[PATCH v4 2/5] tools: add xdp-forwarder

[Yureka Lilian <yureka@cyberchaos.dev>]

The xdp-forwarder's purpose is implementing the functionality needed
within the net-vm (a VM running the Linux drivers for any physical
interfaces on the spectrum system).

In the future, the net-vm will load the included XDP programs on the
passed-through physical interfaces as well as the downstream virtio
interface going into the router (recognized by its special MAC address).

The net-vm needs to multiplex between the physical interfaces, as there
might be several interfaces in the same IOMMU-group.

For this, the XDP program loaded on the physical interfaces
(`prog_physical.o`) applies a VLAN tag corresponding to the interface id
and redirects the packets to the router interface (identified by the
`router_iface` bpf map). In the other direction the XDP program loaded on
the router interface (`prog_router.o`) removes one layer of VLAN tagging
and redirects the packets to the interface read from the VLAN tag.

The helper program `set_router_iface` is used to update the `router_iface`
bpf map to point to the interface passed as argument to the program.

Co-authored-by: Demi Marie Obenour <demiobenour@gmail.com>
Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
---
 pkgs/default.nix                              |   4 +
 release/checks/pkg-tests.nix                  |   1 +
 tools/default.nix                             |  15 +-
 tools/meson.build                             |   4 +
 tools/meson_options.txt                       |   3 +
 tools/xdp-forwarder/include/parsing_helpers.h | 274 ++++++++++++++++++
 tools/xdp-forwarder/include/rewrite_helpers.h | 146 ++++++++++
 tools/xdp-forwarder/meson.build               |  48 +++
 tools/xdp-forwarder/prog_physical.c           |  39 +++
 tools/xdp-forwarder/prog_router.c             |  43 +++
 tools/xdp-forwarder/set_router_iface.c        |  30 ++
 11 files changed, 604 insertions(+), 3 deletions(-)
 create mode 100644 tools/xdp-forwarder/include/parsing_helpers.h
 create mode 100644 tools/xdp-forwarder/include/rewrite_helpers.h
 create mode 100644 tools/xdp-forwarder/meson.build
 create mode 100644 tools/xdp-forwarder/prog_physical.c
 create mode 100644 tools/xdp-forwarder/prog_router.c
 create mode 100644 tools/xdp-forwarder/set_router_iface.c

diff --git a/pkgs/default.nix b/pkgs/default.nix
index 2472218..df3cfdc 100644
--- a/pkgs/default.nix
+++ b/pkgs/default.nix
@@ -42,6 +42,10 @@ let
       appSupport = false;
       hostSupport = true;
     };
+    spectrum-driver-tools = self.callSpectrumPackage ../tools {
+      appSupport = false;
+      driverSupport = true;
+    };
     xdg-desktop-portal-spectrum-host =
       self.callSpectrumPackage ../tools/xdg-desktop-portal-spectrum-host {};
 
diff --git a/release/checks/pkg-tests.nix b/release/checks/pkg-tests.nix
index d7be42b..b1a048f 100644
--- a/release/checks/pkg-tests.nix
+++ b/release/checks/pkg-tests.nix
@@ -14,5 +14,6 @@ import ../../lib/call-package.nix (
   tools = lib.recurseIntoAttrs (callSpectrumPackage ../../tools {
     appSupport = true;
     hostSupport = true;
+    driverSupport = true;
   }).tests;
 }) (_: {})
diff --git a/tools/default.nix b/tools/default.nix
index 201afae..e92f453 100644
--- a/tools/default.nix
+++ b/tools/default.nix
@@ -1,13 +1,16 @@
 # SPDX-License-Identifier: MIT
 # SPDX-FileCopyrightText: 2022-2025 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
 
 import ../lib/call-package.nix (
 { src, lib, stdenv, fetchCrate, fetchurl, runCommand, buildPackages
 , meson, ninja, pkg-config, rustc
 , clang-tools, clippy
 , dbus
+, clang_21, libbpf
 , appSupport ? true
 , hostSupport ? false
+, driverSupport ? false
 }:
 
