125 lines
4.3 KiB
HTML
125 lines
4.3 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
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<HTML>
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<HEAD>
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<title>Title of Article Issue 13</title>
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</HEAD>
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<BODY >
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<H4>
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"Linux Gazette...<I>making Linux just a little more fun!</I>
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"</H4>
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<P> <HR> <P>
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<!--===================================================================-->
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<center>
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<h1>Filtering Advertisements from Web Pages using IPFWADM</h1>
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<H4>By David Rudder
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<a href="mailto:drig@magicweb.com">drig@magicweb.com</a></H4>
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</center>
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<P> <HR> <P>
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Lately, a lot of Web pages have begun selling ad space "banners."
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Wasting valuable bandwidth, these banners often hawk products I don't
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care to hear about. I'd rather not see them, and not have to download
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their contents.
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<p>
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There are two ways of filtering out these banners. The first is to
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deny all pictures that are wider than tall and generally towards the
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top or bottom of the page. The second is to simply block all the
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accesses to and from the web sites that are the notorious advertisers.
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This second approach is the one I'm going to take.
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<p>
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When searching around the web, you will see that many of the banners
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come from the site ad.linkexchange.com. This is the site we will want
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to ban.
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<P> <HR> <P>
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<center><h3>Setting Up Your Firewall</h3></center>
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<p>
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Our first order of business is to set up our firewall. We won't be
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using it for security, although this doesn't prohibit also using the
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firewall for security. First, we recompile the kernel, saying
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"Yes"
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to CONFIG_FIREWALL. This allows us to use the built in kernel
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firewalling.
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<p>
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Then, we need to get the IPFWADM utility. You can find it at:
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<a
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href="http://www.xos.nl/linux/ipfwadm">http://www.xos.nl/linux/ipfwadm
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</a>. Untar, compile and install this utility.
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<p>
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Since we are doing no other firewalling, this should be sufficient.
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</p>
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<P> <HR> <P>
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<center><h3>Blocking Unwanted Sites</h3></center>
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<p>
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Now, we come to the meat of the maneuver. We need now to block access
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to our machine from ad.linkexchange.com. First, block out access to
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the sight, so that our requests don't even make it there.
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<tt>ipfwadm -O -a reject -P tcp -S 0.0.0.0/0 -D ad.linkexchange.com 80</tt><br>
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<p>
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This tells ipfwadm to append a rule to the Output filter. The rule
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says to reject all packets of protocol TCP from anywhere to
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ad.linkexchange.com on port 80. If you don't get this, read Chris
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Kostick's excellent article on IP firewalling at
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<a href="http://www.ssc.com/lj/issue24/1212.html">
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http://www.ssc.com/lj/issue24/1212.html</a>.
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<p>
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The next rule is to keep any stuff from ad.linkexchange.com from
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coming in. Technically, this shouldn't be necessary. If we haven't
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requested it, it shouldn't come. But, better safe than sorry.
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<tt>ipfwadm -I -a reject -P tcp -S ad.linkexchange.com 80 -D 0.0.0.0/0</tt><br>
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<p>
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Now, all access to and from ad.linkexchange.com is rejected.
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<p>
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<em>Note:</em> this will only work when web browsing from that machine. To
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filter for a whole network, do them same but with -F instead of -O and
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-I.
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<P> <HR> <P>
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<center><h3>Testing It Out</h3></center>
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<p>
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To test, visit the site <a href="http://www.reply.net">http://www.reply.net</a>. They have a banner on
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top which should either not appear or appear as a broken icon. Either
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way, no network bandwidth will be wasted downloading the picture, and
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all requests will be rejected immediately.
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</p>
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<P> <HR> <P>
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<center><h3>Filling It Out</h3></center>
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<p>
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Not all banners are so easily dealt with. Many companies, like Netscape, host their
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own banners. You don't want to block access to Netscape, so this
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approach won't work. But, you will find a number of different
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advertisers set up like linkexchange. As you find more, add them to
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the list of rejected sites.
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</p>
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Good luck, and happy filtering!
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<P> <hr> <P>
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<center><H5>Copyright © 1997, David Rudder <BR>
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Published in Issue 13 of the Linux Gazette</H5></center>
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