Network Working Group
Request for Comments: 2931
Updates: 2535
Category: Standards Track
D. Eastlake 3rd
September 2000

DNS Request and Transaction Signatures ( SIG(0)s )

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2000). All Rights Reserved.


Extensions to the Domain Name System (DNS) are described in [RFC 2535] that can provide data origin and transaction integrity and authentication to security aware resolvers and applications through the use of cryptographic digital signatures.

Implementation experience has indicated the need for minor but non- interoperable changes in Request and Transaction signature resource records ( SIG(0)s ). These changes are documented herein.


The contributions and suggestions of the following persons (in alphabetic order) to this memo are gratefully acknowledged:

         Olafur Gudmundsson
         Ed Lewis
         Erik Nordmark
         Brian Wellington

Table of Contents

   1. Introduction.................................................  2
   2. SIG(0) Design Rationale......................................  3
   2.1 Transaction Authentication..................................  3
   2.2 Request Authentication......................................  3
   2.3 Keying......................................................  3
   2.4 Differences Between TSIG and SIG(0).........................  4
   3. The SIG(0) Resource Record...................................  4
   3.1 Calculating Request and Transaction SIGs....................  5
   3.2 Processing Responses and SIG(0) RRs.........................  6
   3.3 SIG(0) Lifetime and Expiration..............................  7
   4. Security Considerations......................................  7
   5. IANA Considerations..........................................  7
   References......................................................  7
   Author's Address................................................  8
   Appendix: SIG(0) Changes from RFC 2535..........................  9
   Full Copyright Statement........................................ 10

1. Introduction

This document makes minor but non-interoperable changes to part of [RFC 2535], familiarity with which is assumed, and includes additional explanatory text. These changes concern SIG Resource Records (RRs) that are used to digitally sign DNS requests and transactions / responses. Such a resource record, because it has a type covered field of zero, is frequently called a SIG(0). The changes are based on implementation and attempted implementation experience with TSIG [RFC 2845] and the [RFC 2535] specification for SIG(0).

Sections of [RFC 2535] updated are all of and parts of 4.2 and 4.3. No changes are made herein related to the KEY or NXT RRs or to the processing involved with data origin and denial authentication for DNS data.

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 [RFC 2119].

2. SIG(0) Design Rationale

SIG(0) provides protection for DNS transactions and requests that is not provided by the regular SIG, KEY, and NXT RRs specified in [RFC 2535]. The authenticated data origin services of secure DNS either provide protected data resource records (RRs) or authenticatably deny their nonexistence. These services provide no protection for glue records, DNS requests, no protection for message headers on requests or responses, and no protection of the overall integrity of a response.

2.1 Transaction Authentication

Transaction authentication means that a requester can be sure it is at least getting the messages from the server it queried and that the received messages are in response to the query it sent. This is accomplished by optionally adding either a TSIG RR [RFC 2845] or, as described herein, a SIG(0) resource record at the end of the response which digitally signs the concatenation of the server's response and the corresponding resolver query.

2.2 Request Authentication

Requests can also be authenticated by including a TSIG or, as described herein, a special SIG(0) RR at the end of the request. Authenticating requests serves no function in DNS servers that predate the specification of dynamic update. Requests with a non- empty additional information section produce error returns or may even be ignored by a few such older DNS servers. However, this syntax for signing requests is defined for authenticating dynamic update requests [RFC 2136], TKEY requests [RFC 2930], or future requests requiring authentication.

2.3 Keying

The private keys used in transaction security belong to the host composing the DNS response message, not to the zone involved. Request authentication may also involve the private key of the host or other entity composing the request or of a zone to be affected by the request or other private keys depending on the request authority it is sought to establish. The corresponding public key(s) are normally stored in and retrieved from the DNS for verification as KEY RRs with a protocol byte of 3 (DNSSEC) or 255 (ANY).

