Re: network setup help

["Jee J.Z." <jz105@york.ac.uk>]

> On Sunday 11 April 2004 12:33 am, Jee J.Z. wrote:
> 
> > Hi Antony,
> 
> Thanks for getting the spelling right - lots of people round here don't :)

Because I try not to annoy you in the very beginning. :)
 
> > > Look at the routing table of each machine the packets are going through,
> > > and then the replies trying to get back again, and see if (a) there is a
> > > path, and (b) it makes sense.
> >
> > Both PC2 and PC3's routing look like:
> > Destination                Gateway                   Genmask
> > Flags      Metric      Ref      Iface
> > 192.168.0.0               0.0.0.0                     255.255.255.0
> > U            0           0        eth1
> > 144.32.xxx.0              0.0.0.0                     255.255.254.0
> > U            0           0        eth0
> > 127.0.0.0                  0.0.0.0                     255.0.0.0
> > U            0           0         lo
> > 0.0.0.0                     144.32.xxx.yyy           0.0.0.0
> > UG           0           0        eth0
> 
> Okay, I'm assuming that 144.32.xxx.yyy is not the address of PC1 - it's 
> something else which is connected to your switch - however that's not too 
> important here.

Right. 144.32.xxx.yyy should be a router on the 144.32.xxx.0/24 network.
 
> Here's what I think is going on:
> 
> PC1 has a simple routing table saying "network 144.32.xxx.0/23 is on eth0, and 
> the deafult gateway is 144.32.xxx.yyy".   That means if you ping 192.168.0.2 
> from PC1 it will send the packets to the default gateway :(

Oh, sorry to get you confused. I am not trying to ping from PC1 to PC3. I am trying to ping from PC2 to PC3 or from PC3 to PC2 to test their connectivity (between PC2 and PC3). Besides the routing table, I am also considering whether my configurations for setting up two NICs on one box are problematic or not at this moment.
 
> Set a network route on PC1 to tell it how to find the 192.168.0.0/24 network, 
> via PC2 as a router:
> 
> route add -net 192.168.0.0 netmask 255.255.255.0 gw 144.32.xxx.b
> 
> where 144.32.xxx.b is the IP address on PC2/eth0
> 
> Once you have done that I think the pings will work, however they will not 
> work the way you would like them to :)
> 
> An echo request packet will come from 144.32.xxx.a (PC1/eth0) to 192.168.0.2 
> and get routed (by your new routing table entry on PC1) via 144.32.xxx.b, 
> where PC2 will decide "192.168.0.2? Oh, that's on my eth1", and will send it 
> to PC2.
> 
> However, if PC2/eth0 is still operational and plugged in, it will think "I 
> have to send an echo response packet to 144.32.xxx.a, and that's on this 
> subnet connected to my eth0", so it will send the reply packet out through 
> eth0.
> 
> This will actually work for pings, but it's not the way you want TCP packets 
> to flow (remember that pings are ICMP packets), and it certainly won't work 
> once you start doing nat on PC2 (which I assume, for some reason, that you do 
> want to do?).

Your descriptions above make sense. And right, I will do filtering and nat on PC2 after I sort the current problem out. Once I set up filter and nat on PC2, PC1 does not need "route add -net 192.168.0.0 netmask 255.255.255.0 gw 144.32.xxx.b
" any more, and PC1 should not know there is a 192.168.0.0 network behind PC2, right?
 
> However, I still remain puzzled about why you have this crazy setup in the 
> first place, and what you're trying to achieve by sending pings from PC1 to 
> PC3 via PC2, so add the routing table entry to PC1, check whether a ping 
> works (with all the cables plugged in, so all the paths shown on your 
> original diagram are available), and then try to explain to us why you are 
> doing things this way and what you want to achieve.

OK, I am trying to set up a simple firewall (just do filtering and nat) between PC3 and the outside world. PC2 is where the firewall locates. And PC1 is just a traffic sender for testing after the firewall is built up. At this moment, the first thing I need to figure out is why the connectivity between PC2/eth1 and PC3/eth1 seems not working. I think the reasons lie in either routing table setup (on PC2 and PC3) or dual NIC configurations. Any suggestions?

Many thanks,
Jee

> Regards,
> 
> Antony.
> 
> -- 
> If at first you don't succeed, destroy all the evidence that you tried.
> 
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