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Re: simple question as always..

  • From: Toshio Shigematsu
  • Date: Tue Mar 10 22:06:53 1998

From: Danny McPherson <[email protected]>
Subject: Re: simple question as always.. 
Date: Mon, 09 Mar 1998 18:33:56 -0700
Message-ID: <[email protected]>


tatsuya> could someone explain to me why I can ping but traceroute
tatsuya> seems to have problem reaching it.

> The combination of a firewall (permitting ICMP echo replies/requests, but 
> filtering your UDP traceroute probes) .. and a traceroute client that 
> doesn't correctly handle (IMO) the response (ICMP Type 3, Code 13: 
> Communication Administratively Prohibited).

tracert with WindowsNT4.0 uses ICMP ECHO messages for probing packets.
Hence one might go through the filtering router boxes. :)

tatsuya> I was told once that traceroute required each router on the
tatsuya> way to KNOW something whereas ping does not.
tatsuya> I just can not recall what this something is and I am
tatsuya> intend to find out.

traceroute ends its probing when it receives a ICMP port unreachable
message. Filtered by an IP gateway like cisco boxes, it receives 
Communication Administratively Prohibited messages and continues 
probing by incrementing IP TTL.

Toshio Shigematsu