Reverse Engineering TrendChip Firmware (ZTE-H108NS).

It all started when I bought a new computer at home for my developer needs. Usually I fall to the other end of the gadget likeness spectrum. Specifically I avoid buying gadget stuff due to the unhealthy amount of time I found myself devoting to their configuration. However this time the conspiracy of things was masterfully executed to the point that I didn’t notice until it was too late (a blog and a github project later).

Actually If I want to be pedantic the first pieces has been fallen into place about 6 months before when my father switched ISP and he ended up with two new modems (ZTE-H108NS). Since I was the one that configured the new connections I preferred to use the existing modems in place. So I ended up with two brand new modems. At the time I thought “Hm? Maybe I should try to optimize my DSL line with a new modem and get rid of my trusty speedtouch 585“. I didn’t realize it then but that was probably the cornerstone that put in motion a series of events.

When finally my new computer arrived it had a feature that my previous 12 year computer was lacking. The mighty new feature was a working sleep/hibernate implementation. I immediately thought “Cool – I will be now able to hibernate my computer – It will not need to stay up all the time.”. Here is the train of thought:

• But what if I need something when I am away and my computer is turned off?

• Well we should try this thing called WOL (Wake on LAN) – Another new feature for me.

Two weeks later after I have studied in detail all possible permutations of BIOS options, the fine differences between S4, power off and hybrid mode and wandered in forums I finally managed to master the WOL from my LAN.

• So what about the WAN? After all if I could use the LAN I could always switch on my computer by pressing the button – no need for WOL at all.

• Wait! said an inner voice and the only advice I can give to myself from now on to not listen again that particular inner voice. If you are going to teach the modem about WOL maybe you should finally ditch the speedtouch 585. and invest some time to the new ZTE-H108NS modem said the voice.

DSL Line Optimization

So I switched modems – I configured the new modem and went to device status page in order to check on the DSL link speed. It was a gapping 12MBit/s exactly the same with the speedtouch 585. Given that my house is 35 meters away from the OTE (Greek ISP provider) building this is unacceptable. I picked up the phone and called my ISP. 15′ later my modem was syncing in 18MBit/s. Not bad.

Further research suggested the following command line (telnet interface of the modem)

#wan dmt2 set snrmoffset 4096 4096
#wan adsl reset

which happily increased my downlink to 22MBit/s. I thought let’s call it a day on the speed front and let’s concentrate back in the energy footprint front.

WOL over WAN

I was able to send the WOL over the internet with a minor arp tables hack. Specifically I had to map another address, let’s say 192.168.1.254 to the broadcast address.

#arp -s 192.168.1.254 FF:FF:FF:FF:FF:FF

Then on Advanced Setup/NAT/Virtual Server I setup a rule for ports 7-9 that goes to the internal ip address 192.168.1.254 which due to the above arp hack ends up as a broadcast packet waking up my computer. The following command wakes up my computer from anywhere in the world.

$wakeonlan -i `host myhost.mydomain.com | awk '{print $NF;}'` de:ad:ca:fe:be:ef

where myhost.mydomain.com is my Dynamic DNS hostname and de:ad:ca:fe:be:ef is the MAC address of my computer.

Dynamic DNS

The situation now begs the question: If the computer is turned off who is gonna maintain the Dynamic DNS entry up to date? Normally this is a job for the modem router because it has intimate knowledge of the external ip address and its changes. I am already a client of namecheap and I would like to use that provider for DDNS. Of course since the Internet community hasn’t succeed so far to standardize the DDNS updates protocol consequently the ZTE-H108NS modem doesn’t have support for namecheap.

Samba woes

In the meantime my wife had an unusual request. She wanted to print like it was the most natural thing of the world. I responded to the request lightly, full of self confidence. I installed samba and cloned the configuration from the backup of my previous setup. Yep that’s right. I am that good. I have a backup. What could possibly go wrong?

Well for starters my new machine was not visible in the network neighborhood. This took a good 2-3 hours to figure it out and it was a lesson to humility. It turns out that:

1. The modem DHCP server sets itself as WINS server.
2. Although nmbd is running the smb.conf in the modem does not enable WINS support. Bummer!
3. There is no option in the GUI to enable the WINS server
4. The modem confuses the workgroup name with the NETBIOS and vice versa.

