Double icmp echo-reply

[bouhdada]

Hello,

I have noticed a strange behaviour for the icmp echo-reply on our firewall. When I ping a host X I am getting 2 echo-reply for one ping request. the host X have a NAT IP Y and the routing and spoofing for that host is correctly configured. I have done the same test for another system with the same configuration and environment and I am getting only one echo-reply for each packet which is normal.

I am pinging from the firewall (unix):

bash-2.05# ping 10.60.15.51
10.60.15.51 is alive
bash-2.05# 10.63.255.52 -> 10.60.15.51 ICMP Echo request (ID: 10487 Sequence nu mber: 0)
10.60.15.51 -> 10.63.255.52 ICMP Echo reply (ID: 10487 Sequence number: 0)
10.60.15.51 -> 10.63.255.52 ICMP Echo reply (ID: 10487 Sequence number: 0)
10.63.255.52 -> 10.60.15.51 ICMP Echo request (ID: 10487 Sequence number: 0)
10.60.15.51 -> 10.63.255.52 ICMP Echo reply (ID: 10487 Sequence number: 0)
10.60.15.51 -> 10.63.255.52 ICMP Echo reply (ID: 10487 Sequence number: 0)


For the 2nd test:

bash-2.05# ping 10.32.36.44
10.32.36.44 is alive
bash-2.05# 10.63.255.52 -> 10.32.36.44 ICMP Echo request (ID: 10489 Sequence nu mber: 0)
10.32.36.44 -> 10.63.255.52 ICMP Echo reply (ID: 10489 Sequence number: 0)
10.63.255.52 -> 10.32.36.44 ICMP Echo request (ID: 10489 Sequence number: 0)
10.32.36.44 -> 10.63.255.52 ICMP Echo reply (ID: 10489 Sequence number: 0)


I have no explanation for this icmp behaviour! Any ideas?

Greetings,
Jalal