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A Firmware Filesystem

DZone's Guide to

A Firmware Filesystem

Someone else's firmware can help organize your own thoughts and projects. By diving into it with binwalk, you can learn how to avoid simple mistakes.

· IoT Zone
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So far, we've extracted the firmware contents of the NC200 cloud camera system from TP-Link, and we've learned about the active kernel version and the bootloader. Now let's look a little more closely at the filesystem. This particular distribution is pretty interesting too, because it looks like we have two of them!

First, let's go to the CPIO encoded filesystem. This one actually looks like a full Unix filesystem:

cclamb@fawkes:~/Work/tplink/nc200/a/rebuilt_fs $ ls
bin dev etc_ro init lib mnt proc sbin sys tmp usr var


The JFFS2 filesystem, on the other hand, seems kind of odd:

cclamb@fawkes:~/Work/tplink/nc200/a/_NC200_2.1.7_Build_160315_Rel.27420.bin.extracted/jffs2-root/fs_1 $ ls
bin config etc lib sbin share www


So what gives? Well, we have a couple of clues. First, on the JFFS2 side, take a look at config/lighttpd.conf:

var.home_dir = "/var/run/lighttpd"
var.log_root = home_dir
var.state_dir = home_dir
var.server_root = "/usr/local/www"
var.conf_dir = "/usr/local/config"


Hmm. We don't have a /usr/local in the JFFS2 distro, but we do in the CPIO distro, though the CPIO distro doesn't have anything in /usr/local. We do have a www and a config directory in the JFFS2 distro though. What else?

Well, on the CPIO side, we have a /etc_ro/rcS file. Take a look at the top of the file:

#!/bin/sh
# mount /usr/local from flash
mount -t jffs2 /dev/mtdblock5 /usr/local
mount -t jffs2 /dev/mtdblock6 /usr/local/config/ipcamera


Whhhhaaaaat? Well, it looks like the JFFS2 filesystem is actually mounted on /usr/local at boot! So, to make things easier, let's copy the contents of the JFFS2 filesystem into the CPIO filesystem at /usr/local (I suggest you use a copy of the CPIO filesystem contents to do this, just in case you mess something up like I did the first time).

Don't worry about the contents of fs_2 — the files there are the same ones as are in the CPIO filesystem.

Why would you design a filesystem like this? Well, notice that most of the application specific stuff is in the JFFS2 filesystems, while typical Unix-y things are in the CPIO filesystem. This kind of design would allow you to update only the application code, without needing to deal with the Linux installation. Bundling everything together might require you to update the Linux code with every application update — something you probably don't want to do. Also notice that of the JFFS2 images that are installed on flash, one is dedicated to the camera, while the other seems to handle mostly configuration and communication. Honestly, a pretty good example of the separation of concerns in practice. Good job TP-Link!

So now we have a complete filesystem to work with. Next, we can look over the Linux configuration, and start to figure out what code is core to this particular device and what isn't.

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Topics:
iot reverse engineering ,iot ,firmware ,filesystem ,tutorial

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