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2005/05/01 blacklist unwanted ip addresses

© May 2005 Tony Lawrence

On Linux, it's easy enough to add an iptables rule to blacklist a particular ip address. You can even automate the process based on certain criteria that you define. However, you don't necessarily want to leave an ip blacklisted forever, because it may be transient (a legitimate user may have that ip address tomorrow), or the condition that triggered your block may have been an error. The ip address may even have been spoofed, thus denying access to legitimate users.

There is a security tools bundle at https://www.apachesecurity.net by the author of Apache Security that can assist with this. It includes a command line perl script "blacklist" that adds entries to your iptables. To prepare for it, your regular iptables startup rules need to add a "BLACKLIST" chain like this:

    iptables -N BLACKLIST
    iptables -A INPUT -p tcp --dport 80 -j BLACKLIST

Then simply make sure that "blacklist start" runs at startup. I copied it to /sbin and added "/sbin/blacklist start" to /etc.rc.local. This is not a daemon; the startup just reads previously stored blocks from a data file and adds them to the BLACKLIST chain.

IP's are blocked for the duration you specify:

blacklist block 300

blocks that ip address for 300 seconds. However, as no daemon is running, there's nothing to unblock unless you run "blacklist unblock_stale" regularly. Therefore, how often you run that determines the lower limit of how long someone will be blocked. I have it running daily only, so any block could last up to 24 hours regardless of the duration set. You may want to run the unblock_stale more often. You can specifically unblock someone with "blacklist unblock <ip>" and clear all blocks with "blacklist clear".

This bundle also includes an "apache-protect" perl script that can monitor apache status and automatically call blacklist when suspicious activity is observed. To use that, you need to enable the apache mod_status module:

# from httpd.conf
LoadModule status_module modules/mod_status.so
ExtendedStatus On
<Location /server-status>
    SetHandler server-status
    Order deny,allow
    Deny from all
    Allow from
    Allow from

This creates a special /server-status page, only visible to the ip addresses listed in "Allow from", that has extended Apache status information:

                     Apache Server Status for unixish.com

   Server Version: Apache
   Server Built: Nov 12 2004 10:10:20

   Current Time: Friday, 22-Apr-2005 08:55:01 EDT
   Restart Time: Thursday, 21-Apr-2005 13:31:56 EDT
   Parent Server Generation: 8
   Server uptime: 19 hours 23 minutes 4 seconds
   Total accesses: 734 - Total Traffic: 1.1 MB
   CPU Usage: u.2 s.29 cu35.21 cs0 - .0512% CPU load
   .0105 requests/sec - 16 B/second - 1583 B/request
   1 requests currently being processed, 8 idle workers


   Scoreboard Key:
   "_" Waiting for Connection, "S" Starting up, "R" Reading Request,
   "W" Sending Reply, "K" Keepalive (read), "D" DNS Lookup,
   "C" Closing connection, "L" Logging, "G" Gracefully finishing,
   "I" Idle cleanup of worker, "." Open slot with no current process

   Srv PID Acc M CPU SS Req Conn Child Slot Client VHost Request
   0-8 1481 0/86/86 _ 5.08 600 0 0.0 0.13 0.13 zenez.com GET
   1-8 1482 0/88/88 _ 4.82 904 0 0.0 0.13 0.13 zenez.com GET
   /tmp/scouw7faqz/cache/77.html HTTP/1.0
   2-8 1483 0/87/87 _ 5.30 1004 0 0.0 0.14 0.14 zenez.com GET
   /tmp/scoprogfaq/cache/68.html HTTP/1.0
   3-8 1484 0/87/87 _ 3.93 720 0 0.0 0.12 0.12 stage.zenez.com
   GET /cgi-bin/scoprogfaq/faq?cmd=maintenance&secret=fed3156b3781
   4-8 1485 0/88/88 _ 3.70 937 0 0.0 0.13 0.13 zenez.com GET
   /robots.txt HTTP/1.0
   5-8 1486 0/86/86 _ 5.22 495 0 0.0 0.11 0.11 zenez.com GET
   /tmp/ou8faqz/cache/125.html HTTP/1.0
   6-8 1487 0/89/89 _ 5.45 21 0 0.0 0.16 0.16 u15181317.onlinehome-server.com
   unixish.com GET /server-status HTTP/1.0
   7-8 1488 0/87/87 _ 4.00 495 0 0.0 0.13 0.13 zenez.com GET
   /robots.txt HTTP/1.0
   8-8 4121 0/36/36 W 1.28 0 0 0.0 0.06 0.06 unixish.com GET
   /server-status HTTP/1.0

    Srv  Child Server number - generation
    PID  OS process ID
    Acc  Number of accesses this connection / this child / this slot
     M   Mode of operation
    CPU  CPU usage, number of seconds
    SS   Seconds since beginning of most recent request
    Req  Milliseconds required to process most recent request
   Conn  Kilobytes transferred this connection
   Child Megabytes transferred this child
   Slot  Total megabytes transferred this slot

   SSL/TLS Session Cache Status:
   cache type: SHMCB, shared memory: 512000 bytes, current sessions: 0
   sub-caches: 32, indexes per sub-cache: 133
   index usage: 0%, cache usage: 0%
   total sessions stored since starting: 0
   total sessions expired since starting: 0
   total (pre-expiry) sessions scrolled out of the cache: 0
   total retrieves since starting: 0 hit, 0 miss
      total removes since starting: 0 hit, 0 miss

The "apache-protect" script processes this information and calls blacklist when indicated. Add apache-protect to cron:

*       *       *       *       *       /sbin/apache-protect

It would definitely be a good idea to add your own ip address to apache-protect's whitelist:

# An overriding threshold list. The value -1 means never
# block. Any other value establishes a threshold for
# the given IP address.
%WHITELIST = ( "" => -1, "" => -1 );

It's easy to use "blacklist" for other conditions, too. I have code in my comments scripts that checks for spamming and refuses the post if it detects unreasonable content; I may also add a call to "blacklist" under such conditions. If a particular site seems to be regularly appearing in your spam email filters, they may as well be blocked permanently. That can be done by adding ip's to the blacklist data file "/etc/blacklist.dat " at startup, perhaps just by maintaining another file of "always block" ip's that you iterate through with "blacklist" at startup. That's simple to add to the blacklist script itself.

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