[DO_NOT_APPLY 1/2] integrate xdp-forwarder into net-vm

Yureka Lilian
2025-08-23T22:22:24Z

Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
---
vm/sys/net/Makefile | 8 +-
vm/sys/net/default.nix | 23 ++--
vm/sys/net/etc/fstab | 1 +
vm/sys/net/etc/mdev/iface | 23 +---
vm/sys/net/etc/nftables.conf | 8 --
vm/sys/net/etc/s6-rc/connman/dependencies | 4 -
vm/sys/net/etc/s6-rc/connman/type | 1 -
vm/sys/net/etc/s6-rc/connman/type.license | 2 -
vm/sys/net/etc/s6-rc/nftables/type | 1 -
vm/sys/net/etc/s6-rc/nftables/type.license | 2 -
vm/sys/net/etc/s6-rc/nftables/up | 6 -
vm/sys/net/xdp-forwarder/README.md | 9 ++
vm/sys/net/xdp-forwarder/default.nix | 35 ++++++
.../xdp-forwarder/include/parsing_helpers.h | 38 +++++++
.../xdp-forwarder/include/rewrite_helpers.h | 103 ++++++++++++++++++
vm/sys/net/xdp-forwarder/load_physical | 4 +
vm/sys/net/xdp-forwarder/load_router | 6 +
vm/sys/net/xdp-forwarder/prog_physical.c | 28 +++++
vm/sys/net/xdp-forwarder/prog_router.c | 34 ++++++
vm/sys/net/xdp-forwarder/set_router_iface.c | 31 ++++++
20 files changed, 308 insertions(+), 59 deletions(-)
delete mode 100644 vm/sys/net/etc/nftables.conf
delete mode 100644 vm/sys/net/etc/s6-rc/connman/dependencies
delete mode 100644 vm/sys/net/etc/s6-rc/connman/type
delete mode 100644 vm/sys/net/etc/s6-rc/connman/type.license
delete mode 100644 vm/sys/net/etc/s6-rc/nftables/type
delete mode 100644 vm/sys/net/etc/s6-rc/nftables/type.license
delete mode 100644 vm/sys/net/etc/s6-rc/nftables/up
create mode 100644 vm/sys/net/xdp-forwarder/README.md
create mode 100644 vm/sys/net/xdp-forwarder/default.nix
create mode 100644 vm/sys/net/xdp-forwarder/include/parsing_helpers.h
create mode 100644 vm/sys/net/xdp-forwarder/include/rewrite_helpers.h
create mode 100755 vm/sys/net/xdp-forwarder/load_physical
create mode 100755 vm/sys/net/xdp-forwarder/load_router
create mode 100644 vm/sys/net/xdp-forwarder/prog_physical.c
create mode 100644 vm/sys/net/xdp-forwarder/prog_router.c
create mode 100644 vm/sys/net/xdp-forwarder/set_router_iface.c

diff --git a/vm/sys/net/Makefile b/vm/sys/net/Makefile
index e681940..9576a92 100644
--- a/vm/sys/net/Makefile
+++ b/vm/sys/net/Makefile
@@ -34,12 +34,11 @@ VM_FILES = \
etc/init \
etc/mdev.conf \
etc/mdev/iface \
- etc/nftables.conf \
etc/passwd \
etc/s6-linux-init/run-image/service/getty-hvc0/run \
etc/s6-linux-init/scripts/rc.init \
etc/sysctl.conf
-VM_DIRS = dev etc/s6-linux-init/env run proc sys var/lib/connman
+VM_DIRS = dev etc/s6-linux-init/env run proc sys

# These are separate because they need to be included, but putting
# them as make dependencies would confuse make.
@@ -59,9 +58,6 @@ build/rootfs.erofs: ../../../scripts/make-erofs.sh $(PACKAGES_FILE) $(VM_FILES)
) | ../../../scripts/make-erofs.sh $@

VM_S6_RC_FILES = \
- etc/s6-rc/connman/dependencies \
- etc/s6-rc/connman/run \
- etc/s6-rc/connman/type \
etc/s6-rc/dbus/notification-fd \
etc/s6-rc/dbus/run \
etc/s6-rc/dbus/type \
@@ -71,8 +67,6 @@ VM_S6_RC_FILES = \
etc/s6-rc/mdevd/notification-fd \
etc/s6-rc/mdevd/run \
etc/s6-rc/mdevd/type \
- etc/s6-rc/nftables/type \
- etc/s6-rc/nftables/up \
etc/s6-rc/ok-all/contents \
etc/s6-rc/ok-all/type \
etc/s6-rc/sysctl/type \
diff --git a/vm/sys/net/default.nix b/vm/sys/net/default.nix
index b5873eb..335c938 100644
--- a/vm/sys/net/default.nix
+++ b/vm/sys/net/default.nix
@@ -1,23 +1,26 @@
# SPDX-License-Identifier: MIT
# SPDX-FileCopyrightText: 2021-2023 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>

