This document is obsolete. Please
refer to RFC 7766.
Internet Engineering Task Force (IETF) R. Bellis Request for Comments: 5966 Nominet UK Updates: 1035, 1123 August 2010 Category: Standards Track ISSN: 2070-1721
DNS Transport over TCP - Implementation Requirements
This document updates the requirements for the support of TCP as a transport protocol for DNS implementations.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
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Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP [RFC0793] is always used for zone transfers and is often used for messages whose sizes exceed the DNS protocol's original 512-byte limit.
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 the blocking of DNS over TCP at the network layer) may result in resolution failure and/or application-level timeouts.
In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below), the normal behaviour of any DNS server needing to send a UDP response that would exceed the 512-byte limit is for the server to truncate the response so that it fits within that limit and then 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.
... 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 DNS Security (DNSSEC) [RFC4033] have shown that truncation at the 512-byte boundary is now commonplace. For example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost invariably larger 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 larger 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 do not implement the algorithms necessary to reassemble fragmented packets. Worse still, some network devices deliberately refuse to handle DNS packets containing EDNS0 options. Other issues relating to UDP transport and packet size are discussed in [RFC5625].
Bellis Standards Track [Page 3]
RFC 5966 DNS over TCP August 2010
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 that may have resolved the packet size issue has been found inadequate.
All general-purpose DNS implementations MUST support both UDP and TCP transport.
o Authoritative server implementations MUST support TCP so that they do not limit the size of responses to what fits in a single UDP packet.
o Recursive server (or forwarder) implementations MUST support TCP so that they do not prevent large responses from a TCP-capable server from reaching its TCP-capable clients.
o Stub resolver implementations (e.g., an operating system's DNS resolution library) MUST support TCP since to do otherwise would limit their interoperability with their own clients and with upstream servers.
Stub resolver implementations MAY omit support for TCP when specifically designed for deployment in restricted environments where truncation can never occur or where truncated DNS responses are acceptable.
... 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.
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. In particular, the server should allow the SOA and AXFR request sequence (which begins a refresh operation) to be made on a single connection. Since the server would be unable to answer queries anyway, a unilateral close or reset may be used instead of a graceful close.
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.
It is therefore RECOMMENDED that the default application-level idle period should be of the order of seconds, but no particular value is specified. In practise, the idle period may vary dynamically, and servers MAY allow dormant connections to remain open for longer periods as resources permit.
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. Similarly, servers MAY impose limits on the number of concurrent TCP connections being handled for any particular client.
Further recommendations for the tuning of TCP stacks to allow higher throughput or improved resiliency against denial-of-service attacks are outside the scope of this document.
RFC 1035 is ambiguous on the question of whether TCP queries may be reordered -- 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 that arrive in a different order to that in which the requests were sent, regardless of the transport protocol in use.
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.
Although there is a 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.
At the time of writing, the vast majority of Top Level Domain (TLD) authority servers and all of the root name servers support TCP and the author knows of no evidence to suggest that TCP-based DoS attacks against existing DNS infrastructure are commonplace.
That notwithstanding, readers are advised to familiarise themselves with [CPNI-TCP].
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 help 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.
The author would like to thank the document reviewers from the DNSEXT Working Group, and in particular, George Barwood, Alex Bligh, Alfred Hoenes, Fernando Gont, Olafur Gudmondsson, Jim Reid, Paul Vixie, and Nicholas Weaver.