DNSOP O. Kolkman Internet-Draft RIPE NCC Expires: August 30, 2004 R. Gieben NLnet Labs March 2004 DNSSEC Operational Practices draft-ietf-dnsop-dnssec-operational-practices-01.txt 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. This Internet-Draft will expire on August 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes a set of practices for operating a DNSSEC aware environment. The target audience is zone administrators deploying DNSSEC that need a guide to help them chose appropriate values for DNSSEC parameters. It also discusses operational matters such as key rollovers, KSK and ZSK considerations and related matters. Kolkman & Gieben Expires August 30, 2004 [Page 1] Internet-Draft DNSSEC Operational Practices March 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 The Use of the Term 'key' . . . . . . . . . . . . . . . . 3 1.2 Keeping the Chain of Trust Intact . . . . . . . . . . . . 3 2. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Time Definitions . . . . . . . . . . . . . . . . . . . . . 4 2.2 Time Considerations . . . . . . . . . . . . . . . . . . . 5 3. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Motivations for the KSK and ZSK Functions . . . . . . . . 7 3.2 Key Security Considerations . . . . . . . . . . . . . . . 8 3.2.1 Key Validity Period . . . . . . . . . . . . . . . . . 8 3.2.2 Key Algorithm . . . . . . . . . . . . . . . . . . . . 8 3.2.3 Key Sizes . . . . . . . . . . . . . . . . . . . . . . 8 3.3 Key Rollovers . . . . . . . . . . . . . . . . . . . . . . 9 3.3.1 Zone-signing Key Rollovers . . . . . . . . . . . . . . 10 3.3.2 Key-signing Key Rollovers . . . . . . . . . . . . . . 13 4. Planning for Emergency Key Rollover . . . . . . . . . . . . . 14 4.1 KSK Compromise . . . . . . . . . . . . . . . . . . . . . . 15 4.2 ZSK Compromise . . . . . . . . . . . . . . . . . . . . . . 15 4.3 Compromises of Keys Anchored in Resolvers . . . . . . . . 16 5. Parental Policies . . . . . . . . . . . . . . . . . . . . . . 16 5.1 Initial Key Exchanges and Parental Policies Considerations . . . . . . . . . . . . . . . . . . . . . . 16 5.2 Storing Keys So Hashes Can Be Regenerated . . . . . . . . 16 5.3 Security Lameness Checks . . . . . . . . . . . . . . . . . 17 5.4 DS Signature Validity Period . . . . . . . . . . . . . . . 17 6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8.1 Normative References . . . . . . . . . . . . . . . . . . . . 18 8.2 Informative References . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19 A. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 19 B. Zone-signing Key Rollover Howto . . . . . . . . . . . . . . . 20 C. Typographic Conventions . . . . . . . . . . . . . . . . . . . 20 D. Document Details and Changes . . . . . . . . . . . . . . . . . 22 D.1 draft-ietf-dnsop-dnssec-operational-practices-00 . . . . . 22 D.2 draft-ietf-dnsop-dnssec-operational-practices-01 . . . . . 22 Intellectual Property and Copyright Statements . . . . . . . . 23 Kolkman & Gieben Expires August 30, 2004 [Page 2] Internet-Draft DNSSEC Operational Practices March 2004 1. Introduction During workshops and early operational deployment tests, operators and system administrators gained experience about operating DNSSEC aware DNS services. This document translates these experiences into a set of practices for zone administrators. At the time of writing, there exists very little experience with DNSSEC in production environments, this document should therefore explicitly not be seen as represented 'Best Current Practices'. The procedures herein are focused on the maintenance of signed zones (i.e. signing and publishing zones on authoritative servers). It is intended that maintenance of zones such as resigning or key rollovers be transparent to any verifying clients on the Internet. The structure of this document is as follows: It begins with discussing some of the considerations with respect to timing parameters of DNS in relation to DNSSEC (Section 2). Aspects of key management such as key rollover schemes are described in Section 3. Emergency rollover considerations are addressed in Section 4. The typographic conventions used in this document are explained in Appendix C. Since this is a document with operational suggestions and there are no protocol specifications, the RFC2119 [5] language does not apply. 1.1 The Use of the Term 'key' It is assumed that the reader is familiar with the concept of asymmetric keys on which DNSSEC is based (Public Key Cryptography [Ref to Schneider?]). Therefore, this document will use the term 'key' rather loosely. Where it is written that 'a key is used to sign data' it is assumed that the reader understands that it is the private part of the key-pair that is used for signing. It is also assumed that the reader understands that the public part of the key-pair is published in the DNSKEY resource record and that it is used in key-exchanges. 