ICMP Type 3 Destination Unreachable
ICMP — Internet Control Message Protocol
An ICMP Type 3 Destination Unreachable message explains why an IPv4 packet could not reach a network, host, protocol, or port.
If, in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, the destination host may send a destination unreachable message to the source host.
Source RFC 792.
Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
┌───────────────┬───────────────┬───────────────────────────────┐
│ Type │ Code │ Checksum │
├───────────────┴───────────────┴───────────────────────────────┤
│ unused │
├───────────────────────────────────────────────────────────────┤
│ Internet Header + 64 bits of Original Data Datagram │
└───────────────────────────────────────────────────────────────┘
Codes
| Code | Name | Sent By | Meaning |
|---|---|---|---|
| 0 | Net Unreachable | Router | No route to destination network |
| 1 | Host Unreachable | Router | Route exists but host not responding |
| 2 | Protocol Unreachable | Destination host | IP protocol not supported |
| 3 | Port Unreachable | Destination host | UDP port not listening |
| 4 | Fragmentation Needed | Router | Packet too big, DF bit set |
| 9 | Net Administratively Prohibited | Router | ACL/policy blocks network |
| 10 | Host Administratively Prohibited | Router | ACL/policy blocks host |
| 13 | Communication Administratively Prohibited | Router or firewall | Administrative filtering blocks traffic |