Top 20 OpenSSH Server Best Security Practices

[Administrator]

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. This page shows how to secure your OpenSSH server running on a Linux or Unix-like system to improve sshd security.

1. Use SSH public key based login

OpenSSH server supports various authentication. It is recommended that you use public key based authentication. First, create the key pair using following ssh-keygen command on your local desktop/laptop:

DSA and RSA 1024 bit or lower ssh keys are considered weak. Avoid them. RSA keys are chosen over ECDSA keys when backward compatibility is a concern with ssh clients. All ssh keys are either ED25519 or RSA. Do not use any other type.

$ ssh-keygen -t key_type -b bits -C "comment"
$ ssh-keygen -t ed25519 -C "Login to production cluster at xyz corp"
$ ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa_aws_$(date +%Y-%m-%d) -C "AWS key for abc corp clients"

Next, install the public key using ssh-copy-id command:

$ ssh-copy-id -i /path/to/public-key-file user@host
$ ssh-copy-id user@remote-server-ip-or-dns-name
$ ssh-copy-id vivek@rhel7-aws-server

When promoted supply user password. Verify that ssh key based login working for you:

ssh vivek@rhel7-aws-server

2. Disable root user login
Before we disable root user login, make sure regular user can log in as root. For example, allow vivek user to login as root using the sudo command.

Allow members of group sudo to execute any command. Add user vivek to sudo group:

$ sudo adduser vivek sudo

Verify group membership with id command

$ id vivek

How to add vivek user to sudo group on a CentOS/RHEL server
Allows people in group wheel to run all commands on a CentOS/RHEL and Fedora Linux server. Use the usermod command to add the user named vivek to the wheel group:

$ sudo usermod -aG wheel vivek
$ id vivek

Test sudo access and disable root login for ssh

Test it and make sure user vivek can log in as root or run the command as root:

$sudo -i
$ sudo /etc/init.d/sshd status
$ sudo systemctl status httpd

3. Disable password based login

All password-based logins must be disabled. Only public key based logins are allowed. Add the following in your sshd_config file:

4. Limit Users' ssh access

By default, all systems user can login via SSH using their password or public key. Sometimes you create UNIX / Linux user account for FTP or email purpose. However, those users can log in to the 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. Only allow root, vivek and jerry user to use the system via SSH, add the following to sshd_config:

AllowUsers vivek jerry

5. Disable Empty Passwords

You need to explicitly disallow remote login from accounts with empty passwords, update sshd_config with the following line:

PermitEmptyPasswords no

6. Use strong passwords and passphrase for ssh users/keys

It cannot be stressed enough how important it is to use strong user passwords and passphrase for your keys. Brute force attack works because user goes to 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):

genpasswd() {
   local l=$1
          [ "$l" == "" ] && l=20
         tr -dc A-Za-z0-9_ < /dev/urandom | head -c ${l} | xargs
}

Run it:

$ genpasswd 16

Output:

uw8CnDVMwC6vOKgW

7. Firewall SSH TCP port # 22

You need to firewall ssh TCP port # 22 by updating iptables/ufw/firewall-cmd or pf firewall configurations. Usually, OpenSSH server must only accept connections from your LAN or other remote WAN sites only. See the ufw command or iptables command examples for more info.

Netfilter (Iptables) Configuration
Update /etc/sysconfig/iptables (Redhat and friends specific file) to accept connection only from 192.168.1.0/24 and 202.54.1.5/29, enter:

-A RH-Firewall-1-INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
-A RH-Firewall-1-INPUT -s 202.54.1.5/29 -m state --state NEW -p tcp --dport 22 -j ACCEPT

If you've dual stacked sshd with IPv6, edit /etc/sysconfig/ip6tables (Redhat and friends specific file), enter:

-A RH-Firewall-1-INPUT -s ipv6network::/ipv6mask -m tcp -p tcp --dport 22 -j ACCEPT

8. Change SSH Port and limit IP binding

By default, SSH listens to all available interfaces and IP address on the system. Limit ssh port binding and change ssh port (many brutes forcing scripts only try to connect to TCP port # 22). To bind to 192.168.1.5 and 202.54.1.5 IPs and port 300, add or correct the following line in sshd_config:

Port 300
ListenAddress 192.168.1.5
ListenAddress 202.54.1.5

A better approach to use proactive approaches scripts such as fail2ban or denyhosts when you want to accept connection from dynamic WAN IP address.

9. Use TCP wrappers (optional)

TCP Wrapper is a host-based Networking ACL system, used to filter network access to the Internet. OpenSSH does support TCP wrappers. Just update your /etc/hosts.allow file as follows to allow SSH only from 192.168.1.2 and 172.16.23.12 IP address:

sshd : 192.168.1.2 172.16.23.12

10. Thwart SSH crackers/brute force attacks

Brute force is a method of defeating a cryptographic scheme by trying a large number of possibilities (combination of users and passwords) using a single or distributed computer network. To prevents brute force attacks against SSH, use the following software:

• 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.
• Explains how to setup DenyHosts under RHEL / Fedora and CentOS Linux.
• Fail2ban is a similar program that prevents brute force attacks against SSH.
• sshguard protect hosts from brute force attacks against ssh and other services using pf.
• security/sshblock block abusive SSH login attempts.
• IPQ BDB filter May be considered as a fail2ban lite.

