DNS Blacklists and Whitelists
RFC 5782

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Internet Research Task Force (IRTF)                            J. Levine
Request for Comments: 5782                          Taughannock Networks
Category: Informational                                    February 2010
ISSN: 2070-1721

                     DNS Blacklists and Whitelists

Abstract

   The rise of spam and other anti-social behavior on the Internet has
   led to the creation of shared blacklists and whitelists of IP
   addresses or domains.  The DNS has become the de-facto standard
   method of distributing these blacklists and whitelists.  This memo
   documents the structure and usage of DNS-based blacklists and
   whitelists, and the protocol used to query them.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Research Task Force
   (IRTF).  The IRTF publishes the results of Internet-related research
   and development activities.  These results might not be suitable for
   deployment.  This RFC represents the consensus of the Anti-Spam
   Research Group of the Internet Research Task Force (IRTF).  Documents
   approved for publication by the IRSG are not a candidate for any
   level of Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5782.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

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Table of Contents

   1. Introduction ....................................................2
   2. Structure of an IP Address DNSBL or DNSWL .......................3
      2.1. IP Address DNSxL ...........................................3
      2.2. IP Address DNSWL ...........................................4
      2.3. Combined IP Address DNSxL ..................................4
      2.4. IPv6 DNSxLs ................................................5
   3. Domain Name DNSxLs ..............................................6
   4. DNSxL Cache Behavior ............................................7
   5. Test and Contact Addresses ......................................7
   6. Typical Usage of DNSBLs and DNSWLs ..............................8
   7. Security Considerations .........................................9
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10

1.  Introduction

   In 1997, Dave Rand and Paul Vixie, well-known Internet software
   engineers, started keeping a list of IP addresses that had sent them
   spam or engaged in other behavior that they found objectionable.
   Word of the list quickly spread, and they started distributing it as
   a BGP feed for people who wanted to block all traffic from listed IP
   addresses at their routers.  The list became known as the Real-time
   Blackhole List (RBL).

   Many network managers wanted to use the RBL to block unwanted e-mail,
   but weren't prepared to use a BGP feed.  Rand and Vixie created a
   DNS-based distribution scheme that quickly became more popular than
   the original BGP distribution.  Other people created other DNS-based
   blacklists either to compete with the RBL or to complement it by
   listing different categories of IP addresses.  Although some people
   refer to all DNS-based blacklists as "RBLs", the term properly is
   used for the Mail Abuse Prevention System (MAPS) RBL, the descendant
   of the original list.  (In the United States, the term RBL is a
   registered service mark of Trend Micro [MAPSRBL].)

   The conventional term is now DNS blacklist or blocklist, or DNSBL.
   Some people also publish DNS-based whitelists or DNSWLs.  Network
   managers typically use DNSBLs to block traffic and DNSWLs to
   preferentially accept traffic.  The structure of a DNSBL and DNSWL
   are the same, so in the subsequent discussion we use the abbreviation
   DNSxL to mean either.

   This document defines the structure of DNSBLs and DNSWLs.  It
   describes the structure, operation, and use of DNSBLs and DNSWLs but
   does not describe or recommend policies for adding or removing

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   addresses to and from DNSBLs and DNSWLs, nor does it recommend
   policies for using them.  We anticipate that management policies will
   be addressed in a companion document.

   This document is a product of the Anti-Spam Research Group (ASRG) of
   the Internet Research Task Force.  It represents the consensus of the
   ASRG with respect to practices to improve interoperability of DNS-
   based blacklists and whitelists.

   Requirements Notation:   The key words "MUST", "MUST NOT",
      "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
      "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
      interpreted as described in [RFC2119], with respect to
      recommendations for improving technical interoperability of
      DNSxLs.

2.  Structure of an IP Address DNSBL or DNSWL

   A DNSxL is a zone in the DNS [RFC1034] [RFC1035].  The zone
   containing resource records identifies hosts present in a blacklist
   or whitelist.  Hosts were originally encoded into DNSxL zones using a
   transformation of their IP addresses, but now host names are
   sometimes encoded as well.  Most DNSxLs still use IP addresses.

