Internet Engineering Task Force F. Dupont Internet-Draft S. Morris Obsoletes: 2845, 4635 (if approved) ISC Intended status: Standards Track P. Vixie Expires: August 23, 2020 Farsight D. Eastlake 3rd Futurewei O. Gudmundsson Cloudflare B. Wellington Akamai February 20, 2020 Secret Key Transaction Authentication for DNS (TSIG) draft-ietf-dnsop-rfc2845bis-07 Abstract This document describes a protocol for transaction level authentication using shared secrets and one way hashing. It can be used to authenticate dynamic updates as coming from an approved client, or to authenticate responses as coming from an approved name server. No recommendation is made here for distributing the shared secrets: it is expected that a network administrator will statically configure name servers and clients using some out of band mechanism. This document obsoletes RFC2845 and RFC4635. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on August 23, 2020. Dupont, et al. Expires August 23, 2020 [Page 1] Internet-Draft DNS TSIG February 2020 Copyright Notice Copyright (c) 2020 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents 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 Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 4 1.3. Document History . . . . . . . . . . . . . . . . . . . . 4 2. Key Words . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Assigned Numbers . . . . . . . . . . . . . . . . . . . . . . 5 4. TSIG RR Format . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. TSIG RR Type . . . . . . . . . . . . . . . . . . . . . . 5 4.2. TSIG Record Format . . . . . . . . . . . . . . . . . . . 6 4.3. MAC Computation . . . . . . . . . . . . . . . . . . . . . 8 4.3.1. Request MAC . . . . . . . . . . . . . . . . . . . . . 8 4.3.2. DNS Message . . . . . . . . . . . . . . . . . . . . . 9 4.3.3. TSIG Variables . . . . . . . . . . . . . . . . . . . 9 5. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 10 5.1. Generation of TSIG on Requests . . . . . . . . . . . . . 10 5.2. Server Processing of Request . . . . . . . . . . . . . . 10 5.2.1. Key Check and Error Handling . . . . . . . . . . . . 11 5.2.2. MAC Check and Error Handling . . . . . . . . . . . . 11 5.2.3. Time Check and Error Handling . . . . . . . . . . . . 12 Dupont, et al. Expires August 23, 2020 [Page 2] Internet-Draft DNS TSIG February 2020 5.2.4. Truncation Check and Error Handling . . . . . . . . . 12 5.3. Generation of TSIG on Answers . . . . . . . . . . . . . . 13 5.3.1. TSIG on Zone Transfer Over a TCP Connection . . . . . 13 5.3.2. Generation of TSIG on Error Returns . . . . . . . . . 14 5.4. Client Processing of Answer . . . . . . . . . . . . . . . 14 5.4.1. Key Error Handling . . . . . . . . . . . . . . . . . 15 5.4.2. MAC Error Handling . . . . . . . . . . . . . . . . . 15 5.4.3. Time Error Handling . . . . . . . . . . . . . . . . . 15 5.4.4. Truncation Error Handling . . . . . . . . . . . . . . 15 5.5. Special Considerations for Forwarding Servers . . . . . . 16 6. Algorithms and Identifiers . . . . . . . . . . . . . . . . . 16 7. TSIG Truncation Policy . . . . . . . . . . . . . . . . . . . 17 8. Shared Secrets . . . . . . . . . . . . . . . . . . . . . . . 17 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 10. Security Considerations . . . . . . . . . . . . . . . . . . . 18 10.1. Issue Fixed in this Document . . . . . . . . . . . . . . 19 10.2. Why not DNSSEC? . . . . . . . . . . . . . . . . . . . . 20 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 11.1. Normative References . . . . . . . . . . . . . . . . . . 20 11.2. Informative References . . . . . . . . . . . . . . . . . 21 Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 23 Appendix B. Change History (to be removed before publication) . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26 1. Introduction 1.1. Background The Domain Name System (DNS, [RFC1034], [RFC1035]) is a replicated hierarchical distributed database system that provides information fundamental to Internet operations, such as name to address translation and mail handling information. This document specifies use of a message authentication code (MAC), generated using certain keyed hash functions, to provide an efficient means of point-to-point authentication and integrity checking for DNS transactions. Such transactions include DNS update requests and responses for which this can provide a lightweight alternative to the secure DNS dynamic update protocol described by [RFC3007]. A further use of this mechanism is to protect zone transfers. In this case the data covered would be the whole zone transfer including any glue records sent. The protocol described by DNSSEC ([RFC4033], [RFC4034], [RFC4035]) does not protect glue records and unsigned records unless SIG(0) (transaction signature) is used. The authentication mechanism proposed in this document uses shared secret keys to establish a trust relationship between two entities. Dupont, et al. Expires August 23, 2020 [Page 3] Internet-Draft DNS TSIG February 2020 Such keys must be protected in a manner similar to private keys, lest a third party masquerade as one of the intended parties (by forging the MAC). There was a need to provide simple and efficient authentication between clients and local servers and this proposal addresses that need. The proposal is unsuitable for general server to server authentication for servers which speak with many other servers, since key management would become unwieldy with the number of shared keys going up quadratically. But it is suitable for many resolvers on hosts that only talk to a few recursive servers. 