[PATCH v5 1/4] tools: add xdp-forwarder

Yureka Lilian
2025-09-23T16:35:43Z

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 | 14 +-
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, 603 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..6132c61 100644
--- a/tools/default.nix
+++ b/tools/default.nix
@@ -1,13 +1,17 @@
# 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 19 (current nixpkgs default) is too old to support -fwrapv-pointer
+, clang_21, libbpf
, appSupport ? true
, hostSupport ? false
+, driverSupport ? false
}:

let
@@ -70,15 +74,18 @@ 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
@@ -88,6 +95,7 @@ stdenv.mkDerivation (finalAttrs: {
mesonFlags = [
(lib.mesonBool "app" appSupport)
(lib.mesonBool "host" hostSupport)
+ (lib.mesonBool "driver" driverSupport)
"-Dhostfsrootdir=/run/virtiofs/virtiofs0"
"-Dtests=false"
"-Dunwind=false"
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 fb520ae..7b46739 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..9b70ce3
--- /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', native: true)
+
+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