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
draft-irtf-asrg-dnsbl-02.txt
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Copyright Notice
Copyright (C) The Internet Society (2005). 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
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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 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, 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
both.
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 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 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
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.
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 127.0.0.0/8 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 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
intervals.
2.4. Test and contact addresses
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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.
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
doms.example.net, and the domain invalid.edu were to be
listed, the entry would be named invalid.edu.doms.example.net.
A few name-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. 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
system.
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
[1].
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",
http://cr.yp.to/djbdns.html.
[3] Mail Abuse Prevention System, "MAPS RBL+", http://mail-
abuse.com/
[4] Michael Tokarev,"rbldnsd: Small Daemon for DNSBLs",
http://www.corpit.ru/mjt/rbldnsd.html.
[5] Senderbase, http://www.senderbase.org.
[6] The South Korean Network Blocking List,
http://korea.services.net.
7. Author's 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 (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
ORGANIZATION HE REPRESENTS OR IS SPONSORED BY (IF ANY), THE
INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE
DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
$Id: draft-irtf-asrg-dnsbl.n,v 2.1 2005/11/18 03:18:14 johnl
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