QNAP VS-2004L en - Manual (Page 166)

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6.3.3 File Services
Enable the SMB/CIFS file service, Web File Manager, and FTP service to access the video
files. These settings are enabled by default.
If the NVR is installed behind the router, enable FTP port mapping to allow the users from
the external network to connect to the NVR via FTP (please refer to
Appendix B ).
Passive FTP Port Range
Use the default port range (55536–56559) or specify a port range larger than 1023. When
using this function, make sure the ports on the router or firewall have been opened.
Respond with external IP address for passive FTP connection request
When passive FTP connection is in use and the NVR is configured behind a router, enable
this function to allow connection to the NVR on WAN. By enabling this function, the FTP
service replies the specified IP address or automatically detects the external IP address so
that the remote computer can connect to the NVR.