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" (secure copy) ed "sftp" (secure file transfer protocol).

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

Installazione
La maggioranza delle installazioni avrà già installato OpenSSH (attraverso il pacchetto ). Questo pacchetto usa le seguenti USE flags:

Dopo aver modificato le USE flags, non dimenticate di ricompilare OpenSSH:

Server
Aggiungere openssh al runlevel default se non è già stato fatto.

Avviare il demone sshd con:

Il server OpenSSH può essere come ogni altro servizio amministrato da OpenRC:

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.) possono essere configurati nei seguenti file:

Per maggiori informazioni vedere il manuale su :

Autenticazione senza password
Utile per l'amministrazione di git

Client
sul client lanciare il seguente comando:

Server
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.

Intrusion Prevention
ssh is a commonly attacked service. sshguard & fail2ban monitor logs and black list remote users who have repeatedly failed to login.

Vedere anche

 * Securing OpenSSH - Documentazione Gentoo developer.
 * OpenSSH gestione delle chiavi, Parte 1 - Documentazione Gentoo, date un'occhiata a tutte e 3 le parti.
 * Gentoo Linux Keychain Guide
 * autossh - Determina quando le connessioni SSH cadono e si auto-riconnette.