Tuesday, February 21, 2012

Recovering a bricked D-Link router

Today my D-Link DSL-2600U ADSL/Wi-Fi router decided to die, without any obvious reason. Only the power and Ethernet LEDs were active, and the computer directly connected to it via Ethernet could not receive an IP address. Fortunately, I had an old ADSL router still around, and thus was able to find information that, in the end, allowed me to recover the DSL-2600U.

The first thing I thought, however, was that a problem is on my local computer. So I started tcpdump, only to find out that the router sends strange packets: a gratious ARP for 192.168.1.1, followed by ARP "Who has 192.168.1.159? Tell 192.168.1.1". It didn't respond to pings even from 192.168.1.2. At this point, it became obvious that it is broken or at least needs its settings to be reset, without any access to the web interface. Pressing the Reset button in the pinhole did not change anything.

When assigning my computer the IP 192.168.1.159, the conversation went a bit further, namely, there appeared a TFTP packet with a string "308nfs6358" in it. Googling for it (via the spare router) gave mostly results about reflashing that same router model that I had, via the COM port, as a last resort. They were useless, as adding a COM port to the router is not a modification that I wanted to do.

Still, I wanted to try to revive the router, and these pages also suggested that TFTP is involved somehow. They even told the reader to put the official firmware to 192.168.1.1 via TFTP after pinging it - but obviously this could not work at this stage.

So, as the router asked for the "308nfs6358" file, and also the I had to provide it via TFTP. On Linux, there is a tftp server, atftpd, so I set it up. It is actually easy. Just create the /tftpboot directory and chmod it to 777, then run:

atftpd --no-fork --daemon

However, it was not an obvious question what to put in that directory. I had a firmware file, downloaded from the official site. When unzipped, this yields DSL-2600U_3.12_6338_12_RU_1.25_20101129.img. However, the web pages containing the reference to the "308nfs6358" file also mentioned "bcm963xx_fs_kernel", so I assumed (maybe wrongly) that I had to split the original firmware file into two.

This OpenWRT page ended up being a useful resource. It contains a program to dump the firmware structure. As the program calculates and verifies CRC of various portions of the firmware file, I guessed that I could modify it to dump these portions. The implementation is obvious: there is a compute_crc32() function that receives the offset and length of the region being checksummed, and it is trivial to dump the bytes into a separate numbered file. So, I ended up with a short 256-byte header and two files. For the reference, here are the offsets and lengths:

rootfs: start=256, length=3063808
kernel: start=3064064, length=560856

As the router was broken anyway, and because I had no better idea, I decided to determine the correct file by trial and error. To be able to understand what happens, I started Wireshark and told it to sniff all packets on the eth0 interface of my computer. I copied one of the files (the shorter one) into /tftpboot/308nfs6358 and power-cycled the router. It downloaded the file via TFTP (as evidenced by Wireshark), and then nothing happened.

With the other file, I had better luck: the router not only downloaded it, but also changed its MAC address, sent some gratious ARP packets and became pingable for some time. Still, it didn't attempt to download anything else. OK, so it means that it waits for me to upload the second file to it, I thought. This didn't work. The correct thing to do was to upload the original unzipped firmware.

So here is the complete sequence of actions that worked:

1. Assign the IP address 192.168.1.159 to the computer.
2. Unzip the firmware, split it.
3. Copy the rootfs (the 3.6 MB part of the firmware) to /tftpboot/308nfs6358
4. Run atftpd as follows: atftpd --no-fork --daemon
5. Power-cycle the router
6. When it changes the MAC address, upload the firmware to it: atftp --no-source-port-checking -p -l  DSL-2600U_3.12_6338_12_RU_1.25_20101129.img -r bcm963xx_fs_kernel 192.168.1.1
7. Try again from the beginning, as the default timeouts are rather short.

This blog post has been sent through the revived router. Still, I guess it won't last long, and I need to buy a replacement.

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