DNS Extensions Working Group G. Barwood Internet-Draft Intended status: Experimental 3 September 2009 Expires: March 2010 DNS Transport draft-barwood-dnsext-dns-transport-04 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 4, 2010. Copyright Notice Copyright (c) 2009 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 (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract Describes an experimental transport protocol for DNS. IP fragmentation is avoided, blind spoofing, amplification attacks and other denial of service attacks are prevented. Latency for a typical DNS query is a single round trip, after a setup exchange that establishes a long term shared secret. No per-client server state is required between transactions. The protocol may have other applications. Barwood Expires March 2010 [Page 1] Internet-Draft DNS Transport September 2009 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Fragmentation. . . . . . . . . . . . . . . . . . . . . . . . 4 2.2 Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Server state . . . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Amplification attacks . . . . . . . . . . . . . . . . . . . 4 2.5 Packet retransmission . . . . . . . . . . . . . . . . . . . 4 2.6 Performance . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2 Setup request . . . . . . . . . . . . . . . . . . . . . . . 5 3.3 Setup response . . . . . . . . . . . . . . . . . . . . . . . 5 3.4 Initial request . . . . . . . . . . . . . . . . . . . . . . 6 3.5 Server response : single page . . . . . . . . . . . . . . . 6 3.6 Server response : multi page . . . . . . . . . . . . . . . . 7 3.7 Follow-up request . . . . . . . . . . . . . . . . . . . . . 8 3.8 Error response . . . . . . . . . . . . . . . . . . . . . . . 8 3.9 Congestion control . . . . . . . . . . . . . . . . . . . . . 9 3.10 Reserved areas . . . . . . . . . . . . . . . . . . . . . . 9 4. Security Considerations . . . . . . . . . . . . . . . . . . . 10 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 7. Normative References . . . . . . . . . . . . . . . . . . . . . 10 A. Implementation of Cookies . . . . . . . . . . . . . . . . . . 11 Authors Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Barwood Expires March 2010 [Page 2] Internet-Draft DNS Transport September 2009 1. Introduction DNSSEC implies that DNS responses may be large, possibly larger than the de facto ~1500 byte internet MTU. Large responses are a challenge for DNS transport. EDNS [RFC2671] was introduced in 1999 to allow larger reponses to be sent over UDP, previously DNS/UDP was limited to a 512 bytes. EDNS is problematic for several reasons: (1) It allows amplification attacks against 3rd parties. DNS/UDP has always been susceptible to these attacks, but EDNS has increased the amplification factor by an order of magnitude. (2) The IP protocol specifies a means by which large IP packets are split into fragments and then re-assembled. However fragmented UDP responses are undesirable for several reasons: o Fragments can easily be spoofed. The DNS ID and port number are only present in the first fragment, and the IP ID is usually easy for an attacker to predict. o In practise fragmentation is not reliable, and large UDP packets may fail to be delivered. o If a single fragment is lost, the entire response must be re-sent. o Re-assembling fragments requires buffer resources, which opens up denial of service attacks. Instead, it is possible to use TCP, but this is undesirable, as TCP imposes increased latency and significant server state that may be vulnerable to denial of service attack. In addition, support for TCP is not universal. Nearly all current DNS traffic is carried by UDP with a maximum size of 512 bytes, and relying on TCP is a risk for the deployment of DNSSEC. Therefore a new protocol to solve these problems is proposed. Barwood Expires March 2010 [Page 3] Internet-Draft DNS Transport September 2009 2. Requirements 2.1 Fragmentation As described in the introduction, fragmentation is undesirable. However, fragmentation is unavoidable if the path MTU is too small. Therefore, we require only that fragmentation does not occur provided the actual path MTU is at least the MTU sent by the client. 2.2 Spoofing Blind spoofing attacks must be prevented. 2.3 Server state No per-client server state should be needed between transactions. 2.4 Amplification attacks Amplification attacks against third parties must be prevented. 2.5 Packet re-transmission Only lost IP packets must be re-transmitted. This reduces problems due to network congestion. 2.6 Performance Each transaction ( for moderate response sizes ) must be performed in 1 RTT, after setup, provided that no IP packets are lost. Barwood Expires March 2010 [Page 4] Internet-Draft DNS Transport September 2009 3. Protocol 3.1 Overview Communication is in two stages. First a long-lived SERVERTOKEN is acquired by the client, using a standard DNS lookup. Subsequent queries are sent using a different port, and are protected by the SERVERTOKEN. Throughout, DNS Payload refers to a DNS Message [RFC1035], not including the 16-bit ID field. 