difference between DROPped pings and non existing hosts

Taylor, Grant gtaylor at riverviewtech.net
Wed Apr 27 00:12:21 CEST 2005

> I would like to know how ICMP distinguishes between DROPped pings and 
> non existing hosts. Both times you don´t get a reply from the 
> destination host but if it doesn´t reply because it doesn´t exist you 
> get the correct destination unreachable message if it drops the requests 
> for example with IPTables you get a timeout. And I haven´t a clue why 
> this is so.

Just because you get an ICMP Host Unreachable message does not mean that the host is not there or that the router can not reach it.  You should probably check the source IP on the ICMP message.  I've had a discussion (with little feed back) on this list to this very effect.  If the host that you are trying to reach wants to it can REJECT the packet with icmp-host-unreachable it can do so in lieu of DROPing the packet.  In this situation you do see an ICMP Host Unreachable message, just from the IP address of the host in question that you are trying to reach, thus you know that someone is playing with you trying to make it so that you don't think the host in question is there.

Grant. . . .

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