Functions for getting packet headers available in sk_buff - c

Question

There are functions in Linux for getting Ethernet header, IP header,and UDP headers likes these
udp_hdr(skb)
ip_hdr
skb_push(skb, ETH_HLEN)
But I could not find any function for getting payload like body of a packet like i.e. which contains body so I can write HTTP or other protocol data. in Linux Device Driver book or after searching couldn't find it. so question is how to compose UDP packet with Ethernet, IP, UDP headers and payload in kernel?

Answer 1

any function for getting payload like body of a packet
You can access payload different ways depending on what you want to do the next time. E.g.:
struct iphdr *iph = ip_hdr(skb);
if (iph->protocol == IPPROTO_UDP) {
struct udphdr *udph = udp_hdr(skb);
// E.g. check for UDP port
struct myl7_header *l7h = (struct myl7_header *)(udph + sizeof(struct udphdr));
// ...
}
Or you can pull the network and transport headers if you want to reconstruct encapsulation further or they are no longer needed (rough example, not with all possible sanity checks):
struct iphdr *iph = ip_hdr(skb);
if (iph->protocol == IPPROTO_UDP) {
struct udphdr *udph = udp_hdr(skb);
struct myl7_header *l7h;
// E.g. check for UDP port
skb_pull(skb, sizeof(struct iphdr));
skb_pull(skb, sizeof(struct udphdr));
l7h = (struct myl7_header *)skb->data;
// tansport protocol payload's length:
// skb->len or skb_tail_pointer(skb) - skb->data
}
I don't really know what you mean by payloadhtml, L7-protocol it not so kernel specific thing, so in general we are talking about transport protocol's payload.
N.B.: ip_hdr(), udp_hdr() functions imply that non-paged (linear) skb is used.
Related: What's the correct way to process all the payload of a sk_buff packet in Linux