1.2. Protocol Overview Secret Key Transaction Authentication makes use of signatures on messages sent between the parties involved (e.g. resolver and server). These are known as "transaction signatures", or TSIG. For historical reasons, in this document they are referred to as message authentication codes (MAC). Use of TSIG presumes prior agreement between the two parties involved (e.g., resolver and server) as to any algorithm and key to be used. The way that this agreement is reached is outside the scope of the document. A DNS message exchange involves the sending of a query and the receipt of one of more DNS messages in response. For the query, the MAC is calculated based on the hash of the contents and the agreed TSIG key. The MAC for the response is similar, but also includes the MAC of the query as part of the calculation. Where a response comprises multiple packets, the calculation of the MAC associated with the second and subsequent packets includes in its inputs the MAC for the preceding packet. In this way it is possible to detect any interruption in the packet sequence. The MAC is contained in a TSIG resource record included in the Additional Section of the DNS message. 1.3. Document History TSIG was originally specified by [RFC2845]. In 2017, two nameservers strictly following that document (and the related [RFC4635]) were discovered to have security problems related to this feature. The implementations were fixed but, to avoid similar problems in the future, the two documents were updated and merged, producing this revised specification for TSIG. While TSIG implemented according to this RFC provides for enhanced security, there are no changes in interoperability. TSIG is on the wire still the same mechanism described in [RFC2845]; only the Dupont, et al. Expires August 23, 2020 [Page 4] Internet-Draft DNS TSIG February 2020 checking semantics have been changed. See Section 10.1 for further details. 2. Key Words The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 3. Assigned Numbers This document defines the following RR type and associated value: TSIG (250) In addition, the document also defines the following DNS RCODEs and associated names: 16 (BADSIG) 17 (BADKEY) 18 (BADTIME) 22 (BADTRUNC) (See [RFC6895] Section 2.3 concerning the assignment of the value 16 to BADSIG.) These RCODES may appear within the "Error" field of a TSIG RR. 4. TSIG RR Format 4.1. TSIG RR Type To provide secret key authentication, we use an RR type whose mnemonic is TSIG and whose type code is 250. TSIG is a meta-RR and MUST NOT be cached. TSIG RRs are used for authentication between DNS entities that have established a shared secret key. TSIG RRs are dynamically computed to cover a particular DNS transaction and are not DNS RRs in the usual sense. As the TSIG RRs are related to one DNS request/response, there is no value in storing or retransmitting them, thus the TSIG RR is discarded once it has been used to authenticate a DNS message. Dupont, et al. Expires August 23, 2020 [Page 5] Internet-Draft DNS TSIG February 2020 4.2. TSIG Record Format The fields of the TSIG RR are described below. As is usual, all multi-octet integers in the record are sent in network byte order (see [RFC1035] 2.3.2). NAME The name of the key used, in domain name syntax. The name should reflect the names of the hosts and uniquely identify the key among a set of keys these two hosts may share at any given time. For example, if hosts A.site.example and B.example.net share a key, possibilities for the key name include .A.site.example, .B.example.net, and .A.site.example.B.example.net. It should be possible for more than one key to be in simultaneous use among a set of interacting hosts. The name may be used as a local index to the key involved and it is recommended that it be globally unique. Where a key is just shared between two hosts, its name actually need only be meaningful to them but it is recommended that the key name be mnemonic and incorporates the names of participating agents or resources as suggested above. TYPE This MUST be TSIG (250: Transaction SIGnature) CLASS This MUST be ANY TTL This MUST be 0 RdLen (variable) RDATA The RDATA for a TSIG RR consists of a number of fields, described below: Dupont, et al. Expires August 23, 2020 [Page 6] Internet-Draft DNS TSIG February 2020 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / Algorithm Name / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Time Signed +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Fudge | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MAC Size | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MAC / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Original ID | Error | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Other Len | / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Other Data / / / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The contents of the RDATA fields are: * Algorithm Name - a octet sequence identifying the TSIG algorithm name in the domain name syntax. (Allowed names are listed in Table 1.) The name is stored in the DNS name wire format as described in [RFC1034]. As per [RFC3597], this name MUST NOT be compressed. * Time Signed - an unsigned 48-bit integer containing the time signed as seconds since 00:00 on 1970-01-01 UTC, ignoring leap seconds. * Fudge - an unsigned 16-bit integer specifying the allowed time difference in seconds permitted in the Time Signed field. * MAC Size - an unsigned 16-bit integer giving the length of MAC field in octets. Truncation is indicated by a MAC size less than the size of the keyed hash produced by the algorithm specified by the Algorithm Name. * MAC - a sequence of octets whose contents are defined by the TSIG algorithm used, possibly truncated as specified by MAC Size. The length of this field is given by the Mac Size. Calculation of the MAC is detailed in Section 4.3. * Original ID - An unsigned 16-bit integer holding the message ID of the original request message. For a TSIG RR on a Dupont, et al. Expires August 23, 2020 [Page 7] Internet-Draft DNS TSIG February 2020 request, it is set equal to the DNS message ID. In a TSIG attached to a response - or in cases such as the forwarding of a dynamic update request - the field contains the ID of the original DNS request. * Error - an unsigned 16-bit integer containing the extended RCODE covering TSIG processing. * Other Len - an unsigned 16-bit integer specifying the length of the "Other Data" field in octets. * Other Data - this unsigned 48-bit integer field will be empty unless the content of the Error field is BADTIME, in which case it will contain the server's current time as the number of seconds since 00:00 on 1970-01-01 UTC, ignoring leap seconds (see Section 5.2.3). 