Adding translation from French version / added "Securing"
drop stuff that is in the default config
|Deletions are marked like this.||Additions are marked like this.|
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|* Decativate using the SSH1 protocol (''Protocol 2'').|
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|* Deactivate using empty passwords (''!PermitEmptyPasswords no'').|
- Configuration files
- Remote login
- Remote commands
- SSH without password
- SSH into Debian from another OS
- SSH and security
ToDo: merge (and translate) this page and the french one (more complete)
SSH stands for Secure Shell and is a protocol for secure remote login and other secure network services over an insecure network1. See Wikipedia - Secure Shell for more general information and ssh, lsh-client or dropbear for the SSH software implementations out of which OpenSSH is the most popular and most widely used2. SSH replaces the unencrypted telnet,rlogin and rsh and adds many features.
In this document we'll be using the OpenSSH command suite, it will also be assumed that the following two variables are defined:
remote_host=<the remote computer> remote_user=<your user name on $remote_host>
So, if you want to use the recipes below, first set these variables to the remote computer name and the user name on that remote computer. Then cut and paste of the commands below should work. remote_host may also be an IP-address.
Installation of the client
Normally the client is installed by default. If not it suffices to run as root:
apt-get install openssh-client
Installation of the server
The server allows to connect remotely and gets installed by running as root:
apt-get install openssh-server
The main configuration files are in the directory /etc/ssh :
ssh_config : client configuration file
sshd_config : server configuration file
In addition this directory contains the private/public key pairs identifying your host :
If you want to login to $remote_host as user $remote_user simply type
and then type in your password.
If the usernames on the local and the remote computer are identical, you can drop the $remote_user@-part and simply write
If this is the first time you login to the remote computer, ssh will ask you whether you are sure you want to connect to the remote computer. Answer 'yes' after you verified the remote computer's fingerprint, type in your password, and ssh will connect you to the remote host.
Using shared keys
One of the function of ssh is using a pair of private/public keys for connecting to a remote host. This method allow to login to a remote host without typing every time ones password. To do this you must generate a pair of private/public keys on your local maschine and deposit the key on the remote host.
To generate the key one uses the program ssh-keygen
ssh-keygen -t rsa
This program generates a pair of private/public keys in the directory ~/.ssh. The program first asks for the destination files for the keys, by default located in ~/.ssh. Afterwards a passphrase is requested.
Note: We recommend to not leave the passphrase empty. An attacker who gets hold of your private key can otherwise connect to the hosts where you deposited you public key since the passphrase is empty. Choose a long and complex passphrase.
You private key is id_rsa (don't give it to someone else), the public key is id_rsa.pub.
You copy your public key to a remote host with the command ssh-copy-id
ssh-copy-id -i ~/.ssh/id_rsa.pub $remote_user@$remote_host
Now you can connect simply to the remote host and the passphase is asked for. Once done, you get connected to the remote host. In case of a new connection the passphrase does not get asked for again during your entire session.
By default a SSH server is relatively secure. With the help of some configuration options and external utilities it is possible to make it even harder for crackers.
Using the latest version of package openssh-server allows to protect against known security holes.
One should edit the file /etc/ssh/sshd_config to change the parameters and then restart the ssh server with
invoke-rc.d ssh restart
Deactivate using passwords for authentication (UsePAM no).
Deactivate using the root account (PermitRootLogin no).
Only allow login by certain users or groups (AllowUsers and AllowGroups)
The options AllowUsers and AllowGroups do not improve the security of a SSH server. But in certain cases their use allows to resist a brute force attack a little longer.
If you just want to run one command on the remote computer, you don't need to login. You can tell ssh to run the command without login, for instance,
ssh $remote_user@$remote_host 'ls *.txt'
lists all files with extension .txt on the remote computer. This works with single tick quotes '...' as shown here, with double tick quotes "...", and without quotes. There may be differences between these three cases, though, not yet documented here.
SSH without password
If you work on a remote computer often, typing in the password each time you use ssh becomes annoying. You can configure ssh such that it does not ask you for a password anymore for that particular connection. You have to generate a private and public encryption key on your local machine and provide the public key to the remote machine.
To generate the keys run
and reply to all questions just with return.
To provide the public key to the remote machine first create there an .ssh directory (if not present already) and then append the public key of your local machine to the authorized_keys file on the remote machine.
ssh $remote_user@$remote_host mkdir -p .ssh cat .ssh/id_rsa.pub | ssh $remote_user@$remote_host 'cat >> .ssh/authorized_keys'
Note that here the cat command within the ssh command takes its input from the pipe.
or you can use
ssh-copy-id -i ~/.ssh/id_rsa.pub $remote_user@$remote_host
From now on, you should be able to login with ssh without password.
REMARK: If the usernames on the local and the remote machine are identical, and if the local and the remote computer have access to the same home-directory of that user, e.g. because they are different clients in the same LAN with a common home directory mounted via nfs, then the private key, the public key, and the authorized_keys file all reside in the same directory. Thus you cannot only login without password from the local to the remote machine but also vice versa. In fact you can login from any computer in the LAN to any other computer. (The username@hostname entry at the end of the public key in the authorized_keys file has no relevance to ssh, you may delete it or change it if you like (I think)).
TROUBLESHOOTING (ssh still asks for a password): Login without password does not work if group or world has write permissions for the home directory on the remote machine. To fix that, run
ssh $remote_user@$remote_host chmod g-w,o-w /home/$remote_user
SOURCE: Mathias Kettner, SSH login without password, http://www.linuxproblem.org/art_9.html, visited 2007-10-06.
SSH into Debian from another OS
Wikipedia has Comparison_of_SSH_clients
SSH and security
Consider using fail2ban which is a log file monitor that automatically bans an ip address after a predefined number of failed login attempts. Guards against brute force attacks.
- Use SSH keys rather than password.
http://lackof.org/taggart/hacking/ssh/ - Good practices for using ssh