Install Fail2ban (Intrusion Prevention) System on RHEL/CentOS 6.3/5.8, Fedora 17/12

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Ravi Saive

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4 Responses

  1. hi, after run yum: yum install fail2ban. I get message:
    No package fail2ban available.

    How can i continue?

    thanks

  2. Cody says:

    (Not sure if my post went through first time – noscript and didn’t see anything about moderation. So if it did go through I apologise; please remove it if so).

    fail2ban is NOT an intrusion prevention system. It monitors log files so technically a breach could have already occurred. Unlikely you suggest? Well, unlikely does not mean impossible. Fact of life. It could be argued that it helps prevent brute force but only if configured properly and only if you are lucky enough.

    But I’ll argue this instead: there is no such thing as an intrusion prevention system. There will always be someone who can better you. Always. You can only make it as secure as possible for your skill level but do not ever rely on software or ANYTHING by itself (key words) or else you’re bound to be disappointed, very disappointed indeed.

    Secondly, as for the command:
    cat /var/log/secure | grep ‘Failed password’ | sort | uniq -c

    Welcome to the useless use of cat award. I would also wonder why sort and uniq (at least as is, and see end of post too). Sure, the sort makes it adjacent for uniq to work, and sure the count is all fine, (but see below) but here’s the thing: even if a failed password attempt happened at the same second (so same yyyy/mm/dd hh:mm:ss) the problem is that each connection has its on PID. So for instance, the part where you see:

    sshd[19079]

    the number between the brackets is the PID of the spawned session. (Also, observe the port portion; that is because of the way networking works and therefore it is going to vary so another thing to keep in mind). Also, in this case, there is no hypothetical situation where the PID is going to be the same as it is the same second. So while yes, it will show you the count, it will be the total even for the same IP (and the total will need to be calculated by you by addition). Now maybe you want that but I’m just informing you in case.

    Tips:

    cat file | grep ‘whatever’ should just be grep ‘whatever’ file because grep (like most Unix – and its derivatives – utilities, they read files because everything in Unix is a file, even the console input, output and error (stdin, stdout, stderr)!). So that part should be:

    grep ‘Failed password’ /var/log/secure

    Also: many utilities will read from ‘-‘. (without the ‘s). I suggest you look at the man pages or even –help output of utilities. Examples include cat and grep.

    If you wanted to just see how many failed passwords (total), just do:

    grep -c ‘Failed password’ /var/log/secure
    (notwithstanding log rotation).

    Contrast the grep -c to grep | wc -l (which is a useless use of wc)

    Please don’t take this to be offensive, I’m just pointing out the mistakes to help others (and alert them to the fallacy of fail2ban preventing intrusion and more than that that there is such a thing) as well as hoping you can learn too. If you’re curious what I mean about useless use, look up the useless use of cat award (via a search engine). Also, one more tip (sort of, sorry for the pun): you can with more advanced command lines strip the pid, the port, and in general only show certain sections. So you could for instance make it so the only part of the line that is visible is the IP then sort and count. Example:

    grep ‘Failed password’ /var/log/secure |cut -d’ ‘ -f12|sort |uniq -c

    … and that could be improved upon …
    but would show you lines in the form of:
    count IP

    Oh, in case html shows it wrong, the – is the option prefix (minus sign) to the commands. So it may appear wrong. Same goes for the ‘ which is the apostrophe (could also in this case be fine with “s).
    Cheers.

  3. Vincent says:

    by default all output is going to the screen in CentOS 6.3 .
    Very annoying.
    Is there anyway to change that?
    The setting says syslog, but i cannot work normally because of the output to screen.
    Thanks for the usefull guide!

    I would be interested in permanent blocking options.

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