Internet Research Task Force (IRTF) J. Levine
Request for Comments: 5782 Taughannock Networks
Category: Informational February 2010
DNS Blacklists and Whitelists
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
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................22. Structure of an IP Address DNSBL or DNSWL .......................32.1. IP Address DNSxL ...........................................32.2. IP Address DNSWL ...........................................42.3. Combined IP Address DNSxL ..................................42.4. IPv6 DNSxLs ................................................53. Domain Name DNSxLs ..............................................64. DNSxL Cache Behavior ............................................75. Test and Contact Addresses ......................................76. Typical Usage of DNSBLs and DNSWLs ..............................87. Security Considerations .........................................98. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................101. 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
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
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 126.96.36.199.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
188.8.131.52.bad.example.com A 127.0.0.2
"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
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
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
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
184.108.40.206.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
220.127.116.11.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:
18.104.22.168.bad.example.com A 127.0.1.1
22.214.171.124.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.
For example, to represent the address:
in the DNSxL ugly.example.com, the entry might be:
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 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
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
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.
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.
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