Top OpenSSH Server Best Security Practices

OpenSSH is the implementation of the SSH protocol. OpenSSH is recommended for remote login, making backups, remote file transfer via scp or sftp, and much more. SSH is perfect to keep confidentiality and integrity for data exchanged between two networks and systems. However, the main advantage is server authentication, through the use of public key cryptography. From time to time there are rumors about OpenSSH zero day exploit. Here are a few things you need to tweak in order to improve OpenSSH server security.

Default Config Files and SSH Port

  • /etc/ssh/sshd_config - OpenSSH server configuration file.
  • /etc/ssh/ssh_config - OpenSSH client configuration file.
  • ~/.ssh/ - Users ssh configuration directory.
  • ~/.ssh/authorized_keys or ~/.ssh/authorized_keys - Lists the public keys (RSA or DSA) that can be used to log into the user’s account
  • /etc/nologin - If this file exists, sshd refuses to let anyone except root log in.
  • /etc/hosts.allow and /etc/hosts.deny : Access controls lists that should be enforced by tcp-wrappers are defined here.
  • SSH default port : TCP 22

Disable OpenSSH Server

Workstations and laptop can work without OpenSSH server. If you need not to provide the remote login and file transfer capabilities of SSH, disable and remove the SSHD server. CentOS / RHEL / Fedora Linux user can disable and remove openssh-server with yum command:
# chkconfig sshd off
# yum erase openssh-server

Debian / Ubuntu Linux user can disable and remove the same with apt-get command:
# apt-get remove openssh-server
You may need to update your iptables script to remove ssh exception rule. Under CentOS / RHEL / Fedora edit the files /etc/sysconfig/iptables and /etc/sysconfig/ip6tables. Once donerestart iptables service:
# service iptables restart
# service ip6tables restart

Only Use SSH Protocol 2

SSH protocol version 1 (SSH-1) has man-in-the-middle attacks problems and security vulnerabilities. SSH-1 is obsolete and should be avoided at all cost. Open sshd_config file and make sure the following line exists:
  1. Protocol 2

Limit Users' SSH Access

By default all systems user can login via SSH using their password or public key. Sometime you create UNIX / Linux user account for ftp or email purpose. However, those user can login to system using ssh. They will have full access to system tools including compilers and scripting languages such as Perl, Python which can open network ports and do many other fancy things. One of my client has really outdated php script and an attacker was able to create a new account on the system via a php script. However, attacker failed to get into box via ssh because it wasn't in AllowUsers.
Only allow root, vivek and jerry user to use the system via SSH, add the following to sshd_config:
  1. AllowUsers root vivek jerry
Alternatively, you can allow all users to login via SSH but deny only a few users, with the following line:
  1. DenyUsers saroj anjali foo
You can also configure Linux PAM allows or deny login via the sshd server. You can allow list of group name to access or deny access to the ssh.

Configure Idle Log Out Timeout Interval

User can login to server via ssh and you can set an idel timeout interval to avoid unattended ssh session. Open sshd_config and make sure following values are configured:
  1. ClientAliveInterval 300
  2. ClientAliveCountMax 0
You are setting an idle timeout interval in seconds (300 secs = 5 minutes). After this interval has passed, the idle user will be automatically kicked out (read as logged out). See how to automatically log BASH / TCSH / SSH users out after a period of inactivity for more details.

Disable .rhosts Files

Don't read the user's ~/.rhosts and ~/.shosts files. Update sshd_config with the following settings:
  1. IgnoreRhosts yes
SSH can emulate the behavior of the obsolete rsh command, just disable insecure access via RSH.
 Disable Host-Based Authentication
To disable host-based authentication, update sshd_config with the following option:
  1. HostbasedAuthentication no

Disable root Login via SSH

There is no need to login as root via ssh over a network. Normal users can use su or sudo (recommended) to gain root level access. This also make sure you get full auditing information about who ran privileged commands on the system via sudo. To disable root login via SSH, update sshd_config with the following line:
1. PermitRootLogin no

Enable a Warning Banner

Set a warning banner by updating sshd_config with the following line:
1. Banner /etc/issue

Change SSH Port and Limit IP Binding

By default SSH listen to all available interfaces and IP address on the system. Limit ssh port binding and change ssh port (by default brute forcing scripts only try to connects to port # 22). To bind to and IPs and to port 300, add or correct the following line:
  1. Port 300
  2. ListenAddress
  3. ListenAddress
A better approach to use proactive approaches scripts such as fail2ban or denyhosts (see below).

 Use Strong SSH Passwords and Passphrase

It cannot be stressed enough how important it is to use strong user passwords and passphrase for your keys. Brute force attack works because you use dictionary based passwords. You can force users to avoid passwords against a dictionary attack and use john the ripper tool to find out existing weak passwords. Here is a sample random password generator (put in your ~/.bashrc):
  1. genpasswd() {
  2. local l=$1
  3. [ "$l" == "" ] && l=20
  4. tr -dc A-Za-z0-9_ < /dev/urandom | head -c ${l} | xargs
  5. }
Run it:
genpasswd 16

Use Public Key Based Authentication

Use public/private key pair with password protection for the private key. See how to use RSAand DSA key based authentication. Never ever use passphrase free key (passphrase key less) login.

Chroot SSHD (Lock Down Users To Their Home Directories)

By default users are allowed to browse the server directories such as /etc/, /bin and so on. You can protect ssh, using os based chroot or use special tools such as rssh. With the release of OpenSSH 4.8p1 or 4.9p1, you no longer have to rely on third-party hacks such as rssh or complicated chroot(1) setups to lock users to their home directories. See this blog post about new ChrootDirectory directive to lock down users to their home directories.

Use TCP Wrappers

TCP Wrapper is a host-based Networking ACL system, used to filter network access to Internet. OpenSSH does supports TCP wrappers. Just update your /etc/hosts.allow file as follows to allow SSH only from :
sshd : 
See this FAQ about setting and using TCP wrappers under Linux / Mac OS X and UNIX like operating systems.

Disable Empty Passwords

You need to explicitly disallow remote login from accounts with empty passwords, update sshd_config with the following line:
  1. PermitEmptyPasswords no

#16: Thwart SSH Crackers (Brute Force Attack)

Brute force is a method of defeating a cryptographic scheme by trying a large number of possibilities using a single or distributed computer network. To prevents brute force attacks against SSH, use the following softwares.
DenyHosts is a Python based security tool for SSH servers. It is intended to prevent brute force attacks on SSH servers by monitoring invalid login attempts in the authentication log and blocking the originating IP addresses.