1.2 Keeping the Chain of Trust Intact Maintaining a valid chain of trust is important because broken chains of trust will result in data being marked as bogus, which may cause entire (sub)domains to become invisible to verifying clients. The administrators of secured zones have to realise that their zone is, to their clients, part of a chain of trust. As mentioned in the introduction, the procedures herein are intended to ensure maintenance of zones, such as resigning or key rollovers, Kolkman & Gieben Expires August 30, 2004 [Page 3] Internet-Draft DNSSEC Operational Practices March 2004 be transparent to the verifying clients on the Internet. Administrators of secured zones will have to keep in mind that data published on an authoritative primary server will not be immediately seen by verifying clients; it may take some time for the data to be transfered to other secondary authoritative nameservers, during which period clients may be fetching data from caching non-authoritative servers. For the verifying clients it is important that data from secured zones can be used to build chains of trust regardless of whether the data came directly from an authoritative server, a caching nameserver or some middle box. Only by carefully using the available timing parameters can a zone administrator assure that the data necessary for verification can be obtained. The responsibility for maintaining the chain of trust is shared by administrators of secured zones in the chain of trust. This is most obvious in the case of a 'key compromise' when a trade off between maintaining a valid chain of trust and the fact that the key has been stolen, must be made. The zone administrator will have to make a tradeoff between keeping the chain of trust intact -thereby allowing for attacks with the compromised key- or to deliberately break the chain of trust thereby making secured subdomains invisible to security aware resolvers. Also see Section 4. 2. Time in DNSSEC Without DNSSEC all times in DNS are relative. The SOA's refresh, retry and expiration timers are counters that are used to determine the time elapsed after a slave server syncronised (or tried to syncronise) with a master server. The Time to Live (TTL) value and the SOA minimum TTL parameter [6] are used to determine how long a forwarder should cache data after it has been fetched from an authoritative server. DNSSEC introduces the notion of an absolute time in the DNS. Signatures in DNSSEC have an expiration date after which the signature is marked as invalid and the signed data is to be considered bogus. 2.1 Time Definitions In this document we will be using a number of time related terms. Within the context of this document the following definitions apply: o "Signature validity period" The period that a signature is valid. It starts at the time specified in the signature inception field of the RRSIG RR and ends at the time specified in the expiration field of the RRSIG RR. Kolkman & Gieben Expires August 30, 2004 [Page 4] Internet-Draft DNSSEC Operational Practices March 2004 o "Signature publication period" Time after which a signature (made with a specific key) is replaced with a new signature (made with the same key). This replacement takes place by publishing the relevant RRSIG in the master zone file. If a signature is published at time T0 and a new signature is published at time T1, the signature publication period is T1 - T0. If all signatures are refreshed at zone (re)signing then the signature publication period is equal signature validity period. o "Maximum/Minimum Zone TTL" The maximum or minimum value of all the TTLs in a zone. 2.2 Time Considerations Because of the expiration of signatures, one should consider the following. o The Maximum Zone TTL of your zone data should be a fraction of your signature validity period. If the TTL would be of similar order as the signature validity period, then all RRsets fetched during the validity period would be cached until the signature expiration time. As a result query load on authoritative servers would peak at signature expiration time. To avoid query load peaks we suggest the TTL on all the RRs in your zone to be at least a few times smaller than your signature validity period. o The signature publication period should be at least one maximum TTL smaller than the signature validity period. Resigning a zone shortly before the end of the signature validity period may cause simultaneous expiration of data from caches. This in turn may lead to peaks in the load on authoritative servers. o The Minimum zone TTL should be long enough to both fetch and verify all the RRs in the authentication chain. 1. During validation, some data may expire before the validation is complete. The validator should be able to keep all data, until is completed. This applies to all RRs needed to complete the chain of trust: DSs, DNSKEYs, RRSIGs, and the final answers i.e. the RR that is returned for the initial query. 2. Frequent verification causes load on recursive nameservers. Data at delegation points, DSs, DNSKEYs and RRSIGs benefit from caching. The TTL on those should be relatively long. Kolkman & Gieben Expires August 30, 2004 [Page 5] Internet-Draft DNSSEC Operational Practices March 2004 We have seen events where data needed for verification of an authentication chain had expired from caches. We suggest the TTL on DNSKEY and DSs to be between ten minutes and one hour. We recommend zone administrators to chose TTLs longer than half a minute. [Editor's Note: this observation could be implementation specific. We are not sure if we should leave this item] o Slave servers will need to be able to fetch newly signed zones well before the data expires from your zone. 