Because requests and replies are highly variable, message authentication SIGs can not be pre-calculated. Thus it will be necessary to keep the private key on-line, for example in software or in a directly connected piece of hardware.

2.4 Differences Between TSIG and SIG(0)

There are significant differences between TSIG and SIG(0).

Because TSIG involves secret keys installed at both the requester and server the presence of such a key implies that the other party understands TSIG and very likely has the same key installed. Furthermore, TSIG uses keyed hash authentication codes which are relatively inexpensive to compute. Thus it is common to authenticate requests with TSIG and responses are authenticated with TSIG if the corresponding request is authenticated.

SIG(0) on the other hand, uses public key authentication, where the public keys are stored in DNS as KEY RRs and a private key is stored at the signer. Existence of such a KEY RR does not necessarily imply implementation of SIG(0). In addition, SIG(0) involves relatively expensive public key cryptographic operations that should be minimized and the verification of a SIG(0) involves obtaining and verifying the corresponding KEY which can be an expensive and lengthy operation. Indeed, a policy of using SIG(0) on all requests and verifying it before responding would, for some configurations, lead to a deadly embrace with the attempt to obtain and verify the KEY needed to authenticate the request SIG(0) resulting in additional requests accompanied by a SIG(0) leading to further requests accompanied by a SIG(0), etc. Furthermore, omitting SIG(0)s when not required on requests halves the number of public key operations required by the transaction.

For these reasons, SIG(0)s SHOULD only be used on requests when necessary to authenticate that the requester has some required privilege or identity. SIG(0)s on replies are defined in such a way as to not require a SIG(0) on the corresponding request and still provide transaction protection. For other replies, whether they are authenticated by the server or required to be authenticated by the requester SHOULD be a local configuration option.

3. The SIG(0) Resource Record

The structure of and type number of SIG resource records (RRs) is given in [RFC 2535] Section 4.1. However all of Section and the parts of Sections 4.2 and 4.3 related to SIG(0) should be considered replaced by the material below. Any conflict between [RFC 2535] and this document concerning SIG(0) RRs should be resolved in favor of this document.

For all transaction SIG(0)s, the signer field MUST be a name of the originating host and there MUST be a KEY RR at that name with the public key corresponding to the private key used to calculate the signature. (The host domain name used may be the inverse IP address mapping name for an IP address of the host if the relevant KEY is stored there.)

For all SIG(0) RRs, the owner name, class, TTL, and original TTL, are meaningless. The TTL fields SHOULD be zero and the CLASS field SHOULD be ANY. To conserve space, the owner name SHOULD be root (a single zero octet). When SIG(0) authentication on a response is desired, that SIG RR MUST be considered the highest priority of any additional information for inclusion in the response. If the SIG(0) RR cannot be added without causing the message to be truncated, the server MUST alter the response so that a SIG(0) can be included. This response consists of only the question and a SIG(0) record, and has the TC bit set and RCODE 0 (NOERROR). The client should at this point retry the request using TCP.

3.1 Calculating Request and Transaction SIGs

A DNS request may be optionally signed by including one SIG(0)s at the end of the query additional information section. Such a SIG is identified by having a "type covered" field of zero. It signs the preceding DNS request message including DNS header but not including the UDP/IP header and before the request RR counts have been adjusted for the inclusions of the request SIG(0).

It is calculated by using a "data" (see [RFC 2535], Section 4.1.8) of (1) the SIG's RDATA section entirely omitting (not just zeroing) the signature subfield itself, (2) the DNS query messages, including DNS header, but not the UDP/IP header and before the reply RR counts have been adjusted for the inclusion of the SIG(0). That is

      data = RDATA | request - SIG(0)

where "|" is concatenation and RDATA is the RDATA of the SIG(0) being calculated less the signature itself.

