ICMP Destination Unreachable
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The Destination Unreachable message is an ICMP message which is generated by the host or its inbound gateway[1] to inform the client that the destination is unreachable for some reason. A Destination Unreachable message may be generated as a result of a TCP, UDP or another ICMP transmission. Unreachable TCP ports notably respond with TCP RST rather than a Destination Unreachable code 3 as might be expected.
The error will not be generated if the original datagram has a multicast destination address. Reasons for this message may include: the physical connection to the host does not exist (distance is infinite); the indicated protocol or port is not active; the data must be fragmented but the 'don't fragment' flag is on.
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Type = 3 | Code | Header Checksum | |||||||||||||||||||||||||||||
| Empty | Next-Hop MTU | ||||||||||||||||||||||||||||||
| IP Header + First 8 Bytes of Original Datagram's Data | |||||||||||||||||||||||||||||||
The type field (bits 0-7) must be set to 3. The code field (bits 8-15) is used to specify the type of error, and can be any of the following:
| Code | Description |
|---|---|
| 0 | Network unreachable error. |
| 1 | Host unreachable error. |
| 2 | Protocol unreachable error (the designated transport protocol is not supported). |
| 3 | Port unreachable error (the designated protocol is unable to inform the host of the incoming message). |
| 4 | The datagram is too big. Packet fragmentation is required but the 'don't fragment' (DF) flag is on. |
| 5 | Source route failed error. |
| 6 | Destination network unknown error. |
| 7 | Destination host unknown error. |
| 8 | Source host isolated error (military use only). |
| 9 | The destination network is administratively prohibited. |
| 10 | The destination host is administratively prohibited. |
| 11 | The network is unreachable for Type Of Service. |
| 12 | The host is unreachable for Type Of Service. |
| 13 | Communication administratively prohibited (administrative filtering prevents packet from being forwarded). |
| 14 | Host precedence violation (indicates the requested precedence is not permitted for the combination of host or network and port). |
| 15 | Precedence cutoff in effect (precedence of datagram is below the level set by the network administrators). |
The Next-Hop MTU field (bits 48-63) contains the MTU of the next-hop network if a code 4 error occurs. The additional data (bits 64-95) is included to allow the client to match the reply with the request that caused the destination unreachable reply.
