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An Overview of Spam Blocking Techniques
Recent analyst estimates indicate that over 60 percent of the world’s email is unsolicited email, or “spam.” Spam is no longer just a simple annoyance. Spam has now become a significant security issue and a massive drain on financial resources. In fact, this deluge of spam costs corporations an estimated $20 billion Today there are a large number of solutions designed to help eliminate the spam problem. These solutions use different techniques for analyzing email and determining if it is indeed spam. Because spam is constantly changing, the most effective spam blocking solutions contain more than one of these techniques to help ensure that all spam, and only spam, is blocked.
The following is an overview of different spam blocking techniques.
Spam History
Spam is a form of abuse of the SimpleMail Transfer Protocol (SMTP), which is Word Filters ________________________________________________
Word filters are a simplistic yet effective way to block the majority of obvious spam. Word filters basis of RFC 524. First proposed in1973, RFC 524 was developed during a simply identify any email that contains certain key words, such as “Viagra,” that are commonly found in spam. Because spammers often work to circumvent word filters by purposely misspelling words, word filters need to be regularly updated with variations of the key words. For example, not a very secure command set, makingit and SMTP susceptible to abuse.
“Viagra” may be purposely misspelled as “V1agra,” so the word filter must be updated to containboth “Viagra” and “V1agra.” Most spam-making tools exploit thesecurity holes in SMTP. They do this by In some circumstances, word filters run the risk of creating false positives. For example, a forging email headers, disguisingsender addresses, and hiding the legitimate email containing the word “Viagra” that is intended for a medical researcher, physician or pharmacist may be inadvertently blocked.
difficult or even impossible to identifythe true sender.
Overall, word filters can be an effective spam blocking technique if they are constantly updated with new key words and phrases, as well as their unique misspellings.
holes, enhancement protocols to thevenerable SMTP have been proposed.
Rule-based Scoring Systems ___________________________________
Most of these enhancement protocolsinvolve features to accurately identify Rule-based scoring systems are a more sophisticated spam blocking technique than word filters.
the sender before accepting the email.
These systems, also known as artificial intelligence (AI) systems, are similar to word filters in that they also check for key words. However, whereas word filters simply just block emails that contain these new protocols to be widelyadopted because anyone who key words, rule-based scoring systems use rules to analyze emails and assign points to each key only be able to accept email fromothers who have also implemented the For example, an email that contains the word “DISCOUNT” in all capital letters might receive +2 points. An email that has the phrase “click here” might receive +1 point. The higher the score, the greater probability the email is spam. If an email reaches a certain score or threshold, it is then will continue to be a problem, drivingorganizations to seek out effective classified as spam. Large quantities of spam and legitimate email are used to determine the appropriate scores for each of the rules in rule-based scoring systems.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
SpamAssassin, an open source spam filter, is an example of a rule-based scoring system. Toidentify spam, SpamAssassin uses a wide range of heuristic tests on mail headers and body text.
Because spammers and their spam-making applications are not static, rule-based scoring systemsface some of the same challenges that word filters face. Rules must be updated regularly in orderfor rule-based scoring systems to remain effective.
For example, if a rule-based scoring system has a rule that assigns points to the word “Viagra,”spammers can easily circumvent this rule by purposely misspelling “Viagra” as many differentways as required to successfully deliver the spam. Rule-based scoring systems, however, ifused properly, can be very effective, eliminating over 90 percent of incoming spam.
Bayesian Filters _____________________________________________
Bayesian filters are personalized to each user and adapt automatically to changes in spam. To determine the likelihood that an email is spam, these filters use Bayesian analysis to compare the words or phrases in the email in question to the frequency of the same words or phrases in the a theory of probability inference,Bayesian analysis uses the intended recipient’s previous emails (both legitimate and spam).
Bayesian filters are very powerful and are regarded as one of the most accurate techniques for blocking spam. Most reports on Bayesian filters have shown accuracy of over 99 percent when thefilter has been “well-trained.” For Bayesian filter training, approximately 200 legitimate emails and200 spam emails from the intended recipient are normally needed. The more emails in thehistorical database of the intended recipient, the more accurate the filters are.
