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Levine 3 Internet-Draft Taughannock Networks 4 Intended status: Standards Track July 28, 2008 5 Expires: January 29, 2009 7 DNS Blacklists and Whitelists 8 draft-irtf-asrg-dnsbl-06 10 Status of this Memo 12 By submitting this Internet-Draft, each author represents that any 13 applicable patent or other IPR claims of which he or she is aware 14 have been or will be disclosed, and any of which he or she becomes 15 aware will be disclosed, in accordance with Section 6 of BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on January 29, 2009. 35 Abstract 37 The rise of spam and other anti-social behavior on the Internet has 38 led to the creation of shared blacklists and whitelists of IP 39 addresses or domains. The DNS has become a de-facto standard method 40 of distributing these blacklists and whitelists. This memo documents 41 the structure and usage of DNS based blacklists and whitelists, and 42 the protocol used to query them. 44 IRTF Notice 46 This document is a product of the Anti-Spam Research Group (ASRG). 47 Comments and discussion may be directed to the ASRG mailing list, 48 asrg@irtf.org. 50 This document represents the consensus of the Anti-Spam Research 51 Group of the Internet Research Task Force. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Structure of an IP address DNSBL or DNSWL . . . . . . . . . . 3 57 2.1. IP address DNSxL . . . . . . . . . . . . . . . . . . . . . 4 58 2.2. IP address DNSWL . . . . . . . . . . . . . . . . . . . . . 4 59 2.3. Combined IP address DNSxL . . . . . . . . . . . . . . . . 5 60 2.4. IPv6 DNSxLs . . . . . . . . . . . . . . . . . . . . . . . 6 61 3. Domain name DNSxLs . . . . . . . . . . . . . . . . . . . . . . 6 62 4. DNSxL cache behavior . . . . . . . . . . . . . . . . . . . . . 7 63 5. Test and contact addresses . . . . . . . . . . . . . . . . . . 7 64 6. Typical usage of DNSBLs and DNSWLs . . . . . . . . . . . . . . 8 65 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 66 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 67 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 68 9.1. Normative References . . . . . . . . . . . . . . . . . . . 10 69 9.2. Informative References . . . . . . . . . . . . . . . . . . 10 70 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 10 71 A.1. Changes since -asrg-dnsbl-05 . . . . . . . . . . . . . . . 11 72 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 73 Intellectual Property and Copyright Statements . . . . . . . . . . 12 75 1. Introduction 77 In 1997, Dave Rand and Paul Vixie, well known Internet software 78 engineers, started keeping a list of IP addresses that had sent them 79 spam or engaged in other behavior that they found objectionable. 80 Word of the list quickly spread, and they started distributing it as 81 a BGP feed for people who wanted to block all traffic from listed IP 82 addresses at their routers. The list became known as the Real-time 83 Blackhole List (RBL). 85 Many network managers wanted to use the RBL to block unwanted e-mail, 86 but weren't prepared to use a BGP feed. Rand and Vixie created a 87 DNS-based distribution scheme that quickly became more popular than 88 the original BGP distribution. Other people created other DNS-based 89 blacklists either to compete with the RBL or to complement it by 90 listing different categories of IP addresses. Although some people 91 refer to all DNS-based blacklists as ``RBLs'', the term properly is 92 used for the MAPS RBL, the descendant of the original list. (In the 93 United States, the term RBL is a registered service mark of Trend 94 Micro[MAPSRBL].) 96 The conventional term is now DNS Blacklist or Blocklist, or DNSBL. 97 Some people also publish DNS-based whitelists or DNSWLs. Network 98 managers typically use DNSBLs to block traffic and DNSWLs to 99 preferentially accept traffic. The structure of a DNSBL and DNSWL 100 are the same, so in the subsequent discussion we use the abbreviation 101 DNSxL to mean either. 103 This document defines the structure of DNSBLs and DNSWLs. It 104 describes the structure, operation, and use of DNSBLs and DNSWLs but 105 does not describe or recommend policies for adding or removing 106 addresses to and from DNSBLs and DNSWLs, nor does it recommend 107 policies for using them. We anticipate that management policies will 108 be addressed in a companion document. 