[CentOS] internet connection tester script

Sat Jan 29 00:14:37 UTC 2011
Nico Kadel-Garcia <nkadel at gmail.com>

On Fri, Jan 28, 2011 at 11:19 AM,  <cpolish at surewest.net> wrote:
> Nico Kadel-Garcia wrote:
>> Yup. That's why it's common to drop at external firewalls and blocked
>> by NAT from reaching inside your network, to protect less thoroughly
>> protected and critical hosts from distributed denial of service (DDOS)
>>  such as the now classic "ping flood" attack. There is generally no
>> good reason to allow external ICMP packets into your local network,
>> except maybe to allow an external monitoring system or VPN connection
>> to verify the presence of a few exposed hosts.

>    Many network security devices block all ICMP messages for
>    perceived security benefits, including the errors that are
>    necessary for the proper operation of PMTUD. This can result in
>    connections that complete the TCP three-way handshake correctly,
>    but then hang when data is transferred. This state is referred
>    to as a black hole connection.

I'm not sure how the lack of PMTUD is worked around: given the number
of environments for which blocking ICMP entirely is standard practice,
it can't be *too* much of an issue, can it? I'm deducing, without
enough proof, that the upstream ISP's routers have to handle ICMP from
the other routers they speak with. But other than that, why should it
be supported again? Do household network devices really need to do the
path optimization supported by this? They're not multi-homed: perhaps
the single gateway that they use helps avoid the issue?