IP Masquerade & MAC Numbers

IP Masquerade & MAC Numbers

Post by JSH » Thu, 18 Feb 1999 04:00:00



I want to set up an aDSL connection. My Linux box would be connected to the
DSL modem via eth0 and to a Windows98 box via eth1, like this:

W --- (eth1) L (eth0) --- ISP

(W is the Windows box on the internal network and L is the masquarade
gateway).

The ISP, however, tracks my MAC number to make sure that I only have one
computer using the DSL connection. Will IP Masquerade replace the MAC number
on packets coming from the internal network with its own before shipping
them out onto the Internet, or will the MAC numbers remain unchanged?

Any help you could give would be appreciated. Thanks!

Dominik Slusarczyk

 
 
 

IP Masquerade & MAC Numbers

Post by Luca Filipoz » Thu, 18 Feb 1999 04:00:00



says...
> I want to set up an aDSL connection. My Linux box would be connected to the
> DSL modem via eth0 and to a Windows98 box via eth1, like this:

> W --- (eth1) L (eth0) --- ISP

> (W is the Windows box on the internal network and L is the masquarade
> gateway).

> The ISP, however, tracks my MAC number to make sure that I only have one
> computer using the DSL connection. Will IP Masquerade replace the MAC number
> on packets coming from the internal network with its own before shipping
> them out onto the Internet, or will the MAC numbers remain unchanged?

> Any help you could give would be appreciated. Thanks!

> Dominik Slusarczyk


The MAC number will be that of the Linux box's eth0 interface. You will
appear to have only one machine connected to the ISP.

This is what happens...
W sends packet to L destined for yahoo, say.
L looks at packet, rewrites IP header (src address and src port) and
emits the packet out of eth0. This all happens at Layer 3. It's when the
packet needs to be sent to another device on the same segment that the
Layer 2 MAC stuff comes in.
L will ask "who has ISP's ip address". ISP will reply with its MAC
address. L will take the Layer 3 packet and stuff it into a Layer 2
packet whose src MAC address is L's eth0 and whose dst address is the
ISP' MAC address.
Thus, everything that leaves eth0 will have eth0's MAC address as src MAC
address.

Hope this helps,

Luca
--


 
 
 

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I am setting up a Linux box to run I.P. Masquerading for other machines
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Does anybody know how to do this?

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