110 Requirements Notation: The key words "MUST", "MUST NOT", 111 "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", 112 "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be 113 interpreted as described in [RFC2119]. 115 2. Structure of an IP address DNSBL or DNSWL 117 A DNSxL is a zone in the DNS[RFC1034][RFC1035]. The zone containing 118 resource records identifies hosts present in a blacklist or 119 whitelist. Hosts were originally encoded into DNSxL zones using a 120 transformation of their IP addresses, but now host names are 121 sometimes encoded as well. Most DNSxLs still use IP addresses. 123 2.1. IP address DNSxL 125 An IPv4 address DNSxL has a structure adapted from that of the rDNS. 126 (The rDNS, reverse DNS, is the IN-ADDR.ARPA[RFC1034] and 127 IP6.ARPA[RFC3596] domains used to map IP addresses to domain names.) 128 Each IPv4 address listed in the DNSxL has a corresponding DNS entry 129 created by reversing the order of the octets of the text 130 representation of the IP address, and appending the domain name of 131 the DNSxL. 133 If, for example, the DNSxL is called bad.example.com, and the IPv4 134 address to be listed is 192.0.2.99, the name of the DNS entry would 135 be 99.2.0.192.bad.example.com. Each entry in the DNSxL MUST have an 136 A record. DNSBLs SHOULD have a TXT record that describes the reason 137 for the entry. DNSWLs MAY have a TXT record that describes the 138 reason for the entry. The A record conventionally has the value 139 127.0.0.2, but MAY have other values as described below in Combined 140 IP address DNSxLs. The TXT record describes the reason that the IP 141 is listed in the DNSxL, and is often used as the text of an SMTP 142 error response when an SMTP client attempts to send mail to a server 143 using the list as a DNSBL, or as explanatory text when the DNSBL is 144 used in a scoring spam filter. The DNS records for this entry might 145 be: 147 99.2.0.192.bad.example.com A 127.0.0.2 148 99.2.0.192.bad.example.com TXT 149 "Dynamic address, see http://bad.example.com?192.0.2.99" 151 Some DNSxLs use the same TXT record for all entries, while others 152 provide a different TXT record for each entry or range of entries 153 that describes the reason that entry or range is listed. The reason 154 often includes the URL of a web page where more information is 155 available. Client software MUST check the A record and MAY check the 156 TXT record. 158 If a range of addresses is listed in the DNSxL, the DNSxL MUST 159 contain an A record (or a pair of A and TXT records) for every 160 address in the DNSxL. Conversely, if an IP address is not listed in 161 the DNSxL, there MUST NOT be any records for the address. 163 2.2. IP address DNSWL 165 Since SMTP has no standard way for a server to advise a client why a 166 request was accepted, TXT records in DNSWLs are not very useful. 167 Some DNSWLs contain TXT records anyway to document the reasons that 168 entries are present. 170 It is possible and occasionally useful for a DNSxL to be used as a 171 DNSBL in one context and a DNSWL in another. For example, a DNSxL 172 that lists the IP addresses assigned to dynamically assigned 173 addresses on a particular network might be used as a DNSWL on that 174 network's outgoing mail server or intranet web server, and used as a 175 DNSBL for mail servers on other networks. 177 2.3. Combined IP address DNSxL 179 In many cases, an organization maintains a DNSxL that contains 180 multiple entry types, with the entries of each type constituting a 181 sublist. For example, an organization that publishes a DNSBL listing 182 sources of unwanted e-mail might wish to indicate why various 183 addresses are included in the list, with one sublist for addresses 184 listed due to sender policy, a second list for addresses of open 185 relays, a third list for hosts compromised by malware, and so forth. 186 (At this point all of the DNSxLs with sublists of which we are aware 187 are intended for use as DNSBLs, but the sublist techniques are 188 equally usable for DNSWLs.) 190 There are three common methods of representing a DNSxL with multiple 191 sublists: subdomains, multiple A records, and bit encoded entries. 192 DNSxLs with sublists SHOULD use both subdomains and one of the other 193 methods. 195 Sublist subdomains are merely subdomains of the main DNSxL domain. 