c - Add data to packet of a specific protocol -

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i'm implementing new protocol called xor. first packets created in user space , sent network. after packet arrive other node in network. need create module intercept every module sent , add header or data. create module put data packet don't have success.

i try things when activate module packets dropped. module doing:

struct xorhdr {     int in;     int out; };  static unsigned int asnfwd_hook(const struct nf_hook_ops *ops, struct sk_buff *skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *)) {     struct iphdr *iph;     struct asnfwd_opt *opt;     __be32 addr = 0;     struct tcphdr *th;     struct udphdr *uh;      /* sanity check */     if (!skb)         goto accept;      /* recover ipv4 header */     iph = ip_hdr(skb);     if (!iph)         goto accept;      if (iph->protocol == 17) {         uh = (struct udphdr *) skb_transport_header(skb);         if ((unsigned int) ntohs(uh->source) == xor_port && (unsigned int) ntohs(uh->dest) == xor_port) {             printk("\n****************************\n");             printk("hook %s\n", (in ? "pre-routing" : "local-out"));             printk("(ogirinal) %pi4 %pi4.\n", &iph->saddr, &iph->daddr);             printk(kern_info "free:%d", skb_headroom(skb));              if (in) {                 struct xorhdr *ptr;                  if (skb->tail + sizeof(struct xorhdr) < skb->end) {                     unsigned char* tail_ptr = skb_tail_pointer(skb);                     ptr = (struct xorhdr *)(tail_ptr);                     printk(kern_info "cbin:%d\n", ptr->in);                     printk(kern_info "cbout:%d\n", ptr->out);                 }              } else {                 if (skb_headroom(skb) > sizeof(struct xorhdr)) {                     struct xorhdr *xorh;                     xorh = kmalloc(sizeof(struct xorhdr), gfp_atomic);                     unsigned char *new_data;                      new_data = skb_tail_pointer(skb);                     if (skb->tail + sizeof(struct xorhdr) < skb->end) {                         xorh->in = 6;                         xorh->out = 5;                         printk(kern_info "cbin:%d\n", xorh->in);                         printk(kern_info "cbout:%d\n", xorh->out);                         skb->tail += sizeof(struct xorhdr);                         skb->len += sizeof(struct xorhdr);                         memcpy(new_data, xorh, sizeof(struct xorhdr));                     }                      kfree(xorh);                      // update ip header pointer                      iph = ip_hdr(skb);                      // calculate udp checksum                     uh = udp_hdr(skb);                     uh->check = 0;                     uh->check = csum_tcpudp_magic(iph->saddr, iph->daddr, ip_payload_len(iph), ipproto_udp, csum_partial(uh, ip_payload_len(iph), skb->csum));                     skb->ip_summed = checksum_none;                      // calculate ip checksum                      ip_send_check (iph);                  }             }             printk("\n****************************\n\n");         }       }      accept:         return nf_accept; }

what i'm doing wrong?

i solved problem moving skb->tail original position doing this: skb->tail -= sizeof(struct xorhdr);. when receive package in other side use data put, , return skb->tail original position.

struct xorhdr {     int in;     int out; };  static unsigned int asnfwd_hook(const struct nf_hook_ops *ops, struct sk_buff *skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *)) {     struct iphdr *iph;     struct asnfwd_opt *opt;     __be32 addr = 0;     struct tcphdr *th;     struct udphdr *uh;      /* sanity check */     if (!skb)         goto accept;      /* recover ipv4 header */     iph = ip_hdr(skb);     if (!iph)         goto accept;      if (iph->protocol == 17) {         uh = (struct udphdr *) skb_transport_header(skb);         if ((unsigned int) ntohs(uh->source) == xor_port && (unsigned int) ntohs(uh->dest) == xor_port) {             printk("\n****************************\n");             printk("hook %s\n", (in ? "pre-routing" : "local-out"));             printk("(ogirinal) %pi4 %pi4.\n", &iph->saddr, &iph->daddr);             printk(kern_info "free:%d", skb_headroom(skb));              if (in) {                 struct xorhdr *ptr;                  if (skb->tail + sizeof(struct xorhdr) < skb->end) {                     unsigned char* tail_ptr = skb_tail_pointer(skb);                     ptr = (struct xorhdr *)(tail_ptr);                     printk(kern_info "cbin:%d\n", ptr->in);                     printk(kern_info "cbout:%d\n", ptr->out);                     skb->tail -= sizeof(struct xorhdr);                      // update ip header pointer                      iph = ip_hdr(skb);                      // calculate udp checksum                     uh = udp_hdr(skb);                     uh->check = 0;                     uh->check = csum_tcpudp_magic(iph->saddr, iph->daddr, ip_payload_len(iph), ipproto_udp, csum_partial(uh, ip_payload_len(iph), skb->csum));                     skb->ip_summed = checksum_none;                      // calculate ip checksum                      ip_send_check (iph);                 }              } else {                 if (skb_headroom(skb) > sizeof(struct xorhdr)) {                     struct xorhdr *xorh;                     xorh = kmalloc(sizeof(struct xorhdr), gfp_atomic);                     unsigned char *new_data;                      new_data = skb_tail_pointer(skb);                     if (skb->tail + sizeof(struct xorhdr) < skb->end) {                         xorh->in = 6;                         xorh->out = 5;                         printk(kern_info "cbin:%d\n", xorh->in);                         printk(kern_info "cbout:%d\n", xorh->out);                         skb->tail += sizeof(struct xorhdr);                         skb->len += sizeof(struct xorhdr);                         memcpy(new_data, xorh, sizeof(struct xorhdr));                     }                      kfree(xorh);                      // update ip header pointer                      iph = ip_hdr(skb);                      // calculate udp checksum                     uh = udp_hdr(skb);                     uh->check = 0;                     uh->check = csum_tcpudp_magic(iph->saddr, iph->daddr, ip_payload_len(iph), ipproto_udp, csum_partial(uh, ip_payload_len(iph), skb->csum));                     skb->ip_summed = checksum_none;                      // calculate ip checksum                      ip_send_check (iph);                  }             }             printk("\n****************************\n\n");         }       }      accept:         return nf_accept; }

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