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How can ping know if my host is down
The Next CEO of Stack OverflowHow does ping know that my packets are filtered?Max ping response time?How can I set a short timeout with the ping command?Ping a Specific PortAndroid/Linux Ping sends another packet before packet timeoutDifference between ping time and time for first ACK answerSending Ping at fixed packet size as jumbo frame is not working in linuxCan't get an nping response through an Alcatel modem - but can get pingCheck a machine's up or down status without using pingPing packet loss even though the target is reachable. Does this mean packets are eventually getting through?
When I ping
a server there are two kinds of result I might get:
- Timeout
- Host is down message
How does the ping
command know if a host is down? In both cases, the host does not send any response to the ping packet, so how can ping
tell the difference?
ping
add a comment |
When I ping
a server there are two kinds of result I might get:
- Timeout
- Host is down message
How does the ping
command know if a host is down? In both cases, the host does not send any response to the ping packet, so how can ping
tell the difference?
ping
add a comment |
When I ping
a server there are two kinds of result I might get:
- Timeout
- Host is down message
How does the ping
command know if a host is down? In both cases, the host does not send any response to the ping packet, so how can ping
tell the difference?
ping
When I ping
a server there are two kinds of result I might get:
- Timeout
- Host is down message
How does the ping
command know if a host is down? In both cases, the host does not send any response to the ping packet, so how can ping
tell the difference?
ping
ping
edited Mar 21 at 13:11
Mark Amery
3041219
3041219
asked Mar 21 at 8:59
Bob5421Bob5421
17117
17117
add a comment |
add a comment |
3 Answers
3
active
oldest
votes
Host is down message:
This indicates that you don't know a route to the desired destination, or a remote router reports that it has no route to the destination.
Timeout:
Indicates the absense of Echo Reply messages. No package were received within the default time.
add a comment |
Differences between responses are not actually determined by ICMP itself but rather indirectly.
ICMP can distingush between the following:
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
But it does so with other network resources. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
If, according to the information in the gateway's routing tables, destination network is unreachable, (e.g., the distance to the network is infinity), the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. It is the Gateways in these networks that can send destination unreachable messages to the source host when the destination host is unreachable, so it's not actually ICMP doing the determinations.
In the case that in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, then the destination host may send a 'destination unreachable' message to the source host.
Finally, if a datagram must be fragmented to be forwarded by a gateway yet the 'Do not Fragment' flag is on, the gateway will discard the datagram and will return a 'destination unreachable' message.
Now to difference between the 2 separate cases: Request Timed Out means that no Echo Reply messages were received within the set time. This can be due to many different causes: ARP request failure, network congestion, packet filtering, routing error, or a even silent discard.
When you get a Reply From [IP address]: 'Destination Host Unreachable,' then the problem occurred at/after a remote router, whose address is indicated by the [IP address]. So it's a router telling you that there is a problem between it and the destination address.
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
|
show 11 more comments
You cant ping if the computer isnt connected to the internet because by pinging you actually request the ip of the host .
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
Host is down message:
This indicates that you don't know a route to the desired destination, or a remote router reports that it has no route to the destination.
Timeout:
Indicates the absense of Echo Reply messages. No package were received within the default time.
add a comment |
Host is down message:
This indicates that you don't know a route to the desired destination, or a remote router reports that it has no route to the destination.
Timeout:
Indicates the absense of Echo Reply messages. No package were received within the default time.
add a comment |
Host is down message:
This indicates that you don't know a route to the desired destination, or a remote router reports that it has no route to the destination.
Timeout:
Indicates the absense of Echo Reply messages. No package were received within the default time.
Host is down message:
This indicates that you don't know a route to the desired destination, or a remote router reports that it has no route to the destination.
Timeout:
Indicates the absense of Echo Reply messages. No package were received within the default time.
answered Mar 21 at 13:41
SantiCartaSantiCarta
364
364
add a comment |
add a comment |
Differences between responses are not actually determined by ICMP itself but rather indirectly.
ICMP can distingush between the following:
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
But it does so with other network resources. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
If, according to the information in the gateway's routing tables, destination network is unreachable, (e.g., the distance to the network is infinity), the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. It is the Gateways in these networks that can send destination unreachable messages to the source host when the destination host is unreachable, so it's not actually ICMP doing the determinations.
In the case that in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, then the destination host may send a 'destination unreachable' message to the source host.
Finally, if a datagram must be fragmented to be forwarded by a gateway yet the 'Do not Fragment' flag is on, the gateway will discard the datagram and will return a 'destination unreachable' message.
Now to difference between the 2 separate cases: Request Timed Out means that no Echo Reply messages were received within the set time. This can be due to many different causes: ARP request failure, network congestion, packet filtering, routing error, or a even silent discard.
When you get a Reply From [IP address]: 'Destination Host Unreachable,' then the problem occurred at/after a remote router, whose address is indicated by the [IP address]. So it's a router telling you that there is a problem between it and the destination address.
