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>9.5. Modem features to restrict usage</H1
><P
>Most modems support the addition of a password. This is not
particularly useful as it has the same strengths and weaknesses of
all other password authentication schemes. We already have
password authentication in the <SPAN
CLASS="ACRONYM"
>BIOS</SPAN
>, in the boot
loader and in <SPAN
CLASS="APPLICATION"
>login</SPAN
>.</P
><P
>Many modems support call-back. The modem is called and a few
seconds after hang-up it calls a pre-configured number. This
limits the locations that can gain access to the console. </P
><P
>Many modems support checking the calling line identification
(CLI) against a predefined list. If the calling number is not on
the list then the call is cleared. The phone line to the modem
must be configured to send CLI, this may incur an additional charge
from the phone company. Not all calling phones can send CLI and
some valid callers may have asked their phone company to suppress
the sending of CLI.</P
><P
>Many modems can be configured to log the calling line
identification. This is useful when tracing misuse.</P
><P
>Many modems support encryption. Some modems allow multiple
keys. This gives a neat solution: only authorized modems can dial
in, but they can do so from any location. The modems usually need
to be of the same make, and perhaps of the same model.</P
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><B
>Encryption dual-use technology</B
></TH
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><TD
> </TD
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><P
>Possessing, using, buying, selling, importing or exporting
modems with encryption features is a serious criminal offense in
some countries.</P
><P
>You must acquiant yourself with the laws in your
jurisdiction and the laws of jurisdictions you may travel
through.</P
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><P
>Many telephone services or <SPAN
CLASS="ACRONYM"
>PBX</SPAN
> lines can
be set to allow only incoming calls. This is useful as it prevents
misuse of the modem should the computer be compromised. A
<SPAN
CLASS="QUOTE"
>"demon dialler"</SPAN
> can call many numbers seeking an
answering modem and the cost of these calls can be
significant.</P
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