root/net/ipv4/netfilter/arpt_mangle.c

DEFINITIONS

1. target
2. checkentry
3. arpt_mangle_init
4. arpt_mangle_fini

/* module that allows mangling of the arp payload */
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_arp/arpt_mangle.h>
#include <net/sock.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Bart De Schuymer <bdschuym@pandora.be>");
MODULE_DESCRIPTION("arptables arp payload mangle target");

static unsigned int
target(struct sk_buff *skb, const struct xt_target_param *par)
{
        const struct arpt_mangle *mangle = par->targinfo;
        const struct arphdr *arp;
        unsigned char *arpptr;
        int pln, hln;

        if (!skb_make_writable(skb, skb->len))
                return NF_DROP;

        arp = arp_hdr(skb);
        arpptr = skb_network_header(skb) + sizeof(*arp);
        pln = arp->ar_pln;
        hln = arp->ar_hln;
        /* We assume that pln and hln were checked in the match */
        if (mangle->flags & ARPT_MANGLE_SDEV) {
                if (ARPT_DEV_ADDR_LEN_MAX < hln ||
                   (arpptr + hln > skb_tail_pointer(skb)))
                        return NF_DROP;
                memcpy(arpptr, mangle->src_devaddr, hln);
        }
        arpptr += hln;
        if (mangle->flags & ARPT_MANGLE_SIP) {
                if (ARPT_MANGLE_ADDR_LEN_MAX < pln ||
                   (arpptr + pln > skb_tail_pointer(skb)))
                        return NF_DROP;
                memcpy(arpptr, &mangle->u_s.src_ip, pln);
        }
        arpptr += pln;
        if (mangle->flags & ARPT_MANGLE_TDEV) {
                if (ARPT_DEV_ADDR_LEN_MAX < hln ||
                   (arpptr + hln > skb_tail_pointer(skb)))
                        return NF_DROP;
                memcpy(arpptr, mangle->tgt_devaddr, hln);
        }
        arpptr += hln;
        if (mangle->flags & ARPT_MANGLE_TIP) {
                if (ARPT_MANGLE_ADDR_LEN_MAX < pln ||
                   (arpptr + pln > skb_tail_pointer(skb)))
                        return NF_DROP;
                memcpy(arpptr, &mangle->u_t.tgt_ip, pln);
        }
        return mangle->target;
}

static bool checkentry(const struct xt_tgchk_param *par)
{
        const struct arpt_mangle *mangle = par->targinfo;

        if (mangle->flags & ~ARPT_MANGLE_MASK ||
            !(mangle->flags & ARPT_MANGLE_MASK))
                return false;

        if (mangle->target != NF_DROP && mangle->target != NF_ACCEPT &&
           mangle->target != ARPT_CONTINUE)
                return false;
        return true;
}

static struct xt_target arpt_mangle_reg __read_mostly = {
        .name           = "mangle",
        .family         = NFPROTO_ARP,
        .target         = target,
        .targetsize     = sizeof(struct arpt_mangle),
        .checkentry     = checkentry,
        .me             = THIS_MODULE,
};

static int __init arpt_mangle_init(void)
{
        return xt_register_target(&arpt_mangle_reg);
}

static void __exit arpt_mangle_fini(void)
{
        xt_unregister_target(&arpt_mangle_reg);
}

module_init(arpt_mangle_init);
module_exit(arpt_mangle_fini);