Dropbear

Dropbear is a relatively small SSH server and client. It runs on a variety of POSIX-based platforms. Dropbear is particularly useful for "embedded" type Linux systems

Enabling dropbear

How to enable Dropbear server

1-Open sdk configuration system

$ make config

2- Enable Dropbear server:

-> File System Configuration
  -> Select target's file system software 
    [*] dropbear-0.52 (SSH Server)

3- When you check Dropbear server, you can enable scp and ssh

-*- dropbear-0.52 (SSH Server)
[*]   Enable dbclient (SSH Client)
[*]   Enable scp

Starting dropbear server

To start the server you can use the script located in /etc/init.d/ named "dropbear"

/etc/init.d/dropbear start

When you run this command, normally you can see some messages about keys, you can get of this log

Commands to use ssh

From host:

$ dbclient user@host

From target:

$ ssh user@host

Commands to use scp

From target or host:

$ scp <file> user@host:/<location>

Setting keys to get ssh connections without password requests

To enable this option you need to add "-s" option at the start server command, it can be added in the devdir fs or in the target fs directly.

If you want to add this option in the devdir, open the file <DEVDIR>/fs/apps/dropbear.x.y/dropbear and add the option "-s" in the flag DROPBEAR_EXTRA_ARGS of the following way:

DROPBEAR_EXTRA_ARGS=-s

After do that, rebuild and reinstall the application, then start the Dropbear server running the following script:

/etc/init.d/dropbear start 

The first time that you run this script, you can see in log some messages about generating public and private keys, you can take the public key to add it in host computer, if Dropbear was started at this point, you can get the public key manually, this process must be explained in the following section.

If you want to add this option in the target fs directly, you need to do the same action in the script /etc/init.d/dropbear. After that, restart the application

/etc/init.d/dropbear restart 

How to get public and private keys of Dropbear in the target manually

You can use "dropbearkey" tool to create public and private keys or see public keys.

To create an pair of keys you can run the following command:

$ dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key

The last command saves in /etc/dropbear/dropbear_rsa_host_key the private key, and prints the public key. If you want to get only the public key as you need to save in the authorized keys in the host, you can run the commmand

$ dropbearkey -y -f /etc/dropbear/dropbear_rsa_host_key | grep ssh-rsa

How to set Dropbear public key in host

When you have the public key generated by Dropbear in your target, it must be placed in the correctly host file location to get connectivity without password request.

The public key must added in the file

/home/<user>/.ssh/authorized_keys

The public key generated by Dropbear must have format similar to:

ssh-rsa AAAAB3NzaC1yc.................C1vSjw2Xcm9KU9mXJtKRj root@davinci

After to add the public key in the host, the ssh server must be reinitialized

/etc/init.d/ssh restart

How to get OpenSSH public and private keys in the host computer

When you run the following command, you will get two files in ~/.ssh/, one is called id_rsa, it's the private key (It mustn't be touched) and another that is named id_rsa.pub, in this file you can find the the public key that must be added in the target in the location explained in the next section.

ssh-keygen -b 4096 -t rsa

How to set public key of OpenSSH in the target

The public key got it in the host, must be added in the target file /home/<user>/.ssh/authorized_keys, It must have format similar to:

ssh-rsa AAAAB3NzaC1yc2EA ................... SVr8uIPvPCYwSsvq5yU= <user>@<host>

After to do that, restart the Dropbear server using the following command:

/etc/init.d/dropbear restart

Commands to use ssh without password request

From host:

$ ssh user@host

From target:

$ dbclient user@host -i /etc/dropbear/dropbear_rsa_host_key

Commands to use scp without password request

Form host:

$ scp <file> user@host:/<location>

Form target:

$ scp <file> user@host:/<location> -i /etc/dropbear/dropbear_rsa_host_key