11. Rate-limit incoming traffic at TCP port # 22 (optional)

Iptables Example
The following example will drop incoming connections which make more than 5 connection attempts upon port 22 within 60 seconds:

#!/bin/bash
inet_if=eth1
ssh_port=22
$IPT -I INPUT -p tcp --dport ${ssh_port} -i ${inet_if} -m state --state NEW -m recent  --set
$IPT -I INPUT -p tcp --dport ${ssh_port} -i ${inet_if} -m state --state NEW -m recent  --update --seconds 60 --hitcount 5 -j DROP

$IPT -A INPUT  -i ${inet_if} -p tcp --dport ${ssh_port} -m state --state NEW -m limit --limit 3/min --limit-burst 3 -j ACCEPT
$IPT -A INPUT  -i ${inet_if} -p tcp --dport ${ssh_port} -m state --state ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -o ${inet_if} -p tcp --sport ${ssh_port} -m state --state ESTABLISHED -j ACCEPT
# another one line example
# $IPT -A INPUT -i ${inet_if} -m state --state NEW,ESTABLISHED,RELATED -p tcp --dport 22 -m limit --limit 5/minute --limit-burst 5-j ACCEPT

12. Use port knocking (optional)

Port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect to the specific port(s). A sample port Knocking example for ssh using iptables:

$IPT -N stage1
$IPT -A stage1 -m recent --remove --name knock
$IPT -A stage1 -p tcp --dport 3456 -m recent --set --name knock2
 
$IPT -N stage2
$IPT -A stage2 -m recent --remove --name knock2
$IPT -A stage2 -p tcp --dport 2345 -m recent --set --name heaven
 
$IPT -N door
$IPT -A door -m recent --rcheck --seconds 5 --name knock2 -j stage2
$IPT -A door -m recent --rcheck --seconds 5 --name knock -j stage1
$IPT -A door -p tcp --dport 1234 -m recent --set --name knock
 
$IPT -A INPUT -m --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT -p tcp --dport 22 -m recent --rcheck --seconds 5 --name heaven -j ACCEPT
$IPT -A INPUT -p tcp --syn -j door

13. Configure idle log out timeout interval

A user can log in to the server via ssh, and you can set an idle timeout interval to avoid unattended ssh session. Open sshd_config and make sure following values are configured:

ClientAliveInterval 300
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.

14. Enable a warning banner for ssh users

Set a warning banner by updating sshd_config with the following line:

Banner /etc/issue

Sample /etc/issue file:

----------------------------------------------------------------------------------------------
You are accessing a XYZ Government (XYZG) Information System (IS) that is provided for authorized use only.
By using this IS (which includes any device attached to this IS), you consent to the following conditions:

+ The XYZG routinely intercepts and monitors communications on this IS for purposes including, but not limited to,
penetration testing, COMSEC monitoring, network operations and defense, personnel misconduct (PM),
law enforcement (LE), and counterintelligence (CI) investigations.

+ At any time, the XYZG may inspect and seize data stored on this IS.

+ Communications using, or data stored on, this IS are not private, are subject to routine monitoring,
interception, and search, and may be disclosed or used for any XYZG authorized purpose.

+ This IS includes security measures (e.g., authentication and access controls) to protect XYZG interests--not
for your personal benefit or privacy.

+ Notwithstanding the above, using this IS does not constitute consent to PM, LE or CI investigative searching
or monitoring of the content of privileged communications, or work product, related to personal representation
or services by attorneys, psychotherapists, or clergy, and their assistants. Such communications and work
product are private and confidential. See User Agreement for details.
----------------------------------------------------------------------------------------------

Above is a standard sample, consult your legal team for specific user agreement and legal notice details.

15. Disable .rhosts files (verification)

Don't read the user's ~/.rhosts and ~/.shosts files. Update sshd_config with the following settings:

Don't read the user's ~/.rhosts and ~/.shosts files. Update sshd_config with the following settings:

SSH can emulate the behavior of the obsolete rsh command, just disable insecure access via RSH.

16. Disable host-based authentication (verification)

To disable host-based authentication, update sshd_config with the following option:

HostbasedAuthentication no

17. Patch OpenSSH and operating systems

It is recommended that you use tools such as yum, apt-get, freebsd-update and others to keep systems up to date with the latest security patches:

18. Chroot OpenSSH (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.

19. Disable OpenSSH server on client computer

Workstations and laptop can work without OpenSSH server. If you do not provide the remote login and file transfer capabilities of SSH, disable and remove the SSHD server. CentOS / RHEL users can disable and remove openssh-server with the yum command:

$ sudo yum erase openssh-server

Debian / Ubuntu Linux user can disable and remove the same with the apt command/apt-get command:

$ sudo 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 done restart iptables service:

# service iptables restart
# service ip6tables restart

20. Bonus tips from Mozilla

#################[ WARNING ]########################
# Do not use any setting blindly. Read sshd_config #
# man page. You must understand cryptography to    #
# tweak following settings. Otherwise use defaults #
####################################################
 
# Supported HostKey algorithms by order of preference.
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
 
# Specifies the available KEX (Key Exchange) algorithms.
KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256
 
# Specifies the ciphers allowed
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
 
#Specifies the available MAC (message authentication code) algorithms
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
 
# LogLevel VERBOSE logs user's key fingerprint on login. Needed to have a clear audit track of which key was using to log in.
LogLevel VERBOSE
 
# Log sftp level file access (read/write/etc.) that would not be easily logged otherwise.
Subsystem sftp  /usr/lib/ssh/sftp-server -f AUTHPRIV -l INFO

You can grab list of cipher and alog supported by your OpenSSH server using the following commands:

# ssh -Q cipher
# ssh -Q cipher-auth
# ssh -Q mac
# ssh -Q kex
# ssh -Q key