[Gllug] OT: Traceroute
Ian Baillie
ian.baillie at westminster.org.uk
Tue Mar 18 18:06:01 UTC 2003
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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