I've been playing for a while with sockets in kernel-space, that's the way i've found to have a kernel thread listening on a UDP port

=== Howto create a UDP Server in kernel-space (for Linux 2.6) ===

First of all, if you are playing with Linux 2.4, see at http://br.kernelnewbies.org/docs/modulos_guia.html#threads, where can be found a clear example on how to use kernel threads and sockets in kernel 2.4

In a second place, '''take into account that kernel-space servers are not the most desiderable thing you can do''', user-space servers are prefered in almost every case.

The code for the client is not provided, please, if you have one post it! ;)

(please inform to topi@phreaker.net if you found a bug on it)

{{{
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <config/modversions.h>
#endif

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/kthread.h>

#include <linux/errno.h>
#include <linux/types.h>

#include <linux/netdevice.h>
#include <linux/ip.h>
#include <linux/in.h>

#include <linux/delay.h>

#define DEFAULT_PORT 2325
#define MODULE_NAME "ksocket"

struct kthread_t
{
	struct task_struct *thread;
	struct socket *sock;
	struct sockaddr_in addr;
	int running;
};

/* function prototypes */
int ksocket_receive(struct socket *sock, struct sockaddr_in *addr, unsigned char *buf, int len);

struct socket *sock;
struct kthread_t *kthread = NULL;

static void ksocket_start(void)
{
	int size, err;
	int bufsize = 1024;
	unsigned char buf[bufsize];

	/* kernel thread initialization */
	lock_kernel();
	kthread->running = 1;
	current->flags |= PF_NOFREEZE;

	/* daemonize (take care with signals, after daemonize() they are disabled) */
	daemonize(MODULE_NAME);
	allow_signal(SIGKILL);
	unlock_kernel();

	/* create a socket */
	if ((err = sock_create(AF_INET, SOCK_DGRAM, IPPROTO_UDP, &kthread->sock)) < 0) {
		printk(KERN_INFO MODULE_NAME": Could not create a datagram socket, error = %d\n", -ENXIO);
		goto out;
	}

	memset(&kthread->addr, 0, sizeof(struct sockaddr));
	kthread->addr.sin_family		= AF_INET;
	kthread->addr.sin_addr.s_addr	= htonl(INADDR_ANY);
	kthread->addr.sin_port		= htons(DEFAULT_PORT);

	if ((err = kthread->sock->ops->bind(kthread->sock, (struct sockaddr *)&kthread->addr, sizeof(struct sockaddr))) < 0) {
		printk(KERN_INFO MODULE_NAME": Could not bind to socket, error = %d\n", -err);
		goto close_and_out;
	}
	//kthread->sock->sk->sk_family = AF_INET;

	printk(KERN_INFO MODULE_NAME": listening on port %d\n", DEFAULT_PORT);

	/* main loop */
	for (;;)
	{
		memset(&buf, 0, bufsize);
		size = ksocket_receive(kthread->sock, &kthread->addr, buf, bufsize);

		if (signal_pending(current))
			break;

		if (size < 0)
			printk(KERN_INFO MODULE_NAME": error getting datagram, sock_recvmsg error = %d\n", size);
		else {
			printk(KERN_INFO MODULE_NAME": received %d bytes\n", size);
			/* data processing */
		}
	}

close_and_out:
	sock_release(kthread->sock);
	kthread->sock = NULL;

out:
	kthread->thread = NULL;
	kthread->running = 0;
}

int ksocket_receive(struct socket* sock, struct sockaddr_in* addr, unsigned char* buf, int len)
{
	struct msghdr msg;
	struct iovec iov;
	mm_segment_t oldfs;
	int size = 0;

	if (sock->sk==NULL) return 0;

	iov.iov_base = buf;
	iov.iov_len = len;

	msg.msg_flags = 0;
	msg.msg_name = addr;
	msg.msg_namelen  = sizeof(struct sockaddr_in);
	msg.msg_control = NULL;
	msg.msg_controllen = 0;
	msg.msg_iov = &iov;
	msg.msg_iovlen = 1;
	msg.msg_control = NULL;

	oldfs = get_fs();
	set_fs(KERNEL_DS);
	size = sock_recvmsg(sock,&msg,len,msg.msg_flags);
	set_fs(oldfs);

	return size;
}

int __init ksocket_init(void)
{
	kthread = kmalloc(sizeof(struct kthread_t), GFP_KERNEL);
	memset(kthread, 0, sizeof(struct kthread_t));

	/* start kernel thread */
	kthread->thread = kthread_run((void *)ksocket_start, NULL, MODULE_NAME);
	if (IS_ERR(kthread->thread)) {
		printk(KERN_INFO MODULE_NAME": unable to start kernel thread\n");
		kfree(kthread);
		kthread = NULL;
		return -ENOMEM;
	}

	return 0;
}

void __exit ksocket_exit(void)
{
	int err;

	if (kthread->thread==NULL)
		printk(KERN_INFO MODULE_NAME": no kernel thread to kill\n");
	else {
		lock_kernel();
		err = kill_proc(kthread->thread->pid, SIGKILL, 1);
		unlock_kernel();

		/* wait for kernel thread to die */
		if (err < 0)
			printk(KERN_INFO MODULE_NAME": unknown error %d while trying to terminate kernel thread\n",-err);
		else {
			while (kthread->running == 1)
				msleep(10);
			printk(KERN_INFO MODULE_NAME": succesfully killed kernel thread!\n");
		}
	}

	/* free allocated resources before exit */
	if (kthread->sock != NULL) {
		sock_release(kthread->sock);
		kthread->sock = NULL;
	}

	kfree(kthread);
	kthread = NULL;

	printk(KERN_INFO MODULE_NAME": module unloaded\n");
}

/* init and cleanup functions */
module_init(ksocket_init);
module_exit(ksocket_exit);

/* module information */
MODULE_DESCRIPTION("kernel thread listening on a UDP socket (code example)");
MODULE_AUTHOR("Toni Garcia-Navarro <topi@phreaker.net>");
MODULE_LICENSE("GPL");
}}}