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Domain Based Spam Prevention? 42

Posted by Cliff
from the more-tools-for-the-spamfighting-toolbox dept.
aralin asks: "Recently I got this idea and wrote a little perl script to extract all the second (third in case of co.uk) level domains from my last month's collection of spam (some 4000 messages). I ran that against a nameserver to find the ones with NS record (valid domains) and made a list for my procmail filter. I get about 10 mails a day that escape to SpamAssassin for various reasons and since I began to check them against my list of domains I caught half of these. The idea is that if they want to sell something, or put a working web bug in my email, they need to provide a valid url with valid domain. If we filter domains from a URL in confirmed spam, then its almost certain any other email referencing such domain is spam as well. What I wanted to ask Slashdot is whether you know about some software project that already uses this form of spam detection as an addition to rule matching and Bayes filters?"
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Domain Based Spam Prevention?

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  • Easily Defeated (Score:5, Insightful)

    by Tom7 (102298) on Wednesday January 28, 2004 @09:09AM (#8112056) Homepage Journal
    Again the arms race problem: This might work for a while, but once the spammers see a certain level of blocking, they can adjust their spam to circumvent it.

    In this case they could start including (hidden, web-bug style) links to popular webmail sites, like hotmail. If you start blocking all messages with links to hotmail, you are probably going to miss some e-mail that you want!
    • Again the arms race problem: This might work for a while, but once the spammers see a certain level of blocking, they can adjust their spam to circumvent it.

      In this case they could start including (hidden, web-bug style) links to popular webmail sites, like hotmail. If you start blocking all messages with links to hotmail, you are probably going to miss some e-mail that you want!

      Also, many times the URLs contained in an e-mail points to a cracked Windoze box, which has been turned into a WWW server b

      • Re:Easily Defeated (Score:3, Informative)

        by forevermore (582201)
        http://shopping.yahoo.com%01@%31%39%32%2e%31%36%38 %2e%31%30%35%28%32%33:3333/porn4all.asp

        Spamassassin already has rules to catch this kind of obfuscation. However, it wouldn't be hard to merely translate these things back into real IP's. After all, the author of this article has already said that he filters on the 2nd (3rd) level domain name, and in an instance like this, there IS no domain name - any good filter would skip over the stuff before the @ and after the :

    • Re:Easily Defeated (Score:2, Informative)

      by gowen (141411)
      There is, as far as I can tell, only one sure way to detect and block spam, and that is the one thing that cannot be forged easily in email headers...

      The "Received: " header added by your server. Filtering on anything the spammer can control means an arms race; filtering on the IP address is the only consistent thing, whether the hosts are complicit with spammers (netvision.il, wideopenwest.net, chello.nl) or just too incompetent/lazy to act on reports of trojanned machines on their network (attbi / comc
    • "If you start blocking all messages with links to hotmail, you are probably going to miss some e-mail that you want!"

      Oh yeah, the email I want to receive is most likely to come from a hotmail account (or yahoo or AOL), right, sure...

      The only solution is to replace email with something else based on "sender pays".

      • Oh yeah, the email I want to receive is most likely to come from a hotmail account (or yahoo or AOL), right, sure...

        I'll assume that was sarcasm. What if one of your clients uses an account on Hotmail, Yahoo! mail, or AOL mail as his or her primary e-mail account? Or do you whitelist only clients who have approached you through a web form?

      • Oh yeah, the email I want to receive is most likely to come from a hotmail account (or yahoo or AOL), right, sure...

        Take it easy. He only said you'd probably miss some e-mail you'd want. He didn't say anything about 'most likely.' And what kind of elitist doesn't accept e-mail from Hotmail? Wow.

  • by Asprin (545477) <gsarnold&yahoo,com> on Wednesday January 28, 2004 @09:13AM (#8112077) Homepage Journal

    Isn't this just like adding a mail client filtering rule to trash all emails with "mydomain.com" in the body?

    Now, having said that, I don't think any mail filter does this explicitly because of problems with legit web page links. All the spammer would need to do is redirect through a page on a hosting service like fortunecity.com or geocities.com.