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
|
show 11 more comments
Differences between responses are not actually determined by ICMP itself but rather indirectly.
ICMP can distingush between the following:
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
But it does so with other network resources. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
If, according to the information in the gateway's routing tables, destination network is unreachable, (e.g., the distance to the network is infinity), the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. It is the Gateways in these networks that can send destination unreachable messages to the source host when the destination host is unreachable, so it's not actually ICMP doing the determinations.
In the case that in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, then the destination host may send a 'destination unreachable' message to the source host.
Finally, if a datagram must be fragmented to be forwarded by a gateway yet the 'Do not Fragment' flag is on, the gateway will discard the datagram and will return a 'destination unreachable' message.
Now to difference between the 2 separate cases: Request Timed Out means that no Echo Reply messages were received within the set time. This can be due to many different causes: ARP request failure, network congestion, packet filtering, routing error, or a even silent discard.
When you get a Reply From [IP address]: 'Destination Host Unreachable,' then the problem occurred at/after a remote router, whose address is indicated by the [IP address]. So it's a router telling you that there is a problem between it and the destination address.
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
|
show 11 more comments
Differences between responses are not actually determined by ICMP itself but rather indirectly.
ICMP can distingush between the following:
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
But it does so with other network resources. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
If, according to the information in the gateway's routing tables, destination network is unreachable, (e.g., the distance to the network is infinity), the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. It is the Gateways in these networks that can send destination unreachable messages to the source host when the destination host is unreachable, so it's not actually ICMP doing the determinations.
In the case that in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, then the destination host may send a 'destination unreachable' message to the source host.
Finally, if a datagram must be fragmented to be forwarded by a gateway yet the 'Do not Fragment' flag is on, the gateway will discard the datagram and will return a 'destination unreachable' message.
Now to difference between the 2 separate cases: Request Timed Out means that no Echo Reply messages were received within the set time. This can be due to many different causes: ARP request failure, network congestion, packet filtering, routing error, or a even silent discard.
When you get a Reply From [IP address]: 'Destination Host Unreachable,' then the problem occurred at/after a remote router, whose address is indicated by the [IP address]. So it's a router telling you that there is a problem between it and the destination address.
Differences between responses are not actually determined by ICMP itself but rather indirectly.
ICMP can distingush between the following:
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable;
3 = port unreachable;
4 = fragmentation needed and DF set;
5 = source route failed.
But it does so with other network resources. Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and 3 may be received from a host.
If, according to the information in the gateway's routing tables, destination network is unreachable, (e.g., the distance to the network is infinity), the gateway may send a destination unreachable message to the internet source host of the datagram. In addition, in some networks, the gateway may be able to determine if the internet destination host is unreachable. It is the Gateways in these networks that can send destination unreachable messages to the source host when the destination host is unreachable, so it's not actually ICMP doing the determinations.
In the case that in the destination host, the IP module cannot deliver the datagram because the indicated protocol module or process port is not active, then the destination host may send a 'destination unreachable' message to the source host.
Finally, if a datagram must be fragmented to be forwarded by a gateway yet the 'Do not Fragment' flag is on, the gateway will discard the datagram and will return a 'destination unreachable' message.
Now to difference between the 2 separate cases: Request Timed Out means that no Echo Reply messages were received within the set time. This can be due to many different causes: ARP request failure, network congestion, packet filtering, routing error, or a even silent discard.
When you get a Reply From [IP address]: 'Destination Host Unreachable,' then the problem occurred at/after a remote router, whose address is indicated by the [IP address]. So it's a router telling you that there is a problem between it and the destination address.
answered Mar 21 at 9:32
OvermindOvermind
1,233514
1,233514
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
|
show 11 more comments
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
3
3
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
no router or other device needs to be involved. Both Windows and Linux produce "host unreachable" if the host is in a local subnet and there is no ARP entry.
– Oh My Goodness
Mar 21 at 15:53
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
If a host is unreachable it cannot produce a receivable-by-initiator answer.
– Overmind
Mar 22 at 7:25
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
The pinging host produces the message. Obviously the destination cannot.
– Oh My Goodness
Mar 22 at 7:34
That comes form the gateway.
– Overmind
Mar 22 at 7:40
That comes form the gateway.
– Overmind
Mar 22 at 7:40
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
No. "Gateway" is a layer 3 construct. There is no routing, and thus no gateway involved at all when pinging a host on the local subnet.
– Oh My Goodness
Mar 22 at 7:53
|
show 11 more comments
You cant ping if the computer isnt connected to the internet because by pinging you actually request the ip of the host .
add a comment |
You cant ping if the computer isnt connected to the internet because by pinging you actually request the ip of the host .
add a comment |
You cant ping if the computer isnt connected to the internet because by pinging you actually request the ip of the host .
You cant ping if the computer isnt connected to the internet because by pinging you actually request the ip of the host .
answered Mar 22 at 12:31
AltairAltair
1
1
add a comment |
add a comment |
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