ip_nat_dumb.c

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/*
 * INET		An implementation of the TCP/IP protocol suite for the LINUX
 *		operating system.  INET is implemented using the  BSD Socket
 *		interface as the means of communication with the user level.
 *
 *		Dumb Network Address Translation.
 *
 * Version:	$Id: ip_nat_dumb.c,v 1.11 2000/12/13 18:31:48 davem Exp $
 *
 * Authors:	Alexey Kuznetsov, <[e-mail]>
 *
 *		This program is free software; you can redistribute it and/or
 *		modify it under the terms of the GNU General Public License
 *		as published by the Free Software Foundation; either version
 *		2 of the License, or (at your option) any later version.
 *
 * Fixes:
 *		Rani Assaf	:	A zero checksum is a special case
 *					only in UDP
 * 		Rani Assaf	:	Added ICMP messages rewriting
 * 		Rani Assaf	:	Repaired wrong changes, made by ANK.
 *
 *
 * NOTE:	It is just working model of real NAT.
 */

#include <linux/config.h>
#include <linux/types.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <linux/netdevice.h>
#include <net/sock.h>
#include <net/ip.h>
#include <net/icmp.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <net/checksum.h>
#include <linux/route.h>
#include <net/route.h>
#include <net/ip_fib.h>


int
ip_do_nat(struct sk_buff *skb)
{
	struct rtable *rt = (struct rtable*)skb->dst;
	struct iphdr *iph = skb->nh.iph;
	u32 odaddr = iph->daddr;
	u32 osaddr = iph->saddr;
	u16	check;

	IPCB(skb)->flags |= IPSKB_TRANSLATED;

	/* Rewrite IP header */
	iph->daddr = rt->rt_dst_map;
	iph->saddr = rt->rt_src_map;
	iph->check = 0;
	iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);

	/* If it is the first fragment, rewrite protocol headers */

	if (!(iph->frag_off & htons(IP_OFFSET))) {
		u16	*cksum;

		switch(iph->protocol) {
		case IPPROTO_TCP:
			cksum  = (u16*)&((struct tcphdr*)(((char*)iph) + (iph->ihl<<2)))->check;
			if ((u8*)(cksum+1) > skb->tail)
				goto truncated;
			check = *cksum;
			if (skb->ip_summed != CHECKSUM_HW)
				check = ~check;
			check = csum_tcpudp_magic(iph->saddr, iph->daddr, 0, 0, check);
			check = csum_tcpudp_magic(~osaddr, ~odaddr, 0, 0, ~check);
			if (skb->ip_summed == CHECKSUM_HW)
				check = ~check;
			*cksum = check;
			break;
		case IPPROTO_UDP:
			cksum  = (u16*)&((struct udphdr*)(((char*)iph) + (iph->ihl<<2)))->check;
			if ((u8*)(cksum+1) > skb->tail)
				goto truncated;
			if ((check = *cksum) != 0) {
				check = csum_tcpudp_magic(iph->saddr, iph->daddr, 0, 0, ~check);
				check = csum_tcpudp_magic(~osaddr, ~odaddr, 0, 0, ~check);
				*cksum = check ? : 0xFFFF;
			}
			break;
		case IPPROTO_ICMP:
		{
			struct icmphdr *icmph = (struct icmphdr*)((char*)iph + (iph->ihl<<2));
			struct   iphdr *ciph;
			u32 idaddr, isaddr;
			int updated;

			if ((icmph->type != ICMP_DEST_UNREACH) &&
			    (icmph->type != ICMP_TIME_EXCEEDED) &&
			    (icmph->type != ICMP_PARAMETERPROB))
				break;

			ciph = (struct iphdr *) (icmph + 1);

			if ((u8*)(ciph+1) > skb->tail)
				goto truncated;

			isaddr = ciph->saddr;
			idaddr = ciph->daddr;
			updated = 0;

			if (rt->rt_flags&RTCF_DNAT && ciph->saddr == odaddr) {
				ciph->saddr = iph->daddr;
				updated = 1;
			}
			if (rt->rt_flags&RTCF_SNAT) {
				if (ciph->daddr != osaddr) {
					struct   fib_result res;
					struct   rt_key key;
					unsigned flags = 0;

					key.src = ciph->daddr;
					key.dst = ciph->saddr;
					key.iif = skb->dev->ifindex;
					key.oif = 0;
#ifdef CONFIG_IP_ROUTE_TOS
					key.tos = RT_TOS(ciph->tos);
#endif
#ifdef CONFIG_IP_ROUTE_FWMARK
					key.fwmark = 0;
#endif
					/* Use fib_lookup() until we get our own
					 * hash table of NATed hosts -- Rani
				 	 */
					if (fib_lookup(&key, &res) == 0) {
						if (res.r) {
							ciph->daddr = fib_rules_policy(ciph->daddr, &res, &flags);
							if (ciph->daddr != idaddr)
								updated = 1;
						}
						fib_res_put(&res);
					}
				} else {
					ciph->daddr = iph->saddr;
					updated = 1;
				}
			}
			if (updated) {
				cksum  = &icmph->checksum;
				/* Using tcpudp primitive. Why not? */
				check  = csum_tcpudp_magic(ciph->saddr, ciph->daddr, 0, 0, ~(*cksum));
				*cksum = csum_tcpudp_magic(~isaddr, ~idaddr, 0, 0, ~check);
			}
			break;
		}
		default:
			break;
		}
	}
	return NET_RX_SUCCESS;

truncated:
	/* should be return NET_RX_BAD; */
	return -EINVAL;
}