 let
@@ -70,15 +73,18 @@ stdenv.mkDerivation (finalAttrs: {
       ./lsvm
       ./start-vmm
       ./subprojects
+    ] ++ lib.optionals driverSupport [
+      ./xdp-forwarder
     ]));
   };
   sourceRoot = "source/tools";
 
   depsBuildBuild = lib.optionals hostSupport [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ meson ninja ]
-    ++ lib.optionals appSupport [ pkg-config ]
-    ++ lib.optionals hostSupport [ rustc ];
-  buildInputs = lib.optionals appSupport [ dbus ];
+    ++ lib.optionals (appSupport || driverSupport) [ pkg-config ]
+    ++ lib.optionals hostSupport [ rustc ]
+    ++ lib.optionals driverSupport [ clang_21 ];
+  buildInputs = lib.optionals appSupport [ dbus ] ++ lib.optionals driverSupport [ libbpf ];
 
   postPatch = lib.optionals hostSupport (lib.concatMapStringsSep "\n" (crate: ''
     mkdir -p subprojects/packagecache
@@ -88,12 +94,15 @@ stdenv.mkDerivation (finalAttrs: {
   mesonFlags = [
     (lib.mesonBool "app" appSupport)
     (lib.mesonBool "host" hostSupport)
+    (lib.mesonBool "driver" driverSupport)
     "-Dhostfsrootdir=/run/virtiofs/virtiofs0"
     "-Dtests=false"
     "-Dunwind=false"
     "-Dwerror=true"
   ];
 
+  hardeningDisable = lib.optionals driverSupport [ "zerocallusedregs" ];
+
   passthru.tests = {
     clang-tidy = finalAttrs.finalPackage.overrideAttrs (
       { name, src, nativeBuildInputs ? [], ... }:
diff --git a/tools/meson.build b/tools/meson.build
index e8b0cf2..059baeb 100644
--- a/tools/meson.build
+++ b/tools/meson.build
@@ -26,3 +26,7 @@ endif
 if get_option('app')
   subdir('xdg-desktop-portal-spectrum')
 endif
+
+if get_option('driver')
+  subdir('xdp-forwarder')
+endif
diff --git a/tools/meson_options.txt b/tools/meson_options.txt
index 75ef1ec..43eba06 100644
--- a/tools/meson_options.txt
+++ b/tools/meson_options.txt
@@ -7,6 +7,9 @@ option('host', type : 'boolean', value : false,
 option('app', type : 'boolean',
   description : 'Build tools for Spectrum App VMs')
 
+option('driver', type : 'boolean', value : false,
+  description : 'Build tools for Spectrum driver VMs')
+
 option('hostfsrootdir', type : 'string', value : '/run/host',
   description : 'Path where the virtio-fs provided by the host will be mounted')
 