'Better no answers than bad answers.' If a properly implemented slave server is not able to contact a master server for an extended period the data will at some point expire and the slave server will not hand out any data. If the server serves a DNSSEC zone than it may well happen that the signatures expire well before the SOA expiration timer counts down to zero. It is not possible to completely prevent this from happening by tweaking the SOA parameters. However, the effects can be minimized where the SOA expiration time is equal or smaller than the signature validity period. The consequence of an authoritative server not being able to update a zone, whilst that zone includes expired signaturs, is that non-secure resolvers will continue to be able to resolve data served by the particular slave servers. Security aware resolvers will experience problems. We suggest the SOA expiration timer being approximately one third or one fourth of the signature validity period. It will allow problems with transfers from the master server to be noticed before the actual signature time out. We suggest that operators of nameservers with slave zones develop 'watch dogs' to spot upcoming signature expirations in slave zones, and take appropriate action. When determining the value for the expiration parameter one has to take the following into account: What are the chances that all my secondary zones expire; How quickly can I reach an administrator and load a valid zone? All these arguments are not DNSSEC specific. 3. Keys In the DNSSEC protocol there is only one type of key, the zone key. With this key, the data in a zone is signed. To make zone re-signing and key rollovers procedures easier to implement, it is possible to use one or more keys as Key Signing Keys (KSK) these keys will only sign the apex DNSKEY RRs in a zone. Other keys can be used to sign all the RRsets in a zone and are referred to as Zone Signing Keys (ZSK). In this document we assume that KSKs are the subset of keys that are used for key exchanges with the parents Kolkman & Gieben Expires August 30, 2004 [Page 6] Internet-Draft DNSSEC Operational Practices March 2004 and potentially for configuration as trusted anchors - the so called Secure Entry Point keys (SEP). In this document we assume a one-to-one mapping between KSK and SEP keys and we assume the SEP flag [4] to be set on KSKs. 3.1 Motivations for the KSK and ZSK Functions Differentiating between the KSK to ZSK functions has several advantages: o Making the KSK stronger (i.e. using more bits in the key material) has little operational impact since it is only used to sign a small fraction of the zone data. o As the KSK is only used to sign a keyset, which is most probably updated less frequently than other data in the zone, it can be stored separately from (and thus in a safer location than) the ZSK. o A KSK can be used for longer periods. o No parent/child interaction is required when ZSKs are updated. The KSK is used less than ZSK, once a keyset is signed with the KSK all the keys in the keyset can be used as ZSK. If a ZSK is compromised, it can be simply dropped from the keyset. The new keyset is then resigned with the KSK. Given the assumption that for KSKs the SEP flag is set, the KSK can be distinguished from a ZSK by examining the flag field in the DNSKEY RR. If the flag field is an odd number it is a KSK if it is an even number it is a ZSK e.g. a value of 256 and a key signing key has 257. The zone-signing key can be used to sign all the data in a zone on a regular basis. When a zone-signing key is to be rolled, no interaction with the parent is needed. This allows for relatively short "Signature Validity Periods". That is, Signature Validity Periods of the order of days. The key-signing key is only to be used to sign the Key RR set from the zone apex. If a key-signing key is to be rolled over, there will be interactions with parties other than the zone administrator such as the registry of the parent zone or administrators of verifying resolvers that have the particular key configured as trusted entry points. Hence, the "Key Usage Time" of these keys can and should be made much longer. Although, given a long enough key, the "Key Usage Time" can be on the order of years we suggest to plan for a "Key Usage Time" of the order of a few months so that a key rollover remains an operational routine. Kolkman & Gieben Expires August 30, 2004 [Page 7] Internet-Draft DNSSEC Operational Practices March 2004 3.2 Key Security Considerations Keys in DNSSEC have a number of parameters which should all be chosen with care, the most important once are: size, algorithm and the key validity period (its lifetime). 3.2.1 Key Validity Period RFC2541 [2] describes a number of considerations with respect to the security of keys. The document deals with the generation, lifetime, size and storage of private keys. In Section 3 of RFC2541 [2] there are some suggestions for a key validity period: 13 months for long-lived keys and 36 days for transaction keys but suggestions for key sizes are not made. If we say long-lived keys are key-signing keys and transactions keys are zone-signing keys, these recommendations will lead to rollovers occurring frequently enough to become part of 'operational habits'; the procedure does not have to be reinvented every time a key is replaced. 3.2.2 Key Algorithm We recommend you choose RSA/SHA-1 as the preferred algorithm for the key. RSA has been developed in an open and transparent manner. As the patent on RSA expired in 2001, its use is now also free. The current known attacks on RSA can be defeated by making your key longer. As the MD5 hashing algorithm is showing (theoretical) cracks, we recommend the usage of SHA1. 