[ale] Can ping but cannot traceroute

[Jason Day]

On Wed, Mar 05, 2003 at 08:27:57AM -0500, Mike Millson wrote:
> I am trying to access the rational site, www.rational.com, and am
> getting the "The operation timed out when attempting to connect to
> www.rational.com" error. I can ping the site, but when I do a
> traceroute, it dies at hop 14 per below. How is it that I am able to
> ping the site but not compete a traceroute? Isn't ping UDP, and isn't
> UDP routed? Any ideas on what the problem is or how I can troubleshoot
> this?

No, ping is not UDP.  Ping works by sending an ICMP ECHO_REQUEST packet
to the given host.  This elicits an ICMP ECHO_RESPONSE (aka "pong") from
the remote host.

Traceroute works by using the IP protocol's "time to live" field.  This
field specifies the maximum number of times the packet may be forwarded
by a gateway.  When you traceroute a host, the traceroute program
constructs a packet (usually UDP by default, but it can also use ICMP
ECHO packets) and sets its TTL field to 1.  The first gateway that gets
the packet will decrement its TTL field, see that it is zero, and return
an ICMP TIME_EXCEEDED response to the originating host.  Traceroute then
prints out the IP address of the gateway that returned this message,
creates a new packet with TTL 2, and so on, until the TIME_EXCEEDED
response comes from the host you're looking for, or some max number of
hops is reached.

The important thing here is that ping and traceroute use different kinds
of packets, and each gateway along the way is capable of filtering each
of these packets.  Some may choose to let ping requests through and deny
traceroute requests.  Or some may block all UDP packets, which would
foil traceroute.

You can try telling traceroute to use ICPM ECHO packets by using the -I
switch.  You might have more luck that way.

HTH,
Jason
-- 
Jason Day                                       jasonday at
http://jasonday.home.att.net                    worldnet dot att dot net
 
"Of course I'm paranoid, everyone is trying to kill me."
    -- Weyoun-6, Star Trek: Deep Space 9
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