diff --git a/tools/xdp-forwarder/include/parsing_helpers.h b/tools/xdp-forwarder/include/parsing_helpers.h
new file mode 100644
index 0000000..da09934
--- /dev/null
+++ b/tools/xdp-forwarder/include/parsing_helpers.h
@@ -0,0 +1,274 @@
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause) */
+/* SPDX-FileCopyrightText: 2021 The xdp-tutorial Authors */
+/* Vendored from https://github.com/xdp-project/xdp-tutorial/blob/d3d3eed6ea9a63d1302bfa8b5a8e93862bfe11f0/common/parsing_helpers.h */
+/*
+ * This file contains parsing functions that are used in the packetXX XDP
+ * programs. The functions are marked as __always_inline, and fully defined in
+ * this header file to be included in the BPF program.
+ *
+ * Each helper parses a packet header, including doing bounds checking, and
+ * returns the type of its contents if successful, and -1 otherwise.
+ *
+ * For Ethernet and IP headers, the content type is the type of the payload
+ * (h_proto for Ethernet, nexthdr for IPv6), for ICMP it is the ICMP type field.
+ * All return values are in host byte order.
+ *
+ * The versions of the functions included here are slightly expanded versions of
+ * the functions in the packet01 lesson. For instance, the Ethernet header
+ * parsing has support for parsing VLAN tags.
+ */
+
+#ifndef __PARSING_HELPERS_H
+#define __PARSING_HELPERS_H
+
+#include <stddef.h>
+#include <linux/if_ether.h>
+#include <linux/if_packet.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/icmp.h>
+#include <linux/icmpv6.h>
+#include <linux/udp.h>
+#include <linux/tcp.h>
+
+/* Header cursor to keep track of current parsing position */
+struct hdr_cursor {
+	void *pos;
+};
+
+/*
+ *	struct vlan_hdr - vlan header
+ *	@h_vlan_TCI: priority and VLAN ID
+ *	@h_vlan_encapsulated_proto: packet type ID or len
+ */
+struct vlan_hdr {
+	__be16 h_vlan_TCI;
+	__be16 h_vlan_encapsulated_proto;
+};
+
+/*
+ * Struct icmphdr_common represents the common part of the icmphdr and icmp6hdr
+ * structures.
+ */
+struct icmphdr_common {
+	__u8 type;
+	__u8 code;
+	__sum16 cksum;
+};
+
+/* Allow users of header file to redefine VLAN max depth */
+#ifndef VLAN_MAX_DEPTH
+#define VLAN_MAX_DEPTH 2
+#endif
+
+#define VLAN_VID_MASK           0x0fff /* VLAN Identifier */
+/* Struct for collecting VLANs after parsing via parse_ethhdr_vlan */
+struct collect_vlans {
+	__u16 id[VLAN_MAX_DEPTH];
+};
+
+static __always_inline int proto_is_vlan(__u16 h_proto)
+{
+	return !!(h_proto == bpf_htons(ETH_P_8021Q) ||
+	          h_proto == bpf_htons(ETH_P_8021AD));
+}
+
+/* Notice, parse_ethhdr() will skip VLAN tags, by advancing nh->pos and returns
+ * next header EtherType, BUT the ethhdr pointer supplied still points to the
+ * Ethernet header. Thus, caller can look at eth->h_proto to see if this was a
+ * VLAN tagged packet.
+ */
+static __always_inline int parse_ethhdr_vlan(struct hdr_cursor *nh,
+                                             void *data_end,
+                                             struct ethhdr **ethhdr,
+                                             struct collect_vlans *vlans)
+{
+	struct ethhdr *eth = nh->pos;
+	int hdrsize = sizeof(*eth);
+	struct vlan_hdr *vlh;
+	__u16 h_proto;
+	int i;
+
+	/* Byte-count bounds check; check if current pointer + size of header
