A customer recently had problems with crawling his Web Application. This problems were solved, but after that, the crawl broke again.
After some investigation, I found out that the IP address of the dedicated WFE for crawling was wrong. SharePoint entered the wrong IP address in the HOSTS file.
I configured the Search Service to use one dedicated WFE for crawling. This is very useful, because the other two WFEs can be used for user access only (they are load balanced).
Here is a picture of that:
I discovered that the HOSTS file was filled with the first entry IPCONFIG shows on the target WFE:
The .178 was added to the HOSTS file, but not reachable from the index server, it should have been the .148. The crawl was impossible.
I configured the search service on the index server to use all WFEs for crawling. This way, it does not touch the HOSTS file and I was able to enter the right address to it manually.
Thanks to http://www.mindsharpblogs.com/daniel/archive/2007/02/01/1518.aspx for guiding me in this direction.