Frequently Asked Questions
Q. How can I prevent packets that I send from being looped back?
A. On Windows 2000
and higher a protocol can set information in the send packet descriptor Flags
field to tell NDIS not to loopback the packet at all. This flag overrides any
other loopback that NDIS may make.
The Flags used to inhibit loopback is the NDIS_FLAGS_DONT_LOOPBACK flag,
defined in NDIS.H in the Windows 2000 and higher DDK.
However, the
NDIS_FLAGS_DONT_LOOPBACK flag does not work correctly on Windows 2000.
On Windows 2000 (only) it is necessary to OR the undocumented value 0x400
with the NDIS_FLAGS_DONT_LOOPBACK flag to inhibit loopback of a send packet.
This value is used in the Windows 2000 DDK MUX driver sample, where it is
defined as:
#define NDIS_FLAGS_SKIP_LOOPBACK_W2K
0x400
Only use
NDIS_FLAGS_SKIP_LOOPBACK_W2K on Windows 2000. It is reserved and may have
other meanings on other platforms.
Please do review the details of using these flags by
examining the Windows XP DDK MUX driver sample.
Unfortunately there are no similar loopback-inhibit flags on Windows 9X/ME or
Windows NT 4.0. On those platforms there is no NDIS mechanism to inhibit
loopback. One approach that may work in some designs is to filter received
packets based on their source link address. If the received packet's source link
address is the same as that of the NIC the packet is being received on, then the
packet is a loopback packet.
For more information see the NDIS Loopback
Discussion.