2.1.  IP Address DNSxL

   An IPv4 address DNSxL has a structure adapted from that of the rDNS.
   (The rDNS, reverse DNS, is the IN-ADDR.ARPA [RFC1034] and IP6.ARPA
   [RFC3596] domains used to map IP addresses to domain names.)  Each
   IPv4 address listed in the DNSxL has a corresponding DNS entry.  The
   entry's name is created by reversing the order of the octets of the
   text representation of the IP address, and appending the domain name
   of the DNSxL.

   If, for example, the DNSxL is called bad.example.com, and the IPv4
   address to be listed is 192.0.2.99, the name of the DNS entry would
   be 99.2.0.192.bad.example.com.  Each entry in the DNSxL MUST have an
   A record.  DNSBLs SHOULD have a TXT record that describes the reason
   for the entry.  DNSWLs MAY have a TXT record that describes the
   reason for the entry.  The contents of the A record MUST NOT be used
   as an IP address.  The A record contents conventionally have the
   value 127.0.0.2, but MAY have other values as described below in
   Section 2.3.  The TXT record describes the reason that the IP address
   is listed in the DNSxL, and is often used as the text of an SMTP
   error response when an SMTP client attempts to send mail to a server
   using the list as a DNSBL, or as explanatory text when the DNSBL is
   used in a scoring spam filter.  The DNS records for this entry might
   be:

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   99.2.0.192.bad.example.com    A      127.0.0.2
   99.2.0.192.bad.example.com    TXT
            "Dynamic address, see http://bad.example.com?192.0.2.99"

   Some DNSxLs use the same TXT record for all entries, while others
   provide a different TXT record for each entry or range of entries
   that describes the reason that entry or range is listed.  The reason
   often includes the URL of a web page where more information is
   available.  Client software MUST check the A record and MAY check the
   TXT record.

   If a range of addresses is listed in the DNSxL, the DNSxL MUST
   contain an A record (or a pair of A and TXT records) for every
   address in the DNSxL.  Conversely, if an IP address is not listed in
   the DNSxL, there MUST NOT be any records for the address.

2.2.  IP Address DNSWL

   Since SMTP has no way for a server to advise a client why a request
   was accepted, TXT records in DNSWLs are not very useful.  Some DNSWLs
   contain TXT records anyway to document the reasons that entries are
   present.

   It is possible and occasionally useful for a DNSxL to be used as a
   DNSBL in one context and a DNSWL in another.  For example, a DNSxL
   that lists the IP addresses assigned to dynamically assigned
   addresses on a particular network might be used as a DNSWL on that
   network's outgoing mail server or intranet web server, and used as a
   DNSBL for mail servers on other networks.

2.3.  Combined IP Address DNSxL

   In many cases, an organization maintains a DNSxL that contains
   multiple entry types, with the entries of each type constituting a
   sublist.  For example, an organization that publishes a DNSBL listing
   sources of unwanted e-mail might wish to indicate why various
   addresses are included in the list, with one sublist for addresses
   listed due to sender policy, a second list for addresses of open
   relays, a third list for hosts compromised by malware, and so forth.
   (At this point, all of the DNSxLs with sublists of which we are aware
   are intended for use as DNSBLs, but the sublist techniques are
   equally usable for DNSWLs.)

   There are three common methods of representing a DNSxL with multiple
   sublists: subdomains, multiple A records, and bit-encoded entries.
   DNSxLs with sublists SHOULD use both subdomains and one of the other
   methods.

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   Sublist subdomains are merely subdomains of the main DNSxL domain.
   If for example, bad.example.com had two sublists 'relay' and
   'malware', entries for 192.0.2.99 would be
   99.2.0.192.relay.bad.example.com or
   99.2.0.192.malware.bad.example.com.  If a DNSxL contains both entries
   for a main domain and for sublists, sublist names MUST be at least
   two characters and contain non-digits, so there is no problem of name
   collisions with entries in the main domain, where the IP addresses
   consist of digits or single hex characters.

