DNSEXT R. Bellis Internet-Draft Nominet UK Updates: 1035, 1123 January 6, 2010 (if approved) Intended status: Standards Track Expires: July 10, 2010 DNS Transport over TCP - Implementation Requirements draft-ietf-dnsext-dns-tcp-requirements-02 Abstract This document updates the requirements for the support of TCP as a transport protocol for DNS implementations. 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 July 10, 2010. Copyright Notice Copyright (c) 2010 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents Bellis Expires July 10, 2010 [Page 1]
Internet-Draft DNS over TCP January 2010 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 BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology used in this document . . . . . . . . . . . . . . . 3 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4 5. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5 6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . . 7 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Bellis Expires July 10, 2010 [Page 2]
Internet-Draft DNS over TCP January 2010 1. Introduction Most DNS [RFC1035] transactions take place over UDP [RFC0792]. The TCP [RFC0793] is used for zone transfers and 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. However, some implementors have taken the text quoted above to mean that TCP support is an optional feature of the DNS protocol. The majority of DNS server operators already support TCP and the default configuration for most software implementations is to support TCP. The primary audience for this document is those implementors whose failure to support TCP restricts interoperability and limits deployment of new DNS features. This document therefore updates the core DNS protocol specifications such that support for TCP is henceforth a REQUIRED part of a full DNS protocol implementation. Whilst this document makes no specific recommendations to operators of DNS servers, it should be noted that failure to support TCP (or blocking of DNS over TCP at the network layer) may result in resolution failure and application-level timeouts. 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 In the absence of EDNS0 (see below) the normal behaviour of any DNS server needing to send a UDP response that exceeds that 512 byte limit is for the server to truncate the response so that it fits within 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 an indication that it should retry over TCP instead. RFC 1123 also says: Bellis Expires July 10, 2010 [Page 3]
Internet-Draft DNS over TCP January 2010 ... 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 that exceed the size of the path MTU causes IP packet fragmentation, which has been found to be unreliable in some circumstances. 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 has arrived, and the only standardised UDP-based mechanism which may have resolved the packet size issue has been found inadequate. 4. Transport Protocol Selection All DNS implementations MUST support both UDP and TCP transport. o Authoritative resolver implementations MUST support TCP so that they may serve any long responses that they are configured to serve. Bellis Expires July 10, 2010 [Page 4]
Internet-Draft DNS over TCP January 2010 o 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. o 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. An exception may be made for proprietary stub resolver implementations. These MAY omit support for TCP if operating in an environment where truncation can never occur, or where DNS lookup failure is acceptable should truncation occur. 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. That requirement is hereby relaxed. 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 (with or without EDNS0) or for other operational reasons, in particular if it already has an open TCP connection to the server. 5. 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 application-level idle period should be of the order of TBD seconds. Servers MAY allow dormant 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. Bellis Expires July 10, 2010 [Page 5]
Internet-Draft DNS over TCP January 2010 To mitigate the risk of unintentional server overload DNS clients MUST take care to minimize the number of concurrent TCP connections made to any individual server. Further recommendations for the tuning of TCP parameters to allow higher throughput or improved resiliency against denial of service attacks are outside the scope of this document. 6. Response re-ordering RFC 1035 is ambiguous on the question of whether TCP queries may be re-ordered - the only relevant text is in Section 4.2.1 which relates to UDP: Queries or their responses may be reordered by the network, or by processing in name servers, so resolvers should not depend on them being returned in order. For the avoidance of future doubt, this requirement is clarified. Client resolvers MUST be able to process responses which arrive in a different order to that in which the requests were sent, regardless of the transport protocol in use. 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" (DoS) attacks. Whilst there is a theoretically higher risk of such attacks against TCP-enabled servers, techniques for the mitigation of DoS attacks at the network level have improved substantially since DNS was first designed. The vast majority of TLD authority servers and all but one of the root name servers already support TCP and the author knows of no evidence to suggest that TCP-based DoS attacks against existing DNS infrastructure are commonplace. 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. Bellis Expires July 10, 2010 [Page 6]
Internet-Draft DNS over TCP January 2010 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, 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. Bellis Expires July 10, 2010 [Page 7]
Internet-Draft DNS over TCP January 2010 Appendix A. Change Log NB: to be removed by the RFC Editor before publication. draft-ietf-dnsext-dns-tcp-requirements-02 Change of title - more focus on implementation and not operation Re-write of some of the security section Added recommendation for minimal concurrent connections Minor editorial nits from Alfred Hoenes draft-ietf-dnsext-dns-tcp-requirements-01 Addition of response ordering section Various minor editorial changes from WG reviewers draft-ietf-dnsext-dns-tcp-requirements-00 Initial draft 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 July 10, 2010 [Page 8]
