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Firewall using libpcap and libnet (perplexing problem)

Posted on 2003-03-17
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Last Modified: 2010-04-21
Hi Experts,

I've got a rather perplexing problem. I want to create a firewall using libpcap (for sniffing) and libnet (for packet injection).

The idea is to have two NICs. To start, I just want to sniff the packets on one NIC and inject them onto the other (and vice-versa). This would create a basic bridge.

The problem comes due to this situation:

NIC A sniffs packet X
NIC B injects packet X
NIC B sniffs the same packet X that it just injected
NIC A injects packet X
LOOP FOREVER

I don't want to change anything about the packets (even MAC address), so I have no way of knowing if a packet is coming from me or not.

Any ideas?!?  Is there a way to set pcap to filter packets that come from me?

Thanks in advance

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Question by:wearyweary
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Expert Comment

by:bryanh
ID: 8175724
>I don't want to change anything about the packets (even
>MAC address), so I have no way of knowing if a packet is
>coming from me or not.

A packet that came from you is one whose source MAC address is the MAC address of your NIC.  

You cannot with an ordinary NIC make an outgoing packet have the MAC of some other NIC, so if you were hoping to make this into an Ethernet bridge, you've got the wrong hardware.  But you probably don't want an Ethernet bridge (an IP bridge is usually all you need), so you're fine.
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Author Comment

by:wearyweary
ID: 8175894
I would like to make it an ethernet bridge for complete transparency, and it is possible to spoof MAC addresses using almost any standard NIC. However this isn't enough due to the case I mentioned.

I'm fine changing the source MAC address to my own if absolutely needed, but I'm not sure if ARP packets will work being altered this way.

Does an ARP packet keep the MAC/IP it is resolving in the payload of the packet, or does it rely on using the source address (the one I'm changing)?

Anyway. An ethernet bridge is not impossible using two NICs and a PC. That I am quite sure of.





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bryanh earned 120 total points
ID: 8176434
I guess you know more about NICs than I do.

But I do know that the ARP packet payload contains the source MAC address and that is the one that gets entered into the table and to which any reply is sent.
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