Internet Draft                                          J. Levine
Expiration: April 22, 2006                   Taughannock Networks
Anti-Spam Research Group                        November 22, 2005

           DNS Based Blacklists and Whitelists for E-Mail

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights


   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 a
   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

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   query them.

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 .................................... 3
     2.3. Combined IP address DNSxLs .......................... 4
     2.4. Test and contact addresses .......................... 5
     2.5. IPv6 DNSxLs ......................................... 5

   3. Domain name DNSxLs ...................................... 5

   4. Typical usage of DNSBLs and DNSWLs ...................... 5

   5. Security Considerations ................................. 6

   6. Informative References .................................. 7

   7. Author's Address ........................................ 7

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's 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.  They 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 MAPS
   RBL, the descendant of the original list.  (In the United
   States, the term RBL is a registered service mark of MAPS[3].)

   The standard term is now DNS Blacklist or Blocklist, or DNSBL.
   Some people also publish DNS-based whitelists or DNSWLs.

   This document describes the structure, operation, and use of
   DNSBLs and DNSWLs but does not describe or recommend policies
   for adding or removing addresses to and from DNSBLs and
   DNSWLs, nor does it recommend policies for using them, nor
   does it take a position on whether the DNS is the best way to

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   distribute such data.

2. Structure of an IP address DNSBL or DNSWL

   Originally, DNSBLs only listed IP addresses, and most DNSBLs
   and DNSWLs still list IP addresses.  A few DNSBLs and DNSWLs
   now list domain names instead.  The structure of a DNSBL and
   DNSWL are the same, so in the subsequent discussion we use the
   abbreviation DNSxL to mean either.

2.1. IP address DNSxL

   An IP address DNSxL has a structure adapted from that of the
   rDNS.  Each IP address listed in the DNSxL has a corresponding
   DNS entry 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, and the IP address to be listed is, the name of the DNS entry would be  Each entry in the DNSxL has an A
   record and often a TXT record.  The A record conventionally
   has the value, but may have other values as
   described below.  The TXT record describes the reason that the
   IP 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.  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.  Some client software only checks
   the A record, some only checks the TXT record, some checks

   If an IP address is not listed in the DNSxL, there is no
   record for the address.  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 were listed, the DNSBL's zone could contain a
   single wildcard for *

2.2. IP address DNSWL

   Since SMTP has no standard 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
   dialup or DHCP users on a particular network might be used as

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   a DNSWL on that network's outgoing mail server or intranet web
   server, and used as a DNSBL for mail servers on other

2.3. Combined IP address DNSxLs

   In many cases, a single organization maintains a variety of
   DNSxLs for different purposes.  There are three common methods
   of representing multiple sublists: subdomains, multiple A
   records, and bit encoded entries.  Most multiple lists use
   both subdomains and one of the other methods.

   Subdomains are merely subdomains of the main DNSxL domain.  If
   for example, had two sublists ugly and smelly,
   entries for would be or  Sublist names consist of
   letters, so there is no problem of name collisions with
   entries in the main domain, where the IP addresses consist of

   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 is the logical OR of the
   bit masks for all of the lists on which the IP appears.  For
   example, the bit masks for the two sublists might be
   and, in which case an entry for an IP on both lists
   would be  With multiple A records, each sublist has
   a different assigned value such as to for
   ten sublists, and there is an A record for each sublist on
   which the IP appears.  There is no widely used convention for
   mapping sublist names to bits or values, beyond the convention
   that all A values are in the range to prevent
   unwanted network traffic if the value is accidentally used as
   an IP address.

   DNSxLs that return multiple A records generally 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.

   The per-record time-to-live and zone refresh intervals of
   DNSBLs and DNSWLs vary greatly depending on the management
   policy of the list.  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

2.4. Test and contact addresses

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   Nearly all IP based DNSxLs contain an entry for for
   testing purposes.  DNSBLs that return multiple values often
   have multiple test addresses so that, for example, the entry
   for returns a A record and corresponding
   TXT record.

   Most DNSxLs also contain an A record at the DNSxL's name that
   points to a web server, so that anyone wishing to learn about
   the DNSBL can check

2.5. IPv6 DNSxLs

   No DNSxL based on IPv6 addresses has, to the best of my
   knowledge, been deployed yet.  The obvious format for one
   would use 32-component hex nibble-reversed IPv6 addresses in
   the same places where IPv4 DNSxLs use four-component decimal
   byte-reversed addresses.  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 contain the listed domain name
   followed by the name of the DNSxL.  If the DNSxL were called, and the domain were to be
   listed, the entry would be named
   A few name-based DNSBLs encode e-mail addresses using a
   convention adopted from DNS SOA records, so an entry for would have the name  There is no consistent
   conventions for a test entry, but some name-based DNSxLs use
   EXAMPLE.COM as a test entry.  Name-based DNSBLs are far less
   common than IP 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 bit fields in the A record representing reputation
   scores and confidence values.

4. Typical usage of DNSBLs and DNSWLs

   DNSxLs can be served either from standard DNS servers, or from
   specialized servers like rbldns[2] and rbldnsd[4] that accept
   lists of IP addresses and 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 AXFR and IXFR or by

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   fetching a file containing addresses and CIDR ranges for the
   specialized servers.

   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 IRC use them
   to check client hosts that attempt to connect to a server.

   Mail servers that test combined lists usually handle them the
   same as single lists and treat any A or TXT record as meaning
   that an IP is listed without distinguishing among the various
   reasons it might have been listed.

   Mail servers typically check a list of DNSBLs and DNSWLs on
   every incoming SMTP connection, with the names of the DNSBLs
   and DNSWLs 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 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.  When using combined DNSxLs, clients
   usually only test for the presence or absence of an IP,
   without regard to the particular value returned.

   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: headers or
   URIs in the body of the message.  The DNSxL results may be
   used to make a binary accept/reject decision, or in a scoring

   Packages that test multiple headers need to be able to
   distinguish among values in lists with sublists since, for
   example, an entry indicating that an IP is assigned to dialup
   users might be treated as a strong indication that a message
   should be rejected if the IP 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.

5. Security Considerations

   Any system manager that uses DNSxLs is entrusting part of his

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   or her server management to the parties that run the lists.  A
   DNSBL manager that decided to list 0/0 (which has actually
   happened) could cause every server that uses the DNSBL to
   reject all mail.  Conversely, if a DNSBL manager removes all
   of the entries (which has also happened), systems that depend
   on the DNSBL will find that their filtering doesn't work as
   they want it to.

   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 IPs or IP blocks send[5,6].

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

6. Informative References

   [1] D. Atkins et al, "Threat Analysis of the Domain Name
   System", RFC 3833, August 2004.

   [2] D. J. Bernstein, rbldns, in "djbdns",

   [3] Mail Abuse Prevention System, "MAPS RBL+", http://mail-

   [4] Michael Tokarev,"rbldnsd: Small Daemon for DNSBLs",

   [5] Senderbase,

   [6] The South Korean Network Blocking List,

7. Author's Address

   John R. Levine
   Taughannock Networks
   PO Box 727
   Trumansburg NY 14886 USA
   Phone: +1 607 330 5711

Full Copyright Statement

   Copyright (C) The Internet Society (2005). All Rights
   Reserved.  This document is subject to the rights, licenses
   and restrictions contained in BCP 78, and except as set forth
   therein, the authors retain all their rights.

   This document and the information contained herein are

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   provided on an "AS IS" basis and THE CONTRIBUTOR, THE

   $Id: draft-irtf-asrg-dnsbl.n,v 2.1 2005/11/18 03:18:14 johnl
   Exp johnl $

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