SSH/de

SSH (Secure SHell) ist ein verschlüsseltes Terminal-Programm, das das klassische telnet Tool auf Unixoiden Betriebssystemen ersetzt.

Zusätzlich zu Terminal-Fernzugriff, welcher durch das Hauptprogramm ssh bereitgestellt wird, enthält die SSH-Programmsammlung inzwischen auch andere Werkzeuge wie scp (häufig in Gebrauch) oder sftp (weniger häufig).

SSH war nicht immer frei. Heute ist allerdings die Implementation von OpenBSD, OpenSSH, die am meisten genutzte und in Gentoo vorinstallierte Version.

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
Der OpenSSH server kann wie jeder andere OpenRC-Service mit "/etc/init.d/ssh start", "/etc/init.d/ssh stop" und "/etc/init.d/ssh restart" kontrolliert werden.

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.

Für eine Konfiguration, die sich besonders auf Sicherheit konzentriert, sollte guide (Englisch) gelesen werden.

Clientkonfiguration
Der "ssh" client und zugehörige Programme ("scp", "sftp", etc.) werden in konfiguriert. Für genauere Informationen dazu sollte die man page "ssh_config" konsultiert werden.

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

For more information read the  manual:

Authentifizierung ohne Passwort
Sehr praktisch für git Server Management.

Handy for git server management

Klient
Auf dem Klient starten Sie:

im Anschluss hieran drücken Sie zum Beispiel 5mal Enter um ihren Schlüssel zu erzeugen. Kopieren Sie vom Klient die Datei ~/.ssh/id_rsa.pub auf den Server in das Verzeichnis des Benutzers, mit welchem Sie sich später Anmelden möchten, ~/.ssh/authorized_keys

On the client run the following command:

Server
Benutzen Sie useradd um einen Benutzer anzulegen

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

Testen auf einem Einzelplatzcomputer
Drücken Sie 5mal Enter zur Erzeugung von Eingabesignalen, welche für die Erzeugung des Schlüssels genutzt werden.

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.

Siehe auch

 * Securing OpenSSH - Gentoo developer documentation.
 * OpenSSH Key Management Part1 - Gentoo documentation, have a look at all 3 parts.
 * Gentoo Linux Keychain Guide
 * autossh - Detects when SSH connections drop and auto-reconnects.