Quick Spam Filter

Current version: 1.2.23 (6 July 2025) [src]

Quick Spam Filter (QSF) is an email classification filter, designed to be small, fast, and accurate, which works to classify incoming email as either spam or non-spam.

To recognise spam, QSF strips the text out of the email (using MIME decoding and HTML stripping) and then splits it into tokens (words, word pairs, URLs, and so on). These tokens are then looked up in a database and analysed using the Bayesian technique to see whether the email should be classified as spam or not.

The database is generated by a process of training - QSF is given two mailboxes, one containing known spam, and the other containing known non-spam, to train itself on. After training, if QSF misfiles any email, the message it got wrong can be fed back into the database, allowing QSF to learn from its mistakes.

For a more in-depth look at the way in which QSF tokenises and classifies messages, please see the "Technical details" section of the manual.

QSF is designed to be run by an MDA, such as procmail. See the FAQ for a quick-start guide.

This software is distributed under the terms of the Artistic License 2.0.

• Source code: qsf-1.2.23.tar.gz [signature]
• Manual for qsf
• Git repository (at Codeberg)
• Issue tracker (at Codeberg)

To install qsf, follow these steps:

curl -O https://ivarch.com/s/qsf-1.2.23.tar.gz
tar xzf qsf-1.2.23.tar.gz
cd qsf-1.2.23
sh ./configure
make
sudo make install

Comments, bug reports, and patches can be sent using the Issue tracker, or through the Contact Form.

News

1.2.23 - 6 July 2025

• fix: correct MD5 library compilation fault on GCC 15 (#1)
• cleanup: rewrite build system to use GNU Automake
• cleanup: reformat source consistently
• cleanup: reformat –version output to match GNU conventions
• docs: restructure manual to read better for newcomers and to follow man-pages(7) conventions
• docs: switch NEWS file to Markdown format
• docs: switch README file to Markdown format
• docs: create a separate ACKNOWLEDGEMENTS document

1.2.15 - 29 March 2021

• bugfix: correct exit status of “qsf -t” to 0 if memory limit exceeded (Zhengdao Wang)
• bugfix: correct compiler warnings and toupper() misfire (Dr. David Alan Gilbert)
• bugfix: report error if “qsf -T” is pointed at directories (Iain Calder)
• cleanup: clean up compiler warnings related to unused vars and type mismatches

1.2.11 - 3 January 2015

• bugfix: Debian #773546 - report error on malformed message (Jameson Graef Rollins)
• bugfix: Debian #651881 - X-Spam-Level corruption on non-ASCII spam (Ian Zimmerman)
• bugfix: MD5Final now correctly clears context (patch from David Binderman)
• bugfix: removed “DESTDIR” / suffix to fix Cygwin installation
• cleanup: mailbox code consolidated into single file
• cleanup: moved acknowledgements out of manual page
• cleanup: better “rpm” and “srpm” build targets

1.2.7 - 28 August 2007

• license change to Artistic 2.0
• pointless option “-l” removed

1.2.6 - 4 February 2007

• bugfix (reversion): removed locking from MySQL as it makes it too slow

1.2.5 - 21 January 2007

• bugfix: fixed random token deletion in list backend
• bugfix: added table locking to MySQL backend to maintain integrity
• bugfix: fixed MySQL database type autodetection for files
• cleanup: rpmlint fixes to spec file

1.2.1 - 25 October 2006

• bugfix: concurrent updates now work properly on all backends
• developers: “make bigbenchmark” generates graph data for gnuplot

1.2.0 - 2 October 2006

• new default backend database “list” (better than btree)
• new option “-H” to set value of X-Spam header (Michal Vitecek)
• new option “-P” to keep a plaintext mapping of hashes to tokens
• new option “-y” to use a deny-list as well as an allow-list
• new allow/deny-list syntax “@domain” to list whole domains

1.1.13 - 15 August 2006

• bugfix: patch from Michal Vitecek to fix segfault in btree backend
• bugfix: fixed MD5 problem on 64-bit systems (eg AMD64)
• bugfix: fixed mmap problem with btree on OpenBSD
• cleanup: better compilation on Solaris
• cleanup: use snprintf more and strcpy/strcat less
• cleanup: fixed problems spotted by Tommy Pettersson

1.1.7 - 8 April 2006

• check Return-Path: header against allow-list as well as From:

1.1.6 - 2 February 2006

• cosmetic fixes to prevent compilation warnings with GCC 4.x
• bugfix: improved MySQL detection when running configure script
• bugfix: if a URL is the first thing in the body it is now still found
• bugfix: handle message/* nested attachments

1.1.2 - 7 July 2005

• allow list matching is now case insensitive
• new btree database mtime is updated after any modification
• spec file fix for FC4

