DNSEXT R. Bellis Internet-Draft Nominet UK Updates: 1123, 1035 October 6, 2009 (if approved) Intended status: Standards Track Expires: April 9, 2010 DNS Transport over TCP draft-ietf-dnsext-dns-tcp-requirements-00 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 April 9, 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 This document updates the requirements for the support of the TCP Bellis Expires April 9, 2010 [Page 1]
Internet-Draft DNS Transport over TCP October 2009 protocol for the transport of DNS traffic. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology used in this document . . . . . . . . . . . . . . . 3 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4 5. Dormant Connection Handling . . . . . . . . . . . . . . . . . . 5 6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . . 6 9.2. Informative References . . . . . . . . . . . . . . . . . . 7 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Bellis Expires April 9, 2010 [Page 2]
Internet-Draft DNS Transport over TCP October 2009 1. Introduction Most DNS [RFC1035] transactions take place over the UDP [RFC0792] protocol. The TCP [RFC0793] protocol is used for zone transfers and is supported by some implementations for the transfer of other packets which exceed the protocol's original 512 byte packet-size limit. Section 6.1.3.2 of [RFC1123] states: DNS resolvers and recursive servers MUST support UDP, and SHOULD support TCP, for sending (non-zone-transfer) queries. This document normatively updates the core DNS protocol specifications such that (except in very limited circumstances) support for the TCP protocol is henceforth REQUIRED. 2. Terminology used in this document 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]. 3. Discussion Some implementors have taken the [RFC1123] text quoted above to mean that TCP support is truly optional for typical DNS operation. However, whilst RFC 1123 predates the current RFC 2119 terminology document it uses exactly the same text: SHOULD - This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course. In the absence of EDNS0 (see below) the normal behaviour of any DNS server needing to send a UDP response that exceeds that 512 limit is for the server to truncate the response at the 512 byte limit and set the TC flag in the response header. When the client receives such a response it takes the TC flag as notice that it should retry over TCP instead. RFC 1123 also says: Bellis Expires April 9, 2010 [Page 3]
Internet-Draft DNS Transport over TCP October 2009 ... it is also clear that some new DNS record types defined in the future will contain information exceeding the 512 byte limit that applies to UDP, and hence will require TCP. Thus, resolvers and name servers should implement TCP services as a backup to UDP today, with the knowledge that they will require the TCP service in the future. Existing deployments of DNSSEC [RFC4033] have shown that truncation at the 512 byte boundary is now commonplace. For example an NXDOMAIN (RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155] is almost invariably longer than 512 bytes. Since the original core specifications for DNS were written the Extension Mechanisms for DNS EDNS0 [RFC2671] have been introduced. These extensions can be used to indicate that the client is prepared to receive UDP responses longer than 512 bytes. An EDNS0 compatible server receiving a request from an EDNS0 compatible client may send UDP packets up to that client's announced buffer size without truncation. However, transport of UDP packets which exceed the size of the path MTU has been found to be unreliable in some circumstances because of IP packet fragmentation. Many firewalls routinely block fragmented IP packets, and some implementations lack the software logic necessary to reassemble a fragmented datagram. Worse still, some devices deliberately refuse to handle DNS packets containing EDNS0 options. Other issues relating to UDP transport and packet size are discussed in [RFC5625]. The MTU most commonly found in the core of the Internet is around 1500 bytes, and even that limit is routinely exceeded by DNSSEC signed responses. The future that was anticipated in RFC 1123 is now here, and the only standardised mechanism which may have resolved the packet size issue has been found inadequate. 4. Transport Protocol Selection On a case by case basis, authoritative DNS server operators MAY elect to disable DNS transport over TCP if all of the conditions below are satisfied: o the server is authoritative Bellis Expires April 9, 2010 [Page 4]
Internet-Draft DNS Transport over TCP October 2009 o the server does not support AXFR o the server does not support DNSSEC o all requests and responses are guaranteed to be <= 512 bytes A general purpose stub resolver implementation (e.g. an operating system's DNS resolution library) MUST support TCP since to do otherwise would limit its interoperability with its own clients and with upstream servers. A proprietary stub resolver implementation MAY omit support for TCP if it is operating in an environment where truncation will not occur, or if it is prepared to accept a DNS lookup failure should truncation occur. A recursive resolver or forwarder MUST support TCP so that it does not prevent long responses from a TCP-capable server from reaching its TCP-capable clients. Otherwise, all DNS implementations MUST support TCP transport. Regarding the choice of when to use UDP or TCP, RFC 1123 says: ... a DNS resolver or server that is sending a non-zone-transfer query MUST send a UDP query first. This requirement is no longer mandatory. A resolver SHOULD send a UDP query first, but MAY elect to send a TCP query instead if it has good reason to expect the response would be truncated if it were sent over UDP, or other operational considerations suggest otherwise. 5. Dormant Connection Handling Section 4.2.2 of [RFC1035] says: If the server needs to close a dormant connection to reclaim resources, it should wait until the connection has been idle for a period on the order of two minutes. Other more modern protocols (e.g. HTTP [RFC2616]) have support for persistent TCP connections and operational experience has shown that long timeouts can easily cause resource exhaustion and poor response under heavy load. Intentionally opening many connections and leaving them dormant can trivially create a "denial of service" attack. This document therefore RECOMMENDS that the idle period should be of the order of TBD seconds. With modern high performance networks 2 to 4 seconds should be sufficient to allow significant numbers (i.e. Bellis Expires April 9, 2010 [Page 5]
Internet-Draft DNS Transport over TCP October 2009 thousands) of concurrent dormant connections without impacting service performance. Servers MAY allow idle connections to remain open for longer periods, but for the avoidance of doubt persistent DNS connections should generally be considered to be as much for the server's benefit as for the client's. Therefore if the server needs to unilaterally close a dormant TCP connection it MUST be free to do so whenever required. 6. Response re-ordering [Potential text to be added regarding whether TCP responses can come back in a different order to requests. I'm not aware whether this is specified anywhere] 7. Security Considerations Some DNS server operators have expressed concern that wider use of DNS over TCP will expose them to a higher risk of "denial of service" attacks. Many large authoritative DNS operators including all but one of the root servers and the vast majority of TLDs already support TCP and attacks against them are infrequent and very rarely successful. Operators of recursive servers should ensure that they only accept connections from expected clients, and do not accept them from unknown sources. In the case of UDP traffic this will protect against reflector attacks [RFC5358] and in the case of TCP traffic it will prevent an unknown client from exhausting the server's limits on the number of concurrent connections. 8. IANA Considerations This document requests no IANA actions. 9. References 9.1. Normative References [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, Bellis Expires April 9, 2010 [Page 6]
Internet-Draft DNS Transport over TCP October 2009 RFC 793, September 1981. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. 9.2. Informative References [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008. [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive Nameservers in Reflector Attacks", BCP 140, RFC 5358, October 2008. [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP 152, RFC 5625, August 2009. Appendix A. Change Log NB: to be removed by the RFC Editor before publication. draft-ietf-dnsext-dns-tcp-requirements-00 Initial draft Bellis Expires April 9, 2010 [Page 7]
Internet-Draft DNS Transport over TCP October 2009 Author's Address Ray Bellis Nominet UK Edmund Halley Road Oxford OX4 4DQ United Kingdom Phone: +44 1865 332211 Email: ray.bellis@nominet.org.uk URI: http://www.nominet.org.uk/ Bellis Expires April 9, 2010 [Page 8]