4.3. MAC Computation When generating or verifying the contents of a TSIG record, the data listed in the rest of this section are passed, in the order listed below, as input to MAC computation. The data are passed in network byte order or wire format, as appropriate, and are fed into the hashing function as a continuous octet sequence with no interfield separator or padding. 4.3.1. Request MAC Only included in the computation of a MAC for a response message (or the first message in a multi-message response), the validated request MAC MUST be included in the MAC computation. If the request MAC failed to validate, an unsigned error message MUST be returned instead. (Section 5.3.2). The request's MAC, comprising the following fields, is digested in wire format: Field Type Description ---------- ----------------------- ---------------------- MAC Length Unsigned 16-bit integer in network byte order MAC Data octet sequence exactly as transmitted Special considerations apply to the TSIG calculation for the second and subsequent messages a response that consists of multiple DNS messages (e.g. a zone transfer). These are described in Section 5.3.1. Dupont, et al. Expires August 23, 2020 [Page 8] Internet-Draft DNS TSIG February 2020 4.3.2. DNS Message A whole and complete DNS message in wire format. When creating a TSIG, this is the message before the TSIG RR has been added to the additional data section and before the DNS Message Header's ARCOUNT field has been incremented to contain the TSIG RR. When verifying an incoming message, this is the message after the TSIG RR and been removed and the ARCOUNT field has been decremented. If the message ID differs from the original message ID, the original message ID is substituted for the message ID. (This could happen, for example, when forwarding a dynamic update request.) 4.3.3. TSIG Variables Also included in the digest is certain information present in the TSIG RR. Adding this data provides further protection against an attempt to interfere with the message. Source Field Name Notes ---------- -------------- ----------------------------------------- TSIG RR NAME Key name, in canonical wire format TSIG RR CLASS (Always ANY in the current specification) TSIG RR TTL (Always 0 in the current specification) TSIG RDATA Algorithm Name in canonical wire format TSIG RDATA Time Signed in network byte order TSIG RDATA Fudge in network byte order TSIG RDATA Error in network byte order TSIG RDATA Other Len in network byte order TSIG RDATA Other Data exactly as transmitted The RR RDLEN and RDATA MAC Length are not included in the input to MAC computation since they are not guaranteed to be knowable before the MAC is generated. The Original ID field is not included in this section, as it has already been substituted for the message ID in the DNS header and hashed. For each label type, there must be a defined "Canonical wire format" that specifies how to express a label in an unambiguous way. For label type 00, this is defined in [RFC4034] Section 6.1. The use of label types other than 00 is not defined for this specification. Dupont, et al. Expires August 23, 2020 [Page 9] Internet-Draft DNS TSIG February 2020 4.3.3.1. Time Values Used in TSIG Calculations The data digested includes the two timer values in the TSIG header in order to defend against replay attacks. If this were not done, an attacker could replay old messages but update the "Time Signed" and "Fudge" fields to make the message look new. This data is named "TSIG Timers", and for the purpose of MAC calculation, they are hashed in their "on the wire" format, in the following order: first Time Signed, then Fudge. 5. Protocol Details 5.1. Generation of TSIG on Requests Once the outgoing record has been constructed, the client performs the keyed hash (HMAC) computation, appends a TSIG record with the calculated MAC to the Additional Data section (incrementing the ARCOUNT to reflect the additional RR), and transmits the request to the server. This TSIG record MUST be the only TSIG RR in the message and MUST be last record in the Additional Data section. The client MUST store the MAC and the key name from the request while awaiting an answer. The digest components for a request are: DNS Message (request) TSIG Variables (request) Note that some older name servers will not accept requests with a nonempty additional data section. Clients SHOULD only attempt signed transactions with servers who are known to support TSIG and share some algorithm and secret key with the client -- so, this is not a problem in practice. 5.2. Server Processing of Request If an incoming message contains a TSIG record, it MUST be the last record in the additional section. Multiple TSIG records are not allowed. If multiple TSIG records are detected or a TSIG record is present in any other position, the DNS message is dropped and a response with RCODE 1 (FORMERR) MUST be returned. Upon receipt of a message with exactly one correctly placed TSIG RR, the TSIG RR is copied to a safe location, removed from the DNS Message, and decremented out of the DNS message header's ARCOUNT. If the TSIG RR cannot be understood, the server MUST regard the message as corrupt and return a FORMERR to the server. Otherwise the server is REQUIRED to return a TSIG RR in the response. Dupont, et al. Expires August 23, 2020 [Page 10] Internet-Draft DNS TSIG February 2020 To validate the received TSIG RR, the server MUST perform the following checks in the following order: 1. Check KEY 2. Check MAC 3. Check TIME values 4. Check Truncation policy 5.2.1. Key Check and Error Handling If a non-forwarding server does not recognize the key or algorithm used by the client (or recognises the algorithm but does not implement it), the server MUST generate an error response with RCODE 9 (NOTAUTH) and TSIG ERROR 17 (BADKEY). This response MUST be unsigned as specified in Section 5.3.2. The server SHOULD log the error. (Special considerations apply to forwarding servers, see Section 5.5.) 