Internet Engineering Task Force M. Sivaraman
Internet-Draft Internet Systems Consortium
Intended status: Experimental S. Kerr
Expires: July 22, 2016 L. Song
Beijing Internet Institute
January 19, 2016

DNS message fragments


This document describes a method to transmit DNS messages over multiple UDP datagrams by fragmenting them at the application layer. The objective is to allow authoriative servers to successfully reply to DNS queries via UDP using multiple smaller datagrams, where larger datagrams may not pass through the network successfully.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on July 22, 2016.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. Introduction

1.1. Background

[RFC1035] describes how DNS messages are to be transmitted over UDP. A DNS query message is transmitted using one UDP datagram from client to server, and a corresponding DNS reply message is transmitted using one UDP datagram from server to client.

The upper limit on the size of a DNS message that can be transmitted thus depends on the maximum size of the UDP datagram that can be transmitted successfully from the sender to the receiver. Typically any size limit only matters for DNS replies, as DNS queries are usually small.

As a UDP datagram is transmitted in a single IP PDU, in theory the size of a UDP datagram (including various lower internet layer headers) can be as large as 64 KiB. But practically, if the datagram size exceeds the path MTU, then the datagram will either be fragmented at the IP layer, or worse dropped, by a forwarder. In the case of IPv6, DNS packets are fragmented by the sender only. If a packet's size exceeds the path MTU, a Packet Too Big (PTB) ICMP message will be received by sender without any clue to the sender to reply again with a smaller sized message, due to the stateless feature of DNS. In addition, IP-level fragmentation caused by large DNS response packet will introduce risk of cache poisoning [Fragment-Poisonous], in which the attacker can circumvent some defense mechanisms (like port, IP, and query randomization [RFC5452]).

As a result, a practical DNS payload size limitation is necessary. [RFC1035] limited DNS message UDP datagram lengths to a maximum of 512 bytes. Although EDNS(0) [RFC6891] allows an initiator to advertise the capability of receiving lager packets (up to 4096 bytes), it leads to fragmentation because practically most packets are limited to 1500 byte size due to host Ethernet interfaces, or 1280 byte size due to minimum IPv6 MTU in the IPv6 stack [RFC3542].

According to DNS specifications [RFC1035], if the DNS response message can not fit within the packet's size limit, the response is truncated and the initiator will have to use TCP as a fallback to re-query to receive large response. However, not to mention the high setup cost introduced by TCP due to additional roundtrips, some firewalls and middle boxes even block TCP/53 which cause no responses to be received as well. It becomes a significant issue when the DNS response size inevitably increases with DNSSEC deployment.

In this memo, DNS message fragmentation attempts to work around middle box misbehavior by splitting a single DNS message across multiple UDP datagrams. Note that to avoid DNS amplification and reflection attacks, DNS cookies [I-D.ietf-dnsop-cookies] is a mandatory requirement when using DNS message fragments.

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 [RFC2119].

1.2. Motivation

It is not a new topic regarding large DNS packets(>512B) issue [I-D.ietf-dnsop-respsize], starting from introduction of IPv6, EDNS(0) [SAC016], and DNSSEC deployment [SAC035]. In current production networks, using DNSSEC with longer DNSKEYs (ZSK>1024B and KSK>2048B) will result in response packets no smaller than 1500B [T-DNS]. Especially during the KSK rollover process, responses to the query of DNSKEY RRset will be enlarged as they contain both the new and old KSK.

When possible, we should avoid dropped packets as this means the client must wait for a timeout, which incurs a high cost. For example, a validator behind a firewall suffers waiting till the timeout with no response, if the firewall drops large EDNS(0) packets and IP fragments. It may even cause disaster when the validator can not recieve response for new trust anchor KSK due to the extreme case of bad middle boxes which also drop TCP/53.

Since UDP requires fewer packets on the wire and less state on servers than TCP, in this memo we propose continuing to use UDP for transmission but fragment the larger DNS packets into smaller DNS packets at the application layer. We would like the fragments to easily go through middle boxes and avoid falling back to TCP.

2. DNS Message Fragmentation Method

2.1. Client Behavior

Clients supporting DNS message fragmentation add an EDNS option to their queries, which declares their support for this feature.

