ethernet interface on Linux drops packets


I'm trying to capture some Ethernet frames with Linux. Some of these packets/frames are invalid and contain corrupted data.

For example an Ethernet frame contains the type 0x0800 which is IPv4, but the following data contain just random bytes. Furthermore, the source and destination MAC are unknown and not predictable.

To receive these frames I wrote a C program that first used raw sockets. This didn't work as expected, so I switched to libpcap.

I open the Ethernet device in promiscuous mode to receive all frames and prevent MAC sided filtering. This works just fine. I can receive frames with any destination MAC address.

Now we get to the big problem:

I send an Ethernet frame with random destination and source MAC. the Type field is 0x1234 and the data is just a few bytes, let's say 0xdeadbeef. This gets padded to the minimum amount of payload. This frame is received by my program using libpcap just fine.

When I send the same frame using a "known" Ethernet type field value like 0x0800, the frame gets dropped and my program can't receive it.

The software runs on an embedded platform, an Altera Cyclone V SoC that has an STMMAC Ethernet module. On a regular PC/Notebook etc. the program runs fine and can even receive these invalid IPv4 frames.

To find out where the packets are dropped, I watched the /sy/class/net/eth0/statistics/rx_* files. There are files that count the number of received packets and bytes as well as the number of dropped packets. these statistics work fine with normal Ethernet traffic. However, sending an invalid frame as described above doesn't result in any change of the statistics. The driver doesn't even count up the "dropped frames" counter. I can't understand this, because as far as I know the statistic of the network interface gets affected as soon as a packet is received by the hardware. The filtering and evaluation process of the network stack should have nothing to do with it. Am I right? Therefore it seems to me that some really low driver code is filtering these packets or maybe even the hardware itself.

Again: With an normal PC on the other side the packets can be received with the program I wrote.

The second problem is with valid TCP packets:

Although I'm using libpcap which provides raw traffic to me, received TCP packets are reassembled to huge packets that are even bigger than the MTU of the receiving network interface. these packets are also received normally with a PC.

Can anyone help me? I need to receive raw Ethernet traffic "as it is" using Linux on the Altera SoC-FPGA platform.

asked on Super User Jun 27, 2016 by GNA • edited Jul 11, 2016 by Toby Speight

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