-import ../../../lib/call-package.nix ({ lseek, src, terminfo, pkgsStatic }:
-pkgsStatic.callPackage (
+import ../../../lib/call-package.nix ({ lseek, src, terminfo, pkgsMusl }:
+pkgsMusl.callPackage (

{ lib, stdenvNoCC, nixos, runCommand, writeClosure
, erofs-utils, jq, s6-rc, util-linux, xorg
-, busybox, connmanMinimal, dbus, execline, kmod, linux_latest, mdevd, nftables
-, s6, s6-linux-init
+, busybox, dbus, execline, kmod, linux_latest, mdevd
+, s6, s6-linux-init, xdp-tools
}:

let
inherit (lib) concatMapStringsSep;
inherit (nixosAllHardware.config.hardware) firmware;

- connman = connmanMinimal;
-
packages = [
- connman dbus execline kmod mdevd s6 s6-linux-init s6-rc
+ dbus execline kmod mdevd s6 s6-linux-init s6-rc xdp-tools
+
+ (pkgsMusl.callPackage ./xdp-forwarder {
+ linux = kernel;
+ })

(busybox.override {
extraConfig = ''
@@ -30,13 +33,13 @@ let
CONFIG_RMMOD n
'';
})
-
- (nftables.override { withCli = false; })
];

# Packages that should be fully linked into /usr,
# (not just their bin/* files).
- usrPackages = [ connman dbus firmware kernel terminfo ];
+ usrPackages = [
+ dbus firmware kernel terminfo
+ ];

packagesSysroot = runCommand "packages-sysroot" {
inherit packages;
diff --git a/vm/sys/net/etc/fstab b/vm/sys/net/etc/fstab
index 6a82ecc..06f26ff 100644
--- a/vm/sys/net/etc/fstab
+++ b/vm/sys/net/etc/fstab
@@ -4,3 +4,4 @@ proc /proc proc defaults 0 0
devpts /dev/pts devpts defaults,gid=4,mode=620 0 0
tmpfs /dev/shm tmpfs defaults 0 0
sysfs /sys sysfs defaults 0 0
+bpffs /sys/fs/bpf bpf defaults 0 0
diff --git a/vm/sys/net/etc/mdev/iface b/vm/sys/net/etc/mdev/iface
index 2306575..9724690 100755
--- a/vm/sys/net/etc/mdev/iface
+++ b/vm/sys/net/etc/mdev/iface
@@ -1,6 +1,7 @@
#!/bin/execlineb -P
# SPDX-License-Identifier: EUPL-1.2+
# SPDX-FileCopyrightText: 2020-2021 Alyssa Ross <hi@alyssa.is>
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>

importas -Si INTERFACE

@@ -8,29 +9,15 @@ ifte

{
# This interface is connected to another VM.
-
- # The other VM's IP is encoded in the NIC-specific portion of the
- # interface's MAC address.
- backtick -E CLIENT_IP {
- awk -F: "{printf \"100.64.%d.%d\\n\", \"0x\" $5, \"0x\" $6}"
- /sys/class/net/${INTERFACE}/address
- }
-
- if { ip address add 169.254.0.1/32 dev $INTERFACE }
- if { ip link set $INTERFACE up }
- ip route add $CLIENT_IP dev $INTERFACE
+ if { load_router $INTERFACE }
+ ip link set $INTERFACE up
}

{
if { test $INTERFACE != lo }
# This is a physical connection to a network device.
- background { s6-rc -bu change connman }
- if { s6-rc -bu change nftables }
- if {
- forx -pE module { nft_counter nft_masq }
- modprobe $module
- }
- nft add rule ip nat postrouting oifname $INTERFACE counter masquerade
+ if { load_physical $INTERFACE }
+ ip link set $INTERFACE up
}