3.2 Setup request The client acquires a SERVERTOKEN for a given Server IP address by sending a special question to the server, with QTYPE = TXT QCLASS = IN QNAME = .DNS.TRANSPORT.LOCAL where is a secret label chosen to prevent spoofing of the response. 3.3 Setup response The server returns a TXT record as answer to the question, which contains a string (there may be other strings) with format +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | SPORT | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SERVERTOKEN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ SPORT is a 16 bit UDP port number, to which requests are sent. SERVERTOKEN is a 32 bit value computed as a hash of the client IP Address and a long-term server secret. The TTL should be at least 1 day. The client associates SERVERTOKEN, SPORT and the client IP address ( for multi-homed clients ) with the Server IP address. If the TXT record is not returned, the server does not have support, and this fact should be cached, with a TTL of at least 1 day. If a copy of the Question is not returned, extra queries need to be sent to prevent spoofing. This should not occur - all known DNS servers return a copy of the question, except for format error responses, which should not occur. Barwood Expires March 2010 [Page 5] Internet-Draft DNS Transport September 2009 3.4 Initial request To make a DNS request, a UDP packet is sent to SPORT, with format: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | COUNT | MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SERVERTOKEN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | QUERYID | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ DATA \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where : COUNT limits the number of pages the server will send. MTU limits the size of the IP packets used to send the response. Must be at least 576 bytes. SERVERTOKEN is a copy of SERVERTOKEN from the setup response. QUERYID is a 64-bit value that identifies the request. DATA is the DNS payload. 3.5 Server response : single page The server checks SERVERTOKEN, and divides the DNS payload into equal size pages, so that the size of each IP packet is not greater than MTU. Servers should use a smaller MTU if the path MTU is less than the MTU supplied by the client. If there is only one page, the UDP response packet has format : +-+-+-+-+-+-+-+-+ | 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | QUERYID | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ DATA \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where : QUERYID is a copy of QUERYID from the request. DATA is the DNS payload. The client checks SERVERTOKEN, and then uses DATA as the normal DNS response. Barwood Expires March 2010 [Page 6] Internet-Draft DNS Transport September 2009 3.6 Server response : multi page If there is more than one page, each UDP response packet has format : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 3 | COUNT | PAGESIZE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PAGE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | TOTAL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | COOKIE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | QUERYID | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ DATA \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where : COUNT is the number of pages sent. PAGESIZE is the size into which the response has been divided. PAGE is the number of this page. TOTAL is the size of the complete DNS payload. COOKIE is used to request further pages ( see section 3.7 ). QUERYID is a copy of QUERYID from the request. DATA is part of the DNS payload. The client allocates an assembly buffer of TOTAL bytes (if not already allocated), and copies DATA into it at offset PAGE x PAGESIZE. Barwood Expires March 2010 [Page 7] Internet-Draft DNS Transport September 2009 3.7 Follow-up request If the client does not receive a page, due to packet loss or not all pages being sent, it sends a request with format : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 4 | COUNT | PAGESIZE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PAGE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | COOKIE | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SERVERTOKEN | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | QUERYID | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \ DATA \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where : COUNT is the number of pages to be sent. PAGESIZE is a copy of PAGESIZE from the server response. PAGE is the 0-based number of the first page to be sent. COOKIE is a copy of COOKIE from the server response. SERVERTOKEN is a copy of the SERVERTOKEN from the setup response. QUERYID identifies the request. DATA is a copy of DATA from the initial request. The server response is the same as in section 3.6. Once a client has received all pages, it processes the complete assembled response as normal. 