DNS Transport over TCP
draft-ietf-dnsext-dns-tcp-requirements-01

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Abstract

This document updates the requirements for the support of the TCP protocol for the transport of DNS traffic.

Table of Contents

1.  Introduction

2.  Terminology used in this document

3.  Discussion

4.  Transport Protocol Selection

5.  Dormant Connection Handling

6.  Response re-ordering

7.  Security Considerations

8.  IANA Considerations

9.  References
    9.1.  Normative References
    9.2.  Informative References

Appendix A.  Change Log

§  Author's Address

1.  Introduction

Most DNS (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) [RFC1035] transactions take place over the UDP (Postel, J., “Internet Control Message Protocol,” September 1981.) [RFC0792] protocol. The TCP (Postel, J., “Transmission Control Protocol,” September 1981.) [RFC0793] protocol is used for zone transfers and is supported by many 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] (Braden, R., “Requirements for Internet Hosts - Application and Support,” October 1989.) 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 truly optional for typical DNS operation.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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 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:

... 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 (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [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 (Laurie, B., Sisson, G., Arends, R., and D. Blacka, “DNS Security (DNSSEC) Hashed Authenticated Denial of Existence,” March 2008.) [RFC5155] is almost invariably longer than 512 bytes.

Since the original core specifications for DNS were written, the Extension Mechanisms for DNS (EDNS0 (Vixie, P., “Extension Mechanisms for DNS (EDNS0),” August 1999.) [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] (Bellis, R., “DNS Proxy Implementation Guidelines,” August 2009.).

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 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 protocols, except as set out below.

On a case by case basis, authoritative DNS server operators MAY elect to disable DNS transport over TCP if all of the following conditions are satisfied:

• the server is authoritative only
• the server does not support AXFR
• 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 can never 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.

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.

5.  Dormant Connection Handling

Section 4.2.2 of [RFC1035] (Mockapetris, P., “Domain names - implementation and specification,” November 1987.) 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 (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.) [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.

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.

Further recommendations for the tuning of TCP parameters to allow higher throughput or improved resiliency against denial of service attacks are (currently) 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" 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 (Damas, J. and F. Neves, “Preventing Use of Recursive Nameservers in Reflector Attacks,” October 2008.) [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

9.2. Informative References

Appendix A.  Change Log

NB: to be removed by the RFC Editor before publication.

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