1.1.0 - 12 May 2005

• per-user databases are now given 10x the weighting of global databases
• improved the pruning algorithm to take token “age” into account
• new number-of-updates counter; each token now has a “last updated” time
• the internal binary tree database is much faster on systems with mmap()
• “make test”, “make benchmark”, and “qsf -B” now support the MySQL backend
• token for words beginning with non-alphanumeric characters
• token for words with more than 3 hyphens or underscores
• token for words between 31 and 59 characters long
• token for FONT tags
• replaced image token with “external image” token
• bugfix: fixed hard-to-trigger SQLite crash after multiple db prunes

1.0.31 - 4 March 2005

• put all internal binary tree db code into a single file
• allow more than one backend to be compiled in at once
• check for gdbm_fdesc, some systems don’t have it
• check for mysql_real_escape_string (if absent, MySQL is too old)
• code cleanup
• configure option “–enable-static” to allow static linking against libs
• developers: new “make benchmark” benchmarks available backends
• developers: benchmarking (“-B”) now reports more information
• packagers: RPM options are now just plain and “static”

1.0.22 - 28 February 2005

• optional argument to “-D” documented in man page
• bugfix: if “-D” is given a filename, check it won’t overwrite a database
• bugfix: don’t output >512kb messages in mark/non-filter mode
• developers: in “-vv” (very verbose) mode, report which backend is in use

1.0.18 - 19 February 2005

• new option “-Q” to pass through messages containing insufficient tokens
• new option “-X” to set a limit on how many tokens can be pruned at once
• filter now adds tokens for more attachment types (DOC, XLS, PDF, images) as well as for single and multiple images

1.0.15 - 5 February 2005

• fixed bug in RPM spec file when looking for MySQL libraries
• fixed bug in “make rpm” and “make srpm” build targets
• updated spec file to use “–with” options (gdbm, mysql, sqlite)

1.0.14 - 26 September 2004

• decode email headers that are expressed in alternate encodings
• minor cleanup of option parsing code
• added “memtest” Makefile target for testing with valgrind
• option “-e” now automatically enables “-a”
• more tests for “make test”; tests no longer depend on “formail”

1.0.9 - 22 September 2004

• added new SQLite backend
• added “-sqlite” package to RPM spec file for “qsf-sqlite” package
• added “–with-sqlite” option to configure script

1.0.6 - 22 June 2004

• added “-v” option to add logging headers to email
• added “-A” option to add “X-Spam-Level” header, like SpamAssassin
• use “-e MSG” to update/query allow list using address read from an email
• included Postfix filtering HOWTO from M. Kölbl

1.0.2 - 28 April 2004

• added tokenisation details and a troubleshooting section to the manual
• added “srpm” and “release” Makefile targets

1.0.1 - 15 March 2004

• corrected default per-user database name back to ~/.qsfdb

1.0.0 - 12 March 2004

• added mailstats.sh to the new “extra/” source directory
• added statstable.sh to the new “extra/” source directory
• scripts from extra/ are installed as documentation in the RPM
• code cleanup using RATS to check for potential problems
• portability fixes in the build process
• word-wrapping in “–help” output
• abort with an error if superfluous command line arguments are given
• abort with an error if an invalid database is supplied
• keyboard interrupt signals blocked during critical db writing parts
• new option “-N” to prevent automatic pruning
• both “-R” and “-D” can take a filename instead of stdin/stdout
• filter to check for gibberish in the sender address
• filter now adds extra tokens for the hostname part of URLs
• filter now checks URL-encoded hostnames (http://%6d%6e%3f/)
• filter now checks for integer hostnames (http://1234567890/)
• filter to look for <img> tags

0.9.25 - 16 January 2004

• new option “-e” to query and update the allow-list directly
• new option “-L” to change the spam threshold level
• option to allow “-g” to be used twice, for two global databases
• removed locks from procmail examples
• added information on using qsf with qmail
• MySQL option allows dynamic linking
• bugfix: prevent memory hogging by limiting number of tokens in one prune

0.9.18 - 6 January 2004

• allow a combination of “-r” “-t” to just output the spam score
• tokeniser improvement: strip multiple spaces/newlines
• tokeniser improvement: add URLs to tokeniser
• filter to add URLs as tokens, with and without “@” parts
• filter for numeric IP addresses in URLs
• filter for .vbs, .vba, .lnk, .com, .bat file extensions

0.9.12 - 1 January 2004

• minor cosmetic fixes for non-Linux systems
• speedup: replaced stdio with low-level read/write in btree db backend
• bugfix in “make index” (only of interest to developers)