If a DNS reply is received that has been fragmented, it will consist of multiple DNS message fragments (each transmitted in a respective UDP packet), and every fragment contain an EDNS option which says how many total fragments there are, and the identifier of the fragment that the current packet represents. The client collects all of the fragments and uses them to reconstruct the full DNS message. Clients MUST maintain a timeout when waiting for the fragments to arrive.

Clients that support DNS message fragments MUST be able to reassemble fragments into a DNS message of any size, up to the maximum of 64KiB.

The client MAY save information about what sizes of packets have been received from a given server. If saved, this information MUST have a limited duration.

If TSIG [RFC2845] is used, then the signature must be checked on each fragment separately.

Any DNSSEC validation is performed on the reassembled DNS message.

2.2. Server Behavior

Servers supporting DNS message fragmentation will look for the EDNS option which declares client support for the feature. If not present, the server MUST NOT use DNS message fragmentation. The server MUST check that DNS cookies are supported. [**FIXME**] Implementation of the first request case, where no existing established cookie is available needs discussion; we want to avoid additional round-trips here. Shane: don't cookies already handle this case?

The server prepares the response DNS message normally. If the message exceeds the maximum UDP payload size specified by the client, then it should fragment the message into multiple UDP datagrams.

Each fragment contains an identical DNS header with TC=1, possibly varying only in the section counts. Setting the TC flag in this way insures that clients which do not support DNS fragments can fallback to TCP transparently.

As many RR are included in each fragment as are possible without going over the desired size of the fragment. An EDNS option is added to every fragment, that includes both the fragment identifier and the total number of fragments. Names are compressed in each fragment separately.

An RRSET may be split across multiple fragments. RRSIG may be sent in a separate fragment from the RRSET that it refers to.

The server needs to know how many total fragments there are to insert into each fragment. A simple approach would be to generate all fragments, and then count the total number at the end, and update the previously-generated fragments with the total number of fragments. Other techniques may be possible.

The server MUST limit the number of fragments that it uses in a reply. Too many packets sent simultaneously can cause network congestion or packet loss, either of which will degrade the overall network performance. (See "Open Issues and Discussion" for remaining work.)

The server should try to minimize the total number of packets sent, however since this is similar to the bin packing problem, which is complexity NP-hard, the server does not have to garantee this. In all cases, the server MUST NOT exceed the maximum fragment size requested by a client.

If TSIG [RFC2845] is used, then the signature must be added to each fragment separately.

If a message cannot be fragmented successfully or the server does not wish to fragment a message, then the FRAGMENT EDNS(0) Option is not included in the reply, and it is truncated as a normal, non-fragmented message. (There are many reasons a server may not wish to fragment a reply, for example if it would result in too many fragments. Also, since fragmentation occurs on an RR boundary, any RR that would cause a fragment to exceed the maximum message size cannot be fragmented. A large TXT record can cause this behavior, for example.)

2.3. Other Notes


ALLOW-FRAGMENTS is an EDNS(0) [RFC6891] option that a client uses to inform a server that it supports fragmented responses.

3.1. Wire Format


3.2. Option Fields

3.2.1. Maximum Fragment Size

The Maximum Fragment Size field is represented as an unsigned 16-bit integer. This is the maximum size used by any given fragment the server returns.

3.3. Presentation Format

As with other EDNS(0) options, the ALLOW-FRAGMENTS option does not have a presentation format.

4. The FRAGMENT EDNS(0) Option

FRAGMENT is an EDNS(0) [RFC6891] option that assists a client in gathering the various fragments of a DNS message from multiple UDP datagrams. It is described in a previous section. Here, its syntax is provided.

4.1. Wire Format


4.2. Option Fields

4.2.1. Fragment Identifier

The Fragment Identifier field is represented as an unsigned 8-bit integer. The first fragment is identified as 1. Values in the range [1,255] can be used to identify the various fragments. Value 0 is used for signalling purposes.

4.2.2. Fragment Count

The Fragment Count field is represented as an unsigned 8-bit integer. It contains the number of fragments in the range [1,255] that make up the DNS message. Value 0 is used for signalling purposes.

4.3. Presentation Format

As with other EDNS(0) options, the FRAGMENT option does not have a presentation format.

5. Network Considerations

5.1. Background

TCP-based application protocols co-exist well with competing traffic flows in the internet due to congestion control methods such as in [RFC5681] that are present in TCP implementations.

