root/net/ipv4/netfilter/nf_nat_proto_tcp.c

DEFINITIONS

1. tcp_unique_tuple
2. tcp_manip_pkt

/* (C) 1999-2001 Paul `Rusty' Russell
 * (C) 2002-2006 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/types.h>
#include <linux/init.h>
#include <linux/ip.h>
#include <linux/tcp.h>

#include <linux/netfilter.h>
#include <linux/netfilter/nfnetlink_conntrack.h>
#include <net/netfilter/nf_nat.h>
#include <net/netfilter/nf_nat_rule.h>
#include <net/netfilter/nf_nat_protocol.h>
#include <net/netfilter/nf_nat_core.h>

static u_int16_t tcp_port_rover;

static bool
tcp_unique_tuple(struct nf_conntrack_tuple *tuple,
                 const struct nf_nat_range *range,
                 enum nf_nat_manip_type maniptype,
                 const struct nf_conn *ct)
{
        return nf_nat_proto_unique_tuple(tuple, range, maniptype, ct,
                                         &tcp_port_rover);
}

static bool
tcp_manip_pkt(struct sk_buff *skb,
              unsigned int iphdroff,
              const struct nf_conntrack_tuple *tuple,
              enum nf_nat_manip_type maniptype)
{
        const struct iphdr *iph = (struct iphdr *)(skb->data + iphdroff);
        struct tcphdr *hdr;
        unsigned int hdroff = iphdroff + iph->ihl*4;
        __be32 oldip, newip;
        __be16 *portptr, newport, oldport;
        int hdrsize = 8; /* TCP connection tracking guarantees this much */

        /* this could be a inner header returned in icmp packet; in such
           cases we cannot update the checksum field since it is outside of
           the 8 bytes of transport layer headers we are guaranteed */
        if (skb->len >= hdroff + sizeof(struct tcphdr))
                hdrsize = sizeof(struct tcphdr);

        if (!skb_make_writable(skb, hdroff + hdrsize))
                return false;

        iph = (struct iphdr *)(skb->data + iphdroff);
        hdr = (struct tcphdr *)(skb->data + hdroff);

        if (maniptype == IP_NAT_MANIP_SRC) {
                /* Get rid of src ip and src pt */
                oldip = iph->saddr;
                newip = tuple->src.u3.ip;
                newport = tuple->src.u.tcp.port;
                portptr = &hdr->source;
        } else {
                /* Get rid of dst ip and dst pt */
                oldip = iph->daddr;
                newip = tuple->dst.u3.ip;
                newport = tuple->dst.u.tcp.port;
                portptr = &hdr->dest;
        }

        oldport = *portptr;
        *portptr = newport;

        if (hdrsize < sizeof(*hdr))
                return true;

        inet_proto_csum_replace4(&hdr->check, skb, oldip, newip, 1);
        inet_proto_csum_replace2(&hdr->check, skb, oldport, newport, 0);
        return true;
}

const struct nf_nat_protocol nf_nat_protocol_tcp = {
        .protonum               = IPPROTO_TCP,
        .me                     = THIS_MODULE,
        .manip_pkt              = tcp_manip_pkt,
        .in_range               = nf_nat_proto_in_range,
        .unique_tuple           = tcp_unique_tuple,
#if defined(CONFIG_NF_CT_NETLINK) || defined(CONFIG_NF_CT_NETLINK_MODULE)
        .range_to_nlattr        = nf_nat_proto_range_to_nlattr,
        .nlattr_to_range        = nf_nat_proto_nlattr_to_range,
#endif
};