+	 * is after data_end.
+	 */
+	if (nh->pos + hdrsize > data_end)
+		return -1;
+
+	nh->pos += hdrsize;
+	*ethhdr = eth;
+	vlh = nh->pos;
+	h_proto = eth->h_proto;
+
+	/* Use loop unrolling to avoid the verifier restriction on loops;
+	 * support up to VLAN_MAX_DEPTH layers of VLAN encapsulation.
+	 */
+	#pragma unroll
+	for (i = 0; i < VLAN_MAX_DEPTH; i++) {
+		if (!proto_is_vlan(h_proto))
+			break;
+
+		if (vlh + 1 > data_end)
+			break;
+
+		h_proto = vlh->h_vlan_encapsulated_proto;
+		if (vlans) /* collect VLAN ids */
+			vlans->id[i] =
+				(bpf_ntohs(vlh->h_vlan_TCI) & VLAN_VID_MASK);
+
+		vlh++;
+	}
+
+	nh->pos = vlh;
+	return h_proto; /* network-byte-order */
+}
+
+static __always_inline int parse_ethhdr(struct hdr_cursor *nh,
+                                        void *data_end,
+                                        struct ethhdr **ethhdr)
+{
+	/* Expect compiler removes the code that collects VLAN ids */
+	return parse_ethhdr_vlan(nh, data_end, ethhdr, NULL);
+}
+
+static __always_inline int parse_ip6hdr(struct hdr_cursor *nh,
+                                        void *data_end,
+                                        struct ipv6hdr **ip6hdr)
+{
+	struct ipv6hdr *ip6h = nh->pos;
+
+	/* Pointer-arithmetic bounds check; pointer +1 points to after end of
+	 * thing being pointed to. We will be using this style in the remainder
+	 * of the tutorial.
+	 */
+	if (ip6h + 1 > data_end)
+		return -1;
+
+	nh->pos = ip6h + 1;
+	*ip6hdr = ip6h;
+
+	return ip6h->nexthdr;
+}
+
+static __always_inline int parse_iphdr(struct hdr_cursor *nh,
+                                       void *data_end,
+                                       struct iphdr **iphdr)
+{
+	struct iphdr *iph = nh->pos;
+	int hdrsize;
+
+	if (iph + 1 > data_end)
+		return -1;
+
+	hdrsize = iph->ihl * 4;
+	/* Sanity check packet field is valid */
+	if(hdrsize < sizeof(*iph))
+		return -1;
+
+	/* Variable-length IPv4 header, need to use byte-based arithmetic */
+	if (nh->pos + hdrsize > data_end)
+		return -1;
+
+	nh->pos += hdrsize;
+	*iphdr = iph;
+
+	return iph->protocol;
+}
+
+static __always_inline int parse_icmp6hdr(struct hdr_cursor *nh,
+                                          void *data_end,
+                                          struct icmp6hdr **icmp6hdr)
+{
+	struct icmp6hdr *icmp6h = nh->pos;
+
+	if (icmp6h + 1 > data_end)
+		return -1;
+
+	nh->pos   = icmp6h + 1;
+	*icmp6hdr = icmp6h;
+
+	return icmp6h->icmp6_type;
+}
+
+static __always_inline int parse_icmphdr(struct hdr_cursor *nh,
+                                         void *data_end,
+                                         struct icmphdr **icmphdr)
+{
+	struct icmphdr *icmph = nh->pos;
+
+	if (icmph + 1 > data_end)
+		return -1;
+
+	nh->pos  = icmph + 1;
+	*icmphdr = icmph;
+
+	return icmph->type;
+}
+
+static __always_inline int parse_icmphdr_common(struct hdr_cursor *nh,
+                                                void *data_end,
+                                                struct icmphdr_common **icmphdr)
+{
+	struct icmphdr_common *h = nh->pos;
+
+	if (h + 1 > data_end)
+		return -1;
+
+	nh->pos  = h + 1;
+	*icmphdr = h;
+
+	return h->type;
+}
+
+/*
