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</tr> </table>A DNSBL (DNS-based Blackhole List, Block List, or Blacklist; see below) is list of IP addresses published through the Internet Domain Name Service in a particular format. DNSBLs are most often used to publish the addresses of computers or networks linked to spamming; most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists.
"DNSBL" is a software mechanism, rather than a specific list or policy. There are dozens of DNSBLs in existence, which use a wide array of standards for listing and delisting of addresses. These may include listing the addresses of zombie computers or other machines being used to send spam, listing the addresses of ISPs who willingly host spammers, or listing addresses which have sent spam to a honeypot system.
Since the creation of the first DNSBL in 1997, the operation and policies of these lists have been frequently controversial, both in Internet advocacy and occasionally in lawsuits. Many mail system operators and users consider DNSBLs a valuable tool to share information about spam sources, but others including prominent Internet activists have objected to them as a form of censorship. In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down.
History of DNSBLs
The first DNSBL was the Real-time Blackhole List (RBL), created in 1997 by Paul Vixie as part of his Mail Abuse Prevention System (MAPS); Dave Rand at Abovenet was its first subscriber. The very first version of the RBL was not published as a DNSBL, but rather a list of networks transmitted via BGP to routers owned by subscribers so that network operators could drop all TCP/IP traffic for machines used to send spam or host spam supporting services, such as a website.
The term "blackhole" refers to a networking black hole, an expression for a link on a network that drops incoming traffic instead of forwarding it normally. The intent of the RBL was that sites using it would refuse traffic from sites which supported spam — whether by actively sending spam, or in other ways. Before an address would be listed on the RBL, volunteers and MAPS staff would attempt repeatedly to contact the persons responsible for it and get its problems corrected. Such effort was considered very important before blackholing all network traffic, but it also meant that spammers and spam supporting ISPs could delay being put on the RBL for long periods while such discussions went on.
Later, the RBL was also released in a DNSBL form and Paul Vixie encouraged the authors of sendmail and other mail software to implement RBL clients. These allowed the mail software to query the RBL and reject mail from listed sites on a per mail server basis instead of blackholing all traffic.
Soon after the advent of the RBL, others started developing their own lists with different policies. One of the first was Alan Brown's Open Relay Behavior-modification System (ORBS). This used automated testing to discover and list mail servers running as open mail relays—exploitable by spammers to carry their spam. ORBS was controversial at the time because many people felt running an open relay was acceptable, and that scanning the Internet for open mail servers could be abusive.
In 2003, a number of DNSBLs have come under denial-of-service attacks. Since no party has admitted to these attacks nor been discovered responsible, their purpose is a matter of speculation. However, many observers believe the attacks are perpetrated by spammers in order to interfere with the DNSBLs' operation or hound them into shutting down. In August 2003, the firm Osirusoft, an operator of several DNSBLs including one based on the SPEWS data set, shut down its lists after suffering weeks of near-continuous attack.
A URI DNSBL is a DNSBL that lists the domain names and IP addresses which are found in the "clickable" links contained in the body of spams, but generally not found inside legitimate messages.
URI DNSBLs were created when it was determined that much spam made it past spam filters during that short time frame between the first use of a spam-sending IP address and the point where that sending IP address was first listed on major sending-IP-based DNSBLs.
In many cases, such elusive spams contain in their links domain names or IP addresses (collectively referred to as a URIs) where that URI was already spotted in previously caught spam and where that URI is not found in non-spam e-mail.
Therefore, when a spam filter extracts all URIs from a message and checks them against a URI DNSBL, then the spam can be blocked even if the sending IP for that spam has not yet been listed on any sending IP DNSBL.
Of the three major URI DNSBLs, the oldest and most popular is SURBL, created and operated primarily by Jeff Chan. After SURBL was created, some of the administrators and contributors to SURBL started the second major URI DNSBL, URIBL. More recently, another current and long-time SURBL administrator, Rob McEwen, started the third major URI DNSBL, ivmURI.
URI DNSBLs are often confused with RHSBLs (Right Hand Side BLs). But they are different. A URI DNSBL lists domain names and IPs found in the body of the message. An RHSBL lists the domain names used in the "from" or "reply-to" e-mail address. RHSBLs are not very effective because most spams either use forged "from" addresses or use "from" addresses containing popular freemail domain names, such as @gmail.com, @yahoo.com, or @hotmail.com addresses. In contrast to marginally effective and not-often-used RHSBLs, URI DNSBLs are very effective and are used by the majority of spam filters.
