/usr/share/fai/dhclient-fai-script is in fai-nfsroot 5.3.6ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #!/bin/sh
# execute perl script to obtain dhcp data
perl /usr/lib/fai/dhclient-perl
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