An alternating space-separated list of the directory from which that untar should start, and the path to the tarball. You can supply either URLs from which we should fetch the tarballs, or paths to files on users.emulab.net. For example:
/somedir http://www.myU.edu/~me/mytarball.tar.gz

or

/somedir /proj/myproj/tarfiles/mytarball.tar.gz
These tarballs will be installed in all slivers in the slice, and you can push out updates to them while the slice is running.