3.2.3 Key Sizes When choosing key sizes, zone administrators will need to take into account how long a key will be used and how much data will be signed during the key publication period. It is hard to give precise recommendations but Lenstra and Verheul [9] supplied the following table with lower bound estimates for cryptographic key sizes. Their recommendations are based on a set of explicitly formulated parameter settings, combined with existing data points about cryptosystems. For details we refer to the original paper. [Editor's Note: DSA???] Kolkman & Gieben Expires August 30, 2004 [Page 8] Internet-Draft DNSSEC Operational Practices March 2004 Year RSA Key Sizes Elliptic Curve Key Size 2000 952 132 2001 990 135 2002 1028 139 2003 1068 140 2004 1108 143 2005 1149 147 2006 1191 148 2007 1235 152 2008 1279 155 2009 1323 157 2010 1369 160 2011 1416 163 2012 1464 165 2013 1513 168 2014 1562 172 2015 1613 173 2016 1664 177 2017 1717 180 2018 1771 181 2019 1825 185 2020 1881 188 2021 1937 190 2022 1995 193 2023 2054 197 2024 2113 198 2025 2174 202 2026 2236 205 2027 2299 207 2028 2362 210 2029 2427 213 For example, should you wish your key to last three years from 2003, check the RSA keysize values for 2006 in this table. In this case 1191. 3.3 Key Rollovers Key rollovers are a fact of life when using DNSSEC. A DNSSEC key cannot be used forever (see RFC2541 [2] and Section 3.2 ). Zone administrators who are in the process of rolling their keys have to Kolkman & Gieben Expires August 30, 2004 [Page 9] Internet-Draft DNSSEC Operational Practices March 2004 take into account that data published in previous versions of their zone still lives in caches. When deploying DNSSEC, this becomes an important consideration; ignoring data that may be in caches may lead to loss of service for clients. The most pressing example of this is when zone material signed with an old key is being validated by a resolver which does not have the old zone key cached. If the old key is no longer present in the current zone, this validation fails, marking the data bogus. Alternatively, an attempt could be made to validate data which is signed with a new key against an old key that lives in a local cache, also resulting in data being marked bogus. To appreciate the situation one could think of a number of authoritative servers that may not be instantaneously running the same version of a zone and a security aware non-recursive resolver that sits behind security aware caching forwarders. Note that KSK rollovers and ZSK rollovers are different. A zone-key rollover can be handled in two different ways: pre-publish (Section Section 3.3.1.1) and double signature (Section Section 3.3.1.2). The pre-publish technique works because the key-signing key stays the same during this ZSK rollover. With this KSK a cache is able to validate the new keyset of a zone. With a KSK rollover a cache can not validate the new keyset, because it does not trust the new KSK. [Editors note: This needs more verbose explanation, nobody will appreciate the situation just yet. Help with text and examples is appreciated] 3.3.1 Zone-signing Key Rollovers For zone-signing key rollovers there are two ways to make sure that during the rollover data still cached can be verified with the new keysets or newly generated signatures can be verified with the keys still in caches. One schema uses double signatures, it is described in Section 3.3.1.2, the other uses key pre-publication (Section 3.3.1.1). The pros, cons and recommendations are described in Section 3.3.1.3. 3.3.1.1 Pre-publish Keyset Rollover This section shows how to perform a ZSK rollover without the need to sign all the data in a zone twice - the so called "prepublish rollover". We recommend this method because it has advantages in the case of key compromise. If the old key is compromised, the new key has already been distributed in the DNS. The zone administrator is then able to quickly switch to the new key and remove the compromised Kolkman & Gieben Expires August 30, 2004 [Page 10] Internet-Draft DNSSEC Operational Practices March 2004 key from the zone. Another major advantage is that the zone size does not double, as is the case with the double signature ZSK rollover. A small "HOWTO" for this kind of rollover can be found in Appendix B. normal pre-roll roll after SOA0 SOA1 SOA2 SOA3 RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2) RRSIG11(SOA3) DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY10 DNSKEY10 DNSKEY10 DNSKEY11 DNSKEY11 DNSKEY11 RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY) RRSIG1 (DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY) normal: Version 0 of the zone: DNSKEY 1 is the key-signing key. DNSKEY 10 is used to sign all the data of the zone, the zone-signing key. pre-roll: DNSKEY 11 is introduced into the keyset. Note that no signatures are generated with this key yet, but this does not secure against brute force attacks on the public key. The minimum duration of this pre-roll phase is the time it takes for the data to propagate to the authoritative servers plus TTL value of the keyset. This equates to two times the Maximum Zone TTL. roll: At the rollover stage (SOA serial 1) DNSKEY 11 is used to sign the data in the zone exclusively (i.e. all the signatures from DNSKEY 10 are removed from the zone). DNSKEY 10 remains published in the keyset. This way data that was loaded into caches from version 1 of the zone can still be verified with key sets fetched from version 2 of the zone. The minimum time that the keyset including DNSKEY 10 is to be published is the time that it takes for zone data from the previous version of the zone to expire from old caches i.e. the time it takes for this zone to propagate to all authoritative servers plus the Maximum Zone