Search for a file

[Panayotis Katsaloulis]

I would like to find a specific file in the ports distribution, when I  
don't already know the name of the package.
In other words, I want to find in which package(s) a specific file  
exists.

For example, in order to use docbook2html command, I  don't know which  
package to install.

In the old days of debian, I used to have apt-file for this job. Is  
there any similar application for macports?
What should I do in this case?