3.8 Error response. If the server encounters an error condition, it sends an error response, with format : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 5 | ERRNUM | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | QUERYID | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where : Barwood Expires March 2010 [Page 8] Internet-Draft DNS Transport September 2009 ERRNUM encodes the error condition, with values 0 Invalid SERVERTOKEN. The client should acquire a new SERVERTOKEN and try again. 1 Invalid COOKIE/PAGESIZE/PAGE. The client should send a new Initial Request. SERVERTOKEN is a copy of the SERVERTOKEN from the setup response. QUERYID is a copy of QUERYID from the request. 3.9 Congestion control Clients should take into account estimated network performance when calculating the COUNT field in requests. Factors are : o The estimated round trip time (RTT). o The time to transmit (PT) 1 packet (due to limited bandwidth). COUNT should not be more than max( RTT / PT, 4 ). For example, if RTT = 150 milli-seconds, the bandwidth is 300 kilobytes per second and the packet size is 1500 bytes, then we have PT = 5 milli-seconds and COUNT = 30. In practice for current DNS purposes, COUNT can simply be set to 4, and this should be sufficient to send the complete response in a single round trip, assuming the MTU is 1500 bytes. Servers may send a smaller number of pages than requested, for policy reasons, or if there is local congestion, etc. 3.10 Reserved areas Unused variable length areas after the defined fields are reserved and must be omitted by the sender and ignored by the receiver. Barwood Expires March 2010 [Page 9] Internet-Draft DNS Transport September 2009 4. Security Considerations Fragmented responses are vulnerable to blind spoofing, therefore fragmented responses should be avoided if possible. A check must be made that the MTU is at least 584, to prevent an attacker generating a large number of IP packets from a single request. Secret values ( the long term server secret, the client secret, QUERYID ) must be generated so that an attacker cannot easily guess them, by using cryptographic hash functions and cryptographic random number generators seeded from data that cannot be guessed by an attacker, such as thermal noise or other random physical fluctuations. The hash function used to compute SERVERTOKEN must be cryptographically secure, although a relatively weak function may be sufficient, since acquiring large numbers of input/output pairs in order to deduce the long term server secret is not easy for an attacker. 5. IANA Considerations None at present. If the protocol were to be become a standard, there could be new registries for : o Strings in setup response ( transport option codes ). o Operation codes ( 1 - 5 have been used ) o Error codes 6. Acknowledgments Mark Andrews, Alex Bligh, Robert Elz, Douglas Otis, Wouter Wijngaards and Nicholas Weaver were each instrumental in creating and refining this specification. 7. Normative References [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. Barwood Expires March 2010 [Page 10] Internet-Draft DNS Transport September 2009 Appendix A : Implementation of Cookies To show how server state is avoided or limited, two possible approaches to the implementation of cookies are shown. These are illustrative, and actual implementations are of course free to take a different approach. (1) The server maintains a DNS database version number, which is incremented when the database is updated. The DNS database is stuctured so that old queries may be replayed, with the database version number being supplied as a parameter. COOKIE is simply the DNS database version number. (2) The server maintains a list of recent multi-page responses: COOKIE DATA ACCESSTIME 1 .... 10:25:11 2 .... 10:25:16 ..... If a response is multi-page, the list is checked to see if there is an existing entry that can be used ( hashing techniques are used to make the search efficient ). Entries that have not been accessed for more than 5 seconds may be deleted. Some care should be taken to ensure that on server restart, old cookie values are not re-used. Periodically, a new range of cookies should be issued, and the new allocation value recorded in permanent storage. Alternatively, the server should wait 10 seconds after restarting before issuing any cookies, or use a new long-term secret to generate SERVERTOKENs. Author's Address George Barwood 33 Sandpiper Close Gloucester GL2 4LZ United Kingdom Phone: +44 452 722670 EMail: george.barwood@blueyonder.co.uk Barwood Expires March 2010 [Page 11]