Is ethernet checksum exposed via AF_PACKET?

Question

As it is implied by this question, it seems that checksum is calculated and verified by ethernet hardware, so it seems highly unlikely that it must be generated by software when sending frames using an AF_PACKET socket, as seem here and here. Also, I don't think it can be received from the socket nor by any simple mean, since even Wireshark doesn't display it.

So, can anyone confirm this? Do I really need to send the checksum myself as shown in the last two links? Will checksum be created and checked automatically by the ethernet adaptor?

Answer 1

No, you do not need to include the CRC.

When using a packet socket in Linux using socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL) ), you must provide the layer 2 header when sending. This is defined by struct ether_header in netinet/if_ether.h and includes the destination host, source host, and type. The frame check sequence is not included, nor is the preamble, start of frame delimiter, or trailer. These are added by the hardware.

Answer 2

On Linux, if you mention socket(AF_PACKET, SOCK_RAW, htobe16(ETH_P_ALL)) similar case, you don't need to calculate ethernet checksum, NIC hardware/driver will do it for you. That means you need to offer whole data link layer frame except checksum before send it to raw socket.