SSH

Secure Shell oder SSH ist ein Netzwerk Protokoll welches den sicheren Datenaustausch zwischen zwei Computern über ein Netzwerk gestattet. SSH bietet dank Verschlüsselung Diskretion und Intigrität der versendeten Daten. Als Verschlüsselungsverfahren wird ein asymmetrisches Kryptosystem (Verschlüsselung durch public und privat Keys) eingesetzt um den zugreifenden Computer und - falls benötigt - auch den zugreifenden Benutzer zu verifizieren.

SSH wird normaler weise benutzt um sich in einem entfernten System einzuloggen und Kommandos auszuführen. Zusätzlich wird Tunneling, das Transmission Control Protocol (TCP) und X11 Verbindungen unterstützt. Datentransfer kann durch die Protokolle SFTP und SCP erreicht werden.

Standardmäßig lauscht der SSH-Server auf dem standart TCP Port 22. Ein SSH Client Programm wird normalerweise benutzt um eine Verbindung zu einem sshd (Secure Shell Daemon) herzustellen. Beide Programme sind für die meisten modernen Betriebssysteme vorhanden. U. a. für Mac OS X, GNU/Linux, Solaris und OpenVMS. Properitäre, freie und open Source Versionen in unterschiedlicher Komplexität und Vollständigkeit existieren.

(Source: Wikipedia:Secure Shell)

OpenSSH

OpenSSH (OpenBSD Secure Shell) ist eine Sammlung von Programmen die eine Verschlüsselte Verbindung über ein Computernetzwerk mit dem SSH Protokoll erlauben. Sie wurde als eine open Source Alternative zu der properitären Secure Shell software suite des Unternehmens SSH Communications Security erstellt. OpenSSH wird als Teil des OpenBSD Projektes entwickelt und von Theo de Raadt geleitet.

OpenSSH wird gelegendlich mit dem ähnlich benannten OpenSSL verwechselt. Beide Projekte haben allerdings verschiedene Zwecke und werden von verschiedenen Teams entwickelt. Der ähnliche Name kommt allein von ähnlichen Zielen.

Installing OpenSSH

# pacman -S openssh

Configuring SSH

Client

Die SSH Klient Konfigurationsdatei liegt in dem Ordner /etc/ssh/ssh_config

Hier eine Beispielkonfiguration:

#       $OpenBSD: ssh_config,v 1.25 2009/02/17 01:28:32 djm Exp $

# This is the ssh client system-wide configuration file.  See
# ssh_config(5) for more information.  This file provides defaults for
# users, and the values can be changed in per-user configuration files
# or on the command line.

# Configuration data is parsed as follows:
#  1. command line options
#  2. user-specific file
#  3. system-wide file
# Any configuration value is only changed the first time it is set.
# Thus, host-specific definitions should be at the beginning of the
# configuration file, and defaults at the end.

# Site-wide defaults for some commonly used options.  For a comprehensive
# list of available options, their meanings and defaults, please see the
# ssh_config(5) man page.

Host *
#   ForwardAgent no
#   ForwardX11 no
#   RhostsRSAAuthentication no
#   RSAAuthentication yes
#   PasswordAuthentication yes
#   HostbasedAuthentication no
#   GSSAPIAuthentication no
#   GSSAPIDelegateCredentials no
#   BatchMode no
#   CheckHostIP yes
#   AddressFamily any
#   ConnectTimeout 0
#   StrictHostKeyChecking ask
#   IdentityFile ~/.ssh/identity
#   IdentityFile ~/.ssh/id_rsa
#   IdentityFile ~/.ssh/id_dsa
#   Port 22
#   Protocol 2,1
#   Cipher 3des
#   Ciphers aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc
#   MACs hmac-md5,hmac-sha1,umac-64@openssh.com,hmac-ripemd160
#   EscapeChar ~
#   Tunnel no
#   TunnelDevice any:any
#   PermitLocalCommand no
#   VisualHostKey no
HashKnownHosts yes
StrictHostKeyChecking ask

It is recommended to change the Protocol line into this:

Protocol 2

That means that only Protocol 2 will be used, since Protocol 1 is considered somewhat insecure.

Daemon

The SSH daemon configuration file can be found and edited in Vorlage:Filename.

An example configuration:

To allow access only for some users add this line:

AllowUsers    user1 user2

You might want to change some lines so that they look as following:

Protocol 2

.

.

.

LoginGraceTime 120

.

.

.

PermitRootLogin no # (put yes here if you want root login)

You could also uncomment the BANNER option and edit Vorlage:Filename for a nice welcome message.

Even though the port ssh is running on could be detected by using a port-scanner like nmap, changing it will reduce the number of log entries caused by automated authentication attempts.

Allowing others in

To let other people ssh to your machine you need to adjust Vorlage:Filename, add the following:

# let everyone connect to you

sshd: ALL



# OR you can restrict it to a certain ip

sshd: 192.168.0.1



# OR restrict for an IP range

sshd: 10.0.0.0/255.255.255.0



# OR restrict for an IP match

sshd: 192.168.1.

Now you should check your Vorlage:Filename for the following line and make sure it looks like this:

ALL: ALL: DENY

That's it. You can SSH out and others should be able to SSH in :).

To start using the new configuration, restart the daemon (as root):

# /etc/rc.d/sshd restart

Managing SSHD Daemon

Just add sshd to the "DAEMONS" section of your Vorlage:Filename:

DAEMONS=(... ... sshd ... ...)

