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Chapter 3     Working with Snort Rules

  The last part is the rule options and contains a message that will be logged

along with the alert.

The next rule isn't quite as bad. It generates alerts for all captured ICMP packets.

Again, this rule is useful to find out if Snort is working.

alert icmp any any  > any any (msg: "ICMP Packet found";)

If you want to test the Snort machine, send a ping packet (which is basically ICMP

ECHO REQUEST packet on UNIX machines). Again, you can use this rule when you

install Snort to make sure that it is working well. As an example, send an ICMP packet to

your gateway address or some other host on the network using the following command:

ping 192.168.2.1

Note that 192.168.2.1 is the IP address of gateway/router or some other host on

the same network where the Snort machine is present. This command should be exe 

cuted on the machine where you installed Snort. The command can be used both on

UNIX and Microsoft Windows machines.

T I P 

I use a slightly modified version of this rule to continuously monitor multiple

Snort sensors just to make sure everybody is up and running. This rule is as follows:

alert icmp 192.168.1.4 any  > 192.168.1.1 any (msg: "HEARTBEAT";)

My Snort sensor IP address is 192.168.1.4 and gateway address is 192.168.1.1. I

run the following command through cron daemon on the Linux machine to trigger

this rule every 10 minutes.

ping  n 1 192.168.1.1

The command sends exactly one ICMP packet to the gateway machine. This packet

causes an alert entry to be created. If there is no alert every 10 minutes, there is

something wrong with the sensor.

3.3 CIDR

Classless Inter Domain Routing or CIDR is defined in RFC 1519. It was intended to

make better use of available Internet addresses by eliminating different classes (like

class A and class B). With the CIDR, you can define any number of bits in the netmask

field, which was not possible with class based networking where the number of bits

was fixed. Using CIDR, network addresses are written using the number of bits in the

netmask at the end of the IP address. For example, 192.168.1.0/24 defines a network

with network address 192.168.1.0 with 24 bits in the netmask. A netmask with 24 bits is