+ * parse_udphdr: parse the udp header and return the length of the udp payload
+ */
+static __always_inline int parse_udphdr(struct hdr_cursor *nh,
+                                        void *data_end,
+                                        struct udphdr **udphdr)
+{
+	int len;
+	struct udphdr *h = nh->pos;
+
+	if (h + 1 > data_end)
+		return -1;
+
+	nh->pos  = h + 1;
+	*udphdr = h;
+
+	len = bpf_ntohs(h->len) - sizeof(struct udphdr);
+	if (len < 0)
+		return -1;
+
+	return len;
+}
+
+/*
+ * parse_tcphdr: parse and return the length of the tcp header
+ */
+static __always_inline int parse_tcphdr(struct hdr_cursor *nh,
+                                        void *data_end,
+                                        struct tcphdr **tcphdr)
+{
+	int len;
+	struct tcphdr *h = nh->pos;
+
+	if (h + 1 > data_end)
+		return -1;
+
+	len = h->doff * 4;
+	/* Sanity check packet field is valid */
+	if(len < sizeof(*h))
+		return -1;
+
+	/* Variable-length TCP header, need to use byte-based arithmetic */
+	if (nh->pos + len > data_end)
+		return -1;
+
+	nh->pos += len;
+	*tcphdr = h;
+
+	return len;
+}
+
+#endif /* __PARSING_HELPERS_H */
diff --git a/tools/xdp-forwarder/include/rewrite_helpers.h b/tools/xdp-forwarder/include/rewrite_helpers.h
new file mode 100644
index 0000000..71aa23e
--- /dev/null
+++ b/tools/xdp-forwarder/include/rewrite_helpers.h
@@ -0,0 +1,146 @@
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause) */
+/* SPDX-FileCopyrightText: 2019 The xdp-tutorial Authors */
+/* Vendored from https://github.com/xdp-project/xdp-tutorial/blob/d3d3eed6ea9a63d1302bfa8b5a8e93862bfe11f0/common/rewrite_helpers.h */
+/*
+ * This file contains functions that are used in the packetXX XDP programs to
+ * manipulate on packets data. The functions are marked as __always_inline, and
+ * fully defined in this header file to be included in the BPF program.
+ */
+
+#ifndef __REWRITE_HELPERS_H
+#define __REWRITE_HELPERS_H
+
+#include <linux/bpf.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/if_ether.h>
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+/* Pops the outermost VLAN tag off the packet. Returns the popped VLAN ID on
+ * success or negative errno on failure.
+ */
+static __always_inline int vlan_tag_pop(struct xdp_md *ctx, struct ethhdr *eth)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	struct ethhdr eth_cpy;
+	struct vlan_hdr *vlh;
+	__be16 h_proto;
+	int vlid;
+
+	if (!proto_is_vlan(eth->h_proto))
+		return -1;
+
+	/* Careful with the parenthesis here */
+	vlh = (void *)(eth + 1);
+
+	/* Still need to do bounds checking */
+	if (vlh + 1 > data_end)
+		return -1;
+
+	/* Save vlan ID for returning, h_proto for updating Ethernet header */
+	vlid = bpf_ntohs(vlh->h_vlan_TCI);
+	h_proto = vlh->h_vlan_encapsulated_proto;
+
+	/* Make a copy of the outer Ethernet header before we cut it off */
+	__builtin_memcpy(&eth_cpy, eth, sizeof(eth_cpy));
+
+	/* Actually adjust the head pointer */
+	if (bpf_xdp_adjust_head(ctx, (int)sizeof(*vlh)))
+		return -1;
+
+	/* Need to re-evaluate data *and* data_end and do new bounds checking
+	 * after adjusting head
+	 */
+	eth = (void *)(long)ctx->data;
+	data_end = (void *)(long)ctx->data_end;
+	if (eth + 1 > data_end)
+		return -1;
+
