Description

This howto outlines a few extra steps that can be taken to further secure a SSH server, chiefly from the constant onslaught of automated brute-force password attacks.

Requirements

An SSH server should already be installed and running. If it is not, do the following:

yum install openssh-server
service sshd start

Doing the Work

Most of the steps below involve editing the global SSH server configuration file located at /etc/ssh/sshd_config, and the access control tcpwrapper configuration files /etc/hosts.allow and /etc/hosts.deny. Use your favorite editor to modify these files as root.

Summary of Steps:

Some or all of the following steps may be taken to secure your SSH server. The steps are sorted roughly in order of entirely sensible precaution to completely paranoid:

  1. Choose a strong password
  2. Install “DenyHosts” to auto-block bad clients
  3. Change the default port
  4. Disable insecure Protocol 1; allowing only Protocol 2
  5. Disable root login
  6. Reduce MaxStartups
  7. Reduce LoginGraceTime
  8. Allow only specific users or groups to connect
  9. Allow only specific IP addresses to connect
  10. Allow only users with keys to connect; no passwords allowed
  11. Bind the ssh server to a specific network interface

Steps Explained:

  1. If you do nothing else, it is of utmost importance to choose strong passwords for all accounts, especially root, since it is the #1 attacked account. It is very enlightening to see the accounts on your system that attackers have been targeting lately; to do this either use the lastb command to quickly see recent failed logins of all types, or, better, parse /var/log/secure to get stats only for failed ssh logins. Three examples:
  2. Centos uses pam_cracklib.so to force normal users to choose semi-strong passwords of 6 characters or more when changing their passwd, but root can still choose weak passwords anytime he wants (but will be warned). Also note that the first user account added to the system after an installation (using firstboot) is allowed to be weak if you ignore the warning.

    You may want to use a utility called “john the ripper” to audit the strength of passwords on your system – if it cracks any too quickly, they’re too weak.

  3. Install Denyhosts which watches the /var/log/secure logfile for invalid ssh login attempts, and if a configurable threshold is crossed, they are automatically blocked by being added to /etc/hosts.deny. Install Denyhosts, and optionally edit the good default configuration in /etc/denyhosts.conf:
  4. yum install denyhosts
    chkconfig denyhosts on
    service denyhosts start

  5. The vast majority of ssh attacks are directed by compromised zombie machines against ssh servers listening on the default port of “22”. By changing this port to something else you greatly reduce the risk of an automated break-in. Edit /etc/ssh/sshd_config and change the line which reads “Port 22” to “Port 39691”, or any other unused port of your choosing, preferably above 1024. A line preceded by a ‘#’ is the commented out default value.
  6. SSH speaks two protocols: The old and insecure Protocol 1, and the newer Protocol 2. Almost all SSH clients now speak Protocol 2, so it is best to disable the older one. Edit sshd_config to include only Protocol 2, and not both:
  7. There is no good reason for root to be allowed to interactively login directly; after connecting as a normal user, you can still “su -“ to root. edit sshd_config:
  8. If root login is required – e.g. for remote system backups – you may alternatively permit root to login, but only if using ssh keys, not interactive password entry:
  9. Finally, it’s also possible to limit root access to only be able to run specific commands, such as backup scripts (beyond the scope of this article):
  10. Limit the maximum number of unauthenticated connections that the ssh server will handle at the same time. The smaller this is, the harder it is for script kiddies to make parallel, coordinated cracking attempts with multiple connections. edit sshd_config and change MaxStartups from the default of “10” to “3:50:10”. The colon separated values tells the ssh server to, “allow 3 users to attempt logging in at the same time, and to randomly and increasingly drop connection attempts between 3 and the maximum of 10”. Note: this should be increased on servers with substantial numbers of valid ssh users logging in.
  11. Reduce the maximum amount of time allowed to successfully login before disconnecting. The default of 2 minutes is too much time to hold open an unauthenticated connection attempt (see above); 30 seconds is more than enough time to log in:
  12. By default, all valid users on the system are allowed to log in. A more secure policy is to only allow a whitelist of users or groups to log in. For example, to allow only the users “john”, “mary”, “joeblow”, “joeschmoe”, “joejoe”, and any username that starts with “joe” to login, add the following line to sshd_config:
  13. Alternatively, you may instead allow only users who are members of certain groups to login. For example, to allow only the members of the “sshusers” group to connect, first make sure the group exists (groupadd sshusers) and add your users to it (usermod -a -G sshusers username), then add the following line to sshd_config:
  14. note: You’ll need to use the groupadd command to create this group: groupadd sshusers

  15. Allow only users from certain IP addresses to connect. Before allowing specific IPs, the default policy must first be set to DENY to be effective. edit /etc/hosts.deny and add the following line:
  16. Next add to /etc/hosts.allow the networks you want to allow. For example, to allow all 254 hosts on the class C network “192.168.1.*”, all 16million hosts from the class A network “10.0.0.0”, and the lonely IP 24.42.69.101, you would add the following to /etc/hosts.allow:
  17. note: You may also allow/deny connections via a firewall, but to maintain sanity it’s best to stick to one method or the other.

  18. To remove the possibility of anybody ever guessing a user’s password, disable password authentication completely, and require that public/private key pairs be used instead. While much more secure than passwords, a user’s private key can still be compromised, especially if not protected by a passphrase. To disable password logins, add the following to sshd_config:
  19. By default, the ssh server listens for connections on ALL interfaces (0.0.0.0). If a ssh server is to only be accessible internally, bind it to a LAN IP. For example: edit sshd_config:

Troubleshooting / How To Test

  • If your changes don’t seem to be working, remember to restart the sshd server, but DO NOT CLOSE THE ACTIVE SSH CONNECTION in case something goes wrong; attempt to make a new connection first, and undo any changes if necessary, or you may find that you’ve remotely locked yourself out of the system.

More Information

Disclaimer

We test this stuff on our own machines, really we do. But you may run into problems, if you do, come to #centoshelp on irc.freenode.net

Added Reading

  • man sshd_config
  • man hosts.allow
  • man host.deny
  • man groupadd
Last Modified: 12 Dec, 2015 at 01:52:01