Something about Firewall hooking and Packet Filtering #2

Hi,

Here the second and last part of my little paper..
First of all, let’s introduce some more specification, to make previous blog entry more clear
The last struct showed, is the _FIREWALL_CONTEXT_T, and as can be seen there is DIRECTION_E that could be a little obscure, so here is reported:

typedef enum _IP_DIRECTION_E {
IP_TRANSMIT,
IP_RECEIVE
} DIRECTION_E, *PDIRECTION_E;

Represents easly a packet is Receiver or Transmitted.

The return values by the filter-routine can be:

FORWARD = 0
DROP = 1
ICMP_ON_DROP = 2

that are proper of FORWARD_ACTION

As previously said, to implement IP_SET_FIREWALL_HOOK_INFO, it’s necessary to write a filter function for \device\IP, so the pointer (to IP) self can be obtained easly by calling IoGetDeviceObjectPointer( )

Now can be installed the filter function, by passing througout IP’s pointer the address of the filtering function self, with IoBuildDeviceIoControlRequest(IOCTL_IP_SET_FIREWALL_HOOK, IpDeviceObject,…..)
It’s important to say also (according to DDK documentation) that IOCTL_PF_SET_EXTENSION_POINTERregisters filter-hook callback to the IP filter driver, to “make known” \device\IP to reroute every packet received or transmitted, and finally this same IOCTL clears the filter function from IP device. All these specifications could be made, by filling up the proper structure of this IOCTL, that will go to constitute the InputBuffer of IoBuildDeviceIoControlRequest:

PF_SET_EXTENSION_HOOK_INFO, that inside have another struct PacketFilterExtensionPtr which specifies the pointer to the filter hook callback, and when ins FALSE clears the filter.


typedef PF_FORWARD_ACTION (*PacketFilterExtensionPtr)(
IN unsigned char *PacketHeader, //Pointer to Ip header of packet
IN unsigned char *Packet, //Points a buffer with informations in the packet
//that filter-hook receives
IN unsigned int PacketLength , //Length of the packet
IN unsigned int RecvInterfaceIndex,//Index number for the interface adapter (InGoing)
IN unsigned int SendInterfaceIndex,//Index number for the interface adapter (OutGoing)
IN IPAddr RecvLinkNextHop, //IP address for the interface adapter that received the packet
IN IPAddr SendLinkNextHop //IP address for the interface adapter that will transmit the packet
);


It’s also important to notice that only on filter function per time can be installed, if others resides functions are stil working this one will not work.

See you to the next post! 🙂