5.2.2. MAC Check and Error Handling Using the information in the TSIG, the server should verify the MAC by doing its own calculation and comparing the result with the MAC received. If the MAC fails to verify, the server MUST generate an error response as specified in Section 5.3.2 with RCODE 9 (NOTAUTH) and TSIG ERROR 16 (BADSIG). This response MUST be unsigned as specified in Section 5.3.2. The server SHOULD log the error. 5.2.2.1. MAC Truncation When space is at a premium and the strength of the full length of a MAC is not needed, it is reasonable to truncate the keyed hash and use the truncated value for authentication. HMAC SHA-1 truncated to 96 bits is an option available in several IETF protocols, including IPsec and TLS. Processing of a truncated MAC follows these rules: 1. If "MAC size" field is greater than keyed hash output length: This case MUST NOT be generated and, if received, MUST cause the DNS message to be dropped and RCODE 1 (FORMERR) to be returned. 2. If "MAC size" field equals keyed hash output length: The entire output keyed hash output is present and used. 3. "MAC size" field is less than the larger of 10 (octets) and half the length of the hash function in use: Dupont, et al. Expires August 23, 2020 [Page 11] Internet-Draft DNS TSIG February 2020 With the exception of certain TSIG error messages described in Section 5.3.2, where it is permitted that the MAC size be zero, this case MUST NOT be generated and, if received, MUST cause the DNS message to be dropped and RCODE 1 (FORMERR) to be returned. 4. Otherwise: This is sent when the signer has truncated the keyed hash output to an allowable length, as described in [RFC2104], taking initial octets and discarding trailing octets. TSIG truncation can only be to an integral number of octets. On receipt of a DNS message with truncation thus indicated, the locally calculated MAC is similarly truncated and only the truncated values are compared for authentication. The request MAC used when calculating the TSIG MAC for a reply is the truncated request MAC. 5.2.3. Time Check and Error Handling If the server time is outside the time interval specified by the request (which is: Time Signed, plus/minus Fudge), the server MUST generate an error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18 (BADTIME). The server SHOULD also cache the most recent time signed value in a message generated by a key, and SHOULD return BADTIME if a message received later has an earlier time signed value. A response indicating a BADTIME error MUST be signed by the same key as the request. It MUST include the client's current time in the time signed field, the server's current time (an unsigned 48-bit integer) in the other data field, and 6 in the other data length field. This is done so that the client can verify a message with a BADTIME error without the verification failing due to another BADTIME error. In addition, the fudge field MUST be set to the fudge value received from the client. The data signed is specified in Section 5.3.2. The server SHOULD log the error. Caching the most recent time signed value and rejecting requests with an earlier one could lead to valid messages being rejected if transit through the network led to UDP packets arriving in a different order to the one in which they were sent. Implementations should be aware of this possibility and be prepared to deal with it, e.g. by retransmitting the rejected request with a new TSIG once outstanding requests have completed or the time given by their time signed plus fudge value has passed. 5.2.4. Truncation Check and Error Handling If a TSIG is received with truncation that is permitted under Section 5.2.2.1 above but the MAC is too short for the local policy Dupont, et al. Expires August 23, 2020 [Page 12] Internet-Draft DNS TSIG February 2020 in force, an RCODE 9 (NOTAUTH) and TSIG ERROR 22 (BADTRUNC) MUST be returned. The server SHOULD log the error. 5.3. Generation of TSIG on Answers When a server has generated a response to a signed request, it signs the response using the same algorithm and key. The server MUST NOT generate a signed response to a request if either the KEY is invalid (e.g. key name or algorithm name are unknown), or the MAC fails validation: see Section 5.3.2 for details of responding in these cases. It also MUST NOT not generate a signed response to an unsigned request, except in the case of a response to a client's unsigned TKEY request if the secret key is established on the server side after the server processed the client's request. Signing responses to unsigned TKEY requests MUST be explicitly specified in the description of an individual secret key establishment algorithm [RFC3645]. The digest components used to generate a TSIG on a response are: Request MAC DNS Message (response) TSIG Variables (response) (This calculation is different for the second and subsequent message in a multi-message answer, see below.) If addition of the TSIG record will cause the message to be truncated, the server MUST alter the response so that a TSIG can be included. This response consists of only the question and a TSIG record, and has the TC bit set and an RCODE of 0 (NOERROR). The client SHOULD at this point retry the request using TCP (as per [RFC1035] 4.2.2). 