Incorrect and variable IP packet checksum value - c

Question

GOAL: crafting a packet from zero and send it.
PROBLEM: after I enabled "validate checksum" on wireshark I noticed that the expected checksum value should be 0xa85d but the checksum function I'm using computes a different value each time. Why the checksum is changing if the packet is always the same? What is the cause of the checksum mismatch between the one I compute and the one expected by wireshark? Thanks
#include <sys/socket.h>
#include <linux/if_packet.h>
#include <net/ethernet.h> /* the L2 protocols */
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <stdio.h>
#include <arpa/inet.h> /* htons */
#include <sys/ioctl.h>
#include <net/if.h> /* ifreq */
#include <string.h>
#include <stdlib.h>
#include <linux/seccomp.h> /* seccomp_data */
#define BUF_SIZE 1024
// https://gist.github.com/leonid-ed/909a883c114eb58ed49f
unsigned short csum(unsigned short *buf, int nwords)
{
unsigned long sum;
for(sum=0; nwords>0; nwords--)
sum += *buf++;
sum = (sum >> 16) + (sum &0xffff);
sum += (sum >> 16);
return (unsigned short)(~sum);
}
int main(){
const char IF[] = "lo"; // modify to change interface
int sockfd, ifindex, tx_len=sizeof(struct ether_header)+sizeof(struct iphdr)+sizeof(struct udphdr);
struct ifreq ifr;
size_t if_name_len;
char buf[BUF_SIZE];
struct ether_header *eh = (struct ether_header *) buf;
struct iphdr *iph = (struct iphdr *) (buf + sizeof(struct ether_header));
struct udphdr *udph = (struct udphdr *) (buf + sizeof(struct ether_header) + sizeof(struct iphdr));
unsigned char *data = buf + sizeof(struct ether_header) + sizeof(struct iphdr) + sizeof(struct udphdr);
u_int16_t src_port, dst_port;
struct sockaddr_ll dst_addr;
struct seccomp_data sec_payload;
const char dmac[] = {0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff};
const char smac[] = {0x00, 0xd0, 0x56, 0xf2, 0xb5, 0x12};
// create raw socket to send/receive ethernet frames that can transport all protocols
if ((sockfd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1) {
perror("socket");
}
// get interface name length
if_name_len = strlen(IF);
if(if_name_len < IF_NAMESIZE) {
strncpy(ifr.ifr_name, IF, strlen(IF));
ifr.ifr_name[if_name_len]=0;
}
// get the interface index number
if(ioctl(sockfd, SIOCGIFINDEX, &ifr) == -1){
perror("ioctl");
}
ifindex = ifr.ifr_ifindex;
// build ethernet header
memcpy(eh->ether_dhost, dmac, ETHER_ADDR_LEN);
memcpy(eh->ether_shost, smac, ETHER_ADDR_LEN);
eh->ether_type = htons(ETH_P_IP);
// add a struct seccomp_data as data
memset(&sec_payload,5,sizeof(struct seccomp_data));
data = (char*)malloc(sizeof(struct seccomp_data));
memcpy(data, (const unsigned char *)&sec_payload, sizeof(struct seccomp_data));
int i;
for(i=0;i<sizeof(sec_payload);i++){
buf[tx_len++] = data[i];
printf("%02X ",data[i]);
}
// build ip header
iph->ihl = 5;
iph->version = 4;
iph->tos = 0;
iph->tot_len = htons(sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(sec_payload));
iph->id = htons(54321);
iph->frag_off = 0;
iph->ttl = 64;
iph->protocol = IPPROTO_UDP;
iph->check = 0;
iph->saddr = inet_addr("127.0.0.1");
iph->daddr = inet_addr("127.0.0.1");
// build udp header
udph->source = htons(8090);
udph->dest = htons(8091);
udph->len = htons(sizeof(struct udphdr) + sizeof(sec_payload));
iph->check = csum((unsigned short *)buf, sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(sec_payload));
memset(&dst_addr,0,sizeof(struct sockaddr_ll));
dst_addr.sll_ifindex = ifr.ifr_ifindex;
dst_addr.sll_halen = ETH_ALEN;
memcpy(dst_addr.sll_addr, dmac, ETH_ALEN);
if (sendto(sockfd, buf, tx_len, 0, (struct sockaddr*)&dst_addr, sizeof(struct sockaddr_ll)) < 0)
printf("Send failed\n");
return 0;
}