Similarly, a SIG(0) can be used to secure a response and the request that produced it. Such transaction signatures are calculated by using a "data" of (1) the SIG's RDATA section omitting the signature itself, (2) the entire DNS query message that produced this response, including the query's DNS header but not its UDP/IP header, and (3) the entire DNS response message, including DNS header but not the UDP/IP header and before the response RR counts have been adjusted for the inclusion of the SIG(0).

   That is

data = RDATA | full query | response - SIG(0)

where "|" is concatenation and RDATA is the RDATA of the SIG(0) being calculated less the signature itself.

Verification of a response SIG(0) (which is signed by the server host key, not the zone key) by the requesting resolver shows that the query and response were not tampered with in transit, that the response corresponds to the intended query, and that the response comes from the queried server.

In the case of a DNS message via TCP, a SIG(0) on the first data packet is calculated with "data" as above and for each subsequent packet, it is calculated as follows:

data = RDATA | DNS payload - SIG(0) | previous packet

where "|" is concatenations, RDATA is as above, and previous packet is the previous DNS payload including DNS header and the SIG(0) but not the TCP/IP header. Support of SIG(0) for TCP is OPTIONAL. As an alternative, TSIG may be used after, if necessary, setting up a key with TKEY [RFC 2930].

Except where needed to authenticate an update, TKEY, or similar privileged request, servers are not required to check a request SIG(0).

Note: requests and responses can either have a single TSIG or one SIG(0) but not both a TSIG and a SIG(0).

3.2 Processing Responses and SIG(0) RRs

If a SIG RR is at the end of the additional information section of a response and has a type covered of zero, it is a transaction signature covering the response and the query that produced the response. For TKEY responses, it MUST be checked and the message rejected if the checks fail unless otherwise specified for the TKEY mode in use. For all other responses, it MAY be checked and the message rejected if the checks fail.

If a response's SIG(0) check succeed, such a transaction authentication SIG does NOT directly authenticate the validity any data-RRs in the message. However, it authenticates that they were sent by the queried server and have not been diddled. (Only a proper SIG(0) RR signed by the zone or a key tracing its authority to the zone or to static resolver configuration can directly authenticate data-RRs, depending on resolver policy.) If a resolver or server does not implement transaction and/or request SIGs, it MUST ignore them without error where they are optional and treat them as failing where they are required.

3.3 SIG(0) Lifetime and Expiration

The inception and expiration times in SIG(0)s are for the purpose of resisting replay attacks. They should be set to form a time bracket such that messages outside that bracket can be ignored. In IP networks, this time bracket should not normally extend further than 5 minutes into the past and 5 minutes into the future.

4. Security Considerations

No additional considerations beyond those in [RFC 2535].

The inclusion of the SIG(0) inception and expiration time under the signature improves resistance to replay attacks.

5. IANA Considerations

No new parameters are created or parameter values assigned by this document.


   [RFC 1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              September 1996.
   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   [RFC 2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic
              Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
              April 1997.
   [RFC 2535] Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

[RFC 2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B.

Wellington, "Secret Key Transaction Signatures for DNS (TSIG)", RFC 2845, May 2000.

   [RFC 2930] Eastlake, D., "Secret Key Establishment for DNS (RR)", RFC
              2930, September 2000.

Author's Address

Donald E. Eastlake 3rd
140 Forest Avenue
Hudson, MA 01749 USA

   Phone: +1-978-562-2827(h)
   Fax:   +1 978-567-7941(h)

Appendix: SIG(0) Changes from RFC 2535

Add explanatory text concerning the differences between TSIG and SIG(0).

Change the data over which SIG(0) is calculated to include the SIG(0) RDATA other than the signature itself so as to secure the signature inception and expiration times and resist replay attacks. Specify SIG(0) for TCP.

Add discussion of appropriate inception and expiration times for SIG(0).

Add wording to indicate that either a TSIG or one or more SIG(0)s may be present but not both.

Reword some areas for clarity.

Full Copyright Statement

Copyright © The Internet Society (2000). All Rights Reserved.

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