5.3.1. TSIG on Zone Transfer Over a TCP Connection A zone transfer over a DNS TCP session can include multiple DNS messages. Using TSIG on such a connection can protect the connection from hijacking and provide data integrity. The TSIG MUST be included on all DNS messages in the response. For backward compatibility, a client which receives DNS messages and verifies TSIG MUST accept up to 99 intermediary messages without a TSIG. The first message is processed as a standard answer (see Section 5.3) but subsequent messages have the following digest components: Prior MAC (running) DNS Messages (any unsigned messages since the last TSIG) Dupont, et al. Expires August 23, 2020 [Page 13] Internet-Draft DNS TSIG February 2020 TSIG Timers (current message) The "Prior MAC" is the MAC from the TSIG attached to the last message containing a TSIG. "DNS Messages" comprises the concatenation (in message order) of all messages after the last message that included a TSIG and includes the current message. "TSIG timers" comprises the "Time Signed" and "Fudge" fields (in that order) pertaining to the message for which the TSIG is being created: this means that the successive TSIG records in the stream will have non-decreasing "Time Signed" fields. Note that only the timers are included in the second and subsequent messages, not all the TSIG variables. This allows the client to rapidly detect when the session has been altered; at which point it can close the connection and retry. If a client TSIG verification fails, the client MUST close the connection. If the client does not receive TSIG records frequently enough (as specified above) it SHOULD assume the connection has been hijacked and it SHOULD close the connection. The client SHOULD treat this the same way as they would any other interrupted transfer (although the exact behavior is not specified here). 5.3.2. Generation of TSIG on Error Returns When a server detects an error relating to the key or MAC in the incoming request, the server SHOULD send back an unsigned error message (MAC size == 0 and empty MAC). It MUST NOT send back a signed error message. If an error is detected relating to the TSIG validity period or the MAC is too short for the local policy, the server SHOULD send back a signed error message. The digest components are: Request MAC (if the request MAC validated) DNS Message (response) TSIG Variables (response) The reason that the request is not included in this MAC in some cases is to make it possible for the client to verify the error. If the error is not a TSIG error the response MUST be generated as specified in Section 5.3. 5.4. Client Processing of Answer When a client receives a response from a server and expects to see a TSIG, it performs the same checks as described in Section 5.2, with the following modifications: Dupont, et al. Expires August 23, 2020 [Page 14] Internet-Draft DNS TSIG February 2020 o If the TSIG RR does not validate, that response MUST be discarded, unless the RCODE is 9 (NOTAUTH), in which case the client SHOULD proceed as described in the following subsections. A message containing an unsigned TSIG record or a TSIG record which fails verification SHOULD NOT be considered an acceptable response; the client SHOULD log an error and continue to wait for a signed response until the request times out. 5.4.1. Key Error Handling If an RCODE on a response is 9 (NOTAUTH), but the response TSIG validates and the TSIG key recognised by the client but different from that used on the request, then this is a Key Error. The client MAY retry the request using the key specified by the server. However, this should never occur, as a server MUST NOT sign a response with a different key to that used to sign the request. 5.4.2. MAC Error Handling If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG), this is a MAC error, and client MAY retry the request with a new request ID but it would be better to try a different shared key if one is available. Clients SHOULD keep track of how many MAC errors are associated with each key. Clients SHOULD log this event. 5.4.3. Time Error Handling If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 18 (BADTIME), or the current time does not fall in the range specified in the TSIG record, then this is a Time error. This is an indication that the client and server clocks are not synchronized. In this case the client SHOULD log the event. DNS resolvers MUST NOT adjust any clocks in the client based on BADTIME errors, but the server's time in the other data field SHOULD be logged. 5.4.4. Truncation Error Handling If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 22 (BADTRUNC) then this is a Truncation error. The client MAY retry with a lesser truncation up to the full HMAC output (no truncation), using the truncation used in the response as a hint for what the server policy allowed (Section 7). Clients SHOULD log this event. Dupont, et al. Expires August 23, 2020 [Page 15] Internet-Draft DNS TSIG February 2020 5.5. Special Considerations for Forwarding Servers A server acting as a forwarding server of a DNS message SHOULD check for the existence of a TSIG record. If the name on the TSIG is not of a secret that the server shares with the originator the server MUST forward the message unchanged including the TSIG. If the name of the TSIG is of a key this server shares with the originator, it MUST process the TSIG. If the TSIG passes all checks, the forwarding server MUST, if possible, include a TSIG of its own, to the destination or the next forwarder. If no transaction security is available to the destination and the message is a query then, if the corresponding response has the AD flag (see [RFC4035]) set, the forwarder MUST clear the AD flag before adding the TSIG to the response and returning the result to the system from which it received the query. 