UDP-based application protocols have no restrictions in lower layers to stop them from flooding datagrams into a network and causing congestion. So applications that use UDP have to check themselves from causing congestion so that their traffic is not disruptive.

In the case of [RFC1035], only one reply UDP datagram was sent per request UDP datagram, and so the lock-step flow control automatically ensured that UDP DNS traffic didn't lead to congestion. When DNS clients didn't hear back from the server, and had to retransmit the question, they typically paced themselves by using methods such as a retransmission timer based on a smoothed round-trip time between client and server.

Due to the message fragmentation described in this document, when a DNS query causes multiple DNS reply datagrams to be sent back to the client, there is a risk that without effective control of flow, DNS traffic could cause problems to competing flows along the network path.

Because UDP does not guarantee delivery of datagrams, there is a possibility that one or more fragments of a DNS message will be lost during transfer. This is especially a problem on some wireless networks where a rate of datagrams can continually be lost due to interference and other environmental factors. With larger numbers of message fragments, the probability of fragment loss increases.

5.2. Implementation Requirements


6. Open Issues and Discussion

  1. Resolver behavior

    We need some more discussion of resolver behavior in general, at least to the point of making things clear to an implementor.
  2. What is the size of fragments?

    Generally speaking the number of fragment increases if fragment size is small (512 bytes, or other empirical value), which makes the mechanism less efficient. If the size can changed dynamically according to negotiation or some detection, it will introduce more cost and round trip time.

  3. We might set specific upper limits for number of fragments.

  4. OPT-RR

    Some OPT-RR seem to be oriented at the entire message, others make more sense per packet. This needs to be sorted out. It looks like only cookies need to be included per fragment.

7. Security Considerations

To avoid DNS amplification or reflection attacks, DNS cookies [I-D.ietf-dnsop-cookies] must be used. The DNS cookie EDNS option is identical in all fragments that make up a DNS message. The duplication of the same cookie values in all fragments that make up the message is not expected to introduce a security weakness in the case of off-path attacks.

8. IANA Considerations

The ALLOW-FRAGMENTS and FRAGMENT EDNS(0) options require option codes to be assigned for them.

9. Acknowledgements

Thanks to Stephen Morris, JINMEI Tatuya, Paul Vixie, Mark Andrews, and David Dragon for reviewing a pre-draft proposal and providing support, comments and suggestions.

10. References

10.1. Normative references

[I-D.ietf-dnsop-cookies] Eastlake, D. and M. Andrews, "Domain Name System (DNS) Cookies", Internet-Draft draft-ietf-dnsop-cookies-09, January 2016.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D. and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E. and T. Jinmei, "Advanced Sockets Application Program Interface (API) for IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More Resilient against Forged Answers", RFC 5452, DOI 10.17487/RFC5452, January 2009.
[RFC5681] Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, DOI 10.17487/RFC5681, September 2009.
[RFC6891] Damas, J., Graff, M. and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891, April 2013.

10.2. Informative references

[Fragment-Poisonous] Herzberg, A. and H. Shulman, "Fragmentation Considered Poisonous", 2012.
[I-D.ietf-dnsop-respsize] Vixie, P., Kato, A. and J. Abley, "DNS Referral Response Size Issues", Internet-Draft draft-ietf-dnsop-respsize-15, February 2014.
[SAC016] ICANN Security and Stability Advisory Committee, "Testing Firewalls for IPv6 and EDNS0 Support", 2007.
[SAC035] ICANN Security and Stability Advisory Committee, "DNSSEC Impact on Broadband Routers and Firewalls", 2008.
[T-DNS] Zhu, L., Hu, Z. and J. Heidemann, "T-DNS: Connection-Oriented DNS to Improve Privacy and Security (extended)", 2007.

Appendix A. Change History (to be removed before publication)

Authors' Addresses

Mukund Sivaraman Internet Systems Consortium 950 Charter Street Redwood City, CA 94063 US EMail: URI:
Shane Kerr Beijing Internet Institute 2/F, Building 5, No.58 Jinghai Road, BDA Beijing, 100176 CN EMail: URI:
Linjian Song Beijing Internet Institute 2/F, Building 5, No.58 Jinghai Road, BDA Beijing, 100176 CN EMail: URI: