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) is a set of computer programs providing encrypted communication sessions over a computer network using the ssh protocol. It was created as an open source alternative to the proprietary Secure Shell software suite offered by SSH Communications Security. OpenSSH is developed as part of the OpenBSD project, which is led by Theo de Raadt.
OpenSSH is occasionally confused with the similarly-named OpenSSL; however, the projects have different purposes and are developed by different teams, the similar name is drawn only from similar goals.
Installing OpenSSH
# pacman -S openssh
Configuring SSH
Client
The SSH client configuration file can be found and edited in Vorlage:Filename.
An example configuration:
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