Update TCP/IP checksum in headers

Question

I am trying to modify the source and destination address in the IP header manually in kernel when sending the packet. After that, I need to recalculate both IP checksum and TCP checksum.

I am doing it in the following way.

iph = ip_hdr(skb);
iph->saddr = mysaddr;
iph->daddr = mydaddr;
tcph= tcp_hdr(skb);
__tcp_v4_send_check(skb, iph->saddr, iph->daddr);
iph->tot_len = htons(skb->len);
ip_send_check(iph);

But at the receiver, the checksum always fails at TCP layer while it can pass IP layer.

I did much debugs, and found that during normal process, when the packet arrives, the skb->ip_summed is generally CHECKSUM_UNNECESSARY, but if I do the modification at the sender, then this value will be CHECKSUM_NONE when arriving at the receiver.

Can anybody give me some suggestion? Thanks.

Shouldn't the assignment to iph->tot_len precede __tcp_v4_send_check? — abligh

xzhao28 xzhao28 · Accepted Answer · 2014-02-21T08:28:04

As for CHECKSUM_UNNECESSARY and CHECKSUM_NONE, too much details about them, can not describe them in simple word, better read:

<<Understanding.Linux.Network.Internals >> / 19.1.1.2. sk_buff structure.

Post my guess here (hope it helpful):

Since at sender side, you see generally CHECKSUM_UNNECESSARY, this might means your network card is probably able to do checksum verification and computing checksum in hardware.

By default, if TCP stack seeing your network card has this hardware capability, it would not bother computing checksum itself in software. But set CHECKSUM_PARTIAL in egress packet, so that network interface driver would instruct its hardware to computing checksum.

In your case, you compute the checksum by yourself, and should set CHECKSUM_NONE, so network interface would not compute the checksum again for you ( if doing so, break your checksum of modified packet ).

you can just use any packet capture tool (like wireshark) with a hub connecting between sender and receiver, to see if the checksum in captured packets are broken.

Update TCP/IP checksum in headers

1 Answers