The Decision

At this point insurmountable pressure has been gathered. I needed to hack the modem in order to fix the following itches ^H^H items.

1. DSL Line setup
2. Arp table setup for WAN WOL support
3. Enable the WINS server in smb.conf
4. namecheap Dynamic DNS support

These problems are not fixable without modifying the root filesystem or uploading custom firmware. Some of them require just a configuration file change but the namecheap Dynamic DNS support requires to modify and rebuild a MIPS executable.

Reverse Engineering -Know your enemy

Let’s try to find some info on this thing. I opened the case and I found these chips:

• Chassis PCB – BZRD0 V1.0
• CPU – U100 TrenChip TC3162U-LQ128G
• SDRAM- EtronTech EM6381165TS-6G
• SPI Flash – winbond 25Q16BVSIG

Let’s enter the modem via telnet:

$ telnet 192.168.1.1
Trying 192.168.1.1...
Connected to 192.168.1.1.
Escape character is '^]'.
 login: admin
Password:
# cat /proc/cpuinfo 
system type             : TrendChip TC3162U SOC
processor               : 0
cpu model               : R3000 V0.1
BogoMIPS                : 330.95
wait instruction        : no
microsecond timers      : no
tlb_entries             : 32
extra interrupt vector  : no
hardware watchpoint     : no
ASEs implemented        :
shadow register sets    : 1
VCED exceptions         : not available
VCEI exceptions         : not available
unaligned accesses      : 187256836
#
# free
              total         used         free       shared      buffers
  Mem:        29196        27936         1260            0         2088
 Swap:            0            0            0
Total:        29196        27936         1260
# ls -l /
drwxr-xr-x    2 17431    10513         389 bin
drwxr-xr-x    4 17431    10513          46 boaroot
drwxr-xr-x    6 17431    10513         535 dev
lrwxrwxrwx    1 17431    10513           8 etc -> /tmp/etc
drwxr-xr-x    4 17431    10513        1507 lib
lrwxrwxrwx    1 17431    10513          11 linuxrc -> bin/busybox
dr-xr-xr-x   64 0        0               0 proc
drwxr-xr-x    2 17431    10513         138 sbin
drwxr-xr-x    7 0        0               0 tmp
drwxr-xr-x    4 17431    10513         107 userfs
drwxr-xr-x    7 17431    10513          67 usr
lrwxrwxrwx    1 17431    10513           8 var -> /tmp/var
# ls /etc/
AppFilter.sh             ipv6_acl.sh              nat_pvc9_0
RT30xxEEPROM.bin         isp0.conf                nat_pvc9_1
UrlFilter.sh             isp1.conf                nat_pvc9_2
Wireless                 isp11.conf               nat_pvc9_3
acl.sh                   l7-protocols             nat_pvc9_4
add_rule_for_dhcp.sh     lanAlias0.conf           nat_pvc9_5
adsl.conf                lan_rip.conf             nat_pvc9_6
adsl.sh                  lanconfig.sh             nat_pvc9_7
autoexec.sh              mac.conf                 ntp.sh
bftpd.conf               nat_pvc0                 passwd
config                   nat_pvc1                 ppp
ddns.conf                nat_pvc10                protocols
defaultWan.conf          nat_pvc10_0              radvd.conf
devInf.conf              nat_pvc10_1              resolv.conf
device_hostindex.conf    nat_pvc10_2              resolv_ipv4.conf
dhcp6c.conf              nat_pvc10_3              resolv_ipv6.conf
dhcp6s.conf              nat_pvc10_4              ripd.conf