To learn more about the power of Bayesian analysis and filters, see “Better Bayesian Filtering,”by Paul Graham at
Black List IP ________________________________________________
> IP ADDRESS: This is a unique
Black list IP is a common spam blocking technique. It has no computational overhead and is easy to implement. This technique simply involves organizations manually keeping a list of the IP addresses of known spammers (a “black list”) so that emails from those addresses are blocked.
the TCP/IP protocol route messagesbased on the IP address of the Because spammers regularly change their IP addresses and use a wide range of IP addresses, black lists are most effective in blocking small amounts of spam for short time periods. They provide aquick fix for blocking one particular source of spam but are ineffective as an overall anti-spam solution.
An alternative to a black list is a white list. That is, a list of IP addresses from which you onlyaccept email. This reverse concept of black lists, however, is impractical because users would onlybe able to receive email from IP addresses that they knew beforehand, making it impossible toreceive email from any new sources.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
RBLs (Realtime Blackhole Lists) ________________________________
RBLs (Realtime Blackhole List), also known as DNSRBLs, check every incoming email’s IP address
against a list of IP addresses in the RBL. If the IP address is part of the RBL, then the email is
identified as spam and blocked.
Unlike the black list IP technique, RBLs are not manually updated by organizations. RBL operatorsmaintain public RBLs and organizations simply subscribe to them.
Many organizations like using RBLs because they not only have low computational overhead butbecause they are normally implemented using a protocol similar to DNS (Domain Name Server),they also have low network overhead.
> FALSE POSITIVE: This is when a
A downside of RBLs is that they may generate false positives. Most RBLs are aggressive and block all reported spam sources. However, many times the spam sources, such as popular ISPs Yahoo, Earthlink or Hotmail, are also the source of legitimate email. In those cases, the legitimate emailis typically never received since it is rejected as soon as its IP address is identified. The RBLs cannot differentiate between when a source is sending spam and when it is sending legitimate email.
It just blocks any email coming from the IP addresses in its list, thereby generating false positivesat times.
RBLs are effective for blocking spam and should be part of an organization’s spam blockingstrategy. With careful selection of which RBLs to use, you can effectively eliminate spamwithout the downside of generating false positives.
DNS MX Record Lookup _______________________________________
This is an effective technique for blocking spam from spammers who use a fake from and/or return
address. Spammers use such fake addresses so that the spam cannot be traced back to them.
To determine if a from address is valid, the system does a lookup on the domain that is used in thefrom address. If the domain does not have a valid DNS MX record, then the from address is notvalid and that email is labeled as spam. Similar lookups can be performed on the return addressof the email as well.
Reverse DNS Lookups ________________________________________
This is an effective spam blocking technique that uses a reverse DNS lookup on the incoming email’s source IP address. If the domain provided by the reverse lookup matches the from address on the email, the email is accepted. If they do not match, the email is rejected.
for the use of email. They typicallydo not have their own email server, Reverse DNS lookups, while popular, often do not work well. They can generate a large number of false positives since many reverse DNS entries are not properly established and many more cannot be properly established. For example, any “vanity” domain name would most likely not have anaccurate reverse DNS lookup. As such, emails from these domains would be rejected, causingunacceptably high false positive rates.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
New Reverse Lookup Systems __________________________________
> DMP (Designated Mailers Protocol):
A number of spam blocking techniques have been proposed that use the DNS system to limit the ability to send spam from forged sender addresses. These techniques improve upon the reverse DNS lookup technique. Examples of these proposed techniques include: or Mail Servers, to send e-mail onbehalf of your domain. This prevents Reverse Mail Exchanger (RMX): Sender Permitted From (SPF): > SPF (Sender Permitted Form): This
Designated Mailers Protocol (DMP): Yahoo! Domain Keys: Microsoft Caller ID for Email: > RMX (Reverse Mail Exchanger): This
These approaches are similar in many respects. Similar to DNS MX records lookup, these reverse lookup solutions define reverse-MX records (“RMX” for RMX, “SPF” for SPF, and “DMP” for DMP) for determining whether email from a particular domain is permitted to originate from a particular IP address. Email addresses that do not originate from the correct RMX/SPF/DMP address rangeare identified as forged and the email itself is tagged as spam.
Like reverse DNS lookups, this technique also has problems with vanity domains, but may bepartially corrected. The general case includes individuals and small companies who want to usetheir own domain rather than their ISP’s, but cannot afford their own static IP address and mailserver. Individuals sending email from a hostless or vanity domain simply configure their mailapplication to send email from their registered domain name. Unfortunately, a lookup of thesender’s IP address will not find the sender’s domain, and a lookup of the sender’s domain may notfind the correct reverse-MX record. The former is particularly common for mobile, dialup, and otherusers that frequently change IP addresses.
