To connect to Dahua DVRs via telnet in Windows, you can use, for example, the PuTTY program.
The default login is root, the password is vizxv. They differ from those that are configured in the web interface, many do not even know about them, and therefore old versions of DVRs are vulnerable, since you can connect to it. In the new version of Dahua firmware, telnet access is disabled by default. Dahua IP cameras essentially have the same operating system, so you can also connect to them.
Telnet works through tcp on port 23.
I will describe some commands when connecting via telnet:
reboot (reboot DVR)
ls -l (view file system)
ls -l /dir (view the contents of a specific directory)
vi /dir/file.txt (example of opening a file in a vim text editor, see also Using the vim text editor)
mount (view mounted partitions)
mount -o rw /mnt/dir (mount a read / write partition)
mount -o ro /mnt/dir (mount a read-only partition)
/unmount /mnt/dir (example of detaching a partition)
cd /dir (change to the specified directory)
cd .. (return to the directory above)
See also my article:
Installing and using john on Linux