    ...although now that I think about it - throwing fortuncity and geocities in your filter list may not be a bad idea either since so little actually goes on there ;) and the interesting stuff is always over their bandwidth limit by the time I get the link. :(
  • by squiggleslash (241428) on Wednesday January 28, 2004 @09:23AM (#8112154) Homepage Journal
    There's a flag somewhere (which I can't find now I'm looking but I can see the rewrite rules in my sendmail.cf) for many of the Sendmail sendmail.cf configuration macros that will block on unresolvable domain names in the MAIL FROM: line (part of the email "envelope").

    I use it, but I'm not happy with it. There are several problems: domain names often are temporarily unresolvable for a variety of reasons (hey, you've been there, you've typed in a perfectly valid website address into your browser, got a message about the name not being resolvable, done it again immediately and it's worked? Right?); and it does encourage Joe Jobs - not necessarily against specific addresses, but against domains.

    Joe Jobs in turn are making ISPs adopt anti-spam practices that require emails with certain addresses only come from certain IPs, which in the absense of a standardized remote-access protocol for SMTP smarthosts makes it much more difficult for people to roam and increases the number of ways in which perfectly valid email may fail to be delivered.

    If I was an ISP, I definitely wouldn't go down this road. I'm wary of doing it anyway, and by-and-large I'm finding the emails that are blocked using this method are ones that would be blocked anyway, or are what appears to be valid emails with temporary DNS problems. It's something I don't intend to use for much longer, I just hate to have to reconfigure sendmail and see what breaks as a result.

    • macros that will block on unresolvable domain names in the MAIL FROM:

      There are two checks for this - one rejects (501) mail that comes from bogus domains (domains which do not exist) and one that sends a temporary failure message (451) for domains which are unresolvable.

      Such rules are necessary for proper operation of a mail server - the MAIL FROM: should always be a resolvable address (with the exception of empty sender) because that's where the bounces should go.

      domain names often are temporarily unr
    • You could try using milter-sender, in addition to your SpamAssassin/Milter-Spamc.

      Milter-Sender attempts a connect to the MX host of record for the purported From address, and if that MX host does not accept mail for that account, your sendmail will not accept mail _from_ that account.

      It's tunable, so you can tell it to wait and try again later, or just pass-thru emails unreachable MX hosts, or just reject them outright.

      It's not a perfect solution for what you're looking for, because as spammer just needs
  • by Glog (303500) on Wednesday January 28, 2004 @09:24AM (#8112171)
    Yep, a company called Cloudmark (http://www.cloudmark.com/products/authority/techn ology/) uses the DNS method you describe as one of its many rules to distinguish spam from regular mail. They call the approach Genetic Classification with the separate rules being called spamGenes. I don't know how much of a classifier (in the true AI sense) they have built but the idea sounds pretty nifty.
  • by FePe (720693)
    I don't know, but the following may be a bad idea: http://www.csc.liv.ac.uk/~ullrich/teaching/MScProj ects/#spam-filter [liv.ac.uk]
  • Already been done (Score:3, Informative)

    by Zocalo (252965) on Wednesday January 28, 2004 @09:44AM (#8112362) Homepage
    You can grab a config file for SpamAssassin here [merchantsoverseas.com] which has hundreds of spam domains listed, all in nicely optimised regular expressions. I did try this sometime back, but it rapidly became clear that this is very much an arms race. Using a new domain to act as a redirector for each spam run is a minimal overhead for a spammer - maybe they need a 0.0002% response rate instead of 0.0001% which is no big deal for the spammer.

    I suppose you could write some scripts to automatically add new domains and expire those beyond a certain age, but I don't see much point. I've been writing custom SpamAssassin rules for a several months now, and for me at least the ones that give the best results by far are the general purpose ones. Sure, if you have a big spam run or something like MyDoom to deal with, then a specific rule can really help, but that seems very much an exception to the rule.

    The rules I have most success with are targeting the obfuscation attempts, which is great because if the spammer omits obfuscation then Bayes has a field day instead. Even if you don't use SpamAssassin, the Wiki [exit0.us] is great for examples of this kind of rule that you can adapt to your own engine if need be. Best of all, this is the kind of stuff that will *always* work, rather than a rule that will at best have a shelf life of a couple of months before it starts to bog down your mail gateway for no benefit.

