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SSH

 
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Article updated at 31 May 2019
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Introduction

SSH can be used for encrypted login over the network or for encrypted file transfer between your host and the module. Our demo images use systemd's socket mechanism to start a ssh server on demand whenever the user tries to connect to the module.

Prerequisites

  • SSH client application.

On Linux, usually SSH is executed from the terminal and comes installed on some distros. For reference, here's how to install it in Debian-based distros (tested on Ubuntu 18.04 LTS):

 
$ sudo apt install openssh-client
 

On Windows, a commonly used SSH client is putty. There is also the option to start a bash instance on Windows 10.

Establishing an SSH Connection

By using the Linux command line SSH client ssh one can establish a connection as follows:

[user@host ~]$ ssh root@192.168.10.2
The authenticity of host '192.168.10.2 (192.168.10.2)' can't be established.
ECDSA key fingerprint is SHA256:7aPobNtSjX9HYLr4HvyvpL7dkDTxi0uAfRRjTVqGbis.
ECDSA key fingerprint is MD5:ae:ef:61:ac:bc:5f:e9:d7:48:ad:17:75:d6:a1:e2:b0.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.10.2' (ECDSA) to the list of known hosts.
root@192.168.10.2's password:
Last login: Tue Nov  3 02:44:09 2015
root@apalis-imx6:~#

Our Ångström-based embedded Linux BSP has default user configured as root and no password.

Our Torizon embedded Linux BSP has default SSH user configured as torizon and password torizon as well. Though the root connection can only be issued from the debug serial port, the torizon user has root privileges and you can execute sudo and su just like on a regular desktop Linux distro.

Passwordless SSH Configuration

During development, it's annoying to type your password every time you need to e.g. ssh into the board or scp a file to it.

To configure passwordless connection you should start a terminal (linux) or bash.exe (Windows 10) and run the following commands:

$ ls $HOME/.ssh

If you already have files named id_rsa and id_rsa.pub you can skip those steps, otherwise you have to create your security keys to be able to use them for ssh connections:

$ ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
...
The key's randomart image is:
...

Now you can copy the keys to the target device and enable ssh login without password. In this sample we are using 192.168.1.114 as the device ip address, please replace it with the actual address/hostname of your device. Please notice that commands after the "#" prompt will be executed on the target device via ssh.

$ scp $HOME/.ssh/id_rsa.pub torizon@192.168.1.114:/home/torizon
torizon@192.168.1.114's password:
id_rsa.pub                                                                            100%  396     0.4KB/s   00:00
$ ssh torizon@192.168.1.114
torizon@192.168.1.114's password:
Last login: Thu May  2 16:09:13 2019 from 192.168.1.3
# cat id_rsa.pub >> .ssh/authorized_keys
# rm id_rsa.pub
# exit
logout
Connection to 192.168.1.114 closed.
# ssh torizon@192.168.1.114
Last login: Fri May  3 09:35:43 2019 from 192.168.1.2

If everything works as expected, the second time you tried to connect to the device you should have been able to do it without typing the password.

Troubleshooting

By default, ssh is configured as a service started by systemd's socket activation method. The connection attempts and current active connections can be gained from the socket unit:

# systemctl status sshd.socket

The command journalctl can be used to print log messages related to the ssh server:

# journalctl -u sshd*

Tips and Known Issues

This section provides insight about ssh configuration as well as known issues.

Password Authentication

The images typically ship with a empty root password (enabled by OpenEmbedded debug-tweaks). The system's user password can be changed by using passwd. In order to keep being able to login as a user, make sure the following ssh daemon configurations are set (in /etc/ssh/sshd_config):

PasswordAuthentication yes
#PermitEmptyPasswords yes

Incompatible Cipher Support

Depending on the SSH flavour and version both on the target (e.g. part of our BSPs) resp. client (e.g. your Linux development workstation) side there are some known cipher incompatibilities which result in the following behaviour:

[user@host ~]$ ssh root@192.168.10.2
ssh_packet_read: Connection closed

Looking more closely the following happens:

[user@host ~]$ ssh -v root@192.168.10.2
OpenSSH_7.1p1, OpenSSL 1.0.2e-fips 3 Dec 2015
debug1: Reading configuration data /home/user/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 56: Applying options for *
debug1: Connecting to 192.168.10.2 [192.168.10.2] port 22.
debug1: Connection established.
debug1: identity file /home/user/.ssh/id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_rsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_dsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_dsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ecdsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ecdsa-cert type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ed25519 type -1
debug1: key_load_public: No such file or directory
debug1: identity file /home/user/.ssh/id_ed25519-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.7
debug1: match: OpenSSH_6.7 pat OpenSSH* compat 0x04000000
debug1: Authenticating to 192.168.10.2:22 as 'root'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client chacha20-poly1305@openssh.com <implicit> none
debug1: kex: client->server chacha20-poly1305@openssh.com <implicit> none
debug1: kex: curve25519-sha256@libssh.org need=64 dh_need=64
debug1: kex: curve25519-sha256@libssh.org need=64 dh_need=64
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:7aPobNtSjX9HYLr4HvyvpL7dkDTxi0uAfRRjTVqGbis
debug1: Host '192.168.10.2' is known and matches the ECDSA host key.
debug1: Found key in /home/user/.ssh/known_hosts:232
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
ssh_packet_read: Connection closed

This can be worked around by either specifying the cipher to be used on the command line as follows:

[user@host ~]$ ssh -oCiphers=aes128-ctr root@192.168.10.2
Last login: Wed Jan 13 08:44:12 2016 from 192.168.10.1
root@apalis-imx6:~# 

Or one can permanently configure a cipher to be used in one of your SSH configuration files as follows:

[user@host ~]$ cat ~/.ssh/config 
Host 192.168.10.*
  Ciphers aes128-ctr
  PreferredAuthentications password
  PubkeyAuthentication no

Please also find further information in the following discussion initiated by us:

https://bbs.archlinux.org/viewtopic.php?id=199640