ICMP messages are used for a basic kind of error reporting between host to host, or host to gateway. Between gateway to gateway, a protocol called Gateway to Gateway protocol (GGP) should normally be used for error reporting. As we have already discussed, the IP protocol is not designed for perfect error handling, but ICMP messages solves some parts of these problems. The big problem from one standpoint is that the headers of the ICMP messages are rather complicated, and differ a little bit from message to message. However, this will not be a big problem from a filtering standpoint most of the time.

The basic form is that the message contains the standard IP header, type, code and a checksum. All ICMP messages contains these fields. The type specifies what kind of error or reply message this packet is, such as for example destination unreachable, echo, echo reply, or redirect message. The code field specifies more information, if necessary. If the packet is of type destination unreachable, there are several possible values on this code field such as network unreachable, host unreachable, or port unreachable. The checksum is simply a checksum for the whole packet.

As you may have noticed, I mentioned the IP header explicitly for the ICMP packet. This was done since the actual IP header is an integral part of the ICMP packet, and the ICMP protocol lives on the same level as the IP protocol in a sense. ICMP does use the IP protocol as if it where a higher level protocol, but at the same time not. ICMP is an integral part of IP, and ICMP must be implemented in every IP implementation.

As already explained, the headers differs a little bit from ICMP type to ICMP type. Most of the ICMP types are possible to group by their headers. Because of this, we will discuss the basic header form first, and then look at the specifics for each group of types that should be discussed.

All packets contain some basic values from the IP headers discussed previously in this chapter. The headers have previously been discussed at some length, so this is just a short listing of the headers, with a few notes about them.

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● Version - This should always be set to 4.

● Internet Header Length - The length of the header in 32 bit words.

● Type of Service - See above. This should be set to 0, as this is the only legit setting according to RFC 792 - Internet Control Message Protocol.

● Total Length - Total length of the header and data portion of the packet, counted in octets.

● Identification , Flags and Fragment offsets - Ripped from the IP protocol.

● Time To Live - How many hops this packet will survive.

● Protocol - which version of ICMP is being used (should always be 1).

● Header Checksum - See the IP explanation.

● Source Address - The source address from whom the packet was sent. This is not entirely true, since the packet can have another source address, than that which is located on the machine in question. The ICMP types that can have this effect will be noted if so.

● Destination Address - The destination address of the packet

There are also a couple of new headers that are used by all of the ICMP types. The new headers are as follows, this time with a few more notes about them:

● Type - The type field contains the ICMP type of the packet. This is always different from ICMP type to type. For example ICMP Destination Unreachable packets will have a type 3 set to it. For a complete listing of the different ICMP types, see the ICMP types appendix. This field contains 8 bits total.

● Code - All ICMP types can contain different codes as well. Some types only have a single code, while others have several codes that they can use. For example, the ICMP Destination Unreachable (type 3) can have at least code 0, 1, 2, 3, 4 or 5 set. Each code has a different meaning in that context then. For a complete listing of the different codes, see the ICMP types appendix. This field is 8 bits in length, total. We will discuss the different codes a little bit more in detail for each type later on in this section.

● Checksum - The Checksum is a 16 bit field containing a one's complement of the ones complement of the headers starting with the ICMP type and down. While calculating the checksum, the checksum field should be set to zero.

At this point the headers for the different packets start to look different also. We will describe the most common ICMP Types one by one, with a brief discussion of its headers and different codes.

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.

The first three fields seen in the image are the same as previously described. The Destination Unreachable type has 16 basic codes that can be used, as seen below in the list.

● Code 0 - Network unreachable - Tells you if a specific network is currently unreachable.

● Code 1 - Host unreachable - Tells you if a specific host is currently unreachable.

● Code 2 - Protocol unreachable - This code tells you if a specific protocol (tcp, udp, etc) can not be reached at the moment.

● Code 3 - Port unreachable - If a port (ssh, http, ftp-data, etc) is not reachable, you will get this message.

● Code 4 - Fragmentation needed and DF set - If a packet needs to be fragmented to be delivered, but the Do not fragment bit is set in the packet, the gateway will return this message.

● Code 5 - Source route failed - If a source route failed for some reason, this message is returned.

● Code 6 - Destination network unknown - If there is no route to a specific network, this message is returned.

● Code 7 - Destination host unknown - If there is no route to a specific host, this message is returned.

● Code 8 - Source host isolated (obsolete) - If a host is isolated, this message should be returned. This code is obsoleted today.

● Code 9 - Destination network administratively prohibited - If a network was blocked at a gateway and your packet was unable to reach it because of this, you should get this ICMP code back.