ipt_tos.c

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/* This is a module which is used for setting the TOS field of a packet. */
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <net/checksum.h>

#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv4/ipt_TOS.h>

static unsigned int
target(struct sk_buff **pskb,
       unsigned int hooknum,
       const struct net_device *in,
       const struct net_device *out,
       const void *targinfo,
       void *userinfo)
{
	struct iphdr *iph = (*pskb)->nh.iph;
	const struct ipt_tos_target_info *tosinfo = targinfo;

	if ((iph->tos & IPTOS_TOS_MASK) != tosinfo->tos) {
		u_int16_t diffs[2];

		/* raw socket (tcpdump) may have clone of incoming
                   skb: don't disturb it --RR */
		if (skb_cloned(*pskb) && !(*pskb)->sk) {
			struct sk_buff *nskb = skb_copy(*pskb, GFP_ATOMIC);
			if (!nskb)
				return NF_DROP;
			kfree_skb(*pskb);
			*pskb = nskb;
			iph = (*pskb)->nh.iph;
		}

		diffs[0] = htons(iph->tos) ^ 0xFFFF;
		iph->tos = (iph->tos & IPTOS_PREC_MASK) | tosinfo->tos;
		diffs[1] = htons(iph->tos);
		iph->check = csum_fold(csum_partial((char *)diffs,