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Documentation/stm32f746g-disco: document Dropbear SSH server setup

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Document the new stm32f746g-disco:dropbear configuration, mirroring
the existing esp32c3-devkit Dropbear documentation.

Signed-off-by: Felipe Moura <moura.fmo@gmail.com>
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Felipe Moura 2026-07-19 23:09:40 -03:00 • committed by Xiang Xiao
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commit 4e33f0e711
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Documentation/platforms/arm/stm32f7/boards/stm32f746g-disco/index.rst
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@ -177,6 +177,60 @@ configuration. This configuration uses USART1 for the serial console.
USART1 is connected to the ST-link virtual com inside board.h to remove
the need of a extra serial connection to use this board.
dropbear
--------
This configuration brings up the `Dropbear <https://matt.ucc.asn.au/dropbear/dropbear.html>`__
SSH server so that an NSH session can be reached over the network. It pulls in
the on-board Ethernet MAC, the DHCP client and ``/dev/urandom`` so the link is
configured automatically at boot, and it persists the SSH host key and the
user database on the board's on-board Micron N25Q QSPI NOR flash, mounted as
LittleFS at ``/data``.
Connect an Ethernet cable to a router/switch before powering on the board.
When it boots it requests an address over DHCP and starts the Dropbear
daemon. Check the assigned address with::
nsh> ifconfig
eth0 Link encap:Ethernet HWaddr 00:e0:de:ad:be:ef at RUNNING mtu 1486
inet addr:192.168.1.xx DRaddr:192.168.1.x Mask:255.255.255.0
The first time the daemon runs it generates an ECDSA host key and stores it at
``/data/dropbear_ecdsa_host_key`` (``CONFIG_NETUTILS_DROPBEAR_HOSTKEY_PATH``), and
the user accounts are read from ``/data/passwd`` (``CONFIG_FSUTILS_PASSWD_PATH``).
Because both files live on the persistent ``/data`` partition, the host key and
the credentials survive reboots, so clients do not see a changing host key.
Add a user from the board before connecting (the password file is created on the
first ``useradd``)::
nsh> useradd admin mypassword
From a host on the same network, open an SSH session and run NSH commands
remotely::
$ ssh admin@192.168.1.xx
admin@192.168.1.xx's password:
nsh>
Each session runs over a pseudo-terminal (``CONFIG_PSEUDOTERM``). The daemon
keeps running after a session ends, so multiple clients can connect over time.
This configuration also enables Dropbear's ``scp`` helper
(``CONFIG_NETUTILS_DROPBEAR_SCP``), so files can be copied to and from the
board from a host on the same network::
$ scp -O localfile.txt admin@192.168.1.xx:/data/localfile.txt
$ scp -O admin@192.168.1.xx:/data/localfile.txt .
Only the legacy scp protocol is supported (no SFTP), so with OpenSSH 9.0 or
newer the ``-O`` flag is required to force the client to use it.
.. note:: This board's serial console (USART1, ``/dev/ttyACM0`` over the
on-board ST-LINK/V2-1 virtual COM port) remains available at the same time
as the SSH sessions and is useful for the initial ``useradd`` and for
recovery if the network is unreachable.
lgvl
----