196 If for example, bad.example.com had two sublists relay and malware, 197 entries for 192.0.2.99 would be 99.2.0.192.relay.bad.example.com or 198 99.2.0.192.malware.bad.example.com. If a DNSxL contains both entries 199 for a main domain and for sublists, sublist names MUST be at least 200 two characters and contain non-digits, so there is no problem of name 201 collisions with entries in the main domain, where the IP addresses 202 consist of digits or single hex characters. 204 To minimize the number of DNS lookups, multiple sublists can also be 205 encoded as bit masks or multiple A records. With bit masks, the A 206 record entry for each IP is the logical OR of the bit masks for all 207 of the lists on which the IP appears. For example, the bit masks for 208 the two sublists might be 127.0.0.2 and 127.0.0.4, in which case an 209 entry for an IP on both lists would be 127.0.0.6: 211 99.2.0.192.bad.example.com A 127.0.0.6 213 With multiple A records, each sublist has a different assigned value 214 such as 127.0.1.1, 127.0.1.2, and so forth, with an A record for each 215 sublist on which the IP appears: 217 99.2.0.192.bad.example.com A 127.0.1.1 218 99.2.0.192.bad.example.com A 127.0.1.2 219 There is no widely used convention for mapping sublist names to bits 220 or values, beyond the convention that all A values SHOULD be in the 221 127.0.0.0/8 range to prevent unwanted network traffic if the value is 222 accidentally used as an IP address. 224 DNSxLs that return multiple A records sometimes return multiple TXT 225 records as well, although the lack of any way to match the TXT 226 records to the A records limits the usefulness of those TXT records. 227 Other combined DNSxLs return a single TXT record. 229 2.4. IPv6 DNSxLs 231 The structure of DNSxLs based on IPv6 addresses is adapted from that 232 of the IP6.ARPA domain defined in [RFC3596]. Each entry MUST be a 233 32-component hex nibble-reversed IPv6 addresses suffixed by the the 234 DNSxL domain. For example, to represent the address: 236 2001:db8:1:2:3:4:567:89ab 238 in the DNSxL ugly.example.com, the entry might be: 240 b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2. 241 ugly.example.com. A 127.0.0.2 242 TXT "Spam received." 244 Combined IPv6 sublist DNSxLs are represented the same way as IPv4 245 DNSxLs, replacing the four octets of IPv4 address with the 32 nibbles 246 of IPv6 address. 248 A single DNSxL could in principle contain both IPv4 and IPv6 249 addresses, since the different lengths prevent any ambiguity. If a 250 DNSxL is represented using traditional zone files and wildcards, 251 there is no way to specify the length of the name that a wildcard 252 matches, so wildcard names would indeed be ambiguous for DNSxLs 253 served in that fashion. 255 3. Domain name DNSxLs 257 A few DNSxLs list domain names rather than IP addresses. They are 258 sometimes called RHSBLs, for right hand side blacklists. The names 259 of their entries MUST contain the listed domain name followed by the 260 name of the DNSxL. If the DNSxL were called doms.example.net, and 261 the domain invalid.edu were to be listed, the entry would be named 262 invalid.edu.doms.example.net: 264 invalid.edu.doms.example.net A 127.0.0.2 265 invalid.edu.doms.example.net TXT "Host name used in phish" 266 A few name-based DNSBLs encode e-mail addresses using a convention 267 adapted from DNS SOA records, with the mailbox name encoded as the 268 first component of the domain name, so an entry for fred@invalid.edu 269 would have the name fred.invalid.edu.doms.example.net: 271 fred.invalid.edu.doms.example.net A 127.0.0.2 273 Name-based DNSBLs are far less common than IP based DNSBLs. There is 274 no agreed convention for wildcards. 276 Name-based DNSWLs can be created in the same manner as DNSBLs, and 277 have been used as simple reputation systems with the values of octets 278 in the A record representing reputation scores and confidence values, 279 typically on a 0-100 or 0-255 scale. 281 4. DNSxL cache behavior 283 The per-record time-to-live and zone refresh intervals of DNSBLs and 284 DNSWLs vary greatly depending on the management policy of the list. 285 The TTL and refresh times SHOULD be chosen to reflect the expected 286 rate of change of the DNSxL. A list of IP addresses assigned to 287 dynamically allocated dialup and DHCP users could be expected to 288 change slowly, so the TTL might be several days and the zone 289 refreshed once a day. On the other hand, a list of IP addresses that 290 had been observed sending spam might change every few minutes, with 291 comparably short TTL and refresh intervals. 