arpt_mangle.c

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/* 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 <[email protected]>");
MODULE_DESCRIPTION("arptables arp payload mangle target");

static unsigned int
target(struct sk_buff *skb, const struct xt_action_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 int 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 -EINVAL;

    if (mangle->target != NF_DROP && mangle->target != NF_ACCEPT &&
       mangle->target != XT_CONTINUE)
        return -EINVAL;
    return 0;
}

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);