OpenSSH

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Introduction

Secure Shell or SSH is a network protocol that allows data to be exchanged over a secure channel between two computers. Encryption provides confidentiality and integrity of data. SSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.

SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; it can transfer files using the associated SFTP or SCP protocols.

An SSH server, by default, listens on the standard TCP port 22. An ssh client program is typically used for establishing connections to an sshd daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, Linux, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.

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 -Sy openssh

Configuring SSH

Client

The SSH client configuration file can be found and edited in Template:Filename.

An example configuration:

Template:File

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 Template:Filename.

An example configuration:

Template:File


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 Template:Filename for a nice welcome message.

Tip: You may want to change the default port from 22 to any higher port (see security through obscurity).

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

Template:Box Note

To let other people ssh to your machine you need to adjust Template: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 Template: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 Template: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 Template:Codeline is your username at the SSH server running at the Template:Codeline. It will ask for your password, and then you're connected! The Template:Codeline flag disables the interactive prompt, and the Template:Codeline flag specifies the local port on wich 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 Template:Filename file as following:

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

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

Install xorg-xauth on the server:

# pacman -Sy xorg-xauth


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


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 /etc/hosts

Mounting a Remote Filesystem with SSHFS

Install sshfs

# pacman -Sy sshfs

Load the Fuse module

# modprobe fuse

Add fuse to the modules array in Template: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 Template:Filename table. This way is can be automatically mounted upon system boot or mounted manually (if Template: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    0 0

Keep Alive

Your ssh session will automatically log out if it is idle. To keep the connection active (alive) add this to ~/.ssh/config or to /etc/ssh/ssh_config on the client.

ServerAliveInterval 5

This will send a "keep alive" signal to the server every 5 seconds. You can usually increase this interval, and I use 120.

Links & References