DNSOP O. Kolkman Internet-Draft RIPE NCC Expires: March 1, 2004 R. Gieben NLnet Labs September 2003 DNSSEC Operational Practices draft-ietf-dnsop-dnssec-operational-practices-00.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 March 1, 2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document intends to describe a set of practices for operating a DNSSEC aware enviroment. Its target audience is zone administrators who are deploying DNSSEC and need a guide to help them chose sensible values for DNSSEC parameters. Is also discusses operational matters like key rollovers, KSK and ZSK considerations and more. Kolkman & Gieben Expires March 1, 2004 [Page 1] Internet-Draft DNSSEC Operational Practices September 2003 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 The use of the term 'key' . . . . . . . . . . . . . . . . . 3 2. Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Time definitions . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Time considerations . . . . . . . . . . . . . . . . . . . . 4 3. Keys . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Motivations for the KSK and ZSK functions . . . . . . . . . 6 3.2 Key security considerations . . . . . . . . . . . . . . . . 7 3.3 Key rollovers . . . . . . . . . . . . . . . . . . . . . . . 8 3.3.1 Zone-signing key rollovers . . . . . . . . . . . . . . . . . 9 3.3.2 Key-signing key rollovers . . . . . . . . . . . . . . . . . 12 4. Planning for emergency key rollover. . . . . . . . . . . . . 13 4.1 KSK compromise . . . . . . . . . . . . . . . . . . . . . . . 13 4.2 ZSK compromise . . . . . . . . . . . . . . . . . . . . . . . 14 4.3 Compromises of keys anchored in resolvers . . . . . . . . . 14 5. Parental policies. . . . . . . . . . . . . . . . . . . . . . 14 5.1 Initial key exchanges and parental policies considerations. . . . . . . . . . . . . . . . . . . . . . . 14 5.2 Storing keys so hashes can be regenerated . . . . . . . . . 15 5.3 Security lameness checks. . . . . . . . . . . . . . . . . . 15 5.4 SIG DS validity period. . . . . . . . . . . . . . . . . . . 15 6. Security considerations . . . . . . . . . . . . . . . . . . 16 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 16 Normative References . . . . . . . . . . . . . . . . . . . . 16 Informative References . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17 A. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 17 B. Zone-signing key rollover howto . . . . . . . . . . . . . . 18 C. Typographic conventions . . . . . . . . . . . . . . . . . . 19 D. Document Details and Changes . . . . . . . . . . . . . . . . 20 D.1 draft-ietf-dnsop-dnssec-operational-practices-00 . . . . . . 21 Intellectual Property and Copyright Statements . . . . . . . 22 Kolkman & Gieben Expires March 1, 2004 [Page 2] Internet-Draft DNSSEC Operational Practices September 2003 1. Introduction During workshops and early operational deployment tests, operators and system administrators gained knowledge about operating DNSSEC aware DNS services. This document describes these practices. The structure of the document is as follows. It starts 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 is 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. Therefore this document will use the term key rather loosely. Wherever we write that 'a key is used to sign data' it is assumed that the reader knows that it is the private part of the key-pair that is used for signing. It is also assumed that the reader will know that the public part of the key-pair is published in the DNSKEY resource record and that it is the public part of a key-pair that is used in key-exchanges. 2. Time in DNSSEC Without DNSSEC all times in DNS are relative. The SOA's refresh, retry and expiration timers are counters that are being used to determine the time elapsed after a slave server synced (or tried to sync) with a master server. The TTL value and the SOA minimum TTL parameter [6] are used to 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 invalid and the signed data is to be considered BAD. 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" Kolkman & Gieben Expires March 1, 2004 [Page 3] Internet-Draft DNSSEC Operational Practices September 2003 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. o "Signature publication period" Time after which a signature made with a 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 on time T0 and a new signature is published on 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 to the period between two consecutive zone signing operations. o "Key publication period" The period for which the public part of the key is published in the DNS. The public part of the key can be published in the DNS while it has not yet been used to sign data. As soon as a public key is published a brute force attack can be attempted to recover the private key. Publishing the public key in advance (and not signing any data with it) does not guard against this attack. [Editor's Note: We don't use this term in the doc yet, is it needed elsewhere and handy to define here? No:1 Yes:0] 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 behavior might become bursty. Kolkman & Gieben Expires March 1, 2004 [Page 4] Internet-Draft DNSSEC Operational Practices September 2003 We suggest the TTL on all the RRs in your zone to be at least an order of magnitude smaller than your signature validity period. o The signature publication period should at least be one maximum TTL smaller than the signature validity period. If a zone is resigned shortly before the end of the signature validity period this may cause simultaneous expiration of data from caches which leads to bursty query behavior and increase the load on authoritative servers. o The Minimum zone TTL should be long enough to fetch and verify all the RRs in the authentication chain. 