DNSEXT Working Group Olafur Gudmundsson
<draft-ietf-dnsext-delegation-signer-02.txt><draft-ietf-dnsext-delegation-signer-03.txt> Updates: RFC 1035, RFC 2535, RFC 3008. Delegation Signer record in parent. Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 Comments should be sent to the authors or the DNSEXT WG mailing list firstname.lastname@example.org This draft expires on February 20,March 26, 2002. Copyright Notice Copyright (C) The Internet Society (2001). All rights reserved. Abstract The Delegation Signer (DS) RR set is stored in a delegating (parent) zone at each delegation point, and indicates the keys used in the delegated (child) zone. The main design goal of the DS RR simplify the operation of secure delegations by eliminating the need to store the same RR in parent and child, as is done with the NS RR set and the KEY set in RFC2535. Secure resolvers need to take an additional step with DS to verify a child's KEY RR set. Operationally this schema is much simpler as operation of the two zones at delegation is now decoupled to great extent. This document updates RFC1035, RFC2535 and RFC3008. 1 - Introduction Familiarity with the DNS system [RFC1035], DNS security extensions [RFC2535] and DNSSEC terminology [RFC3090] is important. When the same data can reside in two administratively different DNS zones, the data frequently gets out of sync. NS record in a zone indicates that this name is a delegation and the NS record lists the authorative servers for the real zone. Based on actual measurements 10-30% of all delegations in the Internet have differing NS sets at parent and child. There are number of reasons for this, including lack of communication between parent and child and bogus name-servers being listed to meet registrar requirements. DNSSEC [RFC2535,RFC3008,RFC3090] specifies that child must have its KEY set signed by the parent to create a verifiable chain of KEYs. There is some debate, where the signed KEY set should reside, parent[Parent] or child[RFC2535]. If the KEY set resides at the child, frequent two way communication is needed between the two parties. First the child needs to transmit the key set to parent and then the parent must sendsends the signed set or signatures to child. If the KEY set resides at the parent the communication is reduced as the child only sends changed key sets to parent. DNSSEC[RFC2535] requires that the parent store NULL key set for unsecure children, this complicates resolution process in many cases as servers for both parent and child need to be queried for KEY set if the child server does not return a KEY set. Storing the KEY record only in the parent zone simplifies this and allows the elimination of the NULL key set. Another complication of the DNSSEC KEY model is that KEY record is used to store DNS zone keys and public keys for other protocols. There are number of potential problems with this including: 1. KEY set maycan become quite large if many applications/protocols store their keys at the zone apex. Possible protocols are IPSEC, HTTP, SMTP, SSH and others that use public key cryptography. 2. Key set may require frequent updates. 3. Probability of compromised/lost keys increases and triggers emergency key rollover procedures. 4. Parent may refuse sign key sets with NON DNS zone keys. 5. Parent may not meet the child's expectations in turnaround time in resigning the key set. Given these and other reasons there is good reason to explore alternatives to using only KEY records to create chain of trust. Some of these problems can be reduced or eliminated by operational rules or protocol changes. To reduce the number of keys at apex, a rule to require applications to store their KEY records at the SRV name for that application is one possibility. Another is to restrict KEY record to DNS keys only and create a new type for all non DNS keys. Third possible solution is to ban the storage of non DNS related keys at zone apex. There are other possible solutions but they are outside the scope of this document. 1.2 - Reserved words The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED", "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be interpreted as described in RFC2119. 2 - DS (Delegation KEY Signer) 2.1 - Delegation Signer Record model This document proposes an alternative to the KEY record chain of trust, that uses a special record that can only reside at the parent. This record will identify the key(s) that child are allowed to self sign its own KEY set. The chain of trust is now established by verifying the parent KEY set, the DS set from the parent and the KEY set at the child. This is cryptographically equivalent to just using KEY records. Communication between the parent and child is greatly reduced, since the child only needs to notify parent about changes in keys that sign its apex KEY RRset. Parent is ignorant of all other keys in the child's apex KEY RRset, and the child maintains full control over the apex KEY set and its content. Child can maintain any policies over its DNS and other KEY