TTL value of any of the data in the previous version of the zone. after: DNSKEY 10 is removed from the zone. The keyset, now only containing DNSKEY 11 is resigned with the DNSKEY 1. The above scheme can be simplified by always publishing the "future" key immediately after the rollover. The scheme would look as follows (we show two rollovers); the future key is introduced in "after" as DNSKEY 12 and again a newer one, numbered 13, in "2nd after": Kolkman & Gieben Expires August 30, 2004 [Page 11] Internet-Draft DNSSEC Operational Practices March 2004 normal roll after 2nd roll 2nd after SOA0 SOA2 SOA3 SOA4 SOA5 RRSIG10(SOA0) RRSIG11(SOA2) RRSIG11(SOA3) RRSIG12(SOA4) RRSIG12(SOA5) DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY10 DNSKEY10 DNSKEY11 DNSKEY11 DNSKEY12 DNSKEY11 DNSKEY11 DNSKEY12 DNSKEY12 DNSKEY13 RRSIG1(DNSKEY) RRSIG1 (DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY) RRSIG12(DNSKEY) RRSIG12(DNSKEY) Note that the key introduced after the rollover is not used for production yet; the private key can thus be stored in a physically secure manner and does not need to be 'fetched' every time a zone needs to be signed. This scheme has the benefit that the key that is intended for future use: immediately during an emergency rollover assuming that the private key was stored in a physically secure manner. 3.3.1.2 Double Signature Zone-signing Key Rollover This section shows how to perform a ZSK key rollover using the double zone data signature scheme, aptly named "double sig rollover". During the rollover stage the new version of the zone file will need to propagate to all authoritative servers and the data that exists in (distant) caches will need to expire, this will take at least the maximum Zone TTL . normal roll after SOA0 SOA1 SOA2 RRSIG10(SOA0) RRSIG10(SOA1) RRSIG11(SOA2) RRSIG11(SOA1) DNSKEY1 DNSKEY1 DNSKEY1 DNSKEY10 DNSKEY10 DNSKEY11 DNSKEY11 RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG1(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG11(DNSKEY) RRSIG11(DNSKEY) normal: Version 0 of the zone: DNSKEY 1 is the key-signing key. DNSKEY 10 is used to sign all the data of the zone, the zone-signing key. Kolkman & Gieben Expires August 30, 2004 [Page 12] Internet-Draft DNSSEC Operational Practices March 2004 roll: At the rollover stage (SOA serial 1) DNSKEY 11 is introduced into the keyset and all the data in the zone is signed with DNSKEY 10 and DNSKEY 11. The rollover period will need to exist until all data from version 0 of the zone has expired from remote caches. This will take at least the maximum Zone TTL of version 0 of the zone. after: DNSKEY 10 is removed from the zone. All the signatures from DNSKEY 10 are removed from the zone. The keyset, now only containing DNSKEY 11, is resigned with DNSKEY 1. At every instance the data from the previous version of the zone can be verified with the key from the current version and vice verse. The data from the current version can be verified with the data from the previous version of the zone. The duration of the rollover phase and the period between rollovers should be at least the "Maximum Zone TTL". Making sure that the rollover phase lasts until the signature expiration time of the data in version 0 of the zone is recommended. However, this date could be considerably longer than the Maximum Zone TTL, making the rollover a lengthy procedure. Note that in this example we assumed that the zone was not modified during the rollover. New data can be introduced in the zone as long as it is signed with both keys. 3.3.1.3 Pros and Cons of the Schemes Prepublish-keyset rollover: This rollover does not involve signing the zone data twice. Instead, just before the actual rollover, the new key is published in the keyset and thus available for cryptanalysis attacks. A small disavantage is that this process requires four steps. Also the prepublish scheme will not work for KSKs as explained in Section 3.3. Double signature rollover: The drawback of this signing scheme is that during the rollover the number of signatures in your zone doubles, this may be prohibitive if you have very big zones. An advantage is that it only requires three steps. 3.3.2 Key-signing Key Rollovers For the rollover of a key-signing key the same considerations as for the rollover of a zone-signing key apply. However we can use a double signature scheme to guarantee that old data (only the apex keyset) in caches can be verified with a new keyset and vice versa. Since only the keyset is signed with a KSK, zone size considerations do not apply. Kolkman & Gieben Expires August 30, 2004 [Page 13] Internet-Draft DNSSEC Operational Practices March 2004 normal roll after SOA0 SOA1 SOA2 RRSIG10(SOA0) RRSIG10(SOA1) RRSIG10(SOA2) DNSKEY1 DNSKEY1 DNSKEY2 DNSKEY2 DNSKEY10 DNSKEY10 DNSKEY10 RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG2(DNSKEY) RRSIG2 (DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) normal: Version 0 of the zone. The parental DS points to DNSKEY1. Before the rollover starts the child will have to verify what the TTL is of the DS RR that points to DNSKEY1 - it is needed during the rollover and we refer to the value as TTL_DS. roll: During the rollover phase the zone administrator generates a second KSK, DNSKEY2. The key is provided to the parent and the child will have to wait until a new DS RR has been generated that points to DNSKEY2. After that DS RR has been published on _all_ servers authoritative for the parents zone, the zone administrator has to wait at least TTL_DS to make