[Gllug] Strange network behaviour

[Alistair Mann]

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Thus spaketh Adrian McMenamin on Sunday 18 May 2003 9:19 pm:
> On Sunday 18 May 2003 20:17, Alistair Mann wrote:
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> > Thus spaketh Adrian McMenamin on Sunday 18 May 2003 7:01 pm:
> > > I have a 10/100 Mb switch and a 10 Mb hub. The 10 Mb hub is connected
> > > to the 100 Mb switch.
> > >
> > > Well, I have connected two other computers to the hub (a Power PC
> > > running Mac OS 9 and a nix box). They work fine, connecting to the
> > > internet via the gateway - which is connected to the switch.
> > >
> > > But when I attach my Thinkpad, also running Linux (RH 8) it cannot
> > > connect to or ping the gateway. But it can connect to and ping another
> > > box connected to the switch.
> > >
> > > All boxes are on the same network - 192.168.62.0/24.
> > >
> > > Even if I disconnect another box from the hub, my Thuinkpad still won't
> > > see the gateway.
> > >
> > > Any ideas?
> >
> > So what you're saying is that the laptop, while connected to the switch,
> > can see anything else connected to the switch; yet it cannot see anything
> > on the slow speed hub, and I assume, cannot see anything else on the hub
> > even when directly connected to it?
>
> Apologies, I didn't make it clear. The laptop is plugged into the hub, not
> the switch and can see all machines on the network (whether plugged into
> the switch or the hub) except the gateway

All the machines can see the gateway, except the laptop, which can't see it 
wherever plugged, but can see all the others, wherever plugged. Okay. 

The obvious one is a firewall issue. Turn them off on both gateway and 
thinkpad and see if that helps. If it does, check if your firewall is geared 
towards access to IPs rather than subnets, and if the IPs so allowed (the 
laptop's frinstance) haven't been mistyped. 

The next most obvious is mismatching subnets. If all the other machines work, 
that suggests the laptop's subnet could be wrong.

Cables must be ok; laptop can ping others. 

Have you ever done complex routing on the laptop? It might associate the 
gateway's address with a different interface than for the rest of the subnet. 
I had this whilst playing around with UML.

After this, I'd check to see if all the machines can see each other when all 
are connected to the hub, thus removing the switch as potential failure point 
(I've experienced dead RJ45 sockets there before). It also forces a known 
interface speed of 10mb/s.

Then I'd take the MacOS's IP, etc, and reuse them on the laptop (taking the 
Mac offline, of course). If it works, there's something peculiar about the 
lapdog's original settings. If it doesn't, that removes the TCP/IP settings 
as suspect.

Are you pinging the gateway by IP? or hostname/FQDN? If the latter, it could 
be ping can't resolve the address.

Beyond here, I'd start looking at iptraf/tcpdumping on the laptop, the unix 
box and the gateway to see if the ping traffic is actually getting to the 
wire.

Cheers,
- -- 
Alistair
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