[slightly OT & long] PPPoE iptables NAT

[Albrecht Dreß]

Hi,

sorry for a slightly off-topic question regarding NAT with a PPPoE network.

I have a home network looking as follows:

         192.168.42.3
         -----------     -------
        | G4 Silver |   |       |---DSL Modem (ppp0)
ISDN---|ippp0  eth0|---|Switch |---more Macs (192.168.42.x)
         -----------     -------

The G4 is running Yellowdog 4.01 with a self-compiled 2.6.11.4 kernel.

The "old" setup was an ISDN connection (no DSL present), and the G4 worked  
as router. Everything was perfect for the "local" net, except that the  
hardware (AVM Fritz) was not supported by MacOS, so I had no internet on  
the G4 (and connected clients) when running OS X.

Therefore, I removed the ISDN card, got DSL, plugged the modem into the  
switch, and now everything works fine with OS X. After a little fiddling  
around with ipfw and natd, I have routing support with OS X client.

Now I tried Linux, and I have internet on the G4 itself, but packets from  
the local net are apparently not routed/forwarded correctly. I did not  
change the NAT rules in iptables (except replacing ippp0 by ppp0, of  
course):

<snip>
[root at antares root]# iptables -t nat -L -n
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
MASQUERADE  all  --  192.168.42.0/24      0.0.0.0/0

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
</snip>

Running tcpdump on both eth0 and ppp0, I can see that e.g. a http (tcp/80)  
request from the local net is going through eth0, and the same packet is  
then passed via ppp0. The response, though, is *not* returned to the  
requesting client.

I am using the kernel-based pppoe modules (pppoe, pppox), not the roaring  
penguin package coming with YDL. Does anyone have an idea why forwarding  
doesn't work with this setup? Any hints how I should configure my box? Or  
is it impossible to use the dsl modem and the local net at the same  
Ethernet interface (but why does it work with OS X, then)? Any pointers?

HELP! I'm really lost here...

Thanks in advance for any help,

Cheers, Albrecht.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  Albrecht Dreß  -  Johanna-Kirchner-Straße 13  -  D-53123 Bonn (Germany)
        Phone (+49) 228 6199571  -  mailto:albrecht.dress at arcor.de
    GnuPG public key:  http://home.arcor.de/dralbrecht.dress/pubkey.asc
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