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9.1 Robot Exclusion

It is extremely easy to make Wget wander aimlessly around a web site, sucking all the available data in progress. ‘wget -r site’, and you're set. Great? Not for the server admin.

As long as Wget is only retrieving static pages, and doing it at a reasonable rate (see the ‘--wait’ option), there's not much of a problem. The trouble is that Wget can't tell the difference between the smallest static page and the most demanding CGI. A site I know has a section handled by a CGI Perl script that converts Info files to HTML on the fly. The script is slow, but works well enough for human users viewing an occasional Info file. However, when someone's recursive Wget download stumbles upon the index page that links to all the Info files through the script, the system is brought to its knees without providing anything useful to the user (This task of converting Info files could be done locally and access to Info documentation for all installed GNU software on a system is available from the info command).

To avoid this kind of accident, as well as to preserve privacy for documents that need to be protected from well-behaved robots, the concept of robot exclusion was invented. The idea is that the server administrators and document authors can specify which portions of the site they wish to protect from robots and those they will permit access.

The most popular mechanism, and the de facto standard supported by all the major robots, is the “Robots Exclusion Standard” (RES) written by Martijn Koster et al. in 1994. It specifies the format of a text file containing directives that instruct the robots which URL paths to avoid. To be found by the robots, the specifications must be placed in ‘/robots.txt’ in the server root, which the robots are expected to download and parse.

Although Wget is not a web robot in the strictest sense of the word, it can download large parts of the site without the user's intervention to download an individual page. Because of that, Wget honors RES when downloading recursively. For instance, when you issue:

 
wget -r http://www.server.com/

First the index of ‘www.server.com’ will be downloaded. If Wget finds that it wants to download more documents from that server, it will request ‘http://www.server.com/robots.txt’ and, if found, use it for further downloads. ‘robots.txt’ is loaded only once per each server.

Until version 1.8, Wget supported the first version of the standard, written by Martijn Koster in 1994 and available at http://www.robotstxt.org/wc/norobots.html. As of version 1.8, Wget has supported the additional directives specified in the internet draft ‘<draft-koster-robots-00.txt>’ titled “A Method for Web Robots Control”. The draft, which has as far as I know never made to an RFC, is available at http://www.robotstxt.org/wc/norobots-rfc.txt.

This manual no longer includes the text of the Robot Exclusion Standard.

The second, less known mechanism, enables the author of an individual document to specify whether they want the links from the file to be followed by a robot. This is achieved using the META tag, like this:

 
<meta name="robots" content="nofollow">

This is explained in some detail at http://www.robotstxt.org/wc/meta-user.html. Wget supports this method of robot exclusion in addition to the usual ‘/robots.txt’ exclusion.

If you know what you are doing and really really wish to turn off the robot exclusion, set the robots variable to ‘off’ in your ‘.wgetrc’. You can achieve the same effect from the command line using the -e switch, e.g. ‘wget -e robots=off url...’.


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