+	/* Copy back the old Ethernet header and update the proto type */
+	__builtin_memcpy(eth, &eth_cpy, sizeof(*eth));
+	eth->h_proto = h_proto;
+
+	return vlid;
+}
+
+/* Pushes a new VLAN tag after the Ethernet header. Returns 0 on success,
+ * -1 on failure.
+ */
+static __always_inline int vlan_tag_push(struct xdp_md *ctx,
+                                         struct ethhdr *eth, int vlid)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	struct ethhdr eth_cpy;
+	struct vlan_hdr *vlh;
+
+	/* First copy the original Ethernet header */
+	__builtin_memcpy(&eth_cpy, eth, sizeof(eth_cpy));
+
+	/* Then add space in front of the packet */
+	if (bpf_xdp_adjust_head(ctx, 0 - (int)sizeof(*vlh)))
+		return -1;
+
+	/* Need to re-evaluate data_end and data after head adjustment, and
+	 * bounds check, even though we know there is enough space (as we
+	 * increased it).
+	 */
+	data_end = (void *)(long)ctx->data_end;
+	eth = (void *)(long)ctx->data;
+
+	if (eth + 1 > data_end)
+		return -1;
+
+	/* Copy back Ethernet header in the right place, populate VLAN tag with
+	 * ID and proto, and set outer Ethernet header to VLAN type.
+	 */
+	__builtin_memcpy(eth, &eth_cpy, sizeof(*eth));
+
+	vlh = (void *)(eth + 1);
+
+	if (vlh + 1 > data_end)
+		return -1;
+
+	vlh->h_vlan_TCI = bpf_htons(vlid);
+	vlh->h_vlan_encapsulated_proto = eth->h_proto;
+
+	eth->h_proto = bpf_htons(ETH_P_8021Q);
+	return 0;
+}
+
+/*
+ * Swaps destination and source MAC addresses inside an Ethernet header
+ */
+static __always_inline void swap_src_dst_mac(struct ethhdr *eth)
+{
+	__u8 h_tmp[ETH_ALEN];
+
+	__builtin_memcpy(h_tmp, eth->h_source, ETH_ALEN);
+	__builtin_memcpy(eth->h_source, eth->h_dest, ETH_ALEN);
+	__builtin_memcpy(eth->h_dest, h_tmp, ETH_ALEN);
+}
+
+/*
+ * Swaps destination and source IPv6 addresses inside an IPv6 header
+ */
+static __always_inline void swap_src_dst_ipv6(struct ipv6hdr *ipv6)
+{
+	struct in6_addr tmp = ipv6->saddr;
+
+	ipv6->saddr = ipv6->daddr;
+	ipv6->daddr = tmp;
+}
+
+/*
+ * Swaps destination and source IPv4 addresses inside an IPv4 header
+ */
+static __always_inline void swap_src_dst_ipv4(struct iphdr *iphdr)
+{
+	__be32 tmp = iphdr->saddr;
+
+	iphdr->saddr = iphdr->daddr;
+	iphdr->daddr = tmp;
+}
+
+#endif /* __REWRITE_HELPERS_H */
diff --git a/tools/xdp-forwarder/meson.build b/tools/xdp-forwarder/meson.build
new file mode 100644
index 0000000..e6d91ca
--- /dev/null
+++ b/tools/xdp-forwarder/meson.build
@@ -0,0 +1,48 @@
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+# SPDX-FileCopyrightText: 2025 Demi Marie Obenour <demiobenour@gmail.com>
+
+libbpf = dependency('libbpf', version : '1.6.2')
+
+executable('set-router-iface', 'set_router_iface.c',
+  dependencies : libbpf,
+  install : true)
+
+clang = find_program('clang')
+
+bpf_o_cmd = [
+  clang.full_path(),
+  '-fno-stack-protector',
+  '-fno-strict-aliasing',
+  '-fwrapv', '-fwrapv-pointer',
+  '-Wall',
+  '-Wextra',
+  '-O2',
+  '-target', 'bpf',
+  '-I', meson.current_source_dir() + '/include',
+  '-g',
+  '-c',
+  '-o', '@OUTPUT@',
+  '-MD',
+  '-MP',
+  '-MF', '@DEPFILE@',
+  '--',
+  '@INPUT@',
+]
+
+prog_router_o = custom_target(
+  input : 'prog_router.c',
+  output : 'prog_router.o',
+  depfile : 'prog_router.o.dep',