   To minimize the number of DNS lookups, multiple sublists can also be
   encoded as bit masks or multiple A records.  With bit masks, the A
   record entry for each IP address is the logical OR of the bit masks
   for all of the lists on which the IP address appears.  For example,
   the bit masks for the two sublists might be 127.0.0.2 and 127.0.0.4,
   in which case an entry for an IP address on both lists would be
   127.0.0.6:

   99.2.0.192.bad.example.com    A      127.0.0.6

   With multiple A records, each sublist has a different assigned value
   such as 127.0.1.1, 127.0.1.2, and so forth, with an A record for each
   sublist on which the IP address appears:

   99.2.0.192.bad.example.com    A      127.0.1.1
   99.2.0.192.bad.example.com    A      127.0.1.2

   There is no widely used convention for mapping sublist names to bits
   or values, beyond the convention that all A values SHOULD be in the
   127.0.0.0/8 range to prevent unwanted network traffic if the value is
   erroneously used as an IP address.

   DNSxLs that return multiple A records sometimes return multiple TXT
   records as well, although the lack of any way to match the TXT
   records to the A records limits the usefulness of those TXT records.
   Other combined DNSxLs return a single TXT record.

2.4.  IPv6 DNSxLs

   The structure of DNSxLs based on IPv6 addresses is adapted from that
   of the IP6.ARPA domain defined in [RFC3596].  Each entry's name MUST
   be a 32-component hex nibble-reversed IPv6 address suffixed by the
   DNSxL domain.  The entries contain A and TXT records, interpreted the
   same way as they are in IPv4 DNSxLs.

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   For example, to represent the address:

     2001:db8:1:2:3:4:567:89ab

   in the DNSxL ugly.example.com, the entry might be:

     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.
                  ugly.example.com. A 127.0.0.2
                                    TXT "Spam received."

   Combined IPv6 sublist DNSxLs are represented the same way as IPv4
   DNSxLs, replacing the four octets of IPv4 address with the 32 nibbles
   of IPv6 address.

   A single DNSxL could in principle contain both IPv4 and IPv6
   addresses, since the different lengths prevent any ambiguity.  If a
   DNSxL is represented using traditional zone files and wildcards,
   there is no way to specify the length of the name that a wildcard
   matches, so wildcard names would indeed be ambiguous for DNSxLs
   served in that fashion.

3.  Domain Name DNSxLs

   A few DNSxLs list domain names rather than IP addresses.  They are
   sometimes called RHSBLs, for right-hand-side blacklists.  The names
   of their entries MUST contain the listed domain name followed by the
   name of the DNSxL.  The entries contain A and TXT records,
   interpreted the same way as they are in IPv4 DNSxLs.

   If the DNSxL were called doms.example.net, and the domain invalid.edu
   were to be listed, the entry would be named
   invalid.edu.doms.example.net:

   invalid.edu.doms.example.net    A      127.0.0.2
   invalid.edu.doms.example.net    TXT    "Host name used in phish"

   Name-based DNSBLs are far less common than IP address based DNSBLs.
   There is no agreed convention for wildcards.

   Name-based DNSWLs can be created in the same manner as DNSBLs, and
   have been used as simple reputation systems with the values of octets
   in the A record representing reputation scores and confidence values,
   typically on a 0-100 or 0-255 scale.  Vouch By Reference [RFC5518] is
   a certification system similar in design and operation to a
   name-based DNSWL.

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4.  DNSxL Cache Behavior

   The per-record time-to-live and zone refresh intervals of DNSBLs and
   DNSWLs vary greatly depending on the management policy of the list.
   The Time to Live (TTL) and refresh times SHOULD be chosen to reflect
   the expected rate of change of the DNSxL.  A list of IP addresses
   assigned to dynamically allocated dialup and DHCP users could be
   expected to change slowly, so the TTL might be several days and the
   zone refreshed once a day.  On the other hand, a list of IP addresses
   that had been observed sending spam might change every few minutes,
   with comparably short TTL and refresh intervals.

5.  Test and Contact Addresses

   IPv4-based DNSxLs MUST contain an entry for 127.0.0.2 for testing
   purposes.  IPv4-based DNSxLs MUST NOT contain an entry for 127.0.0.1.

