difference between DROPped pings and non existing hosts
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.
Quoted 6 lines · Daniel Lopes
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.
Signature · 2 lines
[ Tobias DiPasquale ] 0x636f6465736c696e67657240676d61696c2e636f6d
Quoted 10 lines · Tobias DiPasquale
Quoted 6 lines · Daniel Lopes
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.
Quoted 6 lines
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 :).
Quoted 1 line
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. . . .