OpenSSH
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; file transfer can be accomplished 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, GNU/Linux, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.
(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 Template: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 Template:Filename.
An example configuration:
To allow access only for some users add this line:
AllowUsers user1 user2
To disable root login over SSH, add the following:
PermitRootLogin no
You could also uncomment the BANNER option and edit Template: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 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 a specific IP mask 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
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):
# rc.d restart sshd
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:
# rc.d {start|stop|restart} sshd
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 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 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. Since the current version of SSH supports both SOCKS4 and SOCKS5, you can use either of them.
- 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).
Firefox does not automatically make DNS requests through the socks tunnel. This potential privacy concern can be mitigated by the following steps:
- Type about:config into the Firefox location bar.
- Search for network.proxy.socks_remote_dns
- Set the value to true.
- Restart the browser.
- For Chromium: You can set the SOCKS settings as enviroment variables or as command line options. I recommend to add one of the following functions to your Template:Filename:
function secure_chromium { port=4711 export SOCKS_SERVER=localhost:$port export SOCKS_VERSION=5 chromium & exit }
OR
function secure_chromium { port=4711 chromium --proxy-server="socks://localhost:$port" & exit }
Now open a terminal and just do:
$ secure_chromium
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 Template:Filename on the server.
- Enable the X11Forwarding option in Template:Filename on the server.
- Set the X11DisplayOffset option in Template:Filename on the server to 10.
- Enable the X11UseLocalhost option in Template:Filename on the server.
- Enable the ForwardX11 option in Template: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
If you get "Cannot open display" errors try the following command as the non root user:
$ xhost +
the above command will allow anybody to forward X11 applications. To restrict forwarding to a particular host type:
$ xhost +hostname
where hostname is the name of the particular host you want to forward to. Type "man xhost" for more details.
Be careful with some applications as they check for a running instance on the local machine. Firefox is an example. Either close running Firefox or use the following start parameter to start a remote instance on the local machine
$ firefox -no-remote
Speed up SSH
You can make all sessions to the same host use a single connection, which will greatly speed up subsequent logins, by adding these lines under the proper host in Template:Filename:
ControlMaster auto ControlPath ~/.ssh/socket-%r@%h:%p
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. Please do not do this unless you know what you are doing; arcfour has a number of known weaknesses. To use them, run SSH with the Template:Codeline flag, like this:
# ssh -c arcfour,blowfish-cbc user@server-address
To use them permanently, add this line under the proper host in Template:Filename:
Ciphers arcfour,blowfish-cbc
Another option to improve speed is to enable compression with the Template:Codeline flag. A permanent solution is to add this line under the proper host in Template:Filename:
Compression yes
Login time can be shorten by using the Template:Codeline flag, which bypasses IPv6 lookup. This can be made permanent by adding this line under the proper host in Template:Filename:
AddressFamily inet
Another way of making these changes permanent is to create an alias in Template:Filename:
alias ssh='ssh -C4c arcfour,blowfish-cbc'
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 Template: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 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,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 Template:Filename or to Template: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 Template: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 Template: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.
Troubleshooting
Connection Refused Problem
Is SSH running and listening?
# netstat -tnlp | grep ssh
If the above command doesn't display anything, then SSH is NOT running. Check /var/log/messages
for errors etc.
Are there firewall rules blocking the connection?
Flush your iptables rules to make sure they are not interfering:
rc.d stop iptables
or:
iptables -P INPUT ACCEPT iptables -P OUTPUT ACCEPT iptables -F INPUT iptables -F OUTPUT
Have you allowed SSH in hosts.allow?
Double check you have done this section correctly.
Is the traffic even getting to your computer?
Start a traffic dump on the computer you're having problems with:
tcpdump -lnn -i any port ssh and tcp-syn
This should show some basic information, then wait for any matching traffic to happen before displaying it. Try your connection now. If you don't see any output when you attempt to connect, then something outside of your computer is blocking the traffic (eg, hardware firewall, NAT router etc)
Read from socket failed: Connection reset by peer
Recent versions of openssh sometimes fail with the above error message, due to a bug involving elliptic curve cryptography. In that case, edit the file
~/.ssh/config
or create it, if it doesn't already exist. Add the line
HostKeyAlgorithms ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-rsa,ssh-dss