postinst is in iputils-arping 3:20121221-5ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30  | #!/bin/sh
set -e
if [ "$1" = configure ]; then
    # If we have setcap is installed, try setting cap_net_raw+ep,
    # which allows us to install our binaries without the setuid
    # bit.
    if command -v setcap > /dev/null; then
        if setcap cap_net_raw+ep /usr/bin/arping; then
            echo "Setcap worked! arping is not suid!"
        else
            echo "Setcap failed on /usr/bin/arping, falling back to setuid" >&2
            chmod u+s /usr/bin/arping
        fi
    else
        echo "Setcap is not installed, falling back to setuid" >&2
        chmod u+s /usr/bin/arping
    fi
fi
exit 0
# Local variables:
# mode: shell-script
# tab-width: 4
# indent-tabs-mode: nil
# end:
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