Linux - How To avoid any DoS on SSH listening services

If you have linux server where you published externally SSH port you could get some DoS attack or dictionary based attacks and brute force attacks

In these case you can create e public/private certificate following this old blog article.

In case you do not have relative certificate you would not able to start ssh logging (it is always a good idea disable SSH root login utilizing a different user)

Otherwise you can utilize this took that help you to be aware about dictionary based attacks and brute force attacks.

The main concept is that, this script autocreate an IP blacklist of intruders to block them to continue brute Force attack

Here are more details:

DenyHosts is a script intended to be run by Linux system administrators to help thwart SSH server attacks (also known as dictionary based attacks and brute force attacks).
If you've ever looked at your ssh log (/var/log/secure on Redhat, /var/log/auth.log on Mandrake, etc...) you may be alarmed to see how many hackers attempted to gain access to your server. Hopefully, none of them were successful (but then again, how would you know?). Wouldn't it be better to automatically prevent that attacker from continuing to gain entry into your system?

- Parses /var/log/secure to find all login attempts and filters failed and successful attempts.
- Synchronization mode (new in 2.0) allows DenyHosts daemons the ability to share data via a centralized server to proactively thwart attacks.
- Can be run from the command line, cron or as a daemon (new in 0.9)
- Records all failed login attempts for the user and offending host
- For each host that exceeds a threshold count, records the evil host
- Keeps track of each non-existent user (eg. sdadasd) when a login attempt failed.
- Keeps track of each existing user (eg. root) when a login attempt failed.
- Keeps track of each offending host (with 0.8+ these hosts can be purged if the associated entry in /etc/hosts.deny is expired)
- Keeps track of suspicious logins (that is, logins that were successful for a host that had many login failures)
- Keeps track of the file offset, so that you can reparse the same file (/var/log/secure) continuously (until it is rotated).
- When the log file is rotated, the script will detect it and parse from the beginning.
- Appends /etc/hosts.deny and adds the newly banned hosts
- Optionally sends an email of newly banned hosts and suspicious logins.
- Keeps a history of all user, host, user/host combo and suspicious logins encountered which includes the data and number of corresponding failed login attempts.
- Maintains failed valid and invalid user login attempts in separate files, such that it is easy to see which valid user is under attack (which would give you the opportunity to remove the account, change the password or change it's default shell to something like /sbin/nologin
- Upon each run, the script will load the previously saved data and re-use it to append new failures.
- Resolves IP addresses to hostnames, if available (new in v0.6.0).
- /etc/hosts.deny entries can be expired (purge) at a user specified time (new in 0.8)