Internet Draft J. Levine
Expiration: October 26, 2004 Taughannock Networks
Anti-Spam Research Group April 26, 2004
DNS Based Blacklists and Whitelists for E-Mail
draft-irtf-asrg-dnsbl-00.txt
Status of this Memo
This document is an Internet-Draft and is subject to 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
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This Internet-Draft will expire on October 26, 2004.
This document is intended to evolve, based on comments from
the Anti-Spam Research Group (ASRG). Comments and corrections
are welcome, and may be sent to the ASRG mailing list at
<asrg@ietf.org>.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights
Reserved.
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 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
query them.
Table of Contents
1. Introduction ............................................ 2
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2. Structure of an IP address DNSBL or DNSWL ............... 2
2.1. IP address DNSxL .................................... 3
2.2. IP address DNSWL .................................... 3
2.3. Combined IP address DNSxLs .......................... 3
2.4. Test and contact addresses .......................... 4
3. Domain name DNSxLs ...................................... 4
4. Typical usage of DNSBLs and DNSWLs ...................... 5
5. Security Considerations ................................. 6
6. Informative References .................................. 6
7. Authors' Address ........................................ 6
1. Introduction
In 1997, Paul Vixie, a well known Internet software engineer,
started keeping a list of IP addresses that had sent him spam
or engaged in other behavior that he found objectionable.
Word of the list quickly spread, and he 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).[3]
Many network managers wanted to use the RBL to block unwanted
e-mail, but weren't prepared to block all traffic from lists
in the RBL. 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'', that term
properly is used for the MAPS RBL, the descendant of Vixie's
original list, and 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 DNSBLs and DNSWLs, nor
does it recommend policies for using them, nor does it take a
position whether the DNS is the best way to 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 now list
domain names instead. The structure of a DNSBL and DNSWL are
the same, so in the subsequent discussion we use the
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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 bad.example.com, and the IP 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 has an A
record and often a TXT record. The A record conventionally
has the value 127.0.0.2, 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. 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.
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, 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 DNSBL's zone could contain a
single wildcard for *.2.0.192.bad.example.com.
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 all of the IP addresses assigned
to dialup or DHCP users 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 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.
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Subdomains are merely subdomains of the main DNSxL domain. If
for example, bad.example.com had two sublists ugly and smelly,
entries for 192.0.2.99 would be
99.2.0.192.ugly.bad.example.com or
99.2.0.192.smelly.bad.example.com. 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
digits.
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 127.0.0.1
and 127.0.0.2, in which case an entry for an IP on both lists
would be 127.0.0.3. With multiple A records, each sublist has
a different assigned value such as 127.0.1.1 to 127.0.1.10 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 127/8 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 welll; 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
intervals.
2.4. Test and contact addresses
Nearly all IP based DNSxLs contain an entry for 127.0.0.2 for
testing purposes. DNSBLs that return multiple values often
have multiple test addresses so that, for example, the entry
for 127.0.0.5 returns a 127.0.0.5 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 bad.example.net DNSBL can check http://bad.example.net.
3. Domain name DNSxLs
A few DNSBLs list domain names rather than IP addresses. The
names of their entries contain the listed domain name followed
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by the name of the DNSBL. If the DNSBL 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 few named-based DNSBLS encode e-mail addresses using a
convention adopted from DNS SOA records, so an entry for
fred@invalid.edu would have the name
fred.invalid.edu.doms.example.net.
Name-based DNSBLs are far less common than IP based DNSBLs,
There is no agreed convention for a test entry nor for
wildcards. Name-based DNSWLs could be created in the same
manner as DNSBLs, although to date nobody has done so.
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
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 clients that are trying to connect.
In practice, 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.
Most often they check a list of DNSBLs and DNSWLs on every
incoming SMTP connection, with the names of the DNSBLs and
DNSWLs configured into the server. The server checks 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), it accepts the
connection. 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.
The DNSxL results may be used to make a binary accept/reject
decision, as when they're tested at SMTP time, or may be used
as components in a system that computers an overall score for
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each message. 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.
5. Security Considerations
Any system manager that uses DNSxLs is entrusting part of his
or her server management to the parties that run the lists. A
DNSBL manager that decided to list 0/0 (which has actually
happened) would 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.
As with any other DNS based services, DNSBLs and DNSWLs are
subject to various types of DNS attacks which are described in
[1].
6. Informative References
[1] D. Atkins et al, "Threat Analysis of the Domain Name
System", draft-ietf-dnsext-dns-threats-07
[2] D. J. Bernstein, rbldns, in "djbdns",
http://cr.yp.to/djbdns.html.
[3] Mail Abuse Prevention System, "MAPS RBL", http://mail-
abuse.org/rbl/
[4] Michael Tokarev,"rbldnsd: Small Daemon for DNSBLs",
http://www.corpit.ru/mjt/rbldnsd.html.
7. Authors' Address
John R. Levine
Taughannock Networks
PO Box 727
Trumansburg NY 14886 USA
E-mail: johnl@taugh.com
Phone: +1 607 330 5711
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights
Reserved.
This document and translations of it may be copied and
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