What Does ICMP Code 9 Mean For a Type 8 Packet?

Question

I am currently doing an assignment that requires us to generate ICMP packet with various types and codes. I have consulted the RFCs as well as various other place. Many of the types and codes I get make perfect sense; except for this one.

I somehow generated a packet that had a type of 8 (echo request) and a code of 9. The problem is that type 8 ICMP can only have a single code of 0. See the packet below as a hex dump from wireshark:

0000   a8 39 44 fa 14 e0 94 de 80 6b ab 74 08 00 45 00
0010   00 94 92 7b 40 00 28 01 fb 61 c0 a8 01 40 68 83
0020   9a 20 08 09 ac c9 4a 06 01 27 00 00 00 00 00 00
0030   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0040   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0050   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0060   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0070   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0080   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0090   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00a0   00 00

The reply can also be seen in the next hex dump. It is also strange because it is an ICMP echo reply with a code of 9. An ICMP echo can only have a code of 0.

0000   94 de 80 6b ab 74 a8 39 44 fa 14 e0 08 00 45 00
0010   00 94 98 af 00 00 39 01 24 2e 68 83 9a 20 c0 a8
0020   01 40 00 09 b4 c9 4a 06 01 27 00 00 00 00 00 00
0030   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0040   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0050   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0060   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0070   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0080   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0090   00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00a0   00 00

The question I am asking is how is this possible, what does code 9 mean? Can it be ignored or would it be worth investigating further?

I read in a previous answer that when ICMP tunneling comes in to play the data included in an echo is arbitrary, can this carry over to the header?

Thanks in advance!

Answer 1

If we look at RFC 792 (pages 14-15) it clearly only defines code 0 for ICMP types 0 and 8, this corresponds to IANA's definitions for reply and request. This means above all that 9 does not have a meaning, so it does not mean Communication with Destination Network is Administratively Prohibited as used in the Destination Unreachable message.

The next point is if any other code than 0 is allowed. RFC 792 is quite old and does not really follow the more strict SHOULD/MUST/MAY/... conventions. The remainder of the RFC only states sections like IF code = 0 ... or Code 0 may be received .... It actually never states what should be done if code is not zero. One might intuitively interpret it as the opposite of what is defined, so may *not* be received, another might say that if it's not explicitly disallowed it's okay. There is nothing in the RFC that even hints at what's right, so behavior will likely be very implementation-specific.

There is also RFC 1122 but this does not provide any extra specific limitation on the code field.

Now as for why the reply also uses 9, easiest bet is to look at implementation. A simple echo reply would just take the incoming packet, replace the type 8 with type 0, recalculate the checksum and send back. This is simpler and more efficient than building the reply from scratch, but of course leaves the 9 code.