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I've been trying to figure out how to modify the firmware on a DVR I bought for cheap recently. The built in software isn't great and doesn't offer any option for exporting video other than plugging a USB drive in and running an export through their UI. Ideally I'd like to modify it to export over FTP or NFS or something on a schedule.

You can upgrade the firmware on the device by putting the manufacturer provided upgrade image on a USB drive, plug it in, power on the device, then let it do it's thing to upgrade the firmware.

So I downloaded the firmware and ran binwalk on it. It was able to extract the roofs and I can see that it's running a flavor of embedded Linux. Here is the output that came with the files I extracted:


Scan Time:     2020-05-15 02:16:35
Target File:   /vagrant/rootfs-3531dv100
MD5 Checksum:  18a010179a1e5ae03c260ccc9609ddbc
Signatures:    404

DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
0             0x0             uImage header, header size: 64 bytes, header CRC: 0xCB1642A1, created: 2019-09-26 01:53:07, image size: 9761796 bytes, Data Address: 0x0, Entry Point: 0x0, data CRC: 0x35F26A52, OS: Linux, CPU: ARM, image type: Filesystem Image, compression type: none, image name: "hirootfs"
64            0x40            JFFS2 filesystem, little endian

In the files on the rootfs I found the init scripts that get run. Funny enough there's a call in there to start telnetd that's commented out. There's also a password for the root user set in /etc/passwd.

My question is this: if I modify the init script to uncomment that line so it runs telnetd at boot and generate a new password hash for the root user in /etc/password, how do I package it back up into a bootable image that I can drop onto a USB drive? Will that even work?

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Assuming nothing is cryptographically signed, that should be possible. You simply need to follow your steps in reverse. If you used binwalk's extract option, that made it too easy; you'll want to figure out how to unpack/pack the image manually (maybe the verbose option will provide more info on the steps taken). I realize this is a generic sounding answer, but without knowing the exact firmware structure it is difficult to speculate.

You'll also need to watch out for any checksums/hashes that will need to be modified if you change anything.

However, you could brick the device if you try to flash improper firmware. It may be a good idea to see if it supports any firmware recovery modes, or if there are hardware debug ports on the board to assist in recovery.

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  • I'm pretty new to reverse engineering in general, so forgive me if this is a naive question. There's an RS-485 port on the back of it. Is that a good candidate for a serial debugging port or should I be looking for something else? – jvergeldedios May 15 at 20:25
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    @jvergeldedios if the serial port lets you do anything during the boot process, that's a good sign. Although I'm not sure what RS-485 is used for. There's probably something better on the inside of the device. But that likely involves soldering and having the correct tools to interface – multithr3at3d May 16 at 2:31

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