A robots.txt surprise

Because I don't really like banning MSNBot, MSN Search's web spider, I decided to drop our ban and see if its behavior had improved since last September. The process of doing this has led me to a little surprise about how at least MSNBot matches User-Agent lines in robots.txt.

From looking at our logs, I already knew that MSNBot was still visiting; it pulled robots.txt at least once a day. So all I needed to do was change robots.txt so that it wouldn't be banned.

Since I wanted to note down when I removed the ban, I just added a suffix on the User-Agent string, changing from banning 'msnbot' to banning 'msnbot-reenabled-2006-02-14'. To my surprise nothing happened, so I changed it again, putting 'X-20060222-' on the front. Still nothing happened.

Finally, yesterday evening I changed 'msnbot' to 'mXsXnbXot'. Within 12 hours, MSNBot had started crawling pages here.

The MSNBot web page is rather non-specific about how MSNBot decides whether or not it's excluded; all of their examples certainly use just 'msnbot' as the User-Agent string. A prefix match made sense to me, since it doesn't hose people who put things like 'msnbot/1.0' in their robots.txt, but the rest was surprising.

It turns out that this is actually recommended behavior; the Standard for Robot Exclusion web page says:

The robot should be liberal in interpreting [the User-Agent] field. A case insensitive substring match of the name without version information is recommended.

I don't know how many robots follow this, but MSNBot evidently does. Good for them.