grep -iq ^02:01: /sys/class/net/${INTERFACE}/address
diff --git a/vm/sys/net/etc/nftables.conf b/vm/sys/net/etc/nftables.conf
deleted file mode 100644
index 296d92c..0000000
--- a/vm/sys/net/etc/nftables.conf
+++ /dev/null
@@ -1,8 +0,0 @@
-# SPDX-License-Identifier: EUPL-1.2+
-# SPDX-FileCopyrightText: 2021 Alyssa Ross <hi@alyssa.is>
-
-table nat {
- chain postrouting {
- type nat hook postrouting priority 100;
- }
-}
diff --git a/vm/sys/net/etc/s6-rc/connman/dependencies b/vm/sys/net/etc/s6-rc/connman/dependencies
deleted file mode 100644
index 23bda19..0000000
--- a/vm/sys/net/etc/s6-rc/connman/dependencies
+++ /dev/null
@@ -1,4 +0,0 @@
-# SPDX-License-Identifier: CC0-1.0
-# SPDX-FileCopyrightText: 2020 Alyssa Ross <hi@alyssa.is>
-#
-dbus
diff --git a/vm/sys/net/etc/s6-rc/connman/type b/vm/sys/net/etc/s6-rc/connman/type
deleted file mode 100644
index 5883cff..0000000
--- a/vm/sys/net/etc/s6-rc/connman/type
+++ /dev/null
@@ -1 +0,0 @@
-longrun
diff --git a/vm/sys/net/etc/s6-rc/connman/type.license b/vm/sys/net/etc/s6-rc/connman/type.license
deleted file mode 100644
index 2b3b032..0000000
--- a/vm/sys/net/etc/s6-rc/connman/type.license
+++ /dev/null
@@ -1,2 +0,0 @@
-SPDX-License-Identifier: CC0-1.0
-SPDX-FileCopyrightText: 2020 Alyssa Ross <hi@alyssa.is>
diff --git a/vm/sys/net/etc/s6-rc/nftables/type b/vm/sys/net/etc/s6-rc/nftables/type
deleted file mode 100644
index bdd22a1..0000000
--- a/vm/sys/net/etc/s6-rc/nftables/type
+++ /dev/null
@@ -1 +0,0 @@
-oneshot
diff --git a/vm/sys/net/etc/s6-rc/nftables/type.license b/vm/sys/net/etc/s6-rc/nftables/type.license
deleted file mode 100644
index c49c11b..0000000
--- a/vm/sys/net/etc/s6-rc/nftables/type.license
+++ /dev/null
@@ -1,2 +0,0 @@
-SPDX-License-Identifier: CC0-1.0
-SPDX-FileCopyrightText: 2021 Alyssa Ross <hi@alyssa.is>
diff --git a/vm/sys/net/etc/s6-rc/nftables/up b/vm/sys/net/etc/s6-rc/nftables/up
deleted file mode 100644
index 7d5f141..0000000
--- a/vm/sys/net/etc/s6-rc/nftables/up
+++ /dev/null
@@ -1,6 +0,0 @@
-# SPDX-License-Identifier: EUPL-1.2+
-# SPDX-FileCopyrightText: 2021 Alyssa Ross <hi@alyssa.is>
-
-if { modprobe nft_chain_nat }
-
-nft -f /etc/nftables.conf
diff --git a/vm/sys/net/xdp-forwarder/README.md b/vm/sys/net/xdp-forwarder/README.md
new file mode 100644
index 0000000..2aa6585
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/README.md
@@ -0,0 +1,9 @@
+### xdp-forwarder
+
+The xdp forwarder consists of a redirect_kern.c (which is the actual XDP program, basically just a one-liner) and redirect_user.c (which loads the XDP program and sets up the maps with the interfaces passed on the command line).
+
+It behaves like an ethernet hub between two interfaces.
+
+Temporary repository, to be inlined into the [Spectrum](https://spectrum-os.org/) monorepo.
+
+This work was funded by [NGI Zero](https://www.ngi.eu/ngi-projects/ngi-zero/), an initiative by the Digital Single Market of the European Commission.
diff --git a/vm/sys/net/xdp-forwarder/default.nix b/vm/sys/net/xdp-forwarder/default.nix
new file mode 100644
index 0000000..75b1d66
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/default.nix
@@ -0,0 +1,35 @@
+# SPDX-License-Identifier: MIT
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+{ lib, runCommand, stdenv, llvmPackages, libbpf, linux, bpftools }:
+
+stdenv.mkDerivation {
+ pname = "xdp-forwarder";
+ version = "0";
+
+ src = lib.fileset.toSource {
+ root = ./.;
+ fileset = lib.fileset.fileFilter
+ ({ hasExt, ... }: !(hasExt "nix") && !(hasExt "md")) ./.;
+ };
+
+ buildInputs = [ libbpf ];
+ nativeBuildInputs = [ llvmPackages.clang-unwrapped bpftools ];
+
+ buildPhase = ''
+ bpftool btf dump file ${linux.dev}/vmlinux format c > include/vmlinux.h
+ for prog in physical router; do
+ clang $NIX_CFLAGS_COMPILE -O2 -g -Wall -target bpf -I include -c prog_$prog.c -o prog_$prog.o
+ substituteInPlace load_$prog --replace-fail prog_$prog.o $out/lib/prog_$prog.o
+ done
+ $CC -lbpf -I include -o set_router_iface set_router_iface.c
+ '';
+
+ installPhase = ''
+ for prog in physical router; do
+ install -Dm644 prog_$prog.o $out/lib/prog_$prog.o
+ install -Dm755 load_$prog $out/bin/load_$prog
+ done
+ install -Dm755 set_router_iface $out/bin/set_router_iface
+ '';
+}
diff --git a/vm