293 5. Test and contact addresses 295 IPv4 based DNSxLs MUST contain an entry for 127.0.0.2 for testing 296 purposes. IPv4 based DNSxLs MUST NOT contain an entry for 127.0.0.1. 298 DNSBLs that return multiple values SHOULD have multiple test 299 addresses so that, for example, a DNSBL that can return 127.0.0.5 300 would have a test record for 127.0.0.5 that returns an A record with 301 the value 127.0.0.5, and a corresponding TXT record. 303 IPv6 based DNSxLs MUST contain an entry for ::2, and MUST NOT contain 304 an entry for ::1. 306 Domain name based DNSxLs MUST contain an entry for the [RFC2606] 307 reserved domain name "test" and MUST NOT contain an entry for the 308 reserved domain name "invalid". 310 DNSxLs also MAY contain an A record at the apex of the DNSxL zone 311 that points to a web server, so that anyone wishing to learn about 312 the bad.example.net DNSBL can check http://bad.example.net. 314 The combination of a test address that MUST exist and an address that 315 MUST NOT exist allows a client system to defend against DNSxLs which 316 deliberately or by accident install a wildcard that returns an A 317 record for all queries. DNSxL clients SHOULD periodically check 318 appropriate test entries to ensure that the DNSxLs they are using are 319 still operating. 321 6. Typical usage of DNSBLs and DNSWLs 323 DNSxLs can be served either from standard DNS servers, or from 324 specialized servers like rbldns [RBLDNS] and rbldnsd [RBLDNSD] that 325 accept lists of IP addresses and CIDR ranges and synthesize the 326 appropriate DNS records on the fly. Organizations that make heavy 327 use of a DNSxL usually arrange for a private mirror of the DNSxL, 328 either using the standard AXFR and IXFR or by fetching a file 329 containing addresses and CIDR ranges for the specialized servers. If 330 a /24 or larger range of addresses is listed, and the zone's server 331 uses traditional zone files to represent the DNSxL, the DNSxL MAY use 332 wildcards to limit the size of the zone file. If for example, the 333 entire range of 192.0.2.0/24 were listed, the DNSxL's zone could 334 contain a single wildcard for *.2.0.192.bad.example.com. 336 DNSBL clients are most often mail servers or spam filters called from 337 mail servers. There's no requirement that DNSBLs be used only for 338 mail, and other services such as IRC use them to check client hosts 339 that attempt to connect to a server. 341 A client MUST interpret any returned A record as meaning that an 342 address or domain is listed in a DNSxL. Mail servers that test 343 combined lists most often handle them the same as single lists and 344 treat any A record as meaning that an IP is listed without 345 distinguishing among the various reasons it might have been listed. 346 DNSxL clients SHOULD be able to use bit masks and value range tests 347 on returned A record values in order to select particular sublists of 348 a combined list. 350 Mail servers typically check a list of DNSxLs on every incoming SMTP 351 connection, with the names of the DNSxLs set in the server's 352 configuration. A common usage pattern is for the server to check 353 each list in turn until it finds one with a DNSBL entry, in which 354 case it rejects the connection, or a DNSWL entry in which case it 355 accepts the connection. If the address appears on no list at all 356 (the usual case for legitimate mail), the mail server accepts the 357 connection. In another approach, DNSxL entries are used as inputs to 358 a weighting function that computes an overall score for each message. 360 The mail server uses its normal local DNS cache to limit traffic to 361 the DNSxL servers and to speed up retests of IP addresses recently 362 seen. Long-running mail servers MAY cache DNSxL data internally, but 363 MUST respect the TTL values and discard expired records. 365 An alternate approach is to check DNSxLs in a spam filtering package 366 after a message has been received. In that case, the IP(s) to test 367 are usually extracted from "Received:" header fields or URIs in the 368 body of the message. The DNSxL results can be used to make a binary 369 accept/reject decision, or in a scoring system. 