Black List Sender Email Addresses ______________________________
This is a simple spam blocking technique that is often used. Users create a black list of from
addresses that should be prevented from entering the network and reaching the user’s inbox.
There are a few disadvantages with using this technique. Because spammers can create manyfalse from email addresses, it is difficult to maintain a black list that is always updated with thecorrect emails to block. Also, some spammers do not even use a from address so a black list wouldnot be able to catch these cases. Even a rule to block emails without a from address would not besufficient because some legitimate emails, such as newsletters to which a user may subscribe,may also not include a from address. Black list sender email addresses is effective in temporarilyblocking a small amount of spam but ineffective as an overall anti-spam solution.
As an alternative to black lists, some users set up an email white list consisting of acceptableemail addresses or domains. In this case, users only accept email from users that are listed ontheir white list, while all other email is blocked. This technique poses many challenges as wellsince people want to be able to receive email from people that they might not have entered intotheir white list.
Some techniques will attempt to automatically build the white list from email that you have sentor from your address book. This makes creating the list easier. However, it does not solve theproblems associated when people who legitimately want to send you email have not previouslycorresponded with you via email, have multiple email addresses, or have a new email address.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
Honeypots (Hashing Systems, Fingerprinting) ______________________
> MAIL HASH (also called Message
Honeypots, or decoy email addresses, are used for collecting large amounts of spam. These decoy Digest): This is a number generated
email addresses do not belong to actual end users, but are made public to attract spammers who will think the address is legitimate. Once the spam is collected, identification techniques, such as substantially smaller than the textitself, and is generated by a formula hashing systems or fingerprinting, are used to process the spam and create a database of known spam. Let’s take a closer look at hashing systems and fingerprinting - unlikely that a different set of textwould produce the same hash value.
HASHING SYSTEMS: With hashing systems, each spam email receives an identification number,or “hash,” that corresponds to the contents of the spam. A list of known spam emails and theircorresponding hash is then created. All incoming email is compared to this list of known spam.
If the hashing system determines that an incoming email matches an email in the spam list, then Distributed Checksum Clearinghouse
the email is rejected. This technique works as long as spammers send the same or nearly the (DCC) is a variation on the honeypottechnique. Rather than taking into same email repeatedly. One of the original implementations of this technique was called Razor.
Fingerprinting techniques examine the characteristics, or fingerprint, of emails previously identified as spam and use this information to identify the same or similar email each time one is intercepted. These real time fingerprint checks are continuously updated and provide a method of identifying spam with nearly zero false positives. Fingerprinting techniques can also look spam. For more details, see
specifically at the URLs contained in a message and compare them against URLs of previouslyidentified as spam propagators.
Honeypots with hashing or fingerprinting can be effective provided similar spam emails arewidely sent. If each spam is made unique, these techniques can run into difficulties and fail.
Challenge/Response Systems __________________________________
Challenge/response systems are used to counter spammers who use automated mailing programs
to generate millions of spam emails per day. These systems are designed to slow down spammers
by putting roadblocks up for the incoming spam.
Challenge/response systems, such as those offered by Spam Arrest or MailBlocks, maintain alist of permitted senders. Each time an email from a new sender is sent to a challenge/responsesystem user, the email is temporarily held before delivery. The challenge/response system sendsthe email sender a challenge. This challenge usually consists of a link to a URL or a request thatthe sender copy a numeric code into a box in the reply email. If the sender successfully completesthe “challenge,” the challenge/response system adds him to the list of permitted senders and hisemail is delivered to the intended destination.
Challenge/response systems work under the assumption that spammers using fake sender emailaddresses would never receive the challenge, and spammers using real email addresses wouldnot be able to reply to all of the challenges.
Challenge/response systems have a number of limitations. These limitations include: DEADLOCK: Deadlock is when two people can not communicate with each other because bothare using challenge/response systems. For example, assume Bill and Tom do not know eachother well and have never communicated via email in the past. Bill legitimately needs tocontact Tom and so he sends Tom an email. Tom’s challenge/response system intercepts theemail and sends a challenge to Bill. Because Bill also has a challenge/response system, Bill’ssystem intercepts Tom’s challenge and issues its own challenge. Unfortunately, in a situationwhere both users have challenge/response systems, neither user will ever receive thechallenges and the original email will never get delivered.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
AUTOMATED SYSTEMS: With challenge/response systems, users can not receive email frommailing lists and automated systems such as Yahoo’s “Send To A Friend.” Mailing lists andautomated systems will not be able to respond to the challenge and as a result, their emailswill never get delivered.
