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Date:	Fri, 11 Apr 2014 17:49:28 -0700
From:	Eric Dumazet <>
To:	Arvid Brodin <>
Cc:	"" <>
Subject: Re: TCP checksum problem

On Sat, 2014-04-12 at 02:25 +0200, Arvid Brodin wrote:
> Hi,
> I'm trying to insert a 6-byte HSR header into a newly copied skb,
> between the ethernet header and the IP header. The skb comes from my
> virtual device's ndo_start_xmit() function and is to be sent on to a
> physical interface after modification. Essentially, this is what I'm
> doing:
> 	skb = skb_copy_expand(skb, skb_headroom(skb) + HSR_HLEN, 0,
> 	/* Move ethernet header to make room for the HSR header */
> 	src = skb_mac_header(skb);
> 	WARN_ON_ONCE(src != skb->data);
> 	dst = skb_push(skb, HSR_HLEN);
> 	memmove(dst, src, ETH_HLEN);
> 	skb_reset_mac_header(skb);
> 	/* Fill in the 6-byte header */
> 	hsr_fill_tag(skb, frame, port);
> 	/* Point the skb to the physical interface */
> 	skb->dev = port->dev; 
> 	dev_queue_xmit(skb);
> But something goes wrong with the TCP checksum on the way out. Arp,
> ping etc works fine, but anything using TCP is discarded by the
> receiver due to incorrect TCP checksums.
> Using wireshark and some printks, I've managed to decide that the
> updated TCP checksum is written offset by -6 bytes from where it
> should be (into what wireshark calls the TCP header "acknowledgement
> number" field).
> Any idea why this happens? I've spent a few days on this now and I'm
> running out of ideas...
> This is run on the cadence/macb ethernet device. Before the call to
> skb_copy_expand(), skb->ip_summed is CHECKSUM_PARTIAL, skb->csum_start
> is 120, and skb->csum_offset is 16. The total frame length is 80 bytes
> (on the wire as reported by wireshark - I think this is excluding the
> FCS).

You need to set skb->csum_start to the new offset.

( skb_transport_header(skb) - skb->head )

Check how __tcp_v4_send_check() sets csum_start & csum_offset

(csum_offset you can leave it as is)

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