Understanding the network.pdf - Back to Home

# If any of the hosts fail to respond the script will mail
# a notification message to you.
for i in $hosts;
do
$ping -c 3 $i >/dev/null 2>&1
if [ $? != 0 ]; then
echo $i is down|$mail your@mailaddress-here
fi
done
Although ping is not a glamorous tool in comparison to other network tools, it is
flexible (as you can see from the previous examples). When using ping as a
standalone tool or in a script, keep in mind that a reachability failure or loss of
packets does not automatically indicate that the host is down or there are physical
layer problems with one of your gateways or transport links. A failed ping attempt or
one with a high level of dropped packets can also indicate a routing problem or an
excessive level of network traffic.
Traceroute
To check the route to the destination host or to verify that all of the gateways in the
transit path are up, an additional application, commonly called traceroute, is used.
Traceroute, like ping, uses ICMP echo-request messages to trace the gateway path
one hop at a time. It accomplishes this by sending three ICMP messages with
variable TTL counters. The first ICMP message, which is sent to the host's default
gateway, has a TTL of 1. The message is received by the gateway, and it decreases
the TTL counter to zero and returns an ICMP host unreachable notification message.
Then traceroute sends an ICMP message with a TTL of 2, to reach the next gateway.
This process continues, incrementing the TTL by one, each time a successful ICMP
host unreachable message is received. This continues until the final destination host
is reached. This occurs when the traceroute program receives an ICMP
echo_response from the destination host.
As traceroute maps the transit path, it displays the gateway (hostname or address)
and the round-trip delivery time averages for the ICMP messages sent to each
gateway. The output from a UNIX-based traceroute implementation follows:
%traceroute www.yahoo.com