Recalculating TCP Checksum in Linux Kernel Module - c

Question

I am building a Netfilter module which modifies the TCP payload for packets destined to a specific port. I do not modify neither the IP header nor the TCP header. The module is called at the first point in the Netfilter directly after a packet is received (NF_INET_PRE_ROUTING). Therefore I have to recalculate the TCP checksum field in each modified packet. I already saw some posts here and used there methods to recalculate TCP checksum, but non of these methods worked. Below is the two methods I used:
Method 1:
tcplen = (skb->len - (iph->ihl << 2)); /* tcplen is the length of the
* skb - the ip-header length */
tcph->check = 0;
tcph->check = csum_tcpudp_magic(iph->saddr, iph->daddr,
tcplen, IPPROTO_TCP,
csum_partial(tcph, tcplen, 0));
skb->ip_summed = CHECKSUM_UNNECESSARY;
Method 2:
tcph->check = ~(~tcph->check + ~new_field + new_field);
skb->ip_summed = CHECKSUM_UNNECESSARY;
In the two cases, I get the following error:
verif_dss_csum csum is wrong: 0x874a data_seq 2760801057
dss_csum_added 1 overflowed 0 iterations 1
Any solution for this problem? I am developing my module for Linux Kernel 4.4.83

Answer 1

**The Above code only helps with calculating checksum of pseudo header **
here is the code for calculating IP header Checksum and TCP/UDP Checksum
void UpdateChecksum(char *prefix,struct sk_buff *skb){
struct iphdr *ip_header;
ip_header = ip_hdr(skb);
skb->ip_summed = CHECKSUM_NONE; //stop offloading
skb->csum_valid = 0;
ip_header->check = 0;
ip_header->check = ip_fast_csum((u8 *)ip_header, ip_header->ihl);
if ( (ip_header->protocol == IPPROTO_TCP) || (ip_header->protocol == IPPROTO_UDP) ) {
if(skb_is_nonlinear(skb))
skb_linearize(skb); // very important.. You need this.
if (ip_header->protocol == IPPROTO_TCP) {
struct tcphdr *tcpHdr;
unsigned int tcplen;
tcpHdr = tcp_hdr(skb);
skb->csum =0;
tcplen = ntohs(ip_header->tot_len) - ip_header->ihl*4;
tcpHdr->check = 0;
tcpHdr->check = tcp_v4_check(tcplen, ip_header->saddr, ip_header->daddr, csum_partial((char *)tcpHdr, tcplen, 0));
//printk(KERN_INFO "%s: TCP Len :%d, Computed TCP Checksum :%x : Network : %x\n",prefix,tcplen,tcpHdr->check,htons(tcpHdr->check));
} else if (ip_header->protocol == IPPROTO_UDP) {
struct udphdr *udpHdr;
unsigned int udplen;
udpHdr = udp_hdr(skb);
skb->csum =0;
udplen = ntohs(ip_header->tot_len) - ip_header->ihl*4;
udpHdr->check = 0;
udpHdr->check = udp_v4_check(udplen,ip_header->saddr, ip_header->daddr,csum_partial((char *)udpHdr, udplen, 0));;
//printk(KERN_INFO "%s: UDP Len :%d, Computed UDP Checksum :%x : Network : %x\n",prefix,udplen,udpHdr->check,htons(udpHdr->check));
}
}
}

Answer 2

There is no need to recalculate the ipv4 header checksum, if you change only the tcp payload.
So if I understand you correctly, it should be just:
/* ... */
struct iphdr *ip_header = ip_hdr(skb);
if (ip_header->protocol == IPPROTO_TCP) {
struct tcphdr *tcp_header = tcp_hdr(skb);
if ((unsigned int)ntohs(tcp_header->dest) == some_port) {
unsigned char *data = (char *)tcp_header + tcp_hdrlen(skb);
/* altering TCP-payload */
tcp_header->check = 0;
tcp_header->check = tcp_v4_check(ntohs(ip_header->tot_len) - (ip_header->ihl << 2),
ip_header->saddr, ip_header->daddr,
csum_partial(tcp_header, tcp_header->doff << 2, 0));
/* ... */
/* e.g return NF_ACCEPT */