Книга: Iptables Tutorial 1.2.2
The parameter problem ICMP uses type 12 and it has 2 codes that it uses as well. Parameter problem messages are used to tell the sending host that the gateway or receiving host had problems understanding parts of the IP headers such as errors, or that some required options where missing.
The parameter problem type contains one special header, which is a pointer to the field that caused the error in the original packet, if the code is 0 that is. The following codes are available:
• Code 0 - IP header bad (catchall error) - This is a catchall error message as discussed just above. Together with the pointer, this code is used to point to which part of the IP header contained an error.
• Code 1 - Required options missing - If an IP option that is required is missing, this code is used to tell about it.