Date: Mon, 6 May 1996 11:33:26 -0400 From: Garrett Wollman <wollman@lcs.mit.edu> To: Craig Shrimpton <craigs@venus.os.com> Cc: questions@freebsd.org, gpalmer@freebsd.org Subject: Re: Why must I ifconfig my virtual hosts twice? Message-ID: <9605061533.AA27662@halloran-eldar.lcs.mit.edu> In-Reply-To: <Pine.BSF.3.91.960505142106.13728A-100000@venus.os.com> References: <Pine.BSF.3.91.960505142106.13728A-100000@venus.os.com>
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<<On Sun, 5 May 1996 14:27:38 -0400 (EDT), Craig Shrimpton <craigs@venus.os.com> said:
> First, thanks to everyone that replied. I tried using 0xffffffff and it
> works without complaint but if I use 255.255.255.255 netmask a traceroute to
> the virtual domain will fail. The traceroute will time-out at the last hop
> before the virtual host machine.
That is as it should be. Traceroute works as follows:
ttl = 1
while (we don't have a response from target) {
send-packet-with-IP-time-to-live(target, ttl)
wait-for-response()
read-and-print-response()
ttl++
}
The response that traceroute is waiting for is either an ICMP Time
Exceeded message, which comes from an intermediate hop, or an ICMP
Port Unreachable message, which comes from the final destination.
Traceroute doesn't actually distinguish between the two; it just
prints out the delay. (It does look for host and network unreachable
messages and prints `!H' or `!N'.)
Now, what happens when you run a traceroute to an alias address? Your
machine eventually receives the packet, and sends back a port
unreachable message, but because it appears to come from a different
address than the one traceroute was expecting, traceroute assumes that
it got an intermediate hop and not the final destination. (If you had
a multi-homed host with all the addresses listed in the A record,
traceroute would know to check them all.)
-GAWollman
--
Garrett A. Wollman | Shashish is simple, it's discreet, it's brief. ...
wollman@lcs.mit.edu | Shashish is the bonding of hearts in spite of distance.
Opinions not those of| It is a bond more powerful than absence. We like people
MIT, LCS, ANA, or NSA| who like Shashish. - Claude McKenzie + Florent Vollant
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