1. During validation, some data may expire before validation is complete. The validator should be able to keep all the data, until validation is complete. This applies to all data in 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. 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 at least of the order 10 minutes to an hour and all the other RRs in your zone to be at least 30 seconds. These are absolute minimum, we recommend zone administrators to chose longer ones. [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. 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 counted down to zero. It is not possible to fully prevent this from happening by tweaking the SOA parameters. But the effects can be minimized if the SOA expiration time is of the same of Kolkman & Gieben Expires March 1, 2004 [Page 5] Internet-Draft DNSSEC Operational Practices September 2003 order of magnitude as or smaller than the signature validity period. When a zone cannot be updated while signatures in that zone have expired non-secure resolvers will continue to be able to resolve the data served by the particular slave servers. Only security aware resolvers that receive data with expired signatures will experience problems. We suggest the SOA expiration timer being approximately one third or one fourth of the signature validity period. We also suggest that operators of nameservers with slave zones develop watchdogs to be able to spot these upcoming signature expirations in slave zones, so that appropriate action can be taken. o [Editor's Note: Need examples here] 3. Keys 3.1 Motivations for the KSK and ZSK functions Delegation Signer [7] introduced the concept of key-signing and zone-signing keys.The Key-signing-flag [4] introduced the concept of a key with the Secure Entry Point flag set; a key that is the first key from the zone when following an authentication chain. When using a key-signing key with the SEP flag set (the parent has a DS RR pointing to that DNSKEY) and when using zone-signing keys without the SEP flag set (a practice which we recommend ) one can use the following operational procedures. 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 over no interactions with the parent is needed. This allows for relatively short "Signature Validity Periods" (order of days). The key-signing key (with the SEP flag set) 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 maintainer 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 March 1, 2004 [Page 6] Internet-Draft DNSSEC Operational Practices September 2003 3.2 Key security considerations In RFC2541 [2] a number of considerations with respect to the security of keys are described. That document deals with the generation, lifetime, size and storage of private keys. In Section 3 of RFC2541 [2], Eastlake does have some suggestions: 13 months for long-lived keys and 36 days for transaction keys but suggestions for key sizes are not made. If we read the long-lived key being a key that is used as key-signing key and transaction keys being zone signing keys, then these recommendations are good starting points for an operational procedure. These recommendations will lead to rollovers occurring frequently enough so that they can become part of 'operational habits' and the procedure does not have to be reinvented every time a key is replaced. When choosing a 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. 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 Kolkman & Gieben Expires March 1, 2004 [Page 7] Internet-Draft DNSSEC Operational Practices September 2003 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 Suppose you want your key to last 3 years and the current year is 2003. Add 3 to 2003 equals 2006 and read of the sizes: 1191 for asymmetric keys and 148 bits for elliptic curve keys. Note that adding only a "handful of bits" to the key size will increase the key's resistance against brute force attacks. 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 maintainers who are in the process of rolling their keys have to take into account that data they have 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 which is 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 BAD. 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 BAD. 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. Kolkman & Gieben Expires March 1, 2004 [Page 8] Internet-Draft DNSSEC Operational Practices September 2003 Note that KSK rollovers and ZSK rollovers are different. A zone-key rollover can be handled in two different way: pre-publish and [Editors note: ref please] double-sig. 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 the data still in caches can be verified with the new keysets or the newly generated signatures can be verified with the keys still in caches. One schema uses double signatures, it is described in Section 3.3.1.1, the other uses key pre-publication (Section 3.3.1.2). The pros, cons and recommendations are described in Section 3.3.1.3. 