+  command : bpf_o_cmd,
+  install: true,
+  install_dir: 'lib/xdp')
+
+prog_physical_o = custom_target(
+  input : 'prog_physical.c',
+  output : 'prog_physical.o',
+  depfile : 'prog_physical.o.dep',
+  command : bpf_o_cmd,
+  install: true,
+  install_dir: 'lib/xdp')
+
diff --git a/tools/xdp-forwarder/prog_physical.c b/tools/xdp-forwarder/prog_physical.c
new file mode 100644
index 0000000..2b70654
--- /dev/null
+++ b/tools/xdp-forwarder/prog_physical.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#define VLAN_MAX_DEPTH 1
+
+#include <linux/bpf.h>
+#include <bpf/bpf_endian.h>
+#include "parsing_helpers.h"
+#include "rewrite_helpers.h"
+
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__type(key, int);
+	__type(value, int);
+	__uint(max_entries, 1);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} router_iface SEC(".maps");
+
+SEC("xdp")
+int physical(struct xdp_md *ctx)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+
+	struct hdr_cursor nh;
+	nh.pos = data;
+
+	struct ethhdr *eth;
+	if (parse_ethhdr(&nh, data_end, &eth) < 0)
+		return XDP_DROP;
+
+	if (ctx->ingress_ifindex < 1 || ctx->ingress_ifindex > VLAN_VID_MASK)
+		return XDP_DROP;
+
+	if (vlan_tag_push(ctx, eth, ctx->ingress_ifindex) < 0)
+		return XDP_DROP;
+
+	return bpf_redirect_map(&router_iface, 0, 0);
+}
diff --git a/tools/xdp-forwarder/prog_router.c b/tools/xdp-forwarder/prog_router.c
new file mode 100644
index 0000000..6704b2c
--- /dev/null
+++ b/tools/xdp-forwarder/prog_router.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#define VLAN_MAX_DEPTH 1
+
+#include <linux/bpf.h>
+#include <bpf/bpf_endian.h>
+#include "parsing_helpers.h"
+#include "rewrite_helpers.h"
+
+// The map is actually not used by this program, but just included
+// to keep the reference-counted pin alive before any physical interfaces
+// are added.
+struct {
+	__uint(type, BPF_MAP_TYPE_DEVMAP);
+	__type(key, int);
+	__type(value, int);
+	__uint(max_entries, 1);
+	__uint(pinning, LIBBPF_PIN_BY_NAME);
+} router_iface SEC(".maps");
+
+
+SEC("xdp")
+int router(struct xdp_md *ctx)
+{
+	void *data_end = (void *)(long)ctx->data_end;
+	void *data = (void *)(long)ctx->data;
+
+	struct hdr_cursor nh;
+	nh.pos = data;
+
+	struct ethhdr *eth;
+	int r;
+	if ((r = parse_ethhdr(&nh, data_end, &eth)) < 0)
+		return XDP_DROP;
+
+	int vlid = vlan_tag_pop(ctx, eth);
+	if (vlid < 0) {
+		return XDP_DROP;
+	}
+
+	return bpf_redirect(vlid, 0);
+}
diff --git a/tools/xdp-forwarder/set_router_iface.c b/tools/xdp-forwarder/set_router_iface.c
new file mode 100644
index 0000000..7e6c7a2
--- /dev/null
+++ b/tools/xdp-forwarder/set_router_iface.c
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <net/if.h>
+#include <bpf/bpf.h>
+#include <err.h>
+
+int main(int argc, char **argv)
+{
+	if (argc < 2) {
+		err(EXIT_FAILURE, "missing interface name");
+	}
+
+	int router_idx = if_nametoindex(argv[1]);
+	if (router_idx <= 0) {
+		err(EXIT_FAILURE, "error getting router interface");
+	}
+
+	int map_fd = bpf_obj_get("/sys/fs/bpf/router_iface");
+	if (map_fd < 0) {
+		err(EXIT_FAILURE, "failed to open bpf map");
+	}
+
+	int id = 0;
+	if (bpf_map_update_elem(map_fd, &id, &router_idx, 0) < 0) {
+		err(EXIT_FAILURE, "failed to update bpf map");
+	}
+}
-- 
2.51.0