371 Packages that test multiple header fields MUST be able to distinguish 372 among values in lists with sublists since, for example, an entry 373 indicating that an IP is assigned to dialup users might be treated as 374 a strong indication that a message would be rejected if the IP sends 375 mail directly to the recipient system, but not if the message were 376 relayed through an ISP's mail server. 378 Name-based DNSBLs have been used both to check domain names of e-mail 379 addresses and host names found in mail headers, and to check the 380 domains found in URLs in message bodies. 382 7. Security Considerations 384 Any system manager that uses DNSxLs is entrusting part of his or her 385 server management to the parties that run the lists. A DNSBL manager 386 that decided to list 0/0 (which has actually happened) could cause 387 every server that uses the DNSBL to reject all mail. Conversely, if 388 a DNSBL manager removes all of the entries (which has also happened), 389 systems that depend on the DNSBL will find that their filtering 390 doesn't work as they want it to. 392 Since DNSxL users usually make a query for every incoming e-mail 393 message, the operator of a DNSxL can extract approximate mail volume 394 statistics from the DNS server logs. This has been used in a few 395 instances to estimate the amount of mail individual IPs or IP blocks 396 send[SENDERBASE] [KSN]. 398 As with any other DNS based services, DNSBLs and DNSWLs are subject 399 to various types of DNS attacks which are described in [RFC3833]. 401 8. IANA Considerations 403 This memo includes no request to IANA. 405 9. References 407 9.1. Normative References 409 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 410 STD 13, RFC 1034, November 1987. 412 [RFC1035] Mockapetris, P., "Domain names - implementation and 413 specification", STD 13, RFC 1035, November 1987. 415 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 416 Requirement Levels", BCP 14, RFC 2119, March 1997. 418 [RFC2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS 419 Names", BCP 32, RFC 2606, June 1999. 421 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 422 "DNS Extensions to Support IP Version 6", RFC 3596, 423 October 2003. 425 9.2. Informative References 427 [RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain 428 Name System (DNS)", RFC 3833, August 2004. 430 [RFC3849] Huston, G., Lord, A., and P. Smith, "IPv6 Address Prefix 431 Reserved for Documentation", RFC 3849, July 2004. 433 [RBLDNS] Bernstein, D., "rbldns, in 'djbdns'". 435 [MAPSRBL] "MAPS RBL+". 437 [RBLDNSD] Tokarev, M., "rbldnsd: Small Daemon for DNSBLs". 439 [SENDERBASE] 440 Ironport Systems, "Senderbase". 442 [KSN] Levine, J., "The South Korean Network Blocking List". 444 Appendix A. Change Log 446 *NOTE TO RFC EDITOR: This section may be removed upon publication of 447 this document as an RFC.* 449 A.1. Changes since -asrg-dnsbl-05 451 Pervasive edits to standard language, including RFC2119 terms. 453 Test entries clarified for IPv4, invented for IPv6 and domains. 455 Author's Address 457 John Levine 458 Taughannock Networks 459 PO Box 727 460 Trumansburg, NY 14886 462 Phone: +1 831 480 2300 463 Email: standards@taugh.com 464 URI: http://www.taugh.com 466 Full Copyright Statement 468 Copyright (C) The IETF Trust (2008). 470 This document is subject to the rights, licenses and restrictions 471 contained in BCP 78, and except as set forth therein, the authors 472 retain all their rights. 474 This document and the information contained herein are provided on an 475 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 476 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 477 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 478 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 479 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 480 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 482 Intellectual Property 484 The IETF takes no position regarding the validity or scope of any 485 Intellectual Property Rights or other rights that might be claimed to 486 pertain to the implementation or use of the technology described in 487 this document or the extent to which any license under such rights 488 might or might not be available; nor does it represent that it has 489 made any independent effort to identify any such rights. 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