[Gllug] OT: Traceroute

[Ian Baillie]

Thank you for the references to the man page and google, I was actually
checking to see if any of the 'experts' had any further information, and
would like to thank you all for the information provided.


Ian
 


On Tue, 2003-03-18 at 16:36, Bruce Richardson wrote:
> On Tue, Mar 18, 2003 at 03:19:35PM +0000, Ian wrote:
> > Thanks to both of you.  It did actually reach 30 hops :-(
> > 
> > Strange though, because I can pick up my mail, so it is reaching the
> > mail server...
> > 
> > Can anyone explain this?
> > 
> 
> Tethys explained it perfectly well.  The server is not unreachable, nor
> are the routers connecting you to it (Andrew is in error, there).  Some
> of those routers are not sending back the ICMP messages that traceroute
> requires to compile its list.
> 
> Traceroute is a clever hack that relies on the fact that ip packets have
> a TTL (Time To Live) property.  As an ip packet travels across the
> Internet, each router that it passes through decrements the packet's Hop
> Count by (at least and almost always just) one.  Routers are supposed to
> drop packets when the TTL reaches 0.  They should then send an ICMP
> message back, reporting the failure of the packet.
> 
> The TTL is normally set to something reasonably high (the default varies
> from OS to OS and is usually configurable) but traceroute sends out a
> sequence of packets to the specified destination, the first one having a
> ttl of 1, incrementing the ttl with each subsequent packet.  This means
> that each packet fails at an incrementally further-out point along the
> path to the destination.  As traceroute receives each ICMP error report,
> it performs a DNS lookup on the router that sent it.  This is how the
> traced route is discovered.
> 
> Now, if any of those routers don't send back a reply (in good time), you
> start seeing those * characters.  So they don't mean 'unreachable', they
> just mean that no ICMP messages are coming back.  As Tethys explained,
> this is usually because of a) a firewall blocking ICMP messages or b)
> the router being configured not to send any ICMP messages.
> 
> Now, I've been patient but this is explained on the traceroute man page.
> Not at the same length but with more than enough information for you to
> have filled in the blanks with Google.  Even when I was a newbie to
> Linux, I would never have asked your question without checking those two
> resources first (which means that I wouldn't have needed to ask this
> particular question at all).  So I'd just like to take this opportunity
> to say how much it pisses me off that so many people, on this list and
> elsewhere, don't make use of basic analytical techniques and the brains
> nature endowed them with.  Thank you and good night.
> 
> -- 
> Bruce
> 
> The ice-caps are melting, tra-la-la-la.  All the world is drowning,
> tra-la-la-la-la.  -- Tiny Tim.



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