Problem trying to install iftop via Homebrew

You can remedy things in your current shell by doing:

mkdir -p /usr/local/sbin
export PATH=${PATH}:/usr/local/sbin
brew link iftop

That'll get you past the warnings and let Homebrew install the iftop package. If the iftop package is installing things in to /usr/local/sbin that you're looking to run, you'll need to ensure this is on your $PATH when you open a shell. To do this, edit ~/.bash_profile and add the line:

export PATH=${PATH}:/usr/local/sbin

To the end of the file to prepend /usr/local/sbin to each new shell you open.

Have you tried using:

$ brew link iftop

As the message recommended? Otherwise it's likely you need to be su to successfully create the symlink. Try:

$ ./configure --disable-debug --prefix=/usr/local/Cellar/iftop/0.17 --mandir=/usr/local/Cellar/iftop/0.17/share/man
$ sudo make install