sure that the old DS RR has expired from distant caches. after: DNSKEY1 has been removed. The scenario above puts the responsibility for maintaining a valid chain of trust with the child. It also is based on the premises that the parent only has one DS RR (per algorithm) per zone. St John [The draft has expired] proposed a mechanism where using an established trust relation, the interaction can be performed in-band. In this mechanism there are periods where there are two DS RRs at the parent. [Editors note: We probably need to mention more] 4. Planning for Emergency Key Rollover This section deals with preparation for a possible key compromise. Our advice is to have a documented procedure ready for when a key compromise is suspected or confirmed. [Editors note: We are much in favor of a rollover tactic that keeps the authentication chain intact as long as possible. This means that one has to take all the regular rollover properties into account.] When the private material of one of your keys is compromised it can be used for as long as a valid authentication chain exists. An authentication chain remains intact for: Kolkman & Gieben Expires August 30, 2004 [Page 14] Internet-Draft DNSSEC Operational Practices March 2004 o as long as a signature over the compromised key in the authentication chain is valid, o as long as a parental DS RR (and signature) points to the compromised key, o as long as the key is anchored in a resolver and is used as a starting point for validation. (This is the hardest to update.) While an authentication chain to your compromised key exists, your name-space is vulnerable to abuse by the malicious key holder (i.e. the owner of the compromised key). Zone operators have to make a trade off if the abuse of the compromised key is worse than having data in caches that cannot be validated. If the zone operator chooses to break the authentication chain to the compromised key, data in caches signed with this key cannot be validated. However, if the zone administrator chooses to take the path of a regular roll-over, the malicious key holder can spoof data so that it appears to be valid, note that this kind of attack will usually be localised in the Internet topology. 4.1 KSK Compromise When the KSK has been compromised the parent must be notified as soon as possible using secure means. The keyset of the zone should be resigned as soon as possible. Care must be taken to not break the authentication chain. The local zone can only be resigned with the new KSK after the parent's zone has been updated with the new KSK. Before this update takes place it would be best to drop the security status of a zone all together: the parent removes the DS of the child at the next zone update. After that the child can be made secure again. An additional danger of a key compromise is that the compromised key can be used to facilitate a legitimate DNSKEY/DS and/or nameserver rollover at the parent. When that happens the domain can be in dispute. An out of band and secure notify mechanism to contact a parent is needed in this case. 4.2 ZSK Compromise Primarily because there is no parental interaction required when a ZSK is compromised, the situation is less severe than with with a KSK compromise. The zone must still be resigned with a new ZSK as soon as possible. As this is a local operation and requires no communication between the parent and child this can be achieved fairly quickly. However, one has to take into account that just as with a normal rollover the immediate disappearance from the old compromised key may lead to verification problems. The pre-publication scheme as discussed above minimises such problems. Kolkman & Gieben Expires August 30, 2004 [Page 15] Internet-Draft DNSSEC Operational Practices March 2004 4.3 Compromises of Keys Anchored in Resolvers A key can also be pre-configured in resolvers. If DNSSEC is rolled out as planned the root key should be pre-configured in every secure aware resolver on the planet. [Editors Note: add more about authentication of a newly received resolver key] If trust-anchor keys are compromised, the resolvers using these keys should be notified of this fact. Zone administrators may consider setting up a mailing list to communicate the fact that a SEP key is about to be rolled over. This communication will of course need to be authenticated e.g. by using digital signatures. 5. Parental Policies 5.1 Initial Key Exchanges and Parental Policies Considerations The initial key exchange is always subject to the policies set by the parent (or its registry). When designing a key exchange policy one should take into account that the authentication and authorisation mechanisms used during a key exchange should be as strong as the authentication and authorisation mechanisms used for the exchange of delegation information between parent and child. Using the DNS itself as the source for the actual DNSKEY material, with an off-band check on the validity of the DNSKEY, has the benefit that it reduces the chances of user error. A parental DNSKEY download tool can make use of the SEP bit [4] to select the proper key from a DNSSEC keyset; thereby reducing the chance that the wrong DNSKEY is sent. It can validate the self-signature over a key; thereby verifying the ownership of the private key material. Fetching the DNSKEY from the DNS ensures that the child will not become bogus once the parent publishes the DS RR indicating the child is secure. Note: the off-band verification is still needed when the key-material is fetched by a tool. The parent can not be sure whether the DNSKEY RRs have been spoofed. 