dhcpd                    nat_pvc10_5              route6.sh
dnsmasq.conf             nat_pvc10_6              samba
dproxy.conf              nat_pvc10_7              services
ethertypes               nat_pvc2                 shaper
firewall.conf            nat_pvc3                 snmp
fstab                    nat_pvc4                 snmpd.conf.tmp
fwTCver.conf             nat_pvc5                 sysconfig
fwver.conf               nat_pvc6                 system
group                    nat_pvc7                 trx_config
hosts                    nat_pvc8                 udhcp_lease
igd                      nat_pvc8_0               udhcpd.conf
igmpproxy.conf           nat_pvc8_1               udhcpd.fon.conf
inetd.conf               nat_pvc8_2               udhcpd.fon.leases
init.d                   nat_pvc8_3               usb_modeswitch.conf
inittab                  nat_pvc8_4               usb_modeswitch.d
inittab_no_ra_menu       nat_pvc8_5               usb_modeswitch_cfg.conf
inittab_ra_menu          nat_pvc8_6               usertty
iproute2                 nat_pvc8_7               xml
ipupdate.conf            nat_pvc9                 zebra.conf
# ls /userfs/bin/
auto_mount_dongle   hello               ripd
bftpd               igmpproxy           skbmgr
boa                 inetd               smbd
cfg_manager         iwpriv              smbpasswd
chat                mtd                 snmpd
dhcp6c              nmbd                tcapi
dhcp6s              ntfs-3g             tftpd
dhcrelay            ntpclient           tr69
dnsmasq             pppoe-relay         usb_auto_mount
ethphxcmd           radvd               vconfig
ez-ipupdate         restore_linos_info  zebra
# ls /usr/script/
AppFilterStop.sh              ipv6_filter_forward_start.sh
RebootScript                  ipv6_filter_forward_stop.sh
UrlFilterStop.sh              ipv6rd_start.sh
acl_stop.sh                   lanAlias_start.sh
before_tr069_download.sh      lanAlias_stop.sh
before_web_upgrade.sh         nat_start.sh
br_conf.sh                    nat_stop.sh
ddns.sh                       ntpclient.sh
ddns_run.sh                   ppp_start.sh
dhcp6c_script                 restart_boa.sh
dmz.sh                        samba.sh
dongle_dial_off.sh            samba_add_dir.sh
dongle_dial_on.sh             settime.sh
ether_mac.sh                  spi_fw_start.sh
filter_forward_start.sh       spi_fw_stop.sh
filter_forward_stop.sh        udhcpc.sh
fw_start.sh                   udhcpc_nodef.sh
fw_stop.sh                    upgrade_firmware.sh
getnow.sh                     urlfilter_start.sh
ipaddr_mapping.sh             urlfilter_stop.sh
ipfilter.sh                   vserver.sh
ipfilter_start.sh             wan_start.sh
ipfilter_stop.sh              wan_start_ipv4.sh
ipmacfilter_stop.sh           wan_start_ipv6.sh
ipv6_acl_stop.sh              wan_stop.sh
# mount
/dev/mtdblock3 on / type squashfs (ro)
proc on /proc type proc (rw)
ramfs on /tmp type ramfs (rw)
devpts on /dev/pts type devpts (rw)
usbfs on /proc/bus/usb type usbfs (rw)
# cat /proc/mtd 
dev:    size   erasesize  name
mtd0: 00010000 00010000 "bootloader"
mtd1: 00010000 00010000 "romfile"
mtd2: 000e9560 00010000 "kernel"
mtd3: 004cf000 00010000 "rootfs"
mtd4: 007a0000 00010000 "tclinux"
mtd5: 00040000 00010000 "reservearea"

