ICMP unreachable

Internet Control Message Protocol (ICMP)is used to communicate to the source host if any errors occured during the routing of packets.

An ICMP unreachable message is generated and sent back to the sender to inform it that the destination is unreachable.

The IP header plus the first 8 bytes of the original data is sent back to the source host, below some situations where this behavior is seen:

If the gateway has no next hop information to match the destination address on the IP packet or the distance to such network is infinte(i.e. 255) the gateway may send a destination unreachable message to the source host. Also, when fragmentation is needed for the do not fragment flag is set in the packet an ICMP unreachable is going to be sent back to the source host.

Below some ICMP code types:

Type Name     Reference
---- -------------------------  ---------
  0 Echo Reply     [RFC792]
  1 Unassigned        [JBP]
  2 Unassigned        [JBP]
  3 Destination Unreachable    [RFC792]
  4 Source Quench      [RFC792]
  5 Redirect     [RFC792]

In this case the ICMP code type 3 is used on the returning packet.Below an screenshot of a packet capture

where the ICMP type code 3 is being used by the gateway.