14.2.24. A Different Method for Preventing SSH Scanning Attacks: Using a Custom Service Object with the iptables Module "recent"

The method described in the previous section has a problem in that it permanently blocks access from any client when user mistypes their password several times. It is better to block access temporarily instead of permanently. The iptables module "recent" provides a way to do just that.

In this example, I only use the basic features of the "recent" module you can find more information about the available options for this module at the netfilter How-To page. http://netfilter.org/documentation/HOWTO/netfilter-extensions-HOWTO-3.shtml#ss3.16

To use this module, I create the following custom service object (see Section 5.3.6):

Figure 14.74. Custom Service Object Used to Define Parameters for the iptables Module "recent"

Custom Service Object Used to Define Parameters for the iptables Module "recent"

This module matches packets that have source address that is on the list of the module and was seen within the last 600 seconds. Now we can use this module in a rule:

Figure 14.75. Policy Rules Using the Custom Service Object "recent 10 min"

Policy Rules Using the Custom Service Object "recent 10 min"

These two rules translate into the following iptables script:

# Rule 0 (global)
echo "Rule 0 (global)"
$IPTABLES -A INPUT  -m recent  --rcheck --seconds 600  -j RULE_0
$IPTABLES -A RULE_0  -j LOG  --log-level info --log-prefix "RULE 0 -- DENY "
# Rule 1 (global)
echo "Rule 1 (global)"
$IPTABLES -A INPUT -p tcp -m tcp  --dport 22  -m state --state NEW  -j ACCEPT

Rule 0 blocks any packets that match module "recent," that is, that have source address that is on the module's list and were seen within last 10 minutes. Rule #1 simply permits SSH to the firewall. If everything goes well, no addresses should be on the module recent list, which means rule #0 does not match any packets and SSH access to the firewall is permitted by rule #1. However if any address is placed on the list of the module recent, rule #0 will block access to the firewall from that address for 10 min.

To place addresses of the attacking bots on the list I am using swatch just like in the previous chapter. The configuration file /root/.swatchrc looks like this:

# cat /root/.swatchrc

watchfor /sshd\[\d+\]: Failed password for invalid user (\S+) from (\S+)/
echo bold
exec "/root/swatch/block_ssh_scanner.sh $2"

watchfor /sshd\[\d+\]: Failed password for (\S+) from (\S+)/
echo bold
exec "/root/swatch/block_ssh_scanner.sh $2"

watchfor /sshd\[\d+\]: Did not receive identification string from (\S+)/
echo bold
exec "/root/swatch/block_ssh_scanner.sh $1"

watchfor /sshd\[\d+\]: Invalid user (\S+) from (\S+)/
echo bold
exec "/root/swatch/block_ssh_scanner.sh $2"

When swatch finds log entry that signals a potential SSH scan attack, it calls the script /root/swatch/block_ssh_scanner.sh:




test -f $ADDRDB || touch $ADDRDB

echo $addr >> $ADDRDB

# take last 10 entries from the list, sort and count them, then
# use addresses that appear 3 or more times. This means we'll block
# clients that make 3 mistakes for a short interval of time.
tail -10 $ADDRDB | sort | uniq -c | awk '$1>3 { print $2;}' | while read a
  echo "+$a" > /proc/net/xt_recent/DEFAULT

This script finds addresses that tried wrong password or non-existent user accounts three or more times and adds them to the list "DEFAULT" of the module recent. If such address tries to connect to the firewall one more time, it will be blocked by the rule #0 in the policy. However if they try 10 minutes later, they will be allowed to connect. This means if I mistype my password three times and get blocked, I can still log in 10 minutes later.

Finally, to start swatch and bring this all in motion, I use the following command:

nohup /usr/bin/swatch --daemon --pid-file=$PID_FILE --tail-file=/var/log/auth.log \
    --use-cpan-file-tail < /dev/null &

Swatch should monitor log file /var/log/auth.log on Debian and Ubuntu or /var/log/secure on RedHat, Fedora and other similar systems.


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