Книга: Iptables Tutorial 1.2.2

ICMP Echo Request/Reply

ICMP Echo Request/Reply

I have chosen to speak about both the reply and the request of the ICMP echo packets here since they are so closely related to each other. The first difference is that the echo request is type 8, while echo reply is type 0. When a host receives a type 8, it replies with a type 0.

When the reply is sent, the source and destination addresses switch places as well. After both of those changes has been done, the checksum is recomputed, and the reply is sent. There is only one code for both of these types, they are always set to 0.

? Identifier - This is set in the request packet, and echoed back in the reply, to be able to keep different ping requests and replies together.

? Sequence number - The sequence number for each host, generally this starts at 1 and is incremented by 1 for each packet.

The packets also contains a data part. Per default, the data part is generally empty, but it can contain a userspecified amount of random data.

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