I want to implement a traffic forwarder through netfilter to forward 0.0.0.0:8080 to the intranet 127.0.0.1:80. The problem is that when the checksum is calculated after modifying the TCP and IP data in the NF_INET_PRE_ROUTING stage, some of the calculated checksum values ​​are correct and some are incorrect.

        if (iph->daddr == in_aton("192.168.31.211") && (iph->saddr = in_aton("127.0.0.1")) && ntohs(tcph->source) == 22) {
            // Restore the original source IP address
            iph->saddr = in_aton("192.168.31.126");
            tcph->source = htons(8080);

            // Recalculate the IP checksum
            iph->check = 0;
            iph->check = ip_fast_csum((u8 *)iph, iph->ihl);
            
            // Recalculate the TCP checksum
            tcph->check = 0;
            tcplen = (skb->len - (iph->ihl << 2));
            tcph->check = csum_tcpudp_magic(iph->saddr, iph->daddr, tcplen, IPPROTO_TCP, csum_partial(tcph, tcplen, 0));
            skb->ip_summed = CHECKSUM_NONE;
        }

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Able to calculate the checksum correctly