calling a command from a class

Try adding an eval line in your rule.

Like this:

rule HostRedirects {
  when HTTP_REQUEST {
    set row [findclass [getfield [HTTP::host] ":" 1] $::HostRedirects]
    if { not ($row eq "")}{
      set tmpRow [eval $row]
      HTTP::respond [getfield $tmpRow " " 2] Location [getfield $tmpRow " " 3]
      unset row
      unset tmpRow
      return
    }
  }
}

That should get the HTTP::uri command to evaluate for you. You might also want to add some logging to see what each of your getfield commands are returning if you have further problems.

Colin