Unable to parse HTTP packet using netfilter hooks in kernel module

Question

I am trying to parse HTTP packets using netfilter hooks in my kernel module. I came across this code snippet on How to print data from TCP packets which I modified to make it build. However, I am unable to see any logs related to HTTP in dmesg even though I have loaded the kernel module successfully.

I have tried to debug the issue but haven't been able to figure out what's going wrong, the payload is not matching with HTTP.I suspect that the problem might be related to the fact that HTTP packets are carried over TCP and I am not properly handling the TCP protocol.

Code I am using:

#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/tcp.h>

#define PTCP_WATCH_PORT 80 /* HTTP port */

static struct nf_hook_ops nfho;

static unsigned int ptcp_hook_func(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;        /* IPv4 header */
    struct tcphdr *tcph;      /* TCP header */
    u16 sport, dport;         /* Source and destination ports */
    u32 saddr, daddr;         /* Source and destination addresses */
    unsigned char *user_data; /* TCP data begin pointer */
    unsigned char *tail;      /* TCP data end pointer */
    unsigned char *it;        /* TCP data iterator */

    /* Network packet is empty, seems like some problem occurred. Skip it */
    if (!skb)
        return NF_ACCEPT;

    iph = ip_hdr(skb); /* get IP header */

    /* Skip if it's not TCP packet */
    if (iph->protocol != IPPROTO_TCP)
        return NF_ACCEPT;

    tcph = tcp_hdr(skb); /* get TCP header */

    /* Convert network endianness to host endiannes */
    saddr = ntohl(iph->saddr);
    daddr = ntohl(iph->daddr);
    sport = ntohs(tcph->source);
    dport = ntohs(tcph->dest);

    /* Watch only port of interest */
    if (sport != PTCP_WATCH_PORT)
        return NF_ACCEPT;

    /* Calculate pointers for begin and end of TCP packet data */
    user_data = (unsigned char *)((unsigned char *)tcph + (tcph->doff * 4));
    tail = skb_tail_pointer(skb);

    /* ----- Print all needed information from received TCP packet ------ */

    /* Show only HTTP packets */
    if (user_data[0] != 'H' || user_data[1] != 'T' || user_data[2] != 'T' ||
        user_data[3] != 'P')
    {
        // Control is reaching here always.
        return NF_ACCEPT;
    }

    /* Print packet route */
    printk("print_tcp: %pI4h:%d -> %pI4h:%d\n", &saddr, sport,
           &daddr, dport);

    /* Print TCP packet data (payload) */
    printk("print_tcp: data:\n");
    for (it = user_data; it != tail; ++it)
    {
        char c = *(char *)it;

        if (c == '\0')
            break;

        printk("%c", c);
    }
    printk("\n\n");

    return NF_ACCEPT;
}

static int __init ptcp_init(void)
{
    int res;

    nfho.hook = (nf_hookfn *)ptcp_hook_func; /* hook function */
    nfho.hooknum = NF_INET_PRE_ROUTING;      /* received packets */
    nfho.pf = PF_INET;                       /* IPv4 */
    nfho.priority = NF_IP_PRI_FIRST;         /* max hook priority */

    res = nf_register_net_hook(&init_net,&nfho);
    if (res < 0)
    {
        printk("print_tcp: error in nf_register_hook()\n");
        return res;
    }

    printk("print_tcp: loaded\n");
    return 0;
}

static void __exit ptcp_exit(void)
{
    nf_unregister_net_hook(&init_net,&nfho);
    printk("print_tcp: unloaded\n");
}

module_init(ptcp_init);
module_exit(ptcp_exit);

MODULE_DESCRIPTION("Module for printing HTTP packet data");
MODULE_LICENSE("GPL");