difference between DROPped pings and non existing hosts

difference between DROPped pings and non existing hosts

[Daniel Lopes]

Hi,


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.

Re: difference between DROPped pings and non existing hosts

[Tobias DiPasquale]

On 4/26/05, Daniel Lopes <lopsch@lopsch.com> wrote:
> 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.

In the case where you get a destination unreachable message back, its
the router that is responsible for the network on which the machine
you are trying to ping that is responding with that message. When ICMP
is dropped, the packet makes it to the host and thus the router does
not generate a destination unreachable message to send back to you.

-- 
[ Tobias DiPasquale ]
0x636f6465736c696e67657240676d61696c2e636f6d

Re: difference between DROPped pings and non existing hosts

[Daniel Lopes]

Tobias DiPasquale schrieb:
> On 4/26/05, Daniel Lopes <lopsch@lopsch.com> wrote:
> 
>>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.
> 
> 
> In the case where you get a destination unreachable message back, its
> the router that is responsible for the network on which the machine
> you are trying to ping that is responding with that message. When ICMP
> is dropped, the packet makes it to the host and thus the router does
> not generate a destination unreachable message to send back to you.
> 
Hmm I forgot the hardware address. In the case the router can´t do a 
address resolution he generates a ICMP error message because he won´t be 
able to deliver the packet is that right?
Thank´s so far for the reply :).

Re: difference between DROPped pings and non existing hosts

[Taylor, Grant]

> 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. . . .