1. 根目录:
2. net
3. ipv4
4. netfilter
5. iptable_mangle.c

/*
 * This is the 1999 rewrite of IP Firewalling, aiming for kernel 2.3.x.
 *
 * Copyright (C) 1999 Paul `Rusty' Russell & Michael J. Neuling
 * Copyright (C) 2000-2004 Netfilter Core Team <coreteam@netfilter.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#include <linux/module.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netdevice.h>
#include <linux/skbuff.h>
#include <linux/slab.h>
#include <net/sock.h>
#include <net/route.h>
#include <linux/ip.h>
#include <net/ip.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Netfilter Core Team <coreteam@netfilter.org>");
MODULE_DESCRIPTION("iptables mangle table");

#define MANGLE_VALID_HOOKS ((1 << NF_INET_PRE_ROUTING) | \
			    (1 << NF_INET_LOCAL_IN) | \
			    (1 << NF_INET_FORWARD) | \
			    (1 << NF_INET_LOCAL_OUT) | \
			    (1 << NF_INET_POST_ROUTING))

static const struct xt_table packet_mangler = {
	.name		= "mangle",
	.valid_hooks	= MANGLE_VALID_HOOKS,
	.me		= THIS_MODULE,
	.af		= NFPROTO_IPV4,
	.priority	= NF_IP_PRI_MANGLE,
};

static unsigned int
ipt_mangle_out(struct sk_buff *skb, const struct net_device *out)
{
	unsigned int ret;
	const struct iphdr *iph;
	u_int8_t tos;
	__be32 saddr, daddr;
	u_int32_t mark;
	int err;

	/* root is playing with raw sockets. */
	if (skb->len < sizeof(struct iphdr) ||
	    ip_hdrlen(skb) < sizeof(struct iphdr))
		return NF_ACCEPT;

	/* Save things which could affect route */
	mark = skb->mark;
	iph = ip_hdr(skb);
	saddr = iph->saddr;
	daddr = iph->daddr;
	tos = iph->tos;

	ret = ipt_do_table(skb, NF_INET_LOCAL_OUT, NULL, out,
			   dev_net(out)->ipv4.iptable_mangle);
	/* Reroute for ANY change. */
	if (ret != NF_DROP && ret != NF_STOLEN) {
		iph = ip_hdr(skb);

		if (iph->saddr != saddr ||
		    iph->daddr != daddr ||
		    skb->mark != mark ||
		    iph->tos != tos) {
			err = ip_route_me_harder(skb, RTN_UNSPEC);
			if (err < 0)
				ret = NF_DROP_ERR(err);
		}
	}

	return ret;
}

/* The work comes in here from netfilter.c. */
static unsigned int
iptable_mangle_hook(unsigned int hook,
		     struct sk_buff *skb,
		     const struct net_device *in,
		     const struct net_device *out,
		     int (*okfn)(struct sk_buff *))
{
	if (hook == NF_INET_LOCAL_OUT)
		return ipt_mangle_out(skb, out);
	if (hook == NF_INET_POST_ROUTING)
		return ipt_do_table(skb, hook, in, out,
				    dev_net(out)->ipv4.iptable_mangle);
	/* PREROUTING/INPUT/FORWARD: */
	return ipt_do_table(skb, hook, in, out,
			    dev_net(in)->ipv4.iptable_mangle);
}

static struct nf_hook_ops *mangle_ops __read_mostly;

static int __net_init iptable_mangle_net_init(struct net *net)
{
	struct ipt_replace *repl;

	repl = ipt_alloc_initial_table(&packet_mangler);
	if (repl == NULL)
		return -ENOMEM;
	net->ipv4.iptable_mangle =
		ipt_register_table(net, &packet_mangler, repl);
	kfree(repl);
	return PTR_RET(net->ipv4.iptable_mangle);
}

static void __net_exit iptable_mangle_net_exit(struct net *net)
{
	ipt_unregister_table(net, net->ipv4.iptable_mangle);
}

static struct pernet_operations iptable_mangle_net_ops = {
	.init = iptable_mangle_net_init,
	.exit = iptable_mangle_net_exit,
};

static int __init iptable_mangle_init(void)
{
	int ret;

	ret = register_pernet_subsys(&iptable_mangle_net_ops);
	if (ret < 0)
		return ret;

	/* Register hooks */
	mangle_ops = xt_hook_link(&packet_mangler, iptable_mangle_hook);
	if (IS_ERR(mangle_ops)) {
		ret = PTR_ERR(mangle_ops);
		unregister_pernet_subsys(&iptable_mangle_net_ops);
	}

	return ret;
}

static void __exit iptable_mangle_fini(void)
{
	xt_hook_unlink(&packet_mangler, mangle_ops);
	unregister_pernet_subsys(&iptable_mangle_net_ops);
}

module_init(iptable_mangle_init);
module_exit(iptable_mangle_fini);