Packet Routing for Connection Sharing

corteplaneta · 10-14-2008, 09:19 AM

Hi, I was recently discussing this in a separate thread (ndiswrapper issues), though this is now completely irrelevant from that topics, as I'm using tulip now, not a Windows driver.

Basically, I have 2 linux PC's connected with a crossover cable. "xanderbox" (192.168.1.5) is my PC with 2 NIC's that's connected to the router (eth0, gw 192.168.1.1) & to the other PC, "xravtop" (eth1, xravtop's IP is 192.168.1.8).

To route traffic to the correct NIC, I had to manually add an entry into the routing table that routes all packets from 192.168.1.8 to device eth1, and there is no gateway set.

I cannot access the internet from xravtop, as the packets are only being routed to xanderbox (192.168.1.5).

When I attempt to set the gateway for the route I added to 192.168.1.1, this just blows out the connection between the 2 crossover cable connected PC's, so this is obviously not the solution.

If anyone knows how I am able to route packets as to access the internet from the PC connected via crossover cable, it would be greatly appreciated!

I hope that was clear enough, thanks!

Vit77 · 10-14-2008, 09:50 AM

If I understand you correctly, the topology of your net is following:

Code:

 Router(IP=192.168.1.1) - some active network device
   |
(eth0 = 192.168.1.5)
xanderbox
(eth1 = ?)
   |
(eth1 = 192.168.1.8)
xravtop

If so, I haven't seen the eth1's IP. IP address is assumed not to a PC, but to an interface (LAN card).

Please, clearify the topology and show traceroute output.

robel · 10-14-2008, 12:33 PM

Can you please post a complete 'ifconfig' and 'ip route' on both boxes?

My first thought is that you have two NICs (eth0 and eth1) on the same subnet.

corteplaneta · 10-14-2008, 01:59 PM

Quote:

Originally Posted by Vit77

If I understand you correctly, the topology of your net is following:

Code:

 Router(IP=192.168.1.1) - some active network device
   |
(eth0 = 192.168.1.5)
xanderbox
(eth1 = ?)
   |
(eth1 = 192.168.1.8)
xravtop

If so, I haven't seen the eth1's IP. IP address is assumed not to a PC, but to an interface (LAN card).

Please, clearify the topology and show traceroute output.

I believe this is correct. I am at work right now, or I would have posted the output of those commands. I will post this in 5-6 hours. Thanks!

corteplaneta · 10-14-2008, 05:52 PM

xanderbox:

Code:

bash-3.1# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0A:E6:C5:5C:31  
          inet addr:192.168.1.6  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::20a:e6ff:fec5:5c31/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:54030 errors:0 dropped:0 overruns:0 frame:0
          TX packets:52381 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:25573210 (24.3 MiB)  TX bytes:10752041 (10.2 MiB)
          Interrupt:11 Base address:0xec00 

eth1      Link encap:Ethernet  HWaddr 00:04:5A:81:B4:40  
          inet addr:192.168.1.5  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::204:5aff:fe81:b440/64 Scope:Link
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:301955 errors:1 dropped:0 overruns:0 frame:1
          TX packets:444877 errors:54 dropped:0 overruns:0 carrier:54
          collisions:0 txqueuelen:1000 
          RX bytes:69298182 (66.0 MiB)  TX bytes:512650826 (488.9 MiB)
          Interrupt:10 Base address:0xd000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:2157 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2157 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:244322 (238.5 KiB)  TX bytes:244322 (238.5 KiB)

bash-3.1# ip route
192.168.1.8 dev eth1  scope link 
192.168.1.0/24 dev eth1  proto kernel  scope link  src 192.168.1.5 
192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.6 
127.0.0.0/8 dev lo  scope link 
default via 192.168.1.1 dev eth0

xravtop:

Code:

                                                                                                 
eth0      Link encap:Ethernet  HWaddr 00:0f:1f:be:3a:68
          inet addr:192.168.1.8  Bcast:192.168.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:408399 errors:72 dropped:0 overruns:0 frame:3
          TX packets:284924 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:511000237 (487.3 MB)  TX bytes:32933501 (31.4 MB)
          Interrupt:11

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:42140 errors:0 dropped:0 overruns:0 frame:0
          TX packets:42140 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:3768860 (3.5 MB)  TX bytes:3768860 (3.5 MB)


192.168.1.0/24 dev eth0  proto kernel  scope link  src 192.168.1.8
169.254.0.0/16 dev eth0  scope link  metric 1000
default via 192.168.1.6 dev eth0  metric 100

robel · 10-15-2008, 02:58 AM

You do indeed have two interfaces on the same subnet. This is really not a good idea unless you plan to bundle traffic to and from the 192.168.1.0/24 subnet. I don't think you intend to do the latter.

The simple way to fix this is to use a different subnet (eg. 192.168.2.0/24) between your computers.

Vit77 · 10-15-2008, 07:53 AM

In general, it seems to be possible. But you have to organize your routing correctly.
On the xravtop:

Code:

ip route add default via 192.168.1.6 dev eth0

On the xanderbox:

Code:

ip route add 192.168.1.8/32 via 192.168.1.6 dev eth0
ip route add default via 192.168.1.1 dev eth1

And check if forwarding on xanderbox is on:

Code:

cat /proc/sys/net/ipv4/ip_forward

If it shows '0', then you've got to enable it:

Code:

echo "1" > /proc/sys/net/ipv4/ip_forward

Or, to make it enabled at the boot time, add the line

Code:

net.ipv4.ip_forward = 1

in /etc/sysctl.conf on xanderbox.

With such addresses, your router must have mask 24 on the inner interface.

If you change the subnet xanderbox-xravtop to, for example, 192.168.2.0/24, you will need to add a new route to your router (192.168.1.1), something like this:
for net 192.168.2.0/24 via 192.168.1.5
Syntax depends on the router's vendor and model.

I hope, it'll help you to tune your net up.

corteplaneta · 10-15-2008, 07:03 PM

I put both PC's (eth1 on my main PC, xanderbox) & eth0 on my laptop onto the 192.168.2.x network. I was using a route like this before: "route add 192.168.1.8 netmask 255.255.255.0 dev eth1", but I'm completely baffled as what to change this to now. Sorry, I don't know much about IP--does the netmask change? I simply changed the route to "route add 192.168.2.8 netmask 255.255.255.0 dev eth1" & I once again could ping xravtop from xanderbox (& vice versa) but I still don't have the internet. There is no default gateway set for this route.

EDIT:

I just saw your post Vit77, thanks for the instruction, I'll try this out

!

corteplaneta · 10-15-2008, 07:25 PM

I believe the only problem was that xanderbox didn't have IP forwarding enabled!

Everything works now (internet connection from laptop)...

Thanks for the help everyone!

corteplaneta · 10-19-2008, 11:12 AM

Ok, sorry to bring back an old post I thought solved, but for some reason everything DOES work, but stops working after some time? If I restart my main desktop PC, everything begins to work correctly again? Does anyone have any idea why this would happen?

keefaz · 10-19-2008, 11:26 AM

Maybe check logs on Desktop PC, especially /var/log/syslog

corteplaneta · 10-20-2008, 11:02 PM

Oct 19 16:18:43 xanderbox last message repeated 24 times
Oct 19 16:18:43 xanderbox kernel: eth1: (40770) System Error occurred (0)

This can't be good!