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uClinux STM32F2: Secure communications to the target

Question asked by khusainov.vladimir on Jan 19, 2012
I'd like to show in this thread how to establish a secure connection to an STM32F2 MCU using the 'dropbear' SSH daemon running on the STM32F2 target under uClinux.

Ability to connect to an embedded device via a secure channel in an unsecure network, such as TCP/IP over Ethernet, is becoming a standard requirement for today's embedded applications. With uClinux, this can be done using the Secure Shell (SSH) protocol. SSH defines secure data communications, remote shell services or command execution and other secure network services for two networked computers communicating in an unsecure network.

The best-known application of this protocol is for access to shell accounts on Unix-like operating systems. With embedded Linux, SSH is often realized using the 'dropbear' daemon, which provides a low-memory / low-resource implementation of the SSH server for UNIX. On the client side, any terminal application that implements the SSH client role can be used to connect to a 'drobear' server; example implementations include: PuTTY, SecureCRT and many others.

Here is a sample uClinux session running on the STMF32F that shows a secure connection to the target using 'dropbear'. This session was run on STmicro's STM322G-EVAL board with Emcraft's STM-MEM add-on card (refer to this page for further information on that platform).

After reset, U-boot comes up on the board:

U-Boot 2010.03-dirty (Jan 19 2012 - 19:08:27)

CPU  : STM32 F2 series (Cortex-M3)
Freqs: SYSCLK=120MHz,HCLK=120MHz,PCLK1=30MHz,PCLK2=60MHz
Board: STM3240G-EVAL board + STM-MEM add-on,Rev 2.A
DRAM:  32 MB
Flash:  8 MB
In:    serial
Out:   serial
Err:   serial
Net:   STM32_MAC
Hit any key to stop autoboot:  0


U-Boot proceeds to load a bootable Linux image from
external Flash to RAM and run it:

## Booting kernel from Legacy Image at 60020000 ...
   Image Name:   Linux-2.6.33-arm1
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    2084544 Bytes =  2 MB
   Load Address: 64008000
   Entry Point:  64008001
   Verifying Checksum ... OK
   Loading Kernel Image ... OK
OK

Starting kernel ...


Linux comes on the board showing the following messages relevant to networking and TCP/IP initialization:

Linux version 2.6.33-arm1 (psl@ocean.emcraft.com) (gcc version 4.4.1 (Sourcery G++ Lite 2010q1-188) ) #1 Tue Dec 13 19:42:04 MSK 2011
CPU: ARMv7-M Processor [412fc230] revision 0 (ARMv7M)
CPU: NO data cache, NO instruction cache
Machine: STMicro STM32
...
stm32-eth: found STM32 MAC at 0x40028000, irq 61
stm32_mii_bus: probed
found PHY id 0x20005c90 addr 1
eth0: using MII interface
eth0: attached PHY driver [Generic PHY] (mii_bus:phy_addr=00:01, irq=-1)
TCP cubic registered
NET: Registered protocol family 17
PHY: 00:01 - Link is Up - 100/Full
IP-Config: Guessing netmask 255.255.0.0
IP-Config: Complete:
     device=eth0, addr=172.17.3.176, mask=255.255.0.0, gw=255.255.255.255,
     host=stm3220g-eval, domain=, nis-domain=(none),
     bootserver=172.17.0.1, rootserver=172.17.0.1, rootpath=
Freeing init memory: 932K
init started: BusyBox v1.17.0 (2011-12-13 19:40:43 MSK)
~ #


Once at the shell, I just type 'dropbear' to start the SSH daemon (of course it can be started from an init script too):

~ # dropbear
~ #


Now, I can establish a secure connection to the STMF32F board from any machine connected to the same network and then run various Linux commands from the shell on the target. For instance, here is how I establish a secure connection from my Linux machine:

-bash-3.2$ ssh root@172.17.3.176
root@172.17.3.176's password:
~ # ls
bin    dev    etc    httpd  mnt    proc   root   sys    usr    var
~ # cat /proc/cmdline
stm32_platform=stm3220g-eval console=ttyS2,115200 panic=10 mem=16M ip=172.17.3.176:172.17.0.1:::stm3220g-eval:eth0:off ethaddr=C0:B1:33:88:88:81
~ #

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