ICMP Destination Unreachable
The Destination Unreachable message is an ICMP message which is generated by the host or its inbound gatewayhttp://tools.ietf.org/html/rfc792 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 |
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18 |
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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.
References
- RFC 792
- RFC 1122
- RFC 1812
See also
es:ICMP Destination Unreachable
pl:ICMP Destination Unreachable
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