DELPHI PACKET MANIPLATION AND ROUTING

You can turn on routing on Windows system and it will route the packet without additional efforts from your side.

You have only to make user authentication on the internal interface and NAT on the eternal (if necessary).

Refer Internet Gateway sample for the details about NAT. Install script for this sample also turns on routing (NSIS script function):

Function EnableRouter

SetRebootFlag true

${If} ${IsNT}

WriteRegDWORD HKEY_LOCAL_MACHINE SYSTEMCurrentControlSetServicesTcpipParameters IPEnableRouter 1

${Else}

WriteRegDWORD HKEY_LOCAL_MACHINE SYSTEMCurrentControlSetServicesVxDMstcp EnableRouting 1

WriteRegDWORD HKEY_LOCAL_MACHINE SYSTEMCurrentControlSetServicesVxDMstcp IPEnableRouter 1

${EndIf}

FunctionEnd

Explain what you don’t understand, otherwise it is difficult to advise…

Of course you can implement the routing by yourself. You can read packet from one interface, check/modify and forward to another using NDISAPI interface. You have to support you own routing table and etc.. to implement this properly. IHMO it is doable, but there is easier way…

Personally I prefer to let the MSTCP to do the routing work like it is done in Internet Gateway advanced sample http://www.ntkernel.com/w&p.php?id=31. This is very similar to what you need but the routing operation is performed by MSTCP, so probably it should help you to start.

Internet Gateway is only available in C++.

PassThru is basic packet filtering sample.

Under MSTCP I mean Microsoft TCP/IP stack implementation. You don’t have to use it from your application, but you can use its default behavior to solve the routing task.

Under “using MSTCP default behavior” I mean that you may modify incoming/outgoing packets and let TCP/IP route them. Like in case of NAT, WIndows TCP/IP stack by default routes packets coming from LAN into default gateway interface (connected to the Internet). To implement NAT you can only capture packets from external (Internet connected) network interface:
– For outgoing packets change source IP address to Internet interface assigned one and port allocated/according NAT table
-For incoming packets change destination IP/port according NAT table

This way you use Microsoft TCP/IP routing and don’t have to implement it on your own.

Is that clear now?