0.9.9 - 29 November 2003

• fixed outdated information in the README
• fixed memory management bug in MySQL backend
• minor fixes to “make test” and “make leaktest”
• some speedups in the binary tree database backend

0.9.6 - 15 November 2003

• added MySQL database backend
• added “–global” to change default global database
• added qsf-mysql RPM package using optional MySQL backend

0.9.4 - 21 October 2003

• switched from RPM_BUILD_ROOT to DESTDIR in “make install”
• added locking to binary tree database backend
• data corruption bugfix in binary tree database backend
• dropped support for Berkeley DB
• made the internal binary tree database the default (not GDBM)
• documented the new filters

0.9.0 - 29 August 2003

• switched spam rating calculation to Robinson method
• removed shifting weighting from training (–train)
• removed the “-o” (–add-tokens) option, as it is now redundant
• added support for multiple extra tests/token adding rules
• new rule: GTUBE (see http://www.spamassassin.org/gtube/)
• new rules: detect if an attachment ends in .scr, .pif, .exe
• new rules: detect runs of multiple consonants / vowels
• new rule: detect HTML comments in the middle of words
• changed automatic pruning to only prune after every 500 updates
• added “–enable-profiling” to configure script
• added new –subject-marker option to set subject line
• added token pairing to match pairs of words

0.8.1 - 21 August 2003

• added more documentation, with examples, to the manual
• changed “-a -T” to behave exactly as manual specifies
• some minor (cosmetic) cleanups in the build process

0.7.8 - 18 August 2003

• added “–allowlist” for email address whitelisting

0.7.7 - 31 July 2003

• added automatic database pruning after every 100 updates

0.7.6 - 23 July 2003

• added “–merge” option to merge two databases together
• added C fakemail replacement to speed up “make test”

0.7.4 - 8 July 2003

• increased max HTML tag length from 100 to 500 characters
• decode HTML entities like   and "
• increased minimum token size to 3 characters from 2
• bugfix in header addition

0.7.0 - 5 July 2003

• added logarithmic probability scaling
• changed X-Spam-Rating: values to range from 0 to 100 (>90=spam)
• fixed bug in –prune that forced you to specify your database
• new option to add X-Spam-Token: headers (mainly for debugging)
• added new –benchmark option for developers
• stopped looking at the Received: header (causes too much skew)
• added “Sender:” to the list of headers we look at
• prune function now throws away tokens with less than 4 counts
• added new database-independent storage backend
• exclamation marks now allowed in tokens
• moved upper/lower probability cap back to 3 decimal places, not 4
• new developer option “make indent” to reformat source code
• extra code to allow compilation under MinGW (Windows)

0.5.9 - 27 June 2003

• replaced MD5 code with a public domain version
• binary attachments now have their MD5 checksum used as a token
• maximum token length increased to 34
• bug fixes in mailbox parser (extra null message, missing last message)
• bug fix in shifting weighting code

0.5.4 - 4 June 2003

• improved training method of –train using shifting weightings
• removed GNU getopt code from source tree
• replaced test spam/nonspam mail folders with test token lists

0.5.1 - 11 May 2003

• minor bugfix in –restore

0.5.0 - 10 May 2003

• tokens now stored as MD5 hashes for privacy
• added new “–add-rating” (-r) option to add X-Spam-Rating header
• added new “–prune” (-p) option to prune redundant database entries
• tokeniser now strips HTML where appropriate
• tokeniser now only allows . ’ and - when not at start or end (i.e. URLs)
• added support for Berkeley DB 1.85 and 4
• now seems to build and test OK on Mac OS X
• added “debian” directory from Tom Parker
• added “make rpm” and “make deb” targets to build RPM and Debian packages
• fixed buffer handling mistake in message parser
• build fixes and cleanups, added tests

0.3.1 - 22 January 2003

• allow the use of per-user databases and a system-wide database together
• added variable weighting to the training algorithm
• improved parsing of multipart messages / attachments
• removed use of tolower(), which was causing glibc version problems (!)
• added X-Spam: NO header to non-spam email
• moved all database calls to a wrapper for future portability
• split up message parser for better readability
• split up spam checking functions for better readability
• split up mailbox parsing functions for better readability

0.2.2 - 19 January 2003

• worked around procmail bug: now always exit 0 when filtering

0.2.1 - 17 January 2003

• removed dependency on fmemopen(), which isn’t standard
• found and documented bug in handling with procmail

0.2.0 - 15 January 2003

• non-textual attachments are now skipped
• added –train option
• checked for memory leaks
• tested –train on big mailboxes (around 28000 messages in total)

0.1.0 - 14 January 2003

• first working version

0.0.1 - 11 January 2003

• package created
• first draft of man page written