SSH/it

SSH (Secure SHell) è un tool per la connessione protetta a terminali remoti che rimpiazza il classico telnet sui sistemi operativi Unix-like.

In aggiunta all'accesso remoto provvisto dal binario "ssh" principale, la suite SSH è cresciuta nel tempo fino ad includere altri strumenti come "scp" (di uso molto comune) ed "sftp" (meno comune).

addition to remote terminal access provided by the main ssh binary, the SSH suite of programs has grown to include other tools such as scp (in very common use) and sftp (less common).

In origine, SSH non era free. Comunque, oggi la versione più popolare e standard de-facto di SSH è l'OpenSSH di OpenBSD, che arriva preinstallato su Gentoo.

Installation
Most deployments will already have OpenSSH installed (through the package). This package uses the following USE flags:

After changing USE flags, don't forget to rebuild OpenSSH:

Server
Il server OpenSSH può essere controllato come gni altro servizio manovrato da OpenRC, con /etc/init.d/ssh start, /etc/init.d/ssh stop, e /etc/init.d/ssh restart.

Add openssh to the default runlevel if this hasn't been done already.

Start sshd daemon with:

The OpenSSH server can be controlled like any other OpenRC-managed service:

Create Keys
In order to provide a secure shell, cryptographic keys are used to manage the encryption, decryption and hashing functionalities offered by SSH.

On the first start of the SSH service, system keys will be generated. Keys can be (re)generated using the  command.

To generate the key used for SSH protocol version 1 (which usually is not enabled anymore as it has been deprecated in favor of protocol version 2):

To generate the keys for SSH protocol version 2 (DSA and RSA algorithms):

Server configuration
The SSH server is usually configured in the file, though it is also possible to perform further configuration in OpenRC's, including changing the location of the configuration file. For detailed information on how to configure the server see the sshd_config man page.

E' possibile studiare anche questa  guida per una configurazione focalizzata sulla sicurezza.

Configurazione client
Il client ssh ed i programmi relativi (scp, sftp, ecc.) sono configurati nel file. Per maggiori informazioni riguardo come configurare questi programmi vedere la pagina man su "ssh_config".

The ssh client and related programs (scp, sftp, etc.) can be configured in following files:

For more information read the  manual:

Autenticazione senza password
molto utile per il management di server git

Handy for git server management

Client
sul client lanciare

premere enter 5 volte per generare la vostra chiave portare il file ~/.ssh/id_rsa.pub sul server loggandovi con il vostro utente come ~/.ssh/authorized_keys.

On the client run the following command:

Server
aggiungete uno user con useradd

Make sure an account for the user exists on the server, and then place the clients' file into the  file.

Single Machine Testing
The above procedure can be tested out locally:

Troubleshooting
There are 3 different levels of debug modes that can help troubleshooting issues. With -v ssh prints debugging messages about its progress. This is helpful in debugging connection, authentication, and configuration problems. Multiple -v options increase the verbosity. The maximum is 3.

Death of Long-lived Connections
Many internet access devices perform network address translation (NAT), a process that enables devices on a private network such as that typically found in a home or business place to access foreign networks, such as the internet, despite only having a single IP address on that network. Unfortunately, not all NAT devices are created equal, and some of them incorrectly close long-lived, occasional-use TCP connections such as those used by SSH. This is generally observable as a sudden inability to interact with the remote server, though the ssh client program has not exited.

In order to resolve the issue, OpenSSH clients and servers can be configured to send a 'keep alive', or invisible message aimed at maintaining and confirming the live status of the link.
 * To enable keep alive for all clients connecting to your local server, set ClientAliveInterval 30 (or some other value, in seconds) within the file.
 * To enable keep alive for all servers connected to by your local client, set ServerAliveInterval 30 (or some other value, in seconds) within the file.

X11 Forwarding Not Forwarding or Tunneling!
Problem: After you have made the necessary changes to the configuration files for permitting X11 Forwarding, you find X applications are executing on the server and are not being forwarded to the client.

Solution: What is likely occurring during SSH login into the remote server or host, the  variable is either being unset or is being set after the SSH session sets it.

Test for this scenario as follows after logging in remotely:

You should get something similar to "localhost:10.0" or "localhost2.local:10.0" using server side  setting. If you're getting the usual ":0.0", check to make sure you are not unsetting or initializing the  variable within. If so, remove or comment your custom initialization of  or prevent  from executing during SSH login:

A trick would be to alias this within.