5.2 Storing Keys So Hashes Can Be Regenerated When designing a registry system one should consider if the DNSKEYs and/or the corresponding DSs are stored. Storing DNSKEYs will help during troubleshooting while the overhead of calculating DS records from them is minimal. Having an out-of-band mechanism, such as a Whois database, to find out which keys are used to generate DS Resource Records for specific owners may also help with troubleshooting. Kolkman & Gieben Expires August 30, 2004 [Page 16] Internet-Draft DNSSEC Operational Practices March 2004 5.3 Security Lameness Checks Security Lameness is defined as what happens when a parent has a DS Resource Record pointing to a non-existing DNSKEY RR. During key exchange a parent should make sure that the child's key is actually configured in the DNS before publishing a DS RR in its zone. Failure to do so would render the child's zone being marked as bogus. Child zones should be very careful removing DNSKEY material, specifically SEP keys, for which a DS RR exists. Once a zone is "security lame" a fix (e.g. by removing a DS RR) will take time to propagate through the DNS. 5.4 DS Signature Validity Period Since the DS can be replayed as long as it has a valid signature a short signature validity period over the DS minimises the time a child is vulnerable in the case of a compromise of the child's KSK(s). A signature validity period that is too short introduces the possibility that a zone is marked bogus in case of a configuration error in the signer; there may not be enough time to fix the problems before signatures expire. Something as mundane as operator unavailability during weekends shows the need for DS signature lifetimes longer than 2 days. We recommend the minimum for a DS signature validity period to be a few days. The maximum signature lifetime of the DS record depends on how long child zones are willing to be vulnerable after a key compromise. We consider a signature validity period of around one week to be a good compromise between the operational constraints of the parent and minimising damage for the child. 6. Security Considerations DNSSEC adds data integrity to the DNS. This document tries to assess considerations to operate a stable and secure DNSSEC service. Not taking into account the 'data propagation' properties in the DNS will cause validation failures and may make secured zones unavailable to security aware resolvers. 7. Acknowledgments We, the folk mentioned as authors, only acted as editors. Most of the ideas in this draft were the result of collective efforts during workshops, discussions and try outs. At the risk of forgetting individuals who where the original Kolkman & Gieben Expires August 30, 2004 [Page 17] Internet-Draft DNSSEC Operational Practices March 2004 contributors of the ideas we would like to acknowledge people who where actively involved in the compilation of this document. In random order: Olafur Gudmundsson, Wesley Griffin, Michael Richardson, Scott Rose, Rick van Rein, Tim McGinnis, Gilles Guette and Olivier Courtay, Sam Weiler. Emma Bretherick and Adrian Bedford corrected many of the spelling and style issues. Kolkman and Gieben take the blame for introducing all miscakes(SIC). 8. References 8.1 Normative References [1] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [2] Eastlake, D., "DNS Security Operational Considerations", RFC 2541, March 1999. [3] Lewis, E., "DNS Security Extension Clarification on Zone Status", RFC 3090, March 2001. [4] Lewis, E., Kolkman, O. and J. Schlyter, "KEY RR Key-Signing Key (KSK) Flag", draft-ietf-dnsext-keyrr-key-signing-flag-06 (work in progress), February 2003. 8.2 Informative References [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [6] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, March 1998. [7] Gudmundsson, O., "Delegation Signer Resource Record", draft-ietf-dnsext-delegation-signer-13 (work in progress), March 2003. [8] Arends, R., "Protocol Modifications for the DNS Security Extensions", draft-ietf-dnsext-dnssec-protocol-01 (work in progress), March 2003. [9] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key Sizes", The Journal of Cryptology 14 (255-293), 2001. Kolkman & Gieben Expires August 30, 2004 [Page 18] Internet-Draft DNSSEC Operational Practices March 2004 Authors' Addresses Olaf M. Kolkman RIPE NCC Singel 256 Amsterdam 1016 AB The Netherlands Phone: +31 20 535 4444 EMail: olaf@ripe.net URI: http://www.ripe.net/ Miek Gieben NLnet Labs Kruislaan 419 Amsterdam 1098 VA The Netherlands EMail: miek@nlnetlabs.nl URI: http://www.nlnetlabs.nl Appendix A. Terminology In this document there is some jargon used that is defined in other documents. In most cases we have not copied the text from the documents defining the terms but given a more elaborate explanation of the meaning. Note that these explanations should not be seen as authoritative. Private and Public Keys: DNSSEC secures the DNS through the use of public key cryptography. Public key cryptography is based on the existence of two keys, a public key and a private key. The public keys are published in the DNS by use of the DNSKEY Resource Record (DNSKEY RR). Private keys should remain private i.e. should not be exposed to parties not-authorised to do the actual signing. Signer: The system that has access to the private key material and signs the Resource Record sets in a zone. A signer may be configured to sign only parts of the zone e.g. only those