   DNSBLs that return multiple values SHOULD have multiple test
   addresses so that, for example, a DNSBL that can return 127.0.0.5
   would have a test record for 127.0.0.5 that returns an A record with
   the value 127.0.0.5, and a corresponding TXT record.

   IPv6-based DNSxLs MUST contain an entry for ::FFFF:7F00:2 (::FFFF:
   127.0.0.2), and MUST NOT contain an entry for ::FFFF:7F00:1 (::FFFF:
   127.0.0.1), the IPv4-Mapped IPv6 Address [RFC4291] equivalents of the
   IPv4 test addresses.

   Domain-name-based DNSxLs MUST contain an entry for the [RFC2606]
   reserved domain name "TEST" and MUST NOT contain an entry for the
   reserved domain name "INVALID".

   DNSxLs also MAY contain A and/or AAAA records at the apex of the
   DNSxL zone that point to a web server, so that anyone wishing to
   learn about the bad.example.net DNSBL can check
   http://bad.example.net.

   The combination of a test address that MUST exist and an address that
   MUST NOT exist allows a client system to check that a domain still
   contains DNSxL data, and to defend against DNSxLs that deliberately
   or by accident install a wildcard that returns an A record for all
   queries.  DNSxL clients SHOULD periodically check appropriate test
   entries to ensure that the DNSxLs they are using are still operating.

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6.  Typical Usage of DNSBLs and DNSWLs

   DNSxLs can be served either from standard DNS servers, or from
   specialized servers like rbldns [RBLDNS] and rbldnsd [RBLDNSD] that
   accept lists of IP addresses and Classless Inter-Domain Routing
   (CIDR) ranges and synthesize the appropriate DNS records on the fly.
   Organizations that make heavy use of a DNSxL usually arrange for a
   private mirror of the DNSxL, either using the standard Full Zone
   Transfer (AXFR) and Incremental Zone Transfer (IXFR) or by fetching a
   file containing addresses and CIDR ranges for the specialized
   servers.  If a /24 or larger range of addresses is listed, and the
   zone's server uses traditional zone files to represent the DNSxL, the
   DNSxL MAY use wildcards to limit the size of the zone file.  If for
   example, the entire range of 192.0.2.0/24 were listed, the DNSxL's
   zone could contain a single wildcard for *.2.0.192.bad.example.com.

   DNSBL clients are most often mail servers or spam filters called from
   mail servers.  There's no requirement that DNSBLs be used only for
   mail, and other services such as Internet Relay Chat (IRC) use them
   to check client hosts that attempt to connect to a server.

   A client MUST interpret any returned A record as meaning that an
   address or domain is listed in a DNSxL.  Mail servers that test
   combined lists most often handle them the same as single lists and
   treat any A record as meaning that an IP address is listed without
   distinguishing among the various reasons it might have been listed.
   DNSxL clients SHOULD be able to use bit masks and value range tests
   on returned A record values in order to select particular sublists of
   a combined list.

   Mail servers typically check a list of DNSxLs on every incoming SMTP
   connection, with the names of the DNSxLs set in the server's
   configuration.  A common usage pattern is for the server to check
   each list in turn until it finds one with a DNSBL entry, in which
   case it rejects the connection, or one with a DNSWL entry, in which
   case it accepts the connection.  If the address appears on no list at
   all (the usual case for legitimate mail), the mail server accepts the
   connection.  In another approach, DNSxL entries are used as inputs to
   a weighting function that computes an overall score for each message.

   The mail server uses its normal local DNS cache to limit traffic to
   the DNSxL servers and to speed up retests of IP addresses recently
   seen.  Long-running mail servers MAY cache DNSxL data internally, but
   MUST respect the TTL values and discard expired records.

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   An alternate approach is to check DNSxLs in a spam filtering package
   after a message has been received.  In that case, the IP(s) to test
   are usually extracted from "Received:" header fields or URIs in the
   body of the message.  The DNSxL results can be used to make a binary
   accept/reject decision, or in a scoring system.

   Packages that test multiple header fields MUST be able to distinguish
   among values in lists with sublists because, for example, an entry
   indicating that an IP address is assigned to dialup users might be
   treated as a strong indication that a message would be rejected if
   the IP address sends mail directly to the recipient system, but not
   if the message were relayed through an ISP's mail server.