  • Most of us have probably seen spams pushing various pump-and-dump scams. Many of these are just plain text, bragging that such-and-such a stock is undervalued and will skyrocket in the next few {days|weeks|decades} when the company announces that the {RIAA|FBI|SCOX} have placed a $1 {m|b|tr}illion order for their new whizz-bang {frobnicator|KaZaA-killer|pengiun trap}.

    Usually, there's no URL, because if you were stupid enough to buy the shares, you'd buy them from someone else. Some of these spams, though,

  • Bayes (Score:3, Informative)

    by Gadzinka (256729) <rrw@hell.pl> on Wednesday January 28, 2004 @09:56AM (#8112498) Journal
    If I understand properly how bogofilter tokenizes email, it already collects those domains as spam words.
    • The point is that domain can look many different ways, letters can be substituted with %D5 or other
      ways and after you demangle the domain in the email and compare it with the list you get better match.
  • Joe-Job (Score:2, Insightful)

    by joostje (126457)
    If we filter domains from a URL in confirmed spam, then its almost certain any other email referencing such domain is spam as well.

    OK, the first spammer that wants to irritate you can thus easily block anyone from ever hearing about your website (by running a "joe-job" with your website's URL in it).

  • One, wouldn't a normal Bayesian filter do this automatically? I.e., pick up that url in mail classified as spam and then weight it positively in the future?

    Two, this doesn't help with the strangest category of spam -- email that doesn't refer to a particular product, include a valid reply-to or from address, or contain any valid urls. Those spam emails are the ones that just blow my mind. They suck up bandwidth, cost everyone money and resources, yet they contain only a few random words, none of which c
    • The spammer is probably using them to soften up the bayesian filtering (maybe get you to classify so much as spam that you start getting too many false positives and have to wipe your bayesian db, as I have?) so the real spams get through.
    • 'One, wouldn't a normal Bayesian filter do this automatically? I.e., pick up that url in mail classified as spam and then weight it positively in the future?'

      Yep, that's the case, in SpamAssassin 2.6x at least.
    • Wouldn't just polluting bayes(not the per-user bayes slashdot users are likely to have), but the per-enterprise-appliance-bayes(a small device shared by a bunch of people) or per-webmail-bayes shared by say, yahoo or brightmail. Wouldn't sending two emails, one which increase the chances the other gets through not be a waste, in a spammer's eyes?
      Think of it as a one-two punch against your email box.
  • by cyways (225137) on Wednesday January 28, 2004 @11:00AM (#8113191)
    Many of the spams I see these days use throwaway domains or IP addresses in their URLs, so blocking by domain name seems pretty ineffective. Moreover many of the "websites" to which these spams point are actually compromised machines with proxies that refer traffic to the real site. Given that such compromised machines now surely number in the tens or hundreds of thousands, it wouldn't take much effort to construct messages that use the IP address of a randomly selected proxy in each message's embedded URLs.
  • What happens when the spam simply contains a link to a legit site like Microsoft / RedHat / Apple / Network Associates / Norton etc? You are then going to block all messages that mention these sites? You are going to succeed in cutting yourself off from security mailing lists if nothing else.
  • by macdaddy (38372) on Wednesday January 28, 2004 @12:50PM (#8114322) Homepage Journal
    is that you'll have to use whitelists for all that legit domains. For example receiving spam from spammer.dyndns.com doesn't neccessarily mean that dyndns.com is a spamming domain. You may very well list not-a-spammer.dyndns.com if you chose to block dyndns.com. Likewise for homeunix.org. spammer.homeunix.org isn't the same as not-a-spammer.homeunix.org. You'll have a large and ever-growing whitelist if you use this tactic.

