From 0bc9ea0685cd660b4d5b597414b110cc8cde6752 Mon Sep 17 00:00:00 2001
From: Tizian Maxime Weigt <tizian-maxime.weigt@t-w.dev>
Date: Mon, 27 Oct 2025 16:02:36 +0000
Subject: [PATCH] Adding the xdp based fwd engine

---
 main.c | 123 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 123 insertions(+)
 create mode 100644 main.c

diff --git a/main.c b/main.c
new file mode 100644
index 0000000..e1e365c
--- /dev/null
+++ b/main.c
@@ -0,0 +1,123 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Example of L3 forwarding via XDP and use of bpf FIB lookup helper.
+ *
+ * Copyright (c) 2017-18 David Ahern <dsahern@gmail.com>
+ */
+#define KBUILD_MODNAME "xdp_l3fwd"
+#include <linux/bpf.h>
+#include <linux/in.h>
+#include <linux/if_ether.h>
+#include <linux/if_packet.h>
+#include <linux/if_vlan.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+
+#include <bpf/bpf_helpers.h>
+
+#define IPV6_FLOWINFO_MASK              cpu_to_be32(0x0FFFFFFF)
+
+struct bpf_map_def SEC("maps") xdp_l3fwd_ports = {
+	.type = BPF_MAP_TYPE_DEVMAP,
+	.key_size = sizeof(int),
+	.value_size = sizeof(int),
+	.max_entries = 512,
+};
+
+/* from include/net/ip.h */
+static __always_inline int ip_decrease_ttl(struct iphdr *iph)
+{
+	u32 check = (__force u32)iph->check;
+
+	check += (__force u32)htons(0x0100);
+	iph->check = (__force __sum16)(check + (check >= 0xFFFF));
+	return --iph->ttl;
+}
+
+static __always_inline int xdp_l3fwd_flags(struct xdp_md *ctx, u32 flags)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+	struct bpf_fib_lookup fib_params;
+	struct ethhdr *eth = data;
+	struct ipv6hdr *ip6h;
+	struct iphdr *iph;
+	u16 h_proto;
+	u64 nh_off;
+	int rc;
+
+	nh_off = sizeof(*eth);
+	if (data + nh_off > data_end)
+		return XDP_DROP;
+
+	__builtin_memset(&fib_params, 0, sizeof(fib_params));
+
+	h_proto = eth->h_proto;
+	if (h_proto == htons(ETH_P_IP)) {
+		iph = data + nh_off;
+
+		if (iph + 1 > data_end)
+			return XDP_DROP;
+
+		if (iph->ttl <= 1)
+			return XDP_PASS;
+
+		fib_params.family	= AF_INET;
+		fib_params.tos		= iph->tos;
+		fib_params.l4_protocol	= iph->protocol;
+		fib_params.tot_len	= ntohs(iph->tot_len);
+		fib_params.ipv4_src	= iph->saddr;
+		fib_params.ipv4_dst	= iph->daddr;
+	} else if (h_proto == htons(ETH_P_IPV6)) {
+		struct in6_addr *src = (struct in6_addr *) fib_params.ipv6_src;
+		struct in6_addr *dst = (struct in6_addr *) fib_params.ipv6_dst;
+
+		ip6h = data + nh_off;
+		if (ip6h + 1 > data_end)
+			return XDP_DROP;
+
+		if (ip6h->hop_limit <= 1)
+			return XDP_PASS;
+
+		fib_params.family	= AF_INET6;
+		fib_params.flowinfo	= *(__be32 *)ip6h & IPV6_FLOWINFO_MASK;
+		fib_params.l4_protocol	= ip6h->nexthdr;
+		fib_params.tot_len	= ntohs(ip6h->payload_len);
+		*src			= ip6h->saddr;
+		*dst			= ip6h->daddr;
+	} else {
+		return XDP_PASS;
+	}
+
+	fib_params.ifindex = ctx->ingress_ifindex;
+
+	rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), flags);
+	if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
+		if (!bpf_map_lookup_elem(&xdp_l3fwd_ports, &fib_params.ifindex))
+			return XDP_PASS;
+
+		if (h_proto == htons(ETH_P_IP))
+			ip_decrease_ttl(iph);
+		else if (h_proto == htons(ETH_P_IPV6))
+			ip6h->hop_limit--;
+
+		__builtin_memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
+		__builtin_memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
+		return bpf_redirect_map(&xdp_l3fwd_ports, fib_params.ifindex, 0);
+	}
+
+	return XDP_PASS;
+}
+
+SEC("xdp_l3fwd")
+int xdp_l3fwd_prog(struct xdp_md *ctx)
+{
+	return xdp_l3fwd_flags(ctx, 0);
+}
+
+SEC("xdp_l3fwd_direct")
+int xdp_l3fwd_direct_prog(struct xdp_md *ctx)
+{
+	return xdp_l3fwd_flags(ctx, BPF_FIB_LOOKUP_DIRECT);
+}
+
+char _license[] SEC("license") = "GPL";
\ No newline at end of file