'Make sure intf exists and is not configured.'

@staticmethod
def checkIntf(intf):

    if (('    %s:' % intf) not in quietRun('ip    link    show')):
        showerror(title='Error', message=(('External    interface    ' + intf) + '    does    not    exist!    Skipping.'))
        return False
    ips = re.findall('\\d+\\.\\d+\\.\\d+\\.\\d+', quietRun(('ifconfig    ' + intf)))
    if ips:
        showerror(title='Error', message=(intf + '    has    an    IP    address    and    is    probably    in    use!    Skipping.'))
        return False
    return True