6. Algorithms and Identifiers The only message digest algorithm specified in the first version of these specifications [RFC2845] was "HMAC-MD5" (see [RFC1321], [RFC2104]). Although a review of its security [RFC6151] concluded that "it may not be urgent to remove HMAC-MD5 from the existing protocols", with the availability of more secure alternatives the opportunity has been taken to make the implementation of this algorithm optional. [RFC4635] added mandatory support in TSIG for SHA-1 [FIPS180-4], [RFC3174]. SHA-1 collisions have been demonstrated so the MD5 security considerations apply to SHA-1 in a similar manner. Although support for hmac-sha1 in TSIG is still mandatory for compatibility reasons, existing uses should be replaced with hmac-sha256 or other SHA-2 digest algorithms [FIPS180-4], [RFC3874], [RFC6234]. Use of TSIG between two DNS agents is by mutual agreement. That agreement can include the support of additional algorithms and criteria as to which algorithms and truncations are acceptable, subject to the restriction and guidelines in Section 5.2.2.1 above. Key agreement can be by the TKEY mechanism [RFC2930] or some other mutually agreeable method. Implementations that support TSIG MUST also implement HMAC SHA1 and HMAC SHA256 and MAY implement gss-tsig and the other algorithms listed below. SHA-1 truncated to 96 bits (12 octets) SHOULD be implemented. Dupont, et al. Expires August 23, 2020 [Page 16] Internet-Draft DNS TSIG February 2020 Requirement Name ----------- ------------------------ Optional HMAC-MD5.SIG-ALG.REG.INT Optional gss-tsig Mandatory hmac-sha1 Optional hmac-sha224 Mandatory hmac-sha256 Optional hmac-sha384 Optional hmac-sha512 Table 1 7. TSIG Truncation Policy As noted above, two DNS agents (e.g., resolver and server) must mutually agree to use TSIG. Implicit in such an "agreement" are criteria as to acceptable keys and algorithms and, with the extensions in this document, truncations. Local policies MAY require the rejection of TSIGs, even though they use an algorithm for which implementation is mandatory. When a local policy permits acceptance of a TSIG with a particular algorithm and a particular non-zero amount of truncation, it SHOULD also permit the use of that algorithm with lesser truncation (a longer MAC) up to the full keyed hash output. Regardless of a lower acceptable truncated MAC length specified by local policy, a reply SHOULD be sent with a MAC at least as long as that in the corresponding request. Note if the request specified a MAC length longer than the keyed hash output it will be rejected by processing rules Section 5.2.2.1 case 1. Implementations permitting multiple acceptable algorithms and/or truncations SHOULD permit this list to be ordered by presumed strength and SHOULD allow different truncations for the same algorithm to be treated as separate entities in this list. When so implemented, policies SHOULD accept a presumed stronger algorithm and truncation than the minimum strength required by the policy. 8. Shared Secrets Secret keys are very sensitive information and all available steps should be taken to protect them on every host on which they are stored. Generally such hosts need to be physically protected. If they are multi-user machines, great care should be taken that unprivileged users have no access to keying material. Resolvers often run unprivileged, which means all users of a host would be able to see whatever configuration data is used by the resolver. Dupont, et al. Expires August 23, 2020 [Page 17] Internet-Draft DNS TSIG February 2020 A name server usually runs privileged, which means its configuration data need not be visible to all users of the host. For this reason, a host that implements transaction-based authentication should probably be configured with a "stub resolver" and a local caching and forwarding name server. This presents a special problem for [RFC2136] which otherwise depends on clients to communicate only with a zone's authoritative name servers. Use of strong random shared secrets is essential to the security of TSIG. See [RFC4086] for a discussion of this issue. The secret SHOULD be at least as long as the keyed hash output [RFC2104]. 9. IANA Considerations IANA maintains a registry of algorithm names to be used as "Algorithm Names" as defined in Section 4.2. Algorithm names are text strings encoded using the syntax of a domain name. There is no structure required other than names for different algorithms must be unique when compared as DNS names, i.e., comparison is case insensitive. Previous specifications [RFC2845] and [RFC4635] defined values for HMAC MD5 and SHA. IANA has also registered "gss-tsig" as an identifier for TSIG authentication where the cryptographic operations are delegated to the Generic Security Service (GSS) [RFC3645]. New algorithms are assigned using the IETF Review policy defined in [RFC8126]. The algorithm name HMAC-MD5.SIG-ALG.REG.INT looks like a fully-qualified domain name for historical reasons; other algorithm names are simple (i.e., single-component) names. IANA maintains a registry of RCODES (error codes), including "TSIG Error values" to be used for "Error" values as defined in Section 4.2. New error codes are assigned and specified as in [RFC6895]. 10. Security Considerations The approach specified here is computationally much less expensive than the signatures specified in DNSSEC. As long as the shared secret key is not compromised, strong authentication is provided between two DNS systems, e.g., for the last hop from a local name server to the user resolver, or between primary and secondary nameservers. Recommendations for choosing and maintaining secret keys can be found in [RFC2104]. If the client host has been compromised, the server should suspend the use of all secrets known to that client. If possible, secrets should be stored in encrypted form. Secrets should never be transmitted in the clear over any network. This document Dupont, et al. Expires August 23, 2020 [Page 18] Internet-Draft DNS TSIG February 2020 does not address the issue on how to distribute secrets except that it mentions the possibilities of manual configuration and the use of TKEY [RFC2930]. Secrets SHOULD NOT be shared by more than two entities. This mechanism does not authenticate source data, only its transmission between two