, to avoid resolution failures, UDP requestors MAY retry using TCP or UDP with a smaller EDNS requestor's maximum UDP payload size per local policy.

Recommendations for zone operators and DNS server operators Large DNS responses are the result of zone configuration. Zone operators SHOULD seek configurations resulting in small responses. For example, R9. Use a smaller number of name servers (13 may be too large) R10. Use a smaller number of A/AAAA RRs for a domain name R11. Use minimal-responses configuration: Some implementations have a 'minimal responses' configuration option that causes DNS servers to make response packets smaller, containing only mandatory and required data (). R12. Use a smaller signature / public key size algorithm for DNSSEC. Notably, the signature sizes of ECDSA and EdDSA are smaller than those usually used for RSA.

Considerations

Protocol compliance Prior research has shown that some authoritative servers ignore the EDNS0 requestor's maximum UDP payload size, and return large UDP responses. It is also well known that some authoritative servers do not support TCP transport. Such non-compliant behavior cannot become implementation or configuration constraints for the rest of the DNS. If failure is the result, then that failure must be localized to the non-compliant servers.

IANA Considerations This document has no IANA actions.

Security Considerations

On-path fragmentation on IPv4 If the Don't Fragment (DF) bit is not set, on-path fragmentation may happen on IPv4, and be vulnerable as shown in . To avoid this, recommendation R7 should be used to discard the fragmented responses and retry by TCP. In the future, recommendation R2 could be changed from "MAY" to "SHOULD".

Small MTU network When avoiding fragmentation, a DNS/UDP requestor behind a small-MTU network may experience UDP timeouts which would reduce performance and which may lead to TCP fallback. This would indicate prior reliance upon IP fragmentation, which is universally considered to be harmful to both the performance and stability of applications, endpoints, and gateways. Avoiding IP fragmentation will improve operating conditions overall, and the performance of DNS/TCP has increased and will continue to increase. If a UDP response packet is dropped (for any reason), it increases the attack window for poisoning the requestor's cache.

Acknowledgments The author would like to specifically thank Paul Wouters, Mukund Sivaraman, Tony Finch, Hugo Salgado, Peter van Dijk, Brian Dickson, Puneet Sood, Jim Reid, Petr Spacek, Andrew McConachie, Joe Abley, Daisuke Higashi, Joe Touch and Wouter Wijngaards for extensive review and comments.

