Linux Troubleshooting Make Easy.

Linux Learning & Troubleshooting Make Easy.

OpenSSH Security Tips

#1: Disable 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

Update your iptables script to remove ssh exception rules. 

#2: Only Use SSH Protocol 2

Open sshd_config file and make sure the following line exists.

Protocol 2   

#3: Limit Users' SSH Access

By default all systems user can login via SSH using their password or public key. We can limit users, who can access server via ssh. So attacker failed to get into box via ssh because it wasn't in AllowUsers.

To do that add the following to sshd_config:

AllowUsers root <name1> <name2>

#4: 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:

ClientAliveInterval 300
ClientAliveCountMax 0

#5: Disable .rhosts Files

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

IgnoreRhosts yes

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

#6: Disable Host-Based Authentication

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

HostbasedAuthentication no

#7: 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 to gain root level access. To disable root login via SSH, update sshd_config with the following line:

PermitRootLogin no

#8: Enable a Warning Banner

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

Banner /etc/issue

#9: Firewall SSH Port # 22

You need to firewall ssh port # 22 by updating iptables or pf firewall configurations. Update /etc/sysconfig/iptables to accept connection only from friendly network.

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

#10: 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 in sshd_config file:

Port 300
ListenAddress 192.168.1.5

#11: 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. 

#12: 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.


#13: 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 192.168.1.2

sshd : 192.168.1.2  

#14: Disable Empty Passwords

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

PermitEmptyPasswords no
 

#15: Rate-limit Incoming Port # 22 Connections

Both netfilter and pf provides rate-limit option to perform simple throttling on incoming connections on port # 22.

Check the Link for Details. linux-rate-limit-incoming-ssh



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