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On Monday 5 June 2006 17:06, Leandro Melo de Sales wrote: |
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> Hi, |
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> |
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> today when I was checking the server log I got many external |
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> attempts to connect to my sshd service: |
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> |
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> ... |
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> Jun 5 05:09:45 embedded sshd[4740]: Invalid user barbara from x.y.w.z |
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> Jun 5 05:09:46 embedded sshd[4742]: Invalid user barb from x.y.w.z |
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> Jun 5 05:09:48 embedded sshd[4744]: Invalid user barbie from x.y.w.z |
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> Jun 5 05:09:50 embedded sshd[4746]: Invalid user barbra from x.y.w.z |
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> Jun 5 05:09:51 embedded sshd[4748]: Invalid user barman from x.y.w.z |
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> Jun 5 05:09:53 embedded sshd[4750]: Invalid user barney from x.y.w.z |
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> ... |
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> |
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> this seems to be a brute force attack, but one thing that worried me |
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> is why sshd didn't disconnect the remote host after 3 unsuccessful |
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> attemps? If we see in the log, there are many attemps with time |
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> interval between attemps of 2 or 3 seconds meaning that the sshd |
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> didn't disconnect the remote host after 3 attempts. |
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|
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AFAIK, sshd disconnects when 3 incorrect passwords are tried _for the |
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same account_. |
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|
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> So, first, Am I thinking correct about the sshd attempts? |
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> Second, how can I setup sshd or the entire system to permit just 2 or |
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> 3 attempts of authentication? I was checking the /etc/login.defs file |
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> and I see the following option: |
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> |
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> # |
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> # Max number of login retries if password is bad |
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> # |
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> LOGIN_RETRIES 3 |
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> |
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> but why this didn't work for the above connection attempts? |
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|
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See above. |
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|
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At the very least, you should not permit root login from ssh and choose |
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very strong passwords for the users that are allowed to login or |
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(better) set up public key authentication, although that is not very |
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practical if the same users may log in from random hosts. In this case, |
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one-time passwords could be useful (google for opie or otpw). |
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|
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After that, there are many things you can (and should) do. Some examples |
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include: changing the port on which the ssh daemon listens (not a very |
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effective solution though), using port knocking, using iptables to limit |
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the attemps to no more than two or three per minute, use one of the many |
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denyhosts/fail2ban/captcha modules out there, and so on. Google is your |
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friend here. |
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|
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HTH |
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-- |
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