parties who share some secret. The original source data can come from a compromised zone master or can be corrupted during transit from an authentic zone master to some "caching forwarder." However, if the server is faithfully performing the full DNSSEC security checks, then only security checked data will be available to the client. A fudge value that is too large may leave the server open to replay attacks. A fudge value that is too small may cause failures if machines are not time synchronized or there are unexpected network delays. The RECOMMENDED value in most situations is 300 seconds. For all of the message authentication code algorithms listed in this document, those producing longer values are believed to be stronger; however, while there have been some arguments that mild truncation can strengthen a MAC by reducing the information available to an attacker, excessive truncation clearly weakens authentication by reducing the number of bits an attacker has to try to break the authentication by brute force [RFC2104]. Significant progress has been made recently in cryptanalysis of hash functions of the types used here. While the results so far should not affect HMAC, the stronger SHA-1 and SHA-256 algorithms are being made mandatory as a precaution. See also the Security Considerations section of [RFC2104] from which the limits on truncation in this RFC were taken. 10.1. Issue Fixed in this Document When signing a DNS reply message using TSIG, the MAC computation uses the request message's MAC as an input to cryptographically relate the reply to the request. The original TSIG specification [RFC2845] required that the TIME values be checked before the request's MAC. If the TIME was invalid, some implementations failed to carry out further checks and could use an invalid request MAC in the signed reply. This document makes it a madatory that the request MAC is considered to be invalid until it has been validated: until then, any answer must be unsigned. For this reason, the request MAC is now checked before the TIME value. Dupont, et al. Expires August 23, 2020 [Page 19] Internet-Draft DNS TSIG February 2020 10.2. Why not DNSSEC? This section from the original document [RFC2845] analyzes DNSSEC in order to justify the introduction of TSIG. "DNS has recently been extended by DNSSEC ([RFC4033], [RFC4034] and [RFC4035]) to provide for data origin authentication, and public key distribution, all based on public key cryptography and public key based digital signatures. To be practical, this form of security generally requires extensive local caching of keys and tracing of authentication through multiple keys and signatures to a pre-trusted locally configured key. One difficulty with the DNSSEC scheme is that common DNS implementations include simple "stub" resolvers which do not have caches. Such resolvers typically rely on a caching DNS server on another host. It is impractical for these stub resolvers to perform general DNSSEC authentication and they would naturally depend on their caching DNS server to perform such services for them. To do so securely requires secure communication of queries and responses. DNSSEC provides public key transaction signatures to support this, but such signatures are very expensive computationally to generate. In general, these require the same complex public key logic that is impractical for stubs. A second area where use of straight DNSSEC public key based mechanisms may be impractical is authenticating dynamic update [RFC2136] requests. DNSSEC provides for request signatures but with DNSSEC they, like transaction signatures, require computationally expensive public key cryptography and complex authentication logic. Secure Domain Name System Dynamic Update ([RFC3007]) describes how different keys are used in dynamically updated zones." 11. References 11.1. Normative References [FIPS180-4] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015. [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, . [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . Dupont, et al. Expires August 23, 2020 [Page 20] Internet-Draft DNS TSIG February 2020 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, . [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR) Types", RFC 3597, DOI 10.17487/RFC3597, September 2003, . [RFC4635] Eastlake 3rd, D., "HMAC SHA (Hashed Message Authentication Code, Secure Hash Algorithm) TSIG Algorithm Identifiers", RFC 4635, DOI 10.17487/RFC4635, August 2006, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 11.2. Informative References [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, April 1992, . [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, . [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, DOI 10.17487/RFC2136, April 1997, . [RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000, . [RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic Update", RFC 3007, DOI 10.17487/RFC3007, November 2000, . Dupont, et al. Expires August 23, 2020 [Page 21] Internet-Draft DNS TSIG February 2020 [RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001, . [RFC3645] Kwan, S., Garg, P., Gilroy, J., Esibov, L., Westhead, J., and R. Hall, "Generic Security Service Algorithm for Secret Key Transaction Authentication for DNS (GSS-TSIG)", RFC 3645, DOI 10.17487/RFC3645, October 2003, . [RFC3874] Housley, R., "A 224-bit One-way Hash Function: SHA-224", RFC 3874, DOI 10.17487/RFC3874, September 2004, . [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005, . [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005, . [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, . [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, . [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, March 2011, . [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, May 2011, . [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, April 2013, . Dupont, et al. Expires August 23, 2020 [Page 22] Internet-Draft DNS TSIG