# rpm -qf /sbin/dhclient-script 
dhclient-4.1.1-48.P1.el6.x86_64


Running dhclient with an existing lease file will fail, because:

    BOUND|RENEW|REBIND|REBOOT)
        if [ -z "${interface}" ] || [ -z "${new_ip_address}" ]; then
            exit_with_hooks 2
        fi

        if arping -D -q -c2 -I ${interface} ${new_ip_address}; then
            dhconfig
            exit_with_hooks 0
        else
            # DAD failed, i.e. address is already in use
            exit_with_hooks 1
        fi
        ;;


arping will get the answer from its own interface. Newer dhclient-script versions have:

    BOUND|RENEW|REBIND|REBOOT)
        if [ -z "${interface}" ] || [ -z "${new_ip_address}" ]; then
            exit_with_hooks 2
        fi
        if arping -D -q -c2 -I ${interface} ${new_ip_address}; then
            dhconfig
            exit_with_hooks 0
        else  # DAD failed, i.e. address is already in use
            ARP_REPLY=$(arping -D -c2 -I ${interface} ${new_ip_address} | grep reply | awk '{print toupper($5)}' | cut -d "[" -f2 | cut -d "]" -f1)
            OUR_MACS=$(ip link show | grep link | awk '{print toupper($2)}' | uniq)
            if [[ "${OUR_MACS}" = *"${ARP_REPLY}"* ]]; then
                # the reply can come from our system, that's OK (#1116004#c33)
                dhconfig
                exit_with_hooks 0
            else
                exit_with_hooks 1
            fi
        fi
        ;;