Thanks for coming back with the solution. This is a gotcha indeed.
Please select your post as Best so that the thread is marked as solved.
I have a faint recollection of what you say, but can't find any direct pointers. Contrary, RM0090 says in the mentioned section:
The input frame’s checksum field is ignored during calculation and replaced by the calculated value.
I am not surprised. The Synopsys modules (ETH and OTG) are notoriously badly documented.
(@Amel NASRI , can this please be reviewed/fixed?)
My faint recollection made me to review some older works. This is part of a bootloader I wrote barebones, without relying on lwip or other library, although certainly starting from the lwip implementation. This snippet takes the incoming ETH frame, parses it, and if it's ICMP, generates the reply in place. Note, that the checksum field is explicitly zeroed - this may be simply a coincidence, or I might've stumbled upon the same thing as you and just corrected it and forgot about it [EDIT] Or, most probably, I might've read this thread or a similar one, I was following the lwip forum back then [/EDIT]
{
TIpHeader * iphdr;
uint32_t ipHdrLen;
uint32_t ipLen;
uint8_t * pIpData; // to be passed to upper protocols
uint32_t ipDataLen; // ditto
if (len < (SIZEOF_ETH_HDR + sizeof(TIpHeader))) break; // drop too short packet
iphdr = (TIpHeader *)(ðhdr->data);
if (iphdr->ver != 4) break; // drop non-IPV4
ipHdrLen = iphdr->hdrLen * 4;
ipLen = swap16(iphdr->len);
if (ipHdrLen < 20) break; // invalid len
if (ipHdrLen > len - SIZEOF_ETH_HDR) break; // too big header, not enough data
if (ipLen > len - SIZEOF_ETH_HDR) break; // too big IP packet, not enough data
pIpData = ((uint8_t *)iphdr) + ipHdrLen;
ipDataLen = ipLen - ipHdrLen;
if ((iphdr->offset AND IP_MF) != 0) break; // absolutely no support for fragmenting, RFC or not
#ifndef CHECKSUM_BY_HARDWARE
#error "CHECKSUM_CHECK_IP not implemented, as CHECKSUM_BY_HARDWARE assumed"
#endif
if (!((iphdr->dst[0] == ipAddr[0]) ANL (iphdr->dst[1] == ipAddr[1]) ANL (iphdr->dst[2] == ipAddr[2]) ANL (iphdr->dst[3] == ipAddr[3]))) {
break; // reject if not for us (we should reject eth-broadcasts here too, btw)
}
switch (iphdr->protocol) {
default: break;
case 1: // ICMP
{
TIcmpHeader * icmpHdr;
if (ipDataLen < sizeof(TIcmpHeader)) break; // drop too short packets
icmpHdr = (TIcmpHeader *)pIpData;
if (icmpHdr->type != 8) break; // respond only to pings
if (icmpHdr->code != 0) break;
// we rely on CHECKSUM_BY_HARDWARE to check ICMP checksum, too
// we recycle the packet
icmpHdr->type = 0; // echo response
icmpHdr->checksum = 0; // again we rely on CHECKSUM_BY_HARDWARE to generate ICMP checksum, too
iphdr->checksum = 0; // similarly with IP
iphdr->ttl = 255;
memcpy(iphdr->dst, iphdr->src, sizeof(iphdr->src));
memcpy(iphdr->src, ipAddr, sizeof(ipAddr));
memcpy(ethhdr->dest, ethhdr->src, sizeof(ethhdr->src));
memcpy(ethhdr->src, macAddr, sizeof(macAddr));
EthSendPacket(ethhdr, ipLen + SIZEOF_ETH_HDR); // sending all received length ensures that data is returned untouched
}
JW
