Книга: Iptables Tutorial 1.2.2

TTL equals 0

TTL equals 0

The TTL equals 0 ICMP type is also known as Time Exceeded Message and has type 11 set to it, and has 2 ICMP codes available. If the TTL field reaches 0 during transit through a gateway or fragment reassembly on the destination host, the packet must be discarded. To notify the sending host of this problem, we can send a TTL equals 0 ICMP packet. The sender can then raise the TTL of outgoing packets to this destination if necessary.

The packet only contains the extra data portion of the packet. The data field contains the Internet header plus 64 bits of the data of the IP packet, so that the other end may match the packet to the proper process. As previously mentioned, the TTL equals 0 type can have two codes.

• Code 0 - TTL equals 0 during transit - This is sent to the sending host if the original packet TTL reached 0 when it was forwarded by a gateway.

• Code 1 - TTL equals 0 during reassembly - This is sent if the original packet was fragmented, and TTL reached 0 during reassembly of the fragments. This code should only be sent from the destination host.

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