Linux - SecurityThis forum is for all security related questions.
Questions, tips, system compromises, firewalls, etc. are all included here.
Welcome to LinuxQuestions.org, a friendly and active Linux Community.
You are currently viewing LQ as a guest. By joining our community you will have the ability to post topics, receive our newsletter, use the advanced search, subscribe to threads and access many other special features. Registration is quick, simple and absolutely free. Join our community today!
Note that registered members see fewer ads, and ContentLink is completely disabled once you log in.
it's not a symptom of a DoS attack... it's more like a brute-force attack (if you're seeing tons of these)... keep in mind also that when you have tons of users logging into your box via SSH it's not strange for many of them to get failed reverse DNS lookups (reverse DNS lookups can be easily disabled in your sshd_config file IIRC)... about how many (legitimate) users are accessing your box via ssh??
having your SSH daemon listen on a non-standard port will help keep the brute-force attacks down - maybe enough for your satisfaction, and maybe not... the ideal seems to be to install a solution that scans your log and then runs an IPtables command to the block IPs of obviously bogus connection attempts... more info in the sticky...
With only two authorized ssh users, it may be easier adding these two names to the AllowUsers entry of the /etc/ssh/sshd_config configuration file. All other users will be denied access. The brute force attacks will use system user names, root, common names and dictionary names. Denying root and system users access is the first step to reduce the chance of compromise.
There is a "man 5 sshd_config" manpage which will list all of the options.