SSH is the Secure Shell, a secure replacement for telnet. The OpenSSH client comes in the default Debian install. We've installed the OpenSSH server to provide for remote access to our systems.
Install the SSH client and server packages:
sudo apt-get install ssh openssh-server
Fix it so
root cannot log in, but allow selected commands, which will be authenticated by authprogs:
sudo sed -i -e 's/^PermitRootLogin .*$/PermitRootLogin forced-commands-only/' /etc/ssh/sshd_config
/etc/issue.net to present a warning message to users connecting via SSH:
sudo sh -c 'echo "This system for use by BOOCHTEK employees ONLY. Unauthorized access prohibited." > /etc/issue' sudo cp /etc/issue /etc/issue.net
Configure the SSH daemon to add the warning message.
sudo sed -i -e 's:^#Banner .*$:Banner /etc/issue.net:' /etc/ssh/sshd_config
Disable TCP port forwarding (suggested by http://kitenet.net/~joey/blog/entry/ssh_port_forwarding/):
sudo sh -c '/bin/echo -e "\n# Disable TCP port forwarding.\nAllowTcpForwarding no" >> /etc/ssh/sshd_config'
Require SSH keys; don't allow password authentication. NOTE: Be sure you have set up SSH keys for your accounts first!
sudo sed -i -e 's:^#PasswordAuthentication.*$:PasswordAuthentication no:' /etc/ssh/sshd_config
Only allow "real" users to log in via SSH. NOTE: Be sure you've added all your users to one of the specified groups.
sudo sh -c '/bin/echo -e "\n# Allow only users in these groups to log in. (NOTE: Must be space-separated.)\nAllowGroups users" >> /etc/ssh/sshd_config'
Configure the SSH daemon to automatically log users off if they're idle for more than 30 minutes.
sudo sh -c 'echo "ClientAliveInterval 30m" >> /etc/ssh/sshd_config' sudo sh -c 'echo "ClientAliveCountMax 0" >> /etc/ssh/sshd_config'
Restart to have the settings take effect:
sudo /etc/init.d/ssh restart
NOTE: You can probably run
/etc/init.d/ssh reload instead of
/etc/init.d/ssh restart if you like.
Or just send the daemon a HUP signal to have it reread the configuration file and activate the changes:
sudo kill -HUP `cat /var/run/sshd.pid`
Log into the system as a user via SSH.
Log into the system as a user via sftp and try to transfer any file.
Try logging in as
root via SSH. Make sure the access is denied, and that the attempt is logged.
The SSH server defaults to allowing logins via public-key encryption, so you don't need to enter a password for every login. To allow this for a given user account, first prepare the SSH authorized_keys file:
mkdir -p ~/.ssh chmod 700 ~/.ssh touch ~/.ssh/authorized_keys chmod 600 ~/.ssh/authorized_keys
Generate a public/private key pair on the client (not the server). Be sure to protect the private key with a passphrase and proper file permissions. Then copy the public key from the client to the server. Assuming the client is UNIX-based, your key is named
id_dsa.pub, and you want to get to the
user account on
example.com, you would do something like this:
ssh firstname.lastname@example.org -c 'cat >> ~/.ssh/authorized_keys' < id_dsa.pub
(Many systems now come with an OpenSSH script named
ssh-copy-id that can do this for you.)
If you're coming from Windows, be sure that the
authorized_keys file entry is in the correct format. It's a single line containing space-separated fields: options (optional), key type (ssh-rsa or ssh-dss), the base64-encoded key, and an optional comment.
We install Fail2ban to block attempted brute-force SSH attacks. We also considered BFD, BlockHosts, DenyHosts, and SSHguard. Only Fail2ban and DenyHosts are included in Debian (as of Lenny/5.0). We went with Fail2ban, because it integrates with Shorewall, and supports more than just SSH attacks. Of the others we looked at, SSHguard was a close second, having similar support as Fail2ban.
Installation is pretty straightforward:
sudo apt-get install fail2ban whois
Configuration was also pretty simple. We mainly just told it to use Shorewall and added one of our own networks (osRiver) to ignore.
IGNORE=188.8.131.52/24 sudo sh -c 'cat > /etc/fail2ban/jail.local <<END [DEFAULT] banaction = shorewall ignoreip = 127.0.0.1 $IGNORE [ssh] maxretry = 5 END' sudo /etc/init.d/fail2ban reload
Fail2ban (on Debian) has regexes in the default
AllowGroups, but not
DenyGroups. We don't need them, but others might.
SSH access to the system is logged in
We've disabled direct root login via SSH.
The OpenSSH server and client come built with just about every feature possible to help ensure as secure a connection as possible.
While SSH replaces Telnet, we've kept the
telnet client installed, because it's very helpful in troubleshooting network services by telnetting directly to the port the service runs on. The telnet client should never be used to log into a shell account. The telnet server should never be installed.
molly-guard. Check into whether those might be of use.