As more people use challenge/response systems, these systems end up interfering with thedelivery of legitimate email rather than deterring the unwanted spam.
Computational Challenge Systems
Computational challenge systems add a cost to sending email by requiring the sender’s system
to perform a computation prior to sending the email. Most computational challenge systems use
complex algorithms that are intended to take time to process. The hope is that a high enough cost
would stop people from sending spam to those with computational challenge systems.
How do computational challenge systems work in practice? Let’s assume Derek is using acomputational challenge system to help stop spam. A new friend, Joe, decides to send Derekan email for the first time and therefore is not yet on Derek’s list of acceptable senders. Derek’sserver receives the email and sends a computational challenge (typically a math problem oralgorithm) to Joe’s email client. Derek’s server waits for a response before allowing the emailto be delivered to Derek’s inbox.
As illustrated in the above example, for a single, legitimate user sending emails, the time it takesto complete a computation is unlikely to be noticed. The sender’s system does the challenge andthe email is delivered to the intended recipient. However for someone such as a spammer sendinga lot of email, the small delays add up, making it take a long time and hopefully not worth it, tosend out bulk emails.
A few examples of proposed computational challenge systems are programmer Andy Back’sHashCash program and Microsoft’s Penny Black. These systems, as with all computationalchallenge systems, have limitations. These limitations include: UNEQUAL TAXATION: Computational challenges, whether based on CPU, memory, or network,penalizes users with slower systems. For example, a CPU challenge that takes 10 seconds ona 1Ghz computer would take over 20 seconds on a 500MHz computer.
MAILING LISTS: Legitimate mailing lists, some with thousands or millions of recipients, wouldbe penalized just as significantly as spammers. Computational challenges make mailing listmanagement impractical. Furthermore, any solutions that could be used by mailing lists tobypass the challenge would also allow spammers to bypass the challenge as well, therebydefeating the purpose of having a challenge system.
ROBOT ARMIES: Using Sobig and other spam-supporting viruses, many spammers controlthousands of compromised systems. Spammers can easily distribute any high costs fromchallenge systems across these infected systems (robot armies), making challenge systemsan ineffective way to discourage spammers.
LEGAL ROBOT ARMIES: Spammers generate spam because it brings in significant revenue.
Large spam groups can afford purchasing hundreds of systems for distributing anycomputational cost. This can be done legally, without compromising systems with viruses.
All these limitations make it unlikely that computational challenge systems will be widely adopted.
These systems not only inconvenience legitimate emailers but they also do not appear toeffectively mitigate spam.
Barracuda Networks ■ An Overview of Spam Blocking Techniques
Rate Controls _______________________________________________
Sometimes spammers attempt to cripple email servers by sending a large quantity of email in a
short period of time. This is called a DOS (Denial of Service) attack. With rate controls, a system
administrator can set up parameters that protect the email server from this email flood.
Rate controls can be set up to allow only a certain number of connections from the same IPaddress during a specified time. For example, a rate control time can be set to 30 minutes withonly a certain number of connections to be allowed in that given time period. If the administratorsets this parameter to 50 connections, the firewall will block any correspondence after the first50 connections that comes from a single IP address within a given 30 minute time period.
Rate controls are effective in protecting the network from spammers who attempt to sendhundreds of spam emails at the same time to a specific email server.
Anti-Virus Scanning __________________________________________
Anti-virus scanning can really be viewed as a method of stopping spam since a large amount of
unwanted email is generated by virus programs that attempt to propagate themselves. A virus
scanning solution is certainly an effective tool to include as part of any organization’s overall anti-
spam solution.
Conclusion _________________________________________________
The Barracuda Spam Firewall uses
Spam is a problem that is continuing to grow from day to day, costing corporations billions of dollars in lost productivity. Fortunately though, there are different spam blocking techniques to help counter the various types of spam.
attacks. To learn more about thedefense layers and the spam blocking Because spammers are always trying to bypass anti-spam techniques by changing the methods they use to send spam, it’s best for corporations to protect themselves with a spam blocking solution that uses more than one spam blocking technique. Each one of these techniques hasadvantages, disadvantages, as well as limitations. To minimize the amount of spam that enters anorganization, a spam blocking solution that includes a combination of the most effective techniquesshould be implemented.
Barracuda Networks


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