Here come on let play with the following basic command line usage of the ssh client.
1. Check your SSH Client Version:
Checking for your SSH client is very sare, but sometimes it may be necessary to identify the SSH client that you are currently running and it’s corresponding version number. The SSh client can be identified as follows
$ ssh -V
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2013
$ ssh -V
ssh: SSH Secure Shell 3.2.9.1 (non-commercial version) on i686-pc-linux-gnu
2. Connect and login to remote host:
The First time when you login to the remotehost from a localhost, it will display the host key not found message and you can give “yes” to continue. The host key of the remote host will be added under .ssh2/hostkeys directory of your home directory, as shown below.
localhost$ ssh -l jsmith remotehost.example.com
jsmith@remotehost.example.com password:
remotehost.example.com$
The Second time when you login to the remote host from the localhost, it will prompt only for the password as the remote host key is already added to the known hosts list of the ssh client.
localhost$ ssh -l jsmith remotehost.example.com
jsmith@remotehost.example.com password:
remotehost.example.com$
For some reason, if the host key of the remote host is changed after you logged in for the first time, you may get a warning message as shown below. This could be because of various reasons such as 1) Sysadmin upgraded/reinstalled the SSH server on the remote host 2) someone is doing malicious activity etc., The best possible action to take before saying “yes” to the message below, is to call your sysadmin and identify why you got the host key changed message and verify whether it is the correct host key or not.
localhost$ ssh -l jsmith remotehost.example.com
jsmith @remotehost.example.com's password:
remotehost$
4. Debug SSH Client:
Sometimes it is necessary to view debug messages to troubleshoot any SSH connection issues. For this purpose, pass -v (lowercase v) option to the ssh as shown below.
Example without debug message:
localhost$ ssh -l jsmith remotehost.example.com
warning: Connecting to remotehost.example.com failed: No address associated to the name
localhost$
Example with debug message:
locaclhost$ ssh -v -l jsmith remotehost.example.com
debug: SshConfig/sshconfig.c:2838/ssh2_parse_config_ext: Metaconfig parsing stopped at line 3.
debug: SshConfig/sshconfig.c:637/ssh_config_set_param_verbose: Setting variable 'VerboseMode' to 'FALSE'.
debug: SshConfig/sshconfig.c:3130/ssh_config_read_file_ext: Read 17 params from config file.
debug: Ssh2/ssh2.c:1707/main: User config file not found, using defaults. (Looked for '/home/jsmith/.ssh2/ssh2_config')
debug: Connecting to remotehost.example.com, port 22... (SOCKS not used)
warning: Connecting to remotehost.example.com failed: No address associated to
5. Escape Character: (Toggle SSH session, SSH session statistics etc.)
Escape character ~ get’s SSH clients attention and the character following the ~ determines the escape command.
Toggle SSH Session: When you’ve logged on to the remotehost using ssh from the localhost, you may want to come back to the localhost to perform some activity and go back to remote host again. In this case, you don’t need to disconnect the ssh session to the remote host. Instead follow the steps below.
i. Login to remotehost from localhost: localhost$ssh -l jsmith remotehost
ii. Now you are connected to the remotehost: remotehost$
iii. To come back to the localhost temporarily, type the escape character ~ and Control-Z. When you type ~ you will not see that immediately on the screen until you press and press enter. So, on the remotehost in a new line enter the following key strokes for the below to work: ~
remotehost$ ~^Z
[1]+ Stopped ssh -l jsmith remotehost
localhost$
iv. Now you are back to the localhost and the ssh remotehost client session runs as a typical unix background job, which you can check as shown below:
localhost$ jobs
[1]+ Stopped ssh -l jsmith remotehost
v. You can go back to the remote host ssh without entering the password again by bringing the background ssh remotehost session job to foreground on the localhost
localhost$ fg %1
ssh -l jsmith remotehost
remotehost$