Some notes on what’s going on:

• The CPU is TrendChip 3162U. Looks like there are a lot of products that utilize this CPU and may use the TrendChip firmware.
• It has 32MB of memory.
• Looks like it has 16MB of flash.
• The rootfs is a standard mips uclibc based on busybox.
• The /etc directory is symlinked to /tmp which is a tmpfs.
• The /etc directory is created mostly by scripts living at /usr/script directory.
• The scripts are called by an executable named /userfs/bin/cfg_manager.
• The WEB GUI lives in /boaroot.
• The WEB GUI breaks out to cfg_manager via a socket interface. Not so sure about that.
• USB sticks are mounted under /tmp/mnt/usb1_1 and are automatically exported via samba. Question: Is it possible via a USB hub to mount multiple disks? That would be cool to actually convert a modem to a NAS.
• The flash is partitioned and looks like that some parts of it map directly to the tclinux.bin firmware image.

Reverse Engineering – binwalk

Let’s start with firmware 1.07 named tclinux.bin. Note the the name tclinux.bin is checked by the firmware during update and it is quite possible that the modem will refuse to update its firmware if the file doesn’t have that name.

#apt-get install binwalk
$binwalk tclinux.bin
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
256 0x100 LZMA compressed data, properties: 0x5D, dictionary size: 8388608 bytes, uncompressed size: 3232020 bytes
955744 0xE9560 Squashfs filesystem, little endian, version 3.0, size: 5036741 bytes, 986 inodes, blocksize: 65536 bytes, created: Tue Feb 11 09:50:54 2014

So the first 256 bytes is the header and then something else and much later at 0xE9560 starts a squashfs. Let’s dig a bit more.

$binwalk -e tclinux.bin
$ls _tclinux.bin.extracted/
100 100.7z _100.extracted E9560 E9560.squashfs
$cd _tclinux.bin.extracted
$binwalk 100
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
1430825 0x15D529 LZMA compressed data, properties: 0xD0, dictionary size: 131072 bytes, uncompressed size: 277750168 bytes
2646072 0x286038 Linux kernel version "2.6.22.15 (root@mbjsbbcf01) (gcc version 3.4.6) #1 Tue Feb 11 1c version 3.4.6) #1 Tue Feb 11 10:17:49 CST 2014"

So the firmware image looks like this

• 0-0x100 Header
• secondary bootloader (Couldn’t figure which was it)
• Linux kernel + initrd (Not sure which order)
• 0x09560 squashfs
• padding to 4K

The header has the following fields

• 0x00 magic number = 0x32524448
• 0x04 magic device (or header size – not sure) = 0x100
• 0x08 tclinux size = actual file size in bytes with header included
• 0x0C tclinux checsum = CRC – XOR checksum see below how to calculate
• 0x10 firmware version string = “7.0.1.0\n” in my case
• 0x30 a newline = “\n”
• 0x50 squashfs offset = 0x9560 in my case
• 0x54 squashfs size = size of squashfs size as reported by binwalk 5038080 bytes padded to 4096 (0x1000) sector 5038080 (0x4CE000)
• 0x5C model string = “3 6035 122 74\n” in my case

The program tcrevenge can be used to check an existing firmware file or calculate the checksum and generate a proper header for a new one.

Question: How to compare the live system with the firmware image?

Answer: We can put a usb stick to the modem (sometime requires reboot with the usb stick inside the jack) and use samba to copy files around.

for i in 0 1 2 3 4 5; do echo cat /dev/mtd$i > /tmp/mnt/usb1_1/p$i; done;

Notes:

• p0 (partition 0) is 64k has the primary bootloader. Running strings against it is instructive indeed. It includes a minimal web server and it should be (never done it) possible to recover the modem from a bad firmware image. That means it must have the same CRC algorithm with the normal software for uploading firmware. The contents of this partition do not change with a firmware update.
• p1 is the configuration file as saved by the WEB interface. For some reason it is called romfile.
• p2 is the secondary bootloader and probably the kernel and some initrd image. The contents of the partition are exact match with the firmware image tclinux.bin (header included) up to the squashfs image as extracted by binwalk above.
• p3 is the squashfs starting at 0x9560 of the tclinux.bin.
• p4 holds the tclinux.bin verbatim so the primary bootloader can recover the modem
• p5 is a reserved partition

Question: is it possible to modify the contents of the root (squashfs) filesystem?

Answer: We will need firmware-mod-kit in order to cope with the unusual LZMA compression and the older version number.

$git clone https://code.google.com/p/firmware-mod-kit/

Follow the instructions and build the firmware-mod-kit utilities. To uncompress and extract the filesystem you will need root user capabilities in order to create the device nodes and other weird files.

$sudo ../firmware-mod-kit/src/squashfs-3.0/unsquashfs-lzma E9560.squashfs

created 731 files
created 115 directories
created 91 symlinks
created 48 devices
created 0 fifos
$ls
E9560.squashfs squashfs-root

After making modifications to the root filesystem. Here is how to pack it up again.

rm E9560.sq; ../firmware-mod-kit/src/squashfs-3.0/mksquashfs-lzma squashfs-root squashfs-root.sq

The file squashfs-root.sq contains the modified squashfs root image.

Now we need to extract the kernel so we can assemble back the modified tclinux.bin. Here is a one line to do it:

$dd if=tclinux.bin.orig of=kernel skip=256 count=`binwalk ../tclinux.bin.orig | awk '/Squash/ {print $1 - 256;}'` bs=1

The tclinux.bin.orig in the above command line is the original firmware image.