RRsets for which existing signatures are about to expire. KSK: A Key-Signing Key (KSK) is a key that is used exclusively for signing the apex keyset. The fact that a key is a KSK is only relevant to the signing tool. ZSK: A Zone Signing Key (ZSK) is a key that is used for signing all data in a zone. The fact that a key is a ZSK is only relevant to the signing tool. Kolkman & Gieben Expires August 30, 2004 [Page 19] Internet-Draft DNSSEC Operational Practices March 2004 SEP Key: A KSK that has a parental DS record pointing to it. Note: this is not enforced in the protocol. A SEP Key with no parental DS is security lame. Anchored Key: A DNSKEY configured in resolvers around the globe. This Key is hard to update, hence the term anchored. Bogus: [Editors Note: a reference here] An RRset in DNSSEC is marked "Bogus" when a signature of a RRset does not validate against the DNSKEY. Even if the key itself was not marked Bogus. A cache may choose to cache Bogus data for various reasons. Singing the Zone File: The term used for the event where an administrator joyfully signs its zone file while producing melodic sound patterns. Zone Administrator: The 'role' that is responsible for signing a zone and publishing it on the primary authoritative server. Appendix B. Zone-signing Key Rollover Howto Using the pre-published signature scheme and the most conservative method to assure oneself that data does not live in distant caches here follows the "HOWTO". [WES: has some comments about this] Key notation: Step 0: The preparation: Create two keys and publish both in your keyset. Mark one of the keys as "active" and the other as "published". Use the "active" key for signing your zone data. Store the private part of the "published" key, preferably off-line. Step 1: Determine expiration: At the beginning of the rollover make a note of the highest expiration time of signatures in your zone file created with the current key marked as "active". Wait until the expiration time marked in Step 1 has passed Step 2: Then start using the key that was marked as "published" to sign your data i.e. mark it as "active". Stop using the key that was marked as "active", mark it as "rolled". Step 3: It is safe to engage in a new rollover (Step 1) after at least one "signature validity period". Appendix C. Typographic Conventions The following typographic conventions are used in this document: Key notation: A key is denoted by KEYx, where x is a number, x could be thought of as the key id. RRset notations: RRs are only denoted by the type. All other information - owner, class, rdata and TTL - is left out. Thus: example.com 3600 IN A 192.168.1.1 is reduced to: A. RRsets are a list of RRs. A example of this would be: A1,A2, specifying the RRset containing two A records. This could again be abbreviated to just: A. Kolkman & Gieben Expires August 30, 2004 [Page 20] Internet-Draft DNSSEC Operational Practices March 2004 Signature notation: Signatures are denoted as RRSIGx(RRset), which means that RRset is signed with DNSKEYx. Zone representation: Using the above notation we have simplified the representation of a signed zone by leaving out all unnecessary details such as the names and by representing all data by "SOAx" SOA representation: SOA's are represented as SOAx, where x is the serial number. Using this notation the following zone : example.net. 600 IN SOA ns.example.net. ernie.example.net. ( 10 ; serial 450 ; refresh (7 minutes 30 seconds) 600 ; retry (10 minutes) 345600 ; expire (4 days) 300 ; minimum (5 minutes) ) 600 RRSIG SOA 5 2 600 20130522213204 ( 20130422213204 14 example.net. cmL62SI6iAX46xGNQAdQ... ) 600 NS a.iana-servers.net. 600 NS b.iana-servers.net. 600 RRSIG NS 5 2 600 20130507213204 ( 20130407213204 14 example.net. SO5epiJei19AjXoUpFnQ ... ) 3600 DNSKEY 256 3 5 ( EtRB9MP5/AvOuVO0I8XDxy0... ) ; key id = 14 3600 DNSKEY 256 3 5 ( gsPW/Yy19GzYIY+Gnr8HABU... ) ; key id = 15 3600 RRSIG DNSKEY 5 2 3600 20130522213204 ( 20130422213204 14 example.net. J4zCe8QX4tXVGjV4e1r9... ) 3600 RRSIG DNSKEY 5 2 3600 20130522213204 ( 20130422213204 15 example.net. keVDCOpsSeDReyV6O... ) 600 NSEC a.example.net. NS SOA TXT RRSIG DNSKEY NSEC 600 RRSIG NSEC 5 2 600 20130507213204 ( 20130407213204 14 example.net. obj3HEp1GjnmhRjX... ) a.example.net. 600 IN TXT "A label" 600 RRSIG TXT 5 3 600 20130507213204 ( 20130407213204 14 example.net. IkDMlRdYLmXH7QJnuF3v... ) 600 NSEC b.example.com. TXT RRSIG NSEC 600 RRSIG NSEC 5 3 600 20130507213204 ( 20130407213204 14 example.net. Kolkman & Gieben Expires August 30, 2004 [Page 21] Internet-Draft DNSSEC Operational Practices March 2004 bZMjoZ3bHjnEz0nIsPMM... ) ... is reduced to the following represenation: SOA10 RRSIG14(SOA10) DNSKEY14 DNSKEY15 RRSIG14(KEY) RRSIG15(KEY) The rest of the zone data has the same signature as the SOA record, i.e a RRSIG created with DNSKEY 14. Appendix D. Document Details and Changes This section is to be removed by the RFC editor if and when the document is published. $Header: /var/cvs/dnssec-key/ draft-ietf-dnsop-dnssec-operational-practices.xml,v 1.22 2004/05/12 08:29:11 dnssec Exp $ D.1 draft-ietf-dnsop-dnssec-operational-practices-00 Submission as working group document. This document is a modified and updated version of draft-kolkman-dnssec-operational-practices-00. D.2 draft-ietf-dnsop-dnssec-operational-practices-01 changed the definition of "Bogus" to reflect the one in the protocol draft. Bad to Bogus Style and spelling corrections KSK - SEP mapping made explicit. 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