This memo describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. It specifies a small change to the way routers generate one type of ICMP message. For a path that passes through a router that has not been so changed, this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by current practice. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes Path MTU Discovery (PMTUD) for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC 1981. The Domain Name System's wire protocol includes a number of fixed fields whose range has been or soon will be exhausted and does not allow requestors to advertise their capabilities to responders. This document describes backward-compatible mechanisms for allowing the protocol to grow. This document updates the Extension Mechanisms for DNS (EDNS(0)) specification (and obsoletes RFC 2671) based on feedback from deployment experience in several implementations. It also obsoletes RFC 2673 ("Binary Labels in the Domain Name System") and adds considerations on the use of extended labels in the DNS. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460. The Domain Name System (DNS) is defined in literally dozens of different RFCs. The terminology used by implementers and developers of DNS protocols, and by operators of DNS systems, has sometimes changed in the decades since the DNS was first defined. This document gives current definitions for many of the terms used in the DNS in a single document. This document obsoletes RFC 7719 and updates RFC 2308. This document specifies Datagram Packetization Layer Path MTU Discovery (DPLPMTUD). This is a robust method for Path MTU Discovery (PMTUD) for datagram Packetization Layers (PLs). It allows a PL, or a datagram application that uses a PL, to discover whether a network path can support the current size of datagram. This can be used to detect and reduce the message size when a sender encounters a packet black hole. It can also probe a network path to discover whether the maximum packet size can be increased. This provides functionality for datagram transports that is equivalent to the PLPMTUD specification for TCP, specified in RFC 4821, which it updates. It also updates the UDP Usage Guidelines to refer to this method for use with UDP datagrams and updates SCTP. The document provides implementation notes for incorporating Datagram PMTUD into IETF datagram transports or applications that use datagram transports. This specification updates RFC 4960, RFC 4821, RFC 6951, RFC 8085, and RFC 8261. IPv6 specifies the Fragment Header, which is employed for the fragmentation and reassembly mechanisms. The Fragment Header contains an "Identification" field that, together with the IPv6 Source Address and the IPv6 Destination Address of a packet, identifies fragments that correspond to the same original datagram, such that they can be reassembled together by the receiving host. The only requirement for setting the Identification field is that the corresponding value must be different than that employed for any other fragmented datagram sent recently with the same Source Address and Destination Address. Some implementations use a simple global counter for setting the Identification field, thus leading to predictable Identification values. This document analyzes the security implications of predictable Identification values, and provides implementation guidance for setting the Identification field of the Fragment Header, such that the aforementioned security implications are mitigated. The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP. Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control. This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage. This document describes IP fragmentation and explains how it introduces fragility to Internet communication. This document also proposes alternatives to IP fragmentation and provides recommendations for developers and network operators. Fraunhofer Institute for Secure Information Technology SIT, Darmstadt, Germany Fraunhofer Institute for Secure Information Technology SIT, Darmstadt, Germany Fraunhofer Institute for Secure Information Technology SIT, Darmstadt, Germany Fraunhofer Institute for Secure Information Technology SIT, Darmstadt, Germany Fraunhofer Institute for Secure Information Technology SIT, Darmstadt, Germany cz.nic JPRS APNIC Labs APNIC Labs ISC Salesforce Aiven Verisign The DNS uses glue records to allow iterative clients to find the addresses of name servers that are contained within a delegated zone. Authoritative Servers are expected to return all available glue records for in-domain name servers in a referral response. If message size constraints prevent the inclusion of all glue records for in-domain name servers, the server must set the TC flag to inform the client that the response is incomplete, and that the client should use another transport to retrieve the full response. This document updates RFC 1034 to clarify correct server behavior. RFC1034 provided a description of how to cache negative responses. It however had a fundamental flaw in that it did not allow a name server to hand out those cached responses to other resolvers, thereby greatly reducing the effect of the caching. This document addresses issues raise in the light of experience and replaces RFC1034 Section 4.3.4. [STANDARDS-TRACK] This RFC is the revised specification of the protocol and format used in the implementation of the Domain Name System. It obsoletes RFC-883. This memo documents the details of the domain name client - server communication. This document describes a DNS RR which specifies the location of the server(s) for a specific protocol and domain. [STANDARDS-TRACK] Google Akamai Technologies Akamai Technologies This document specifies the "SVCB" and "HTTPS" DNS resource record (RR) types to facilitate the lookup of information needed to make connections to network services, such as for HTTP origins. SVCB records allow a service to be provided from multiple alternative endpoints, each with associated parameters (such as transport protocol configuration), and are extensible to support future uses (such as keys for encrypting the TLS ClientHello). They also enable aliasing of apex domains, which is not possible with CNAME. The HTTPS RR is a variation of SVCB for use with HTTP [HTTP]. By providing more information to the client before it attempts to establish a connection, these records offer potential benefits to both performance and privacy. TO BE REMOVED: This document is being collaborated on in Github at: https://github.com/MikeBishop/dns-alt-svc (https://github.com/MikeBishop/dns-alt-svc). The most recent working version of the document, open issues, etc. should all be available there. The authors (gratefully) accept pull requests. This document is part of a family of documents that describe the DNS Security Extensions (DNSSEC). The DNS Security Extensions are a collection of new resource records and protocol modifications that add data origin authentication and data integrity to the DNS. This document describes the DNSSEC protocol modifications. This document defines the concept of a signed zone, along with the requirements for serving and resolving by using DNSSEC. These techniques allow a security-aware resolver to authenticate both DNS resource records and authoritative DNS error indications. This document obsoletes RFC 2535 and incorporates changes from all updates to RFC 2535. [STANDARDS-TRACK] The Domain Name System Security (DNSSEC) Extensions introduced the NSEC resource record (RR) for authenticated denial of existence. This document introduces an alternative resource record, NSEC3, which similarly provides authenticated denial of existence. However, it also provides measures against zone enumeration and permits gradual expansion of delegation-centric zones. [STANDARDS-TRACK]

Weaknesses of IP fragmentation "Fragmentation Considered Poisonous" proposed effective off-path DNS cache poisoning attack vectors using IP fragmentation. "IP fragmentation attack on DNS" and "Domain Validation++ For MitM-Resilient PKI" proposed that off-path attackers can intervene in path MTU discovery to perform intentionally fragmented responses from authoritative servers. stated the security implications of predictable fragment identification values. DNSSEC is a countermeasure against cache poisoning attacks that use IP fragmentation. However, DNS delegation responses are not signed with DNSSEC, and DNSSEC does not have a mechanism to get the correct response if an incorrect delegation is injected. This is a denial-of-service vulnerability that can yield failed name resolutions. If cache poisoning attacks can be avoided, DNSSEC validation failures will be avoided. In Section 3.2 (Message Side Guidelines) of UDP Usage Guidelines we are told that an application SHOULD NOT send UDP datagrams that result in IP packets that exceed the Maximum Transmission Unit (MTU) along the path to the destination. A DNS message receiver cannot trust fragmented UDP datagrams primarily due to the small amount of entropy provided by UDP port numbers and DNS message identifiers, each of which being only 16 bits in size, and both likely being in the first fragment of a packet, if fragmentation occurs. By comparison, TCP protocol stack controls packet size and avoids IP fragmentation under ICMP NEEDFRAG attacks. In TCP, fragmentation should be avoided for performance reasons, whereas for UDP, fragmentation should be avoided for resiliency and authenticity reasons.