How a DNSBL works
To operate a DNSBL requires three things: a domain to host it under, a nameserver for that domain, and a list of addresses to publish.
It is possible to serve a DNSBL using any general-purpose DNS server software. However this is typically inefficient for zones containing large numbers of addresses, particularly DNSBLs which list entire Classless Inter-Domain Routing netblocks. DNSBL-specific software — such as Michael J. Tokarev's rbldnsd, Daniel J. Bernstein's rbldns, or the DNS Blacklist Plug-In for Simple DNS Plus — is faster, uses less memory, and is easier to configure for this purpose.
The hard part of operating a DNSBL is populating it with addresses. DNSBLs intended for public use usually have specific, published policies as to what a listing means, and must be operated accordingly to attain or sustain public confidence.
When a mail server receives a connection from a client, and wishes to check that client against a DNSBL (let's say, dnsbl.example.net), it does more or less the following:
Looking up an address in a DNSBL is thus similar to looking it up in reverse-DNS. The differences are that a DNSBL lookup uses the "A" rather than "PTR" record type, and uses a forward domain (such as dnsbl.example.net above) rather than the special reverse domain in-addr.arpa.
There is an informal protocol for the addresses returned by DNSBL queries which match. Most DNSBLs return an address in the 127.0.0.0/8 IP loopback network. The address 127.0.0.2 indicates a generic listing. Other addresses in this block may indicate something specific about the listing—that it indicates an open relay, proxy, spammer-owned host, etc. For details see an Internet draft by the Anti-Spam Research Group.
An URI DNSBL query (and an RHSBL query) is fairly straightforward. Just prepend the domain name to the DNS list host as follows:
Generally if an A record is returned the name is listed.
Different DNSBLs have different policies. DNSBL policies differ from one another on three fronts:
Uses of DNSBLs
DNSBLs can be used in rule based spam blocking software like Spamassassin where different black list are given point scores that can be mitigated by white rules to reduce false positives. They can also be used by MTAs like Exim, Sendmail and Postfix to outright block email if the senders IP address or host name is listed in a DNSBL.
One way to do this is to first check white lists and pass the email if the server is on a white list. A technique developed by Junk Email Filter uses Yellow Lists and NoBL lists to mitigate the false positives that often occur when using multiple black lists. Yellow lists are host names and IP addresses of servers that are known to be a source of mixed spam and non-spam. Examples would be yahoo, hotmail, and gmail. If the forward confirmed rDNS FCrDNS resolves to one of these hosts then the IP address contains no information as to if the message is or isn't spam. Thus other DNSBL tests should not occur because the IP carries no useful information.
Here's a list of different kinds of DNS Lists.
White List - IP is a trusted email source Black List - 100% spam. Message should be rejected. Yellow List - Message comes from a mixed source and IP should not be tested further NoBL list - IP is not a spam source but may be a ham source
Messages should first be checked for yellow listing. If listed, then no further checking is needed. Then the message should be white list tested. If it is found listed in a white list, the message should be accepted as good. Then the NoBL lists should be tested. If listed then black list tests should be bypassed. Finally the black list testing should apply. The message is rejected if it is found in a black list. To be safe one might score the blacklists and reject if listed on multiple lists.
Email users who find their messages blocked from mail servers that use DNSBLs often object vociferously, sometimes to the extent of attacking the existence of the lists themselves. The following lists are controversial:
Although many have voiced objections to specific DNSBLs, few people object to the principle that mail-receiving sites should be able to reject undesired mail systematically. One who does is John Gilmore, who deliberately operates an open mail relay. Gilmore accuses DNSBL operators of violating antitrust law.
A number of parties, such as the Electronic Frontier Foundation and Peacefire, have raised concerns about some use of DNSBLs by ISPs. One joint statement issued by a group including EFF and Peacefire addressed "stealth blocking", in which ISPs use DNSBLs or other spam-blocking techniques without informing their clients."
Spammers have pursued lawsuits against DNSBL operators on similar grounds: