Vector PC-Duo Gateway Manual page 37

Gateway Operation
NOTE: At this first connection, the Host implicitly trusts the PC-Duo Gateway because it
is on the known Gateways list. For even higher level of authentication, use SSL protocol
with valid certificates to confirm the identity of the PC-Duo Gateway.
On all subsequent connection attempts when the Host and PC-Duo Gateway are not in
the same domain, the shared secret password will be presented and accepted for
authentication (because it is known only to the Host and PC-Duo Gateway). No
configuration change is required and the Host security remains unchanged for all other
requests.
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