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Old 01-21-2007, 10:27 PM   #1
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LAN in Debian


My network connection was like this:

................................................PC(with cable)
Internet-Cable Modem-WiFi Router

Everything was working fine, but I want to route all the traffic from the Cable mdoem through my PC and then to the router.

I have two NIC's, so I only need help with configuration. I have read tutorials on the web, but they haven't given a sufficient answer.

Thank you.

Last edited by kuratkull; 01-21-2007 at 10:29 PM.
Old 01-22-2007, 09:21 AM   #2
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Hint -- Put ASCII art in 'code' blocks.

If I understand you correctly, this is what you have:
                           \              /                     +--------+  
(^^^^^^^^^^)   +-------+    \            /  ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ | Laptop |
( Internet )===| Cable |   +--------------+   +------------+    | (Wifi) |
(          )   | Modem |---|up   WiFi   dn|---|eth0  (eth1)|    +--------+    
 vvvvvvvvvv  A +-------+ B |    Router    | C |     PC     |  D
                           +--------------+   +------------+
and this is what you want, w/ the PC & the Router swapping places:
                                            \              /     +--------+  
(^^^^^^^^^^)   +-------+                     \            /  ~ ~ | Laptop |
( Internet )===| Cable |   +------------+   +--------------+     | (Wifi) |
(          )   | Modem |---|eth0    eth1|---|up   WiFi (dn)|     +--------+       
 vvvvvvvvvv  A +-------+ B |     PC     | C |    Router    |  D
                           +------------+   +--------------+
You have the 2nd NIC for the PC, & I assume you know how to do the cabling, so let's consider the configuration.

Your biggest problems, in approximate order of diffculty:
  1. Firewalling on your PC after the change.
    1. From the outside
    2. From the WLAN
  2. Forwarding packets across the PC
  3. Re-configuring (C)
  4. Re-configuring (B)
Because of 1., I would not do what you are suggesting w/o putting a NAT'ing router or firewall (I use SmoothWall Express 2.0) between my LAN & the outside world. Furthermore, when I do turn my WLAN on, it also is connected through my "Smoothie".

I presume you understand the consequences & can do the necessary firewalling.

Item 2. is easy, but often overlooked. The command is:
echo 1  > /proc/sys/net/ipv4/ip_forward
You must either run it manually or put it in an appropriate start-up script.

If you are lucky, both B & C are currently using DHCP, the Cable Modem & the Router acting as servers, while the Router & the PC are the respective clients. Also C & D are configured as separate private networks, say & If so, you need only restart the DHCP clients, if necessary by re-booting.

On the other hand, if either your ISP requires PPPoE, or you are using static IP's on your LAN, (or both); things are more complicated. The bad news is, I don't do either of those, so I can't help much with them. The good news is, you probably have been through setting them up & may remember how.

One last piece of good news: there should be no need to change D -- it has been moved intact.


art, ascii, asciiart, diagram, router

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