GEN:SID 1:368
Message ICMP PING BSDtype
Summary This event is generated when an ICMP echo request is made from a Berkeley Systems Development (BSD) host.
Impact Information gathering.  An ICMP echo request can determine if a host is active.
Detailed Information An ICMP echo request is used by the ping command to elicit an ICMP echo reply from a listening live host.  An echo request that originates from a host running a BSD TCP/IP networking stack such as FreeBSD, NetBSD, or OpenBSD, will contain a unique payload in the message request.  
Affected Systems All
Attack Scenarios An attacker may attempt to determine live hosts in a network prior to launching an attack.
Ease of Attack Simple
Corrective Action Block inbound ICMP echo requests.
Additional References Arachnids:
http://www.whitehats.com/info/IDS152
Rule References arachnids: 152