Older blog entries for jwb (starting at number 12)

avriettea

If you think mysql didn't install itself in the right place, just pass some more parameters to the configure script. You can also change the path to the database files at runtime with the -h,--datadir option. Agree completely that mysql can seem like a bad joke after using pg, but then again mysql doesn't have the operational headaches that come with postgresql: vacuum, reindex, and so forth. I tend to use both systems.

flaming

My colleagues at work had the genius idea to install a mail filtering system called MailMarshal in front of their Exchange server. I would not recommend this product. It blocks mails containing GIF files, but the same image converted to PNG is not blocked. Also if the GIF is buried in one extra level of MIME hierarchy, it will be allowed. Finally it seems more interested in the memo From: header than the SMTP envelope sender.

The punch line is that I cannot send mail even to people within my own company without resorting to a comical rfc822-in-rfc822 tunneling system. It is the mail analog to ipip tunneling through firewalls. And it proves once again that control freaks don't understand networks.

Unfortunately I was unable to control my frustration after a week of trying to diagnose this mail problem, and flamed the administrators (gently). This never helps the situation but I always do it. I am broken in that regard.

Incremental apt-get update

I'd like to meddle with the way Debian distributed software. It is very silly that apt-get update downloads several megabytes of data, just to get the incremental changes since the last apt-get update. I believe it should be easy to implement an incremental update that sends only the changes since time t. This would require that each upload have a serial number, but either that already exists somewhere in the apt system, or it could be hacked in without too much trouble.

By my caculations a daily apt-get update could be reduced from several megabytes to tens of kilobytes.

BitTorrent

After reading numerous enthusiastic endorsements of BitTorrent, I downloaded the client and tried to get Mandrake 9.1 images from the network. I must say I was not impressed. The estimated time to complete the download was 42 hours, at about 10KB/sec. Normally I can download at around 135KB/sec. Worse, my client was uploading at around 20KB/sec! On my assymetrical connection, uploads degrade downloads. Is this something BitTorrent does not account for? The client should limit uploads to at most, say, 10% of downloads. Otherwise there is little reason for the user to participate. Certainly if uploads were going to be 200% of downloads I would never use it.

Also their FAQ desperately needs a "How do run this farking program?" section. /usr/bin/btdownloadprefetched.py is not obvious.

Someone mailed me to request my spam filter system, so I packaged it up slightly with some command line arguments and documentation. You may download it here:

http://atari.saturn5.com/~jwb/spam-2.0.tar.gz

I added a --gram-length=n option, so you can play with that dimension of the system.

workrave

I recommend you try Workrave, to help keep your hands useful in later life.

n-grams in spam filter

I modified my spam filter to compare the performance of unigrams, digrams, and trigrams. The undesired corpus contains 353 mails; the desired corpus holds 3352. When using trigrams the vocabulary is over 1.1 million terms. My system is the same as Paul Grahams, except I do not double the document frequency of terms in the good corpus, and I consider mail as spam if its probability exceeds 50%.

The unigram system identified all but eight mails from the spam corpus, with zero false positives. The digram and trigram systems both identified all but three, also with no false positives. Of course the trigram system takes much longer for the analysis, so I believe I will use digrams for the present system.

The system works so well that I will write a small C library for use by mail clients. I think spam filtering has no effect on spammers until it becomes widespread. So I will try to spread it widely.

tk: I have tried using 2-grams and 3-grams in my spam filter. Of course this tends to bloat the vocabulary and therefore the time required for analysis. Later I will attempt to characterize the effect of term length on filter performance. My parser it something of a hack, so any extensions to the term length will probably result in repulsive code.

spam

The spam filter is getting better as the training corpus grows. It now has 300 messages, which explains why I need this software. The mail parsing code I posted earlier was not optimal. It turns out that parsing the unencoded content of binary attachments helps to trap viruses and other windows programs. In both the bodies and the headers it is necessary to ignore very short words, and to remove non-word characters. This means that:

To: jwbaker@acm.org

Is reduced to just

jwbakeracmorg

Which is perfectly fine for these purposes. I'm still tweaking the implementation. The latest iteration found five spams buried in my "good" corpus, but decided some things in my spam corpus were good. There are some odd effects currently. For example, my mail goes through mail.saturn5.com comparatively recently. Before it went via a different server, and I still have many spams from that server. Therefore the spam corpus disproportionately represents the old mail server. The term for the new mail server "mailsaturn5com" can tend to make a spam appear legitimate.