   Name-based DNSBLs have been used both to check domain names of e-mail
   addresses and host names found in mail headers, and to check the
   domains found in URLs in message bodies.

7.  Security Considerations

   Any system manager that uses DNSxLs is entrusting part of his or her
   server management to the parties that run the lists, and SHOULD
   ensure that the management policies for the lists are consistent with
   the policies the system manager intends to use.  Poorly chosen DNSBLs
   might block addresses that send mail that the system manager and the
   system's users wish to receive.  The management of DNSBLs can change
   over time; in some cases, when the operator of a DNSBL has wished to
   shut it down, he has either removed all entries from the DNSBL or
   installed a wildcard to list 0/0, which would produce unexpected and
   unwanted results for anyone using the DNSBL.

   The A records in a DNSxL zone (other than the ones at the apex of the
   zone) represent blacklist and/or whitelist entries rather than IP
   addresses.  Should a client attempt to use the A records as IP
   addresses, e.g., attempt to use a DNSxL entry name as a web or FTP
   server, peculiar results would ensue.  If the operator of the DNSxL
   were to disregard the advice in Section 2.3 and put values in the A
   records outside of the 127/8 range, the peculiar results might not be
   limited to the host misusing the records.  Conversely, if a system
   attempts to use a zone that is not a DNSxL as a blacklist or
   whitelist, yet more peculiar results will ensue.  This situation has
   been observed in practice when an abandoned DNSBL domain was re-
   registered and the new owner installed a wildcard with an A record
   pointing to a web server.  To avoid this situation, systems that use
   DNSxLs SHOULD check for the test entries described in Section 5 to
   ensure that a domain actually has the structure of a DNSxL, and
   SHOULD NOT use any DNSxL domain that does not have correct test
   entries.

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   Since DNSxL users usually make a query for every incoming e-mail
   message, the operator of a DNSxL can extract approximate mail volume
   statistics from the DNS server logs.  This has been used in a few
   instances to estimate the amount of mail individual IP addresses or
   IP blocks send [SENDERBASE] [KSN].

   As with any other DNS-based services, DNSBLs and DNSWLs are subject
   to various types of DNS attacks, which are described in [RFC3833].

8.  References

8.1.  Normative References

   [RFC1034]     Mockapetris, P., "Domain names - concepts and
                 facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]     Mockapetris, P., "Domain names - implementation and
                 specification", STD 13, RFC 1035, November 1987.

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2606]     Eastlake, D. and A. Panitz, "Reserved Top Level DNS
                 Names", BCP 32, RFC 2606, June 1999.

   [RFC3596]     Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
                 "DNS Extensions to Support IP Version 6", RFC 3596,
                 October 2003.

   [RFC4291]     Hinden, R. and S. Deering, "IP Version 6 Addressing
                 Architecture", RFC 4291, February 2006.

   [RFC5518]     Hoffman, P., Levine, J., and A. Hathcock, "Vouch By
                 Reference", RFC 5518, April 2009.

8.2.  Informative References

   [RFC3833]     Atkins, D. and R. Austein, "Threat Analysis of the
                 Domain Name System (DNS)", RFC 3833, August 2004.

   [RBLDNS]      Bernstein, D., "rbldns, in 'djbdns'",
                 <http://cr.yp.to/djbdns.html>.

   [MAPSRBL]     "MAPS RBL+", <http://mail-abuse.com/>.

   [RBLDNSD]     Tokarev, M., "rbldnsd: Small Daemon for DNSBLs",
                 <http://www.corpit.ru/mjt/rbldnsd.html>.

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   [SENDERBASE]  Ironport Systems, "Senderbase",
                 <http://www.senderbase.org>.

   [KSN]         Levine, J., "The South Korean Network Blocking List",
                 <http://korea.services.net>.

Author's Address

   John Levine
   Taughannock Networks
   PO Box 727
   Trumansburg, NY  14886

   Phone: +1 607 330 5711
   EMail: standards@taugh.com
   URI:   http://www.taugh.com

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