Trouble programming network access filter gateway

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I am programming a real-time network access filter gateway as a
requirement of my course. The main purpose of the gateway is to block
access to some black-listed websites (i.e. block some HTTP requests).

I am using WinPCap 3.0 library and using VC++ 6.0 for development.
WinPCap is good for developing network analysis tools, but there is
one feature of the library which allows one to send raw packets to the
network adapter ( ).

Now my pseudo code for capturing request packets goes like this

 open network adapter connected to internal network (e.g. LAN);
 capture all request packets;
 if tcp request
    if http request
      parse http header and get domain name;
      lookup the domain name in the blocked list;
      if blocked
        drop the request packet(s);
        send customized response back;
      else         allow the request;
 send captured request packets to the network adapter connected to
the external network (e.g. Internet);
 open network adapter connected to external network;
 capture all response packets;
 send captured responses to the adapter connected to the internal

Now I am trying to capture packets from the internal network adapter
using a filter expression
( ) in a
promiscuous mode. The expression looks like "eth src xx:xx:xx:xx:xx:xx
and eth dst yy:yy:yy:yy:yy:yy", where "xx:xx:....:xx" is MAC address
of the adapter where the requests are coming from (e.g. router) and
"yy:yy:...:yy" is MAC address of the adapter on the gateway connected
to internal network. similarly I follow the similar filter expression
for the response packet capturing.

Now the main issue is I don't see any response coming from the
external network even if I transfer all the captured packets from
internal network adapter to the  external network adapter. Do I have
to change the MAC layer addresses when I transfer all the packets from
internal network to the external network ?

I will appreciate any guidelines or references to the similar

Thank you.

Sachin Shah

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