3.3.1.1 A double signature zone-signing key rollover This section shows how to perform a ZSK key rollover using the double zone data signature scheme. 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) Kolkman & Gieben Expires March 1, 2004 [Page 9] Internet-Draft DNSSEC Operational Practices September 2003 normal: Version 0 of the zone: DNSKEY 1 is a key-signing key. DNSKEY 10 is used to sign all the data of the zone, it is the zone-signing key. 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 the 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 the 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". To be on the safe side one could make sure that the rollover phase lasts until the signature expiration time of the data in version 0 of the zone. But this date could be considerable longer than the Maximum Zone TTL, making the rollover a lengthly procedure. Note that in this example we assumed that the zone did not get modified during the rollover. New data can be introduced in the zone as long as it is signed with both keys. 3.3.1.2 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. We recommend this method because it has advantages in the case of key compromises. If the old key gets compromised the new key is already distributed in the DNS. The zone administrator is then able to quickly switch to the new key and remove the compromised 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) Kolkman & Gieben Expires March 1, 2004 [Page 10] Internet-Draft DNSSEC Operational Practices September 2003 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 a key-signing key. DNSKEY 10 is used to sign all the data of the zone, its the zone-signing key. pre-roll: DNSKEY 11 is introduced in the keyset. Note that no signatures are generated with this key yet, but this will not prevent 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 on the keyset. This would boil down 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 that includes 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 a bit by always publishing the "future" key immediately after the rollover. The scheme would look like this (we show 2 rollovers); the future key is introduced in "after" as DNSKEY 12 and again a newer one, numbered 13, in "2nd after": normal roll after 2nd roll 2nd after SOA0 SOA2 SOA3 SOA4 SOA5 RRSIG10(SOA0) RRSIG11(SOA2) RRSIG11(SOA3) RRSIG12(SOA4) RRSIG12(SOA5) Kolkman & Gieben Expires March 1, 2004 [Page 11] Internet-Draft DNSSEC Operational Practices September 2003 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, can immediately be used during an emergency rollover under the assumption that it was stored in a physically secure manner. 3.3.1.3 Pros and cons of the schemes A double signature rollover: The drawback of this signing scheme is that during the rollover the number of signatures in your zone doubles, which may be prohibitive if you have very big zones. An advantage is that it only requires three steps. 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 is useless for KSKs as explained in Section 3.3. 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, size considerations do not apply. normal roll after SOA0 SOA1 SOA2 RRSIG10(SOA0) RRSIG10(SOA1) RRSIG10(SOA2) DNSKEY1 DNSKEY1 DNSKEY2 DNSKEY2 DNSKEY10 DNSKEY10 DNSKEY10 Kolkman & Gieben Expires March 1, 2004 [Page 12] Internet-Draft DNSSEC Operational Practices September 2003 RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG2(DNSKEY) RRSIG2 (DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY) 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 has as a result 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 by 'blackhats' for as long as a valid authentication chain exists. A authentication chain remains intact for: as long as a signature over the compromised key in the authentication chain is valid, as long as a parental DS RR (and signature) points to the compromised key, 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 "blackhat". 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 can not be validated. On the other hand if the zone administrator chooses to take the path of a regular roll-over the "blackhat" can spoof data so that it appears to be valid, note that this kind of attack will usually be localized in the Internet topology. 4.1 KSK compromise When the KSK has been compromised the parent must be notified as soon as possible and through 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. Kolkman & Gieben Expires March 1, 2004 [Page 13] Internet-Draft DNSSEC Operational Practices September 2003 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 legitemate 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 Mainly 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. One has to take into account though 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 minimizes that problem. 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 that key is compromised all the resolvers 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 authorization mechanisms used during a key exchange should be as strong as the authentication and authorization 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 changes of operator error. A parental DNSKEY Kolkman & Gieben Expires March 1, 2004 [Page 14] Internet-Draft DNSSEC Operational Practices September 2003 download tool can make use of the SEP bit [4] to select the proper key from a DNSSEC keyset; thereby reducing the change that the wrong DNSKEY is sent. It can validate the self-signature over a key; thereby verifying the ownership of the private key material. Besides, by fetching the DNSKEY from the DNS one can be sure that the child will not become invisible once the parent indicates the child is secure by publishing the DS RR. Note: the off-band verification is still needed when the keymaterial is fetched by a tool. The parent can not be sure if the DNSKEY RRs where not spoofed. 