The tcrevenge program will output the header and the necessary padding and a nice command line suggestion to create the new tclinux.bin.

$./tcrevenge -k kernel -s squashfs-root.sq -o header -p padding
Creating necessary squashfs paddingfile padding 1352
Magic number: 0x32524448 at 0x00
Magic device: 0x00000100 at 0x04
tclinux.bin size: 5993824 (0x005B7560) at 0x08
tclinux.bin checksum: 0xD4BEA4AA at 0x0C
Firmware version at 0x10: 7.0.1.0
squashfs offset: 955488 (0x000E9460) at 0x50
squashfs size: 5038080 (0x004CE000) at 0x54
Model at 0x5C: 3 6035 122 74
Writing header to header. Create image with
         cat header kernel squashfs-root.sq padding > tclinux.bin
$

Update 29/3/2015 15:30: The disassembly of the MIPS executables that led to the reverse engineering and the C reimplementation of the CRC algorithm in tcrevenge is the subject of another article.

Modifications

Now that we can build a new modified firmware at request let’s proceed with the actual modifications we would like to achieve.

DSL Line setup

That’s an easy one and it does not require firmware changes at all. The configuration file is called romfile.cfg. It is an XML file and has a section named Autoexec.

<Autoexec>
<Entry cmd1="w dmt2 db tlb 2b"
cmd2="wan ghs set multi_number 3 3" cmd3="wan dmt2 set largeD 2"
cmd4="w dmt eoc dyingasp off" cmd5="w dmt2 set lpr off"
cmd6="echo 1 &gt; /proc/tc3162/port_reverse"
cmd7="wan dmt2 set snrmoffset 4096 4096"/>
</Autoexec>

Note the extra cmd7 added by me manually. This is enough to boost modem sync up to 22 MBit/s in my case. I didn’t find any script in /usr/script that generated /etc/autoexec.sh but the  /userfs/bin/cfg_manager matches autoexec. So probably /etc/autoexec.sh is generated by the cfg_manager binary. Here is how /etc/autoexec.sh looks like:

# cat autoexec.sh 
w dmt2 db tlb 2b
wan ghs set multi_number 3 3
wan dmt2 set largeD 2
w dmt eoc dyingasp off
w dmt2 set lpr off
echo 1 > /proc/tc3162/port_reverse
wan dmt2 set snrmoffset 4096 4096

Note that you have to see that inside the modem via telnet since /etc/autoexec.sh is automatically generated on boot time and it is not available in the expanded squashfs partition.

Arp table setup for WAN WOL support

It took a bit of trial and error but finally I managed to guess the correct script to edit. It is not immediately obvious because scripts are being run by the mysterious cfg_manager binary. The right script is /usr/script/nat_start.sh and here is the patch.

diff -ur ../_tclinux.bin.extracted.orig/E9560/usr/script/nat_start.sh E9560/usr/script/nat_start.sh
--- ../_tclinux.bin.extracted.orig/E9560/usr/script/nat_start.sh        2014-02-11 04:50:45.000000000 +0200
+++ E9560/usr/script/nat_start.sh       2015-02-27 20:37:59.908163151 +0200
@@ -62,3 +62,5 @@
 #              /usr/script/ipaddr_mapping.sh add $i $j
 #      fi
 #done
+
+/sbin/arp -s 192.168.1.254 FF:FF:FF:FF:FF:FF

WINS support

This one was easier. The /usr/script/samba.sh is generating the /etc/samba/smb.conf. Here is the patch that enables WINS support.

diff -ur ../_tclinux.bin.extracted.orig/E9560/usr/script/samba.sh E9560/usr/script/samba.sh
--- ../_tclinux.bin.extracted.orig/E9560/usr/script/samba.sh    2015-02-06 23:18:17.953887683 +0200
+++ E9560/usr/script/samba.sh   2015-02-06 22:49:31.920892757 +0200
@@ -17,6 +17,7 @@
 netbios name = $NETBIOS_NAME
 server string = Samba Server
 workgroup = $WORKGROUP
+wins support = yes
 security = user
 guest account = $GUEST
 log file = /var/log.samba

To be continued…

What is missing now is adding support for namecheap DDNS which belongs to another article.