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Documentation/sim: document dropbear configuration

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Describe how to build and use the Dropbear SSH server configuration on
the simulator, including host TAP network setup, user creation and the
volatile /tmp host key/passwd caveat.

Signed-off-by: Felipe Moura <moura.fmo@gmail.com>
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Felipe Moura 2026-07-11 11:12:16 -03:00 • committed by Xiang Xiao
parent b466c449b8
commit cd6ed0dbf9
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Documentation/platforms/sim/sim/boards/sim/index.rst
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@ -645,6 +645,57 @@ test is used to verify the uClibc++ port to NuttX.
To really use this example, I will have to think of some way to postpone
running C++ static initializers until NuttX has been initialized.
dropbear
--------
This configuration runs the `Dropbear <https://matt.ucc.asn.au/dropbear/dropbear.html>`__
SSH server (see :doc:`/applications/netutils/dropbear/index`) on the simulator,
so it can be tested without real hardware. NSH starts the daemon automatically
at boot (``CONFIG_NSH_DROPBEAR``).
The simulator reaches the host network through the TAP device
(``CONFIG_SIM_NETDEV``); see :doc:`/platforms/sim/network_linux` for the
details. In short, build and prepare the host with:
.. code:: console
$ ./tools/configure.sh sim:dropbear
$ make
$ # necessary on recent Linux distributions
$ sudo setcap cap_net_admin+ep ./nuttx
$ # replace eth0 with your Ethernet or wireless interface
$ sudo ./tools/simhostroute.sh eth0 on
$ ./nuttx
The defconfig preconfigures the simulation address ``10.0.1.2`` with default
router ``10.0.1.1``, matching the network that ``simhostroute.sh`` creates on
the host, so no manual ``ifconfig`` is needed. Add a user account (the
password file is created on the first ``useradd``):
.. code:: console
nsh> useradd admin mypassword
Then open an SSH session from the host — Dropbear listens on port 2222
(``CONFIG_NETUTILS_DROPBEAR_PORT``) — and run NSH commands remotely over a
pseudo-terminal (``CONFIG_PSEUDOTERM``):
.. code:: console
$ ssh -p 2222 admin@10.0.1.2
admin@10.0.1.2's password:
nsh>
.. note::
The host key (``/tmp/dropbear_ecdsa_host_key``) and the user database
(``/tmp/passwd``) live on the volatile ``/tmp`` file system, so both are
recreated on every run: the ``useradd`` step must be repeated after each
boot, and OpenSSH clients will warn that the host key changed between
runs. For local testing the check can be relaxed with::
$ ssh -p 2222 -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no admin@10.0.1.2
fb
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