netdev - How to do TCP tx checksums

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Date:	Mon, 16 Nov 2015 16:20:35 -0800
From:	Ben Greear <greearb@...delatech.com>
To:	netdev <netdev@...r.kernel.org>
Subject: How to do TCP tx checksums

Hello!

I'm hacking on (my already hacked) pktgen, trying to get it to send TCP frames.

And, having issues getting checksums to work properly.

I'm trying this:
	struct iphdr *iph = ip_hdr(skb);
	struct net_device *odev = pkt_dev->odev;

	if (pkt_dev->flags & F_TCP) {
		if (odev->features & NETIF_F_V4_CSUM) {
			skb->ip_summed = CHECKSUM_PARTIAL;
		} else {
			skb->ip_summed = CHECKSUM_NONE;
		}
		skb->csum = 0;
		__tcp_v4_send_check(skb, iph->saddr, iph->daddr);

I added an export so I could call that __tcp_v4_send_check method
w/out having to put a fake socket struct on the stack.

But, the receiving NIC reports 100% checksum failure, so obviously
I'm not doing it correct.

Any suggestions on what I might be doing wrong or the proper method(s)
to call?

Thanks,
Ben

-- 
Ben Greear <greearb@...delatech.com>
Candela Technologies Inc  http://www.candelatech.com

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