usage with minimal impact on parent. Thus if child wants to have frequent key rollover for its DNS keys parent does not need to be aware of it as the child can use one key to only sign its apex KEY set and other keys to sign the other record sets in the zone. This model fits well with slow roll out of DNSSEC and islands of security model. In the islands of security model someone that trusts "good.example." can preconfigure a key from "good.example." as a trusted keys and from then on trusts any data that is signed by that key or has a chain of trust to that key. If "example." starts advertising DS records, "good.example." does not have to change operations, by suspending self-signing. DS records can also be used to identify trusted keys instead of KEY records. One further advantage is the information stored in the parent is minimized, as only records for secure delegations are needed. The main disadvantage of this approach that verifying delegations KEY set requires twice as many signature verification operations. There is no impact on the number of signatures verified for other RR sets. 2.2 Protocol change A DS RR set MUST appear at eachEach secure delegation fromin a secure zone.zone MUST contain a DS RR set. If a DS RR set accompanies the NS RR set, the intent is to state that the child zone is secured. If an NS RR set exists without a DS RR set the intent is to state that the child zone is unsecure. The public keys indicated in theDS RR set are the keys the child has informed the parent, the child allows to sign the childsets MUST NOT appear at non delegations or at zone apex KEY RR set. Barring emergency, the intent of the DS RR set it to indicate to state the child'sAPEX. In a zone keyset signing keys. Ifthat uses DS, insecure delegations MUST have the child's APEX is not signed by any KEY indicatedNODS[TBD] bit set in the DS RR set than any of number of problems may have occurred, and are described later.NXT record. This is required to differenciate this delegation from Secure RFC2535 delegation. Updates RFC2535 sections 2.3.4 and 3.4, as well as RFC3008 section 2.7: Delegating zones MUST NOT store KEY records for delegations. The only records that can appear at delegation in parent are NS, SIG, NXT and DS. Zone MUST self sign its apex KEY set, it SHOULD sign it with a key that corresponds to a DS record in the parent. The KEY used to sign the apex KEY RRset MAY sign other RRsets in the zone. If child apex KEY RRset is not signed with one of the keys specified in the DS record the child is locally secure[RFC3090] and SHOULD only be considered secure if the resolver has been configured to trust the key used. Authorative server foranswering a zonequery with DS recordsthe OK bit[OKbit] set, MUST include the DS records and NXT record along with signatures in answers for a delegation, when the OKbit[okbit] is set in the querydelegation and ifspace is available in answer.available. DS and NXT records SHOULD have lower priority than address records but higher priority than KEY records.KEY. Caching servers SHOULD return the DS recordand parent NXT record(s) in the additional section under the same condition. 2.2.1 - Comments on protocol change Over the years there has been various discussions on that the delegation model in DNS is broken as there is no real good way to assert if delegation exists. In RFC2535 version of DNSSEC the authentication of a delegation is the NS bit in the NXT bitmap at the delegation point. Something more explicit is needed and the DS record addresses this for secure delegations. DS record is the first DNS record that can only appear on the upper side of a delegation. NS records appear at both sides as do SIG and NXT. All other records can only appear at the lower side. This will cause some problems as servers authorative for parent mayparent, reject DS record even if the server understands unknown types, or will not hand them out unless explicitly asked. Similarly a nameserver acting as a authorative for child and as a caching recursive server may never return the DS record. A caching server that supports unkown types, does not care from which side DS record comes from and thus does not have to be changed if it supports unknown types.changed. Different TTL values on the child's NS set and parents DS set maycan cause the DS set to expire before the NS set. In this case an non-DS aware server would ask the child server for the DS set and get NXDOMAIN answer. DS aware server will know to ask a parent DNS server for the DS record.Secure resolvers need to know about the DS record and how to interpret it. In the worst case, introducing the DS record, doubles the signatures that need to be checked to validate a KEY set. Note: The working group must determine if the tradeoff between more work in resolvers is justified by the operational simplification of this model. The author thinks this is a small price to pay to have a cleaner delegations structure. One argument, put forward is that DNS should be optimized for read when ever possible, and on the face of it adding the DS record makes reading data from DNS more expensive. The operational complexities and legal hurdles related to KEY records in either parents or children may prevent DNSSEC deployment.2.3 Wire format of DS record The DS (type=TDB) record consists of algorithm, key tag and SHA-1 digest of the public key KEY record allowed to sign the child's delegation. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | key tag | algorithm | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SHA-1 digest | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | (20 bytes) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The key tag is calculated as specified in RFC2535, Algorithm MUST be an algorithm number assigned in the range 1..251. The SHA-1 digest is calculated over the canonical name of the delegation followed by the RDATA of the KEY record. The size of the DS RDATA is 23 bytes, regardless of key size. 2.3.1 Justifications for fields The algorithm and key tag fields are here to allow resolvers to quickly identify the candidate KEY records to examine. The key tag adds some greater assurance than SHA-1 digest on its own. SHA-1 is a strong cryptographic checksum, it is real hard for attacker to generate a KEY record that has the same SHA-1 digest. Combining the name of the key and the key data as input to the digest provides stronger assurance of the binding. This format allows concise representation of the keys that child will use, thus keeping down the size of the answer for the delegation, reducing the probability of packet overflow. The SHA-1 hash is strong enough to uniquely identify the key. This is similar to the PGP footprint. DS record is also well suited to lists trusted keys for islands of security in configuration files. 2.4 Presentation format of DS record The presentation format of DS record consists of 2 numbers followed by digest presented in hex. foo.example DS 12345 3 123456789abcdef67890 2.5 Transition issues for installed base RFC2535 compliant resolver will assume that all DS secured delegations are locally secure. This is a bad thing, thus it might be necessary for a transition period to support both DS and SIG@Child. The cost is one or more signatures in the answer for KEY records and that early adopters have to use cumbersome communications that DS solves. 2.6 Backwards compatibilty with RFC2535 SIG@child and RFC1035 This section needs work, it needs listdocuments how a resolver determines the type of all cases and find if there are any where resolvers get confused ordelegation. RFC1035 delegation has: RFC1035 NS RFC2535 adds the following two cases: Secure RFC2535: NS + NXT + SIG(NXT) NXT bit map contains: NS SIG NXT Insecure RFC2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT) NXT bit map contains: NS SIG KEY NXT KEY must be null-key. DS adds the following two states: Secure DS: NS + DS + SIG(DS) + NXT + SIG(NXT) NXT bit map contains: NS SIG NXT DS Insecure DS: NS + NXT + SIG(NXT) NXT bit map contains: NS SIG KEY NXT NODS If the NODS bit is not used, a resover can not determine what the security statusif this is a DS delegation zone. Thus is not able to determine if this delegtion is a secure RFC2535 or a insecure DS. 2.6.1 NODS support in servers NODS is a virtual type, servers MUST refuse to store any record of child is.this type. No special processing is required on answers. 3 Resolver Example To create a chain of trust resolver goes from trusted KEY to DS to KEY. Assume the key for domain "example." is trusted. In zone "example." we have example. KEY <stuff> secure.example. DS tag=10243 alg=3 <foofoo> secure.example. NS ns1.secure.example. NS ns1.secure.example.ns2.secure.example. secure.example. NXT NS SIG NXT DS unsecure.example. secure.example. SIG(NXT) secure.example. SIG(DS) unsecure.example NS ns1.unsecure.example. unsecure.example NS ns2.unsecure.example. unsecure.example. NXT NS SIG NXT NODS .example. unsecure.example. SIG(NXT) In zone "secure.example." we have secure.example. SOA <soa stuff> secure.example. NS ns1.secure.example. NS ns1.secure.example. secure.example. KEY <tag=12345 size=1024 alg=3> KEY <tag=54321 size=512 alg=5> KEY <tag=32145 size=1024 alg=3> secure.example. SIG(KEY) <key-tag=12345 alg=3> secure.example. SIG(SOA) <key-tag=54321 alg=5> secure.example. SIG(NS) <key-tag=54321 alg=5> In this example the trusted key for "example." signs the DS record for "secure.example.", making that a trusted record. The DS record states what key is expected to sign the KEY RRset at "secure.example". Here "secure.example." has three different KEY records and the KEY identified in the DS record signs the KEY set, thus the KEY set is validated and trusted. Note that one of the other keys in the keyset actually signs the zone data, and resolvers will trust the signatures as the key appears in the KEY set. This example has only one DS record for the child but there no reason to outlaw multiple DS records. More than one DS record is needed during signing key rollover. It is strongly recommended that the DS