Details of requestor's maximum UDP payload size discussions There are many discussions for default path MTU size and requestor's maximum UDP payload size. The minimum MTU for an IPv6 interface is 1280 octets (see Section 5 of ). So, we can use it as the default path MTU value for IPv6. The corresponding minimum MTU for an IPv4 interface is 68 (60 + 8) . Most of the Internet and especially the inner core has an MTU of at least 1500 octets. Maximum DNS/UDP payload size for IPv6 on MTU 1500 ethernet is 1452 (1500 minus 40 (IPv6 header size) minus 8 (UDP header size)). To allow for possible IP options and distant tunnel overhead, the authors' recommendation of default maximum DNS/UDP payload size is 1400. defines that "A security-aware name server MUST support the EDNS0 message size extension, MUST support a message size of at least 1220 octets". Then, the smallest number of the maximum DNS/UDP payload size is 1220. In order to avoid IP fragmentation, proposed that the UDP requestors set the requestor's payload size to 1232, and the UDP responders compose UDP responses so they fit in 1232 octets. The size 1232 is based on an MTU of 1280, which is required by the IPv6 specification , minus 48 octets for the IPv6 and UDP headers. analyzed the result of and reported that their measurements suggest that in the interior of the Internet between recursive resolvers and authoritative servers the prevailing MTU is at 1,500 and there is no measurable signal of use of smaller MTUs in this part of the Internet, and proposed that their measurements suggest setting the EDNS0 requestor's UDP payload size to 1472 octets for IPv4, and 1452 octets for IPv6.

Minimal-responses Some implementations have a "minimal responses" configuration setting/option that causes a DNS server to make response packets smaller, containing only mandatory and required data. Under the minimal-responses configuration, a DNS server composes responses containing only nessesary RRs. For delegations, see . In case of a non-existent domain name or non-existent type, the authority section will contain an SOA record and the answer section is empty. (defined in Section 2 of ). Some resource records (MX, SRV, SVCB, HTTTPS) require additional A, AAAA, and SVCB records in the Additional Section defined in , and . In addition, if the zone is DNSSEC signed and a query has the DNSSEC OK bit, signatures are added in the answer section, or the corresponding DS RRSet and signatures are added in the authority section. Details are defined in and .

Known Implementations This section records the status of known implementations of these best practices defined by this specification at the time of publication, and any deviation from the specification. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors.

BIND 9 BIND 9 does not implement the recommendations 1 and 2 in . BIND 9 on Linux sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with a fallback to IP_PMTUDISC_DONT. BIND 9 on systems with IP_DONTFRAG (such as FreeBSD), IP_DONTFRAG is disabled. Accepting PATH MTU Discovery for UDP is considered harmful and dangerous. BIND 9's settings avoid attacks to path MTU discovery. For recommendation 3, BIND 9 will honor the requestor's size up to the configured limit (max-udp-size). The UDP response packet is bound to be between 512 and 4096 bytes, with the default set to 1232. BIND 9 supports the requestor's size up to the configured limit (max-udp-size). In the case of recommendation 4, and the send fails with EMSGSIZE, BIND 9 set the TC bit and try to send a minimal answer again. In the first recommendation of , BIND 9 uses the edns-buf-size option, with the default of 1232. BIND 9 does implement recommendation 2 of . For recommendation 3, after two UDP timeouts, BIND 9 will fallback to TCP.

Knot DNS and Knot Resolver Both Knot servers set IP_PMTUDISC_OMIT to avoid path MTU spoofing. UDP size limit is 1232 by default. Fragments are ignored if they arrive over an XDP interface. TCP is attempted after repeated UDP timeouts. Minimal responses are returned and are currently not configurable. Smaller signatures are used, with ecdsap256sha256 as the default.

PowerDNS Authoritative Server, PowerDNS Recursor, PowerDNS dnsdist IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT default EDNS buffer size of 1232, no probing for smaller sizes no handling of EMSGSIZE Recursor: UDP timeouts do not cause a switch to TCP. "Spoofing nearmisses" do.

PowerDNS Authoritative Server the default DNSSEC algorithm is 13 responses are minimal, this is not configurable

Unbound Unbound sets IP_MTU_DISCOVER to IP_PMTUDISC_OMIT with fallback to IP_PMTUDISC_DONT. It also disables IP_DONTFRAG on systems that have it, but not on Apple systems. On systems that support it Unbound sets IPV6_USE_MIN_MTU, with a fallback to IPV6_MTU at 1280, with a fallback to IPV6_USER_MTU. It also sets IPV6_MTU_DISCOVER to IPV6_PMTUDISC_OMIT with a fallback to IPV6_PMTUDISC_DONT. Unbound requests UDP size 1232 from peers, by default. The requestors size is limited to a max of 1232. After some timeouts, Unbound retries with a smaller size, if that is smaller, at size 1232 for IPv6 and 1472 for IPv4. This does not do anything since the flag day change to 1232. Unbound has minimal responses as an option, default on.