Re: host availability

In article <ip-dnZhnhJmPxFnenZ2dnUVZ_tCdnZ2d@xxxxxxxxxxx>,
zee <Nospam@xxxxxxxxxx> wrote:

>"Walter Roberson" <roberson@xxxxxxxxxxxx> wrote in message
>news:2cYwf.66317$tl.13822@xxxxxxxxxxx
>> In article <8eednQAOHqeY3lnenZ2dnUVZ_sednZ2d@xxxxxxxxxxx>,
>> zee <Nospam@xxxxxxxxxx> wrote:
>>>I would like to get a perl, python script that would do the following:

>>>Tell me when a host from a list of hosts went up or down, such as
>>>google.com, my default gateway, comcast's mail server etc.

>> Why does it matter to you whether comcast's mail server was up or not,
>> if you can't -reach- the mail server?

>> The answer to your question as stated is "No, that cannot be done."

>Oh, come on. I can't do a local log into /var/log/messages if I can't ping
>the mail server at comcast?

Review what you asked for. You asked for the script to tell you whether
a host was "up or down". ping can't tell you that.

If you don't get an answer to a ping request, then the meaning is...
that you didn't get an answer to the ping request.

Firewalls, network congestion, committed access rates, automated
protections against Denial of Service attacks, routing problems, dead
router, dead switch, faulty power supplies, device in the middle
happened to be rebooting, packet get corrupted in transmission and was
discarded, dns servers got poisoned, dns servers got corrupted data,
load balancing mispredicted availability, rogue system on the net sent
you an icmp redirect and your equipment -believed- it, target host
packet buffers got full and host discarded packet, remote host admin
happened to reset the interface right then, remote system got trojaned
and the replacement program ignores pings, remote system has an
operating system bug in its network stack and can't send packets to
-your- IP, remote system has a SCSI bug glitch and goes into a strange
state, NIC goes bad... and more.

I've had nearly all of those happen to me at various times. And as a
systems administrator, there have been times when I've sat down at a
system console, and it has taken me an hour or more to figure out
whether the system is down or whether it's just not talking to me. If
I, right *at* the equipment, cannot tell whether the system is up, then
how the $#@ is YOUR system going to remotely figure out whether the
system is up or not?


A successful ping tells you that the network able to pass icmp
echo and echo reply packets, at least as far along as the device
that answered the ping (which could be anywhere from your -own-
network on outward, since the response could have been forged
or could have been generated by a different device.)

Unfortunately, an unsuccessful ping doesn't tell you that
the remote system is down. And the success or lack of success
of the ping doesn't tell you whether you will be able to
send your email message or do your google search. comcast
has -several- mail servers, and google has -many- systems answering
queries, ao failure of -one- IP to answer doesn't tell you
that the service is kaput. Besides, they might just be ignoring
*your* machine.


Here's something for you think about: I have tested the Windows
program Look@Lan, which will periodically do ping sweeps over
pre-assigned IP ranges, and will do some nice reporting
of what it finds (or doesn't.) Complete with a popup to indicate
which systems became accessible or inaccessible on the last sweep.

What the results showed was that there were some systems that
pretty much never were detected as going down, but that at least
a third of the systems donen't a answer at in
.

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