Why a host not ping-able?

Why a host not ping-able?

[Woody Wu]

Hi, list

I am not sure this has something with kernel.  But the system I just
generated cannot be reached from ping.  It can ping outside, but if I
ping it from outside, I just get "Destination Host Unreachable".

I think there is not firewall in between, and the two host in the test
are in the same ethernet sub network.  Is that possible the strange
behavior caused by somehow misconfigured kernel?

-- 
woody
I can't go back to yesterday - because I was a different person then.

Re: Why a host not ping-able?

[Alan Cox]

On Tue, 4 Dec 2012 06:17:25 +0000 (UTC)
Woody Wu <narkewoody@gmail.com> wrote:

> Hi, list
> 
> I am not sure this has something with kernel.  But the system I just
> generated cannot be reached from ping.  It can ping outside, but if I
> ping it from outside, I just get "Destination Host Unreachable".
> 
> I think there is not firewall in between, and the two host in the test
> are in the same ethernet sub network.  Is that possible the strange
> behavior caused by somehow misconfigured kernel?

Very unlikely. More likely you have a routing problem somewhere. tcpdump
or similar network monitoring tools are probably what you need.

Re: Why a host not ping-able?

[Rob Landley]

On 12/04/2012 12:17:25 AM, Woody Wu wrote:
> Hi, list
> 
> I am not sure this has something with kernel.

It doesn't.

> But the system I just
> generated cannot be reached from ping.  It can ping outside, but if I
> ping it from outside, I just get "Destination Host Unreachable".

Some distributions' default firewall rules respond to icmp packets with  
a host unreachable packet. This is a system/network administration  
thing (iptables) and nothing to do with kernel development.

> I think there is not firewall in between,

There's a firewall  built into linux, read the man page for the  
"iptables" command. The "iptables-save" command dumps your entire  
current ruleset to stdout so you can see what's in there. (You might  
have to run it as root, I forget.)

https://help.ubuntu.com/community/IptablesHowTo

Rob

Re: Why a host not ping-able?

[Woody Wu]

On 2012-12-04, Rob Landley <rob@landley.net> wrote:
> On 12/04/2012 12:17:25 AM, Woody Wu wrote:
>> Hi, list
>> 
>> I am not sure this has something with kernel.
>
> It doesn't.
>
>> But the system I just
>> generated cannot be reached from ping.  It can ping outside, but if I
>> ping it from outside, I just get "Destination Host Unreachable".
>
> Some distributions' default firewall rules respond to icmp packets with  
> a host unreachable packet. This is a system/network administration  
> thing (iptables) and nothing to do with kernel development.
>
>> I think there is not firewall in between,
>
> There's a firewall  built into linux, read the man page for the  
> "iptables" command. The "iptables-save" command dumps your entire  
> current ruleset to stdout so you can see what's in there. (You might  
> have to run it as root, I forget.)
>
> https://help.ubuntu.com/community/IptablesHowTo
>

This 'distribution' I am running is a tailor linux 3.4.19 on an embedded
system (Samsung S3C2410).  Because this is a very small system, I
believe I did not add any ip-table options in the kernel. And, I did not
install any user space firware packet for it.  So maybe this is really a
firewall play in between.

Now I run tcpdump on the target system with 'tcpdump -i eth0'. Then when
I ping it from another Linux box, I see nothing from the tcpdump output.
Does this confirm there is a firewall blocked the ping packets?

Thanks in advance.


-- 
woody
I can't go back to yesterday - because I was a different person then.