dired-isearch-filenames is a variable defined in `dired-aux.el'.
Its value is

Non-nil to Isearch in file names only.
If t, Isearch in Dired always matches only file names.
If `dwim', Isearch matches file names when initial point position is on
a file name. Otherwise, it searches the whole buffer without restrictions.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.