To start/restart/stop the daemon, use the following:

# /etc/rc.d/sshd {start|stop|restart}

Connecting to the server

To connect to a server, run:

$ ssh -p port user@server-address

Tips and Tricks

Encrypted Socks Tunnel

This is highly useful for laptop users connected to various unsafe wireless connections. The only thing you need is an SSH server running at a somewhat secure location, like your home or at work. It might be useful to use a dynamic DNS service like DynDNS so you don't have to remember your IP-address.

Step 1: Start the Connection

You only have to execute this single command in your favorite terminal to start the connection:

$ ssh -ND 4711 user@host

where Vorlage:Codeline is your username at the SSH server running at the Vorlage:Codeline. It will ask for your password, and then you're connected! The Vorlage:Codeline flag disables the interactive prompt, and the Vorlage:Codeline flag specifies the local port on which to listen on (you can choose any port number if you want).

One way to make this easier is to put an alias line in your Vorlage:Filename file as following:

alias sshtunnel="ssh -ND 4711 -v user@host"

It's nice to add the verbose Vorlage:Codeline flag, because then you can verify that it's actually connected from that output. Now you just have to execute the Vorlage:Codeline command :)

Step 2: Configure your Browser (or other programs)

The above step is completely useless if you don't configure your web browser (or other programs) to use this newly created socks tunnel.

• For Firefox: Edit → Preferences → Advanced → Network → Connection → Setting:

Check the "Manual proxy configuration" radio button, and enter "localhost" in the "SOCKS host" text field, and then enter your port number in the next text field (I used 4711 above).

Make sure you select SOCKS4 as the protocol to use. This procedure will not work for SOCKS5.

Enjoy your secure tunnel!

X11 Forwarding

To run graphical programs through a SSH connection you can enable X11 forwarding. An option needs to be set in the configuration files on the server and client (here "client" means your (desktop) machine your X11 Server runs on, and you will run X applications on the "server").

Install xorg-xauth on the server:

# pacman -S xorg-xauth

• Enable the AllowTcpForwarding option in Vorlage:Filename on the server.

• Enable the X11Forwarding option in Vorlage:Filename on the server.

• Set the X11DisplayOffset option in Vorlage:Filename on the server to 10.

• Enable the X11UseLocalhost option in Vorlage:Filename on the server.

• Enable the ForwardX11 option in Vorlage:Filename on the client.

To use the forwarding, log on to your server through ssh:

# ssh -X -p port user@server-address

If you receive errors trying to run graphical applications try trusted forwarding instead:

# ssh -Y -p port user@server-address

You can now start any X program on the remote server, the output will be forwarded to your local session:

# xclock

Speed up SSH

Changing the ciphers used by SSH to less cpu-demanding ones can improve speed. In this aspect, the best choices are arcfour and blowfish-cbc. To use them, run SSH with the Vorlage:Codeline flag, like this:

# ssh -c arcfour,blowfish-cbc user@server-address

To use them permanently, add this line under the proper host in Vorlage:Filename:

Ciphers arcfour,blowfish-cbc

Another option to improve speed is to enable compression with the Vorlage:Codeline flag. A permanent solution is to add this line under the proper host in Vorlage:Filename:

Compression yes

Login time can be shorten by using the Vorlage:Codeline flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in Vorlage:Filename:

AddressFamily inet

Another way of making these changes permanent is to create an alias in Vorlage:Filename:

alias ssh='ssh -C4c arcfour,blowfish-cbc'

Finally, you can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding those line under the proper host in Vorlage:Filename:

ControlMaster auto

ControlPath ~/.ssh/socket-%r@%h:%p

Trouble Shooting

make sure your DISPLAY string is resolveable on the remote end:

ssh -X user@server-address

server$ echo $DISPLAY

localhost:10.0

server$ telnet localhost 6010

localhost/6010: lookup failure: Temporary failure in name resolution   

can be fixed by adding localhost to Vorlage:Filename.

Mounting a Remote Filesystem with SSHFS

Install sshfs

# pacman -S sshfs

Load the Fuse module

# modprobe fuse

Add fuse to the modules array in Vorlage:Filename to load it on each system boot.

Mount the remote folder using sshfs

# mkdir ~/remote_folder

# sshfs USER@remote_server:/tmp ~/remote_folder

The command above will cause the folder /tmp on the remote server to be mounted as ~/remote_folder on the local machine. Copying any file to this folder will result in transparent copying over the network using SFTP. Same concerns direct file editing, creating or removing.

When we’re done working with the remote filesystem, we can unmount the remote folder by issuing:

# fusermount -u ~/remote_folder

If we work on this folder on a daily basis, it is wise to add it to the Vorlage:Filename table. This way is can be automatically mounted upon system boot or mounted manually (if Vorlage:Codeline option is chosen) without the need to specify the remote location each time. Here is a sample entry in the table:

sshfs#USER@remote_server:/tmp /full/path/to/directory fuse    defaults,auto,allow_other    0 0

Keep Alive

Your ssh session will automatically log out if it is idle. To keep the connection active (alive) add this to Vorlage:Filename or to Vorlage:Filename on the client.

ServerAliveInterval 120

This will send a "keep alive" signal to the server every 120 seconds.

Conversely, to keep incoming connections alive, you can set

ClientAliveInterval 120

(or some other number greater than 0) in Vorlage:Filename on the server.

Save connection data in .ssh/config

Whenever you want to connect to a server, you usually have to type at least its address and your username. To save that typing work for servers you regularly connect to, you can use the Vorlage:Filename file as shown in the following example:

Now you can simply connect to the server by using the name you specified:

$ ssh myserver

To see a complete list of the possible options, check out ssh_config's manpage on your system or the ssh_config documentation on the official website.

See Also

• Using SSH Keys

Links & References

• A Cure for the Common SSH Login Attack

• Using your browser as SSH client

• Defending against brute force ssh attacks