5.2 Storing keys so hashes can be regenerated When designing a registry system one should consider if the DNSKEYs or the corresponding DSs are stored. Storing DNSKEYs will help during troubleshooting while the overhead of calculating DS records from them is minimal. Having a 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. 5.3 Security lameness checks. Security lameness is defined as the event that a parent has a DS Resource Record that points to a non-existing DNSKEY RR. At key exchange a parent should make sure that the childs 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 marked "BAD". Child zones should be very careful removing DNSKEY material, specifically SEP keys, for which a DS RR exist. 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 SIG DS validity period. Since the DS can be replayed as long as it has a valid signature a short signature validity period over the DS minimizes the time a child is vulnerable in the case of a compromise of the child's KSK. A signature validity period that is too short introduces the possibility that a zone is marked BAD 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 weekends show the need for a DS signature lifetimes longer than 2 days. We recommend the minimum for a DS signature validity period to be about a few Kolkman & Gieben Expires March 1, 2004 [Page 15] Internet-Draft DNSSEC Operational Practices September 2003 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 the order of one week a good compromise between the operational constraints of the parent and minimizing 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. 7. Acknowledgments We, the folk mentioned as authors, only acted as editors. Most of the ideas in this draft where the result of collective efforts during workshops and discussions and try outs. At the risk of forgetting individuals who where the original contributors of the ideas we like to acknowledge people who where actively involved in the compilation of this document. In alphabetical order: Olafur Gudmundsson, Wesley Griffin, Michael Richardson, Scott Rose, Rick van Rein, Tim McGinnis. Kolkman and Gieben take the blame for all mistakes. 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. 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 Kolkman & Gieben Expires March 1, 2004 [Page 16] Internet-Draft DNSSEC Operational Practices September 2003 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. Authors' Addresses Olaf M. Kolkman RIPE NCC Singel 256 Amsterdam 1016 AB NL Phone: +31 20 535 4444 EMail: olaf@ripe.net URI: http://www.ripe.net/ Miek Gieben NLnet Labs Kruislaan 419 Amsterdam 1098 VA NL 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 give 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 2 keys, a public key and a private key. The public keys are published in the DNS by use of the DNSKEY Resource Record Kolkman & Gieben Expires March 1, 2004 [Page 17] Internet-Draft DNSSEC Operational Practices September 2003 (DNSKEY RR). Private keys are supposed to remain private i.e. should not be exposed to parties not-authorized 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 for exclusively 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. BAD: [Editors Note: a reference here] A RRset in DNSSEC is marked "bad" when a signature of a RRset does not validate against the DNSKEY. Even is the key itself was not marked BAD. BAD data is not cached. Singing the Zone File: The term used for the event where an administrator joyfully signs its zone file while producing melodic sound patterns. 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] STEP 0, the preparation: Create two keys and publish them 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 zonefile created with the current key currently marked as "active". Wait until the expiration time marked in STEP 1 Kolkman & Gieben Expires March 1, 2004 [Page 18] Internet-Draft DNSSEC Operational Practices September 2003 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 abreviated to just: A. Signature notation: Signatures are denoted as SIGx(RRset), which means that RRset is signed with KEYx. Zone representation: Using the above notation we have simplify the representation of a signed zone by leaving out all unneeded details such as the names and by just representing all data by "SOAx" SOA representation: Soa's are represented as SOA x, 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. Kolkman & Gieben Expires March 1, 2004 [Page 19] Internet-Draft DNSSEC Operational Practices September 2003 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. 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 Kolkman & Gieben Expires March 1, 2004 [Page 20] Internet-Draft DNSSEC Operational Practices September 2003 document is published. $Header: /var/cvs/dnssec-key/ draft-ietf-dnsop-dnssec-operational-practices.xml,v 1.5 2003/10/10 09:49:07 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. 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