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  Using Expressionlists in SquidGuard

You will never be able to add a possible "bad" pages to your domains and urls files. To achieve additional blocking expressionlists can be used.
In an expressionlist you enter words that are mostly likely part of unwanted domains and urls. Each domains and url will be matched against the entries found in the configured expression lists.
Attention: Only use expressionlists if you are sure you do not block away innocent sites. Additionally keep in mind that using these lists can result in a noticable performance impact.

  1. Adding expression lists to the configuration

    Expression lists are included in the dest tag. The example below shows the relevant part of the squidGuard configuration file:

    Using expression lists to block porn
     dest porn {
          domainlist       porn/domains
          urllist          porn/urls
          expressionlist   porn/expressions

    You can add an expression list to each defined destination if you find this necessary (reminder: performance!). SquidGuard expects the expression file relative to the defined dbhome just as the domains and urls files.

  2. Syntax of expression lists

    The expressionlist file format is lines with regular expressions as described in regex(5). Of most interrest is:

    . Matches any single character (use "\." to match a ".").
    [abc] Matches one of the characters ("[abc]" matches a single "a" or "b" or "c").
    [c-g] Matches one of the characters in the range ("[c-g]" matches a single "c" or "d" or "e" or "f" or "g".
    "[a-z0-9]" matches any single letter or digit.
    "[-/.:?]" matches any single "-" or "/" or "." or ":" or "?".).
    ? None or one of the preceding ("words?" will match "word" and "words".
    "[abc]?" matches a single "a" or "b" or "c" or nothing (i.e. "")).
    * None or more of the preceding ("words*" will match "word", "words" and "wordsssssss". ".*" will match anything including nothing).
    + One or more of the preceding ("xxx+" will match a sequence of 3 or more "x").
    (expr1|expr2) One of the expressions, which in turn may contain a similar construction ("(foo|bar)" will match "foo" or "bar".
    "(foo|bar)? will match "foo" or "bar" or nothing (i.e. "")).
    $ The end of the line ("(foo|bar)$" will match "foo" or "bar"only at the end of a line).
    \x Disable the special meaning of x where x is one of the special regex characters ".?*+()^$[]{}\"  ("\." will match a single ".", "\\" a single "\" etc.)

    The entries to the expression lists are cleartext. Thus a start to block possible sexual material by expression match could look like:


    See here a very extensive example how to use expression lists to block porn request in searchengine (thanks to Denis Immoos [http://wg.edubs.ch/], http://dimmoos.wordpress.com/): Examples/08.expressionlist. In order to prevent over-blocking consider the following entries in a white list: Examples/09.whiteexpression.
    If you want to block incoming files with extensions like txt.com (as it used by some viruses) check Examples/10.antivirus-explist.
    Check and test it carefully if you want to use parts or the complete list.

    If everything is set up to your liking make the changes active by issuing the command:

    squid -k reconfigure

  3. Some notes and hints

    • Unless you build your expressions very very carefully there is a high risk you will have annoyed users on your neck. Typically you might accidentally block "Essex", "Sussex", "breastcancer", "www.x.org" etc. in your eagerness for blocking pornographic material. In practice you would probably replace some of the words in the example above with some more clearly pornographic related words that I don't find appropriate to list here.

    • While the size of the domain and urllists only has marginal influence on the performance, too many large or complex expressions will quickly degrade the performance of squidGuard. Though it may depend heavily on the performance of the regex library you link with.

    • There is a set of sample files for a group of supposedly pornographic sites under samples/dest/adult in the source tree that you can use as a start if porn blocking is one of your tasks. Please note: These lists are very, very old and not intended for production systems. Use them to test your squidGuard. For an initial setup the lists will work, but we recommend that you review these lists before using them. Those domains and urls have been collected automagically by a robot. No manual evaluation of the corresponding contents has been performed. Therefor there is a chance some nonpornographic sites have sliped in.

    • To avoid publishing to your users a complete guide to banned sites, you probably want to have some or all of these files protected by for instance:
      chmod 640 /wherever/filter/db/dest/adult/*
      chown cache_effective_user /wherever/filter/db/dest/adult/*
      chgrp cache_effective_group /wherever/filter/db/dest/adult/*
      where cache_effective_user and cache_effective_group are the values for the corresponding tags as defined in squid.conf.

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