set be kept small. Resolver determines the security status of "unsecure.example." by examining the parent size NXT for this name. 3.1 Resolver cost estimates for DS records From a RFC2535 resolver point of view for each delegation followed to chase down an answer one KEY record has to be verified and possibly some other records based on policy, for example the contents of the NS set. Once the resolver gets to the appropriate delegation validating the answer may require verifying one or more signatures. A simple A record lookup requires at least N delegations to be verified and 1 RRset. For a DS enabled resolver the cost is 2N+1. For MX record the cost where the target of the MX record is in the same zone as the MX record the costs are N+2 and 2N+2. In the case of negative answer the same ratios hold true. Resolver may require an extra query to get the DS record and this may add to the overall cost of the query, but this is never worse than chasing down NULL KEY records from the parent in RFC2535 DNSSEC. DS adds processing overhead on resolvers, increases the size of delegation answers but much less than SIG@Parent. 4 Acknowledgments Number of people have over the last few years contributed number of ideas that are captured in this document. The core idea of using one key to only sign key set, comes from discussions with Bill Manning and Perry Metzger on how to put in a single root key in all resolvers. Alexis Yushin, Brian Wellington, Jakob Schlyter, Scott Rosen, Edward Lewis, Dan Massey, Lars-Johan Liman, Mark Kosters, Olaf Kolman, Miek Gieben, Havard Eidnes, Donald Eastlake 3rd., Randy Bush, David Blacka, Rob Austein, Derek Atkins, Roy Arends, and others have provided useful comments. 4 - Security Considerations: This document proposes a change to the validation chain of KEY records in DNS. The change in is not believed to reduce security in the overall system, in RFC2535 DNSSEC child must communicate keys to parent and prudent parents will require some authentication on that handshake. The modified protocol will require same authentication but allows the child to exert more local control over its own KEY set. There is a possibility that an attacker can generate an valid KEY that matches all the DS fields thus starting to forge data from the child. This is considered impractical as on average more than 2^80 keys must be generated before one is found that will match. DS record is a change to DNSSEC protocol and there is some installed base of implementations, as well as text books on how to set up secured delegations. Implementations that do not understand DS record will not be able to follow the KEY to DS to KEY chain and consider all zone secured that way insecure. 5 - IANA Considerations: IANA needs to allocate RR type code for DS from the standard RR type space. IANA needs to allocate RR type code for the virtual NODS record from the standard RR type space. Note: SINK (40) was never implemented and that type code can be reused for NODS. References: [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification'', STD 13, RFC 1035, November 1987. [RFC2535] D. Eastlake, ``Domain Name System Security Extensions'', RFC 2535, March 1999. [RFC3008] B. Wellington, ``Domain Name System Security (DNSSEC) Signing Authority'', RFC 3008, November 2000. [RFC3090] E. Lewis `` DNS Security Extension Clarification on Zone Status'', RFC 3090, March 2001. [OKbit] D. Conrad, ``Indicating Resolver Support of DNSSEC'', work in progress <draft-ietf-dnsext-dnssec-okbit-02.txt>, April 2001. [Parent] R. Gieben, T. Lindgreen, ``Parent stores the child's zone KEYs'', work in progress <draft-ietf-dnsext-parent-stores- zones-keys-01.txt>, May 2001. Author Address Olafur Gudmundsson 3826 Legation Street, NW Washington, DC, 20015 USA <email@example.com> Appendix A: Changes from Prior versions Changes from version 02 Added text outlawing DS at non delegations. Added table showing the contents of DS, SIG@child, and RFC1034 delegations. Added the NODS type/bit definition to distiguish insecure DS delegation from secure SIG@child one. Added the requirement that NXT be returned with referal answers. Minor text edits. Changes from version 01 Deleted KEY size field as it did not contribute anything but complexity. Number of wordsmith changes to make document more readable. The word CAN was used when SHOULD was intended. Deleted section 2.4 "Justifications for compact format" moved relevant text to section 2.2. Reverse alphabetized the acknowledgments section. Reorganized sections 1 and 2 for readability. Changes from version 00 Changed name from DK to DS based on working group comments. Dropped verbose format based on WG comments. Added text about TTL issue/problem in caching servers. Added text about islands of security and clarified the cost impact. Major editing of arguments and some reordering of text for clarity. Added section on transition issues. Full Copyright Statement Copyright (C) The Internet Society (2001). All Rights Reserved. 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