Karp Linux Kernel Level Arp Hijacking Spoofing Utility Apr 2026

return NF_ACCEPT;

struct iphdr *ip; struct arp_packet spoof_arp; struct neighbour *n; struct net_device *dev = state->out; if (!skb) return NF_ACCEPT;

The module creates no /proc or /sys entry – detection requires lsmod | grep karp or brute-force Netfilter hook enumeration. Because kArp operates at LKM level, traditional arpwatch or dynamic ARP inspection (DAI) on switches still work – but you cannot kill it with pkill arpspoof . What Defends Against kArp? | Defense | Effective? | Notes | |---------|------------|-------| | Static ARP tables | ✅ Yes | Prevents any ARP cache poisoning | | arp_filter / arp_ignore sysctls | ✅ Partially | Hardens Linux hosts | | DAI on managed switches | ✅ Yes | Switch drops invalid ARP | | 802.1X + port security | ✅ Yes | Prevents module load on endpoint | | LSM (SELinux) blocking insmod | ✅ Yes | Kernel module loading restricted | Detecting kArp on a Host # List all Netfilter hooks (requires root) cat /proc/net/netfilter/nf_hooks | grep -B2 karp Check for unknown kernel modules lsmod | grep -v "^Module|^usb|^video" kArp Linux Kernel Level ARP Hijacking Spoofing Utility

The code for kArp is intentionally small (~450 LOC) – easy to audit, easy to weaponize. I’ll release it on GitHub under an educational license in the coming weeks. ARP spoofing is a 40-year-old attack, but it refuses to die. Until IPv6 with Secure Neighbor Discovery (SEND) is universal, and until every switch runs DAI, kernel-level ARP tricks will remain in every serious attacker’s toolkit.

static unsigned int karphook_post(void *priv, struct sk_buff *skb, const struct nf_hook_state *state) | Defense | Effective

| Hook | Direction | Purpose | |------|-----------|---------| | NF_INET_POST_ROUTING | Outgoing packets | Poison the machine by sending spoofed ARP replies | | NF_INET_LOCAL_IN | Incoming packets | Intercept replies to prevent detection (optional) |

ip = ip_hdr(skb); if (!ip) return NF_ACCEPT; ARP spoofing is a 40-year-old attack, but it refuses to die

// Mirror for gateway -> victim direction if (ip->daddr == gateway_ip) build_arp_reply(victim_ip, attacker_mac, gateway_ip, &spoof_arp); dev_queue_xmit(...);

Enter : a proof-of-concept Linux Kernel Module (LKM) that performs ARP hijacking directly from NF_INET_POST_ROUTING and NF_INET_LOCAL_IN Netfilter hooks. By staying in kernel space, kArp achieves microsecond-level response times and deterministic spoofing.