February 2020 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . Appendix A. Acknowledgments This document consolidates and updates the earlier documents by the authors of [RFC2845] (Paul Vixie, Olafur Gudmundsson, Donald E. Eastlake 3rd and Brian Wellington) and [RFC4635] (Donald E. Eastlake 3rd). The security problem addressed by this document was reported by Clement Berthaux from Synacktiv. Note for the RFC Editor (to be removed before publication): the first 'e' in Clement is a fact a small 'e' with acute, unicode code U+00E9. I do not know if xml2rfc supports non ASCII characters so I prefer to not experiment with it. BTW I am French too so I can help if you have questions like correct spelling... Peter van Dijk, Benno Overeinder, Willem Toroop, Ondrej Sury, Mukund Sivaraman and Ralph Dolmans participated in the discussions that prompted this document. Mukund Sivaraman, Martin Hoffman and Tony Finch made extremely helpful suggestions concerning the structure and wording of the updated document. Appendix B. Change History (to be removed before publication) RFC EDITOR: Please remove this appendix before publication. draft-dupont-dnsop-rfc2845bis-00 [RFC4635] was merged. Authors of original documents were moved to Acknowledgments (Appendix A). Section 2 was updated to [RFC8174] style. Spit references into normative and informative references and updated them. Added a text explaining why this document was written in the Abstract and at the beginning of the introduction. Clarified the layout of TSIG RDATA. Dupont, et al. Expires August 23, 2020 [Page 23] Internet-Draft DNS TSIG February 2020 Moved the text about using DNSSEC from the Introduction to the end of Security Considerations. Added the security clarifications: 1. Emphasized that MAC is invalid until it is successfully validated. 2. Added requirement that a request MAC that has not been successfully validated MUST NOT be included into a response. 3. Added requirement that a request that has not been validated MUST NOT generate a signed response. 4. Added note about MAC too short for the local policy to Section 5.3.2. 5. Changed the order of server checks and swapped corresponding sections. 6. Removed the truncation size limit "also case" as it does not apply and added confusion. 7. Relocated the error provision for TSIG truncation to the new Section 5.2.4. Moved from RCODE 22 to RCODE 9 and TSIG ERROR 22, i.e., aligned with other TSIG error cases. 8. Added Section 5.4.4 about truncation error handling by clients. 9. Removed the limit to HMAC output in replies as a request which specified a MAC length longer than the HMAC output is invalid according to the first processing rule in Section 5.2.2.1. 10. Promoted the requirement that a secret length should be at least as long as the HMAC output to a SHOULD [RFC2119] key word. 11. Added a short text to explain the security issue. draft-dupont-dnsop-rfc2845bis-01 Improved wording (post-publication comments). Specialized and renamed the "TSIG on TCP connection" (Section 5.3.1) to "TSIG on zone transfer over a TCP connection". Dupont, et al. Expires August 23, 2020 [Page 24] Internet-Draft DNS TSIG February 2020 Added a SHOULD for a TSIG in each message (was envelope) for new implementations. draft-ietf-dnsop-rfc2845bis-00 Adopted by the IETF DNSOP working group: title updated and version counter reset to 00. draft-ietf-dnsop-rfc2845bis-01 Relationship between protocol change and principle of assuming the request MAC is invalid until validated clarified. (Jinmei Tatuya) Cross reference to considerations for forwarding servers added. (Bob Harold) Added text from [RFC3645] concerning the signing behavior if a secret key is added during a multi-message exchange. Added reference to [RFC6895]. Many improvements in the wording. Added RFC 2845 authors as co-authors of this document. draft-ietf-dnsop-rfc2845bis-02 Added a recommendation to copy time fields in BADKEY errors. (Mark Andrews) draft-ietf-dnsop-rfc2845bis-03 Further changes as a result of comments by Mukund Sivaraman. Miscellaneous changes to wording. draft-ietf-dnsop-rfc2845bis-04 Major restructing as a result of comprehensive review by Martin Hoffman. Amongst the more significant changes: * More comprehensive introduction. * Merged "Protocol Description" and "Protocol Details" sections. * Reordered sections so as to follow message exchange through "client "sending", "server receipt", "server sending", "client receipt". Dupont, et al. Expires August 23, 2020 [Page 25] Internet-Draft DNS TSIG February 2020 * Added miscellaneous clarifications. draft-ietf-dnsop-rfc2845bis-05 Make implementation of HMAC-MD5 optional. Require that the Fudge field in BADTIME response be equal to the Fudge field received from the client. Added comment concerning the handling of BADTIME messages due to out of order packet reception. draft-ietf-dnsop-rfc2845bis-06 Wording changes and minor corrections after feedback. draft-ietf-dnsop-rfc2845bis-07 Updated text about use of hmac-sha1 using suggestion from Tony Finch. Corrected name of review policy used for new algorithms. Authors' Addresses Francis Dupont Internet Software Consortium 950 Charter Street Redwood City, CA 94063 United States of America Email: Francis.Dupont@fdupont.fr Stephen Morris Internet Software Consortium 950 Charter Street Redwood City, CA 94063 United States of America Email: sa.morris8@gmail.com Dupont, et al. Expires August 23, 2020 [Page 26] Internet-Draft DNS TSIG February 2020 Paul Vixie Farsight Security Inc 177 Bovet Road, Suite 180 San Mateo, CA 94402 United States of America Email: paul@redbarn.org Donald E. Eastlake 3rd Futurewei Technologies 2386 Panoramic Circle Apopka, FL 32703 United States of America Email: d3e3e3@gmail.com Olafur Gudmundsson Cloudflare San Francisco, CA 94107 United States of America Email: olafur+ietf@cloudflare.com Brian Wellington Akamai United States of America Email: bwelling@akamai.com Dupont, et al. Expires August 23, 2020 [Page 27]