Looping Back NDIS Packets

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If the NDIS_NBL_FLAGS_IS_LOOPBACK_PACKET flag in the NblFlags member of the NET_BUFFER_LIST structure is set, the packet is a loopback packet. Protocol drivers and filter drivers can check this flag to determine if a packet is a loopback packet.

NDIS loops packets back if all of the following three conditions are satisfied:

  1. The underlying miniport adapter media type is NdisMedium802_3 or NdisMedium802_5.

  2. Any one of the following three conditions is satisfied:

    1. A protocol binding set the NDIS_PACKET_TYPE_PROMISCUOUS setting with the OID_GEN_CURRENT_PACKET_FILTER OID to specify its packet filter (and, for Windows 8 and later, did not set NDIS_PACKET_TYPE_NO_LOCAL in the same OID) and either of the following is true:

      • There is more than one binding to the miniport adapter.
      • There is a filter module attached to the miniport adapter and the filter module registered a receive handler.

    2. A protocol binding set the NDIS_PACKET_TYPE_ALL_LOCAL setting with the OID_GEN_CURRENT_PACKET_FILTER OID to specify its packet filter and either of the following is true.

      • There is more than one binding to the miniport adapter.
      • There is a filter module attached to the miniport adapter and the filter module registered a receive handler.

    3. The caller sets the NDIS_SEND_FLAGS_CHECK_FOR_LOOPBACK flag in the SendFlags parameter of the NdisSendNetBufferLists function.

  3. The packet is acceptable as determined by the packet filter set with the OID_GEN_CURRENT_PACKET_FILTER OID for the miniport adapter. The following are some examples:

    • If the packet is a direct packet, the destination address in the packet must match the MAC address of the miniport adapter.
    • If the packet is a multicast packet, the packet filter must have NDIS_PACKET_TYPE_ALL_MULTICAST set or the destination address matches one of the multicast address in the miniport adapter's multicast address list and the packet filter has NDIS_PACKET_TYPE_MULTICAST set.
    • If the packet is a broadcast packet, the miniport adapter's packet filter must have NDIS_PACKET_TYPE_BROADCAST set.
    • The miniport adapter's packet filter has NDIS_PACKET_TYPE_PROMISCUOUS or NDIS_PACKET_TYPE_ALL_LOCAL set.

A protocol binding receives loopback packets if either of the following is true:

  1. The protocol binding is the original sender of the packet and NDIS_SEND_FLAGS_CHECK_FOR_LOOPBACK is set.

  2. The protocol binding does not set NDIS_PACKET_TYPE_NO_LOCAL in the packet filter.

A protocol binding will not receive loopback packets if either of the following is true:

  1. The protocol binding sets NDIS_PACKET_TYPE_NO_LOCAL in the packet filter and it is not the original sender for the packet.

  2. The protocol binding is the original sender but NDIS_SEND_FLAGS_CHECK_FOR_LOOPBACK is not set in the SendFlags parameter in a call to the NdisSendNetBufferLists function.

The following figure shows the loopback algorithm logic flow.

Flowchart that shows the loopback algorithm logic flow.