The goal is still zero false positives.

victory The best part of a spam filter is the tiny feeling of victory when a spam takes a detour into the spam folder with 100% confidence, and helps to make sure that any spams like it will follow. Muhhhhahhahhaha

spam

I implemented Paul Graham's probabilistic filtering for spam. At first I attempted to simply use rainbow from the libbow distribution, but rainbow has many problems. It is painfully academic, its HTML parsing code always segfaults, it is miserably slow, and it cannot test more than one document per launch. Therefore did I apt-get remove libbow.

My implementation, in Perl, attempts to understand the MIME and RFC822 structures for better tokenizing. Here is the meat of the lexer:

sub lex_header {
    my ($h, $w) = @_;
    my (@t);
    
    @t = $h->tags();
    
    foreach my $t (@t) {
        my ($n);
        
        $n = $h->count($t);
        
        for (my $i = 0; $i < $n; $i++) {
            foreach my $tok (split(/\s/, $h->get($t, $i))) {
              $w->{$tok}++;
            }
        }
    }
}

 sub lex_entity {
    my ($r, $w) = @_;
    my ($t, $h, $n);
    
    lex_header($r->head(), $w);
    
    $t = $r->effective_type();
    $h = $r->bodyhandle();
    
    if (defined($h)) {
        $h = $h->open("r");
    
        if ($t eq 'text/plain' || $t eq 'text/html') {
            while ($_ = $h->getline()) {
                chomp();
                
                if (!/[^A-Za-z0-9\+\=]/) {
                    next;
                }
                
                foreach my $tok (split(/\b/, $_)) {
                    $w->{$tok}++;
                }
            }
        }
    }
    
    $n = $r->parts();
    
    for (my $i = 0; $i < $n; $i++) {
        lex_entity($r->parts($i), $w);
    }
}

This is a decidedly informal approach to tokenizing a mail, but I think it works. Especially important is the line that rejects base64-encoded text in a text/plain entity. See if you can spot it.

After training the system on a spam corpus of 142 mails and a legit corpus of 3332 mails, the filter can reject about half of incoming spam and has not yet produced a false positive. Hopefully the rejection ratio improves as the spam corpus grows.

One damning bit of trivia arose from this experiment. The mail servers at my previous employer Critical Path are all 99% reliable indicators of spam. One of Critical Path's features is supposed to be spam filtering. Evidently it doesn't work overly well.

CD Changer UI

I have a 3-disc CD changer. It is pretty useless. You play the first CD, then the second, then the third. To play the fourth, you eject the third. To play the fifth, you eject the fourth and so on. The first and second discs stay in there forever. If I really wanted to program four hours of music, I would plug in the iPod.

Gene Kan

I knew Gene Kan, barely. He and Yaroslav are friends with my friends, and are occassional visitors. Gene was in my office a few weeks before his death, installing an email server in a machine room. Why would a man expecting death build an email server to last a decade? I do not know.

Role-based access control

There is a SIG in the ACM for role-based access control (RBAC). They meet every year for a symposium. A lot of papers are presented. I cannot see anything in all the RBAC literature which is more functional than putting users into groups the old-fashioned Unix way. My machines using regular Unix security have many role accounts: httpd, nntpd, ftpd, lpd, uucp, daemon, nobody, and so forth.

The main problem with the Unix authorization model is the lack of delegation of authority, but this is a separate issue from RBAC.