    IMHO a better method would be to use the WHOIS information for a given domain name to match it to other spamming domains. I used to maintain the largest list of Alan Ralsky's [spamhaus.org] spamming domains. My list was enormous. Alan had a bad habit (good for us anti-spammers though) of using identical or very similar WHOIS information in each of his spamming domains. This was the case with probably 90% of his spamming domains. He frequently used the same nameservers as well. I think a crafty programmer could come up with a way to use a Bayesian filter to identify spam by the WHOIS records of the domains in a given message that's been marked as spam. This would be a worthwhile project to me. Best of luck.

  • Do what I do and filter out any email which has HTML. I get enough email already. These people are the first to go.
    • And while that might work for you, it won't for those of us who actually need to distingiush spam from ham.

      I, for one, am not going to miss a business opportunity (as in a job, not transfereing money out of Nigeria), because the poor guy with the money and the standard Outlook setup sends me a HTML mail.

      I might also just stop reading email, y'know .. ?

      By the way - I send MIME-multipart mails with both the text and HTML version. And I reply above the quote. So shoot me.
  • I modified qmail and capture a list of all domains into a database. I easily blacklist the spammer's domains through a web interface I made. It has been pretty effective for me. I'm blocking about 100 emails a minute *after* four RBL Blacklists. Plus a few other techniques, and I am blocking about 83% of all email *before* spam assassin.
  • As I sit with several domain names available at my disposal, I got to thinking that this may be the way one could combat spam - registering your own domainname. Let me explain.

    So I have the domain "blah.com" and I want to register for an Ebay account. Instead of simply giving "me@blah.com", I'd instead register "ebay@blah.com" which would just point to my inbox. Now I can easily filter mail appropriately as it comes through. Not only that, but I can tell which places gave my email address out to spaming co
    • You don't even need additional 2nd-level domains to do this, just add a 3rd-level domain for this purpose.

      For instance, suppose my normal address is me@mydomain.com, but when I give out my address on websites, I use something like amazon@replies.mydomain.com. In your DNS, just set up an MX record for the subdomain. If you use sendmail, it's easy to add a mailertable entry on the final delivery server like this:

      replies.mydomain.com local:myreplymailbox

      Make sure replies.mydomain.com also appears in /e

    • I've been doing this for years. Your own domain + sendmail aliases is a wonderful, wonderful thing. You'd be surprised, however, how very little spam comes from registering on websites (out of the hundreds of emails I created over the years, I can count on one hand how many received spam. Note: these were from non-messageboard type sites).

      Most spam comes from having your address posted on some websites. Even newsgroups don't seem to be heavilly crawled by spammers. I did a test last year, posted to a few
    • http://tmda.net

      It lets you do dated addresses that expire after a period of time. It also lets you generate cryptographically signed addresses through the web interface (you-keyword-kht9840w@youraddress.com), so they can't just make them up.

      It also allows you to do challenge response where people have to prove they aren't lying about their email address.
  • the configuration for exim4 written for sa-exim by default goes a little further: it looks up not only a reverse-dns but also checks for an MX record. the only problem is that if the ISP's configuration is piss-poor broken, e.g. they pretend to be a host for which they themselves do not have a DNS record (yes i have seen it happen), you will get a response sent to postmaster@sendersdomain. ... and if, as most people do not, you don't _have_ an alias postmaster@sendersdomain, then the sender will get - to t
  • ...to get the spamvertised ISP's hat color and adjust spam scores.

    A while ago, I made a SpamAssassin patch [gmane.org] which resolves any URL found within an email and tests the resulting IP addresses against blacklists which are otherwise used to block unwanted email. A lot of Chinese bulletproof servers' IP addresses are listed on the Spamhaus Block List (SBL) [spamhaus.org] and/or SPEWS [spews.org] as well as on certain *.blackholes.us [blackholes.us] lists.

  • I don't know if that will work. It's too easy to obscure [1109654166] URLs.
  • by H0B0 (748255)
    all users of cable modems in north america are required to use a 24.xxx IP. So, why arent all email servers required to use a predetermined range of IPs, eg. 25, 64, or one of the yet-to-be-assigned ranges. If all spammers were kept within one range, it would be far easier to stop spammers (they couldnt rent a fly-by-night IP in a different range, for example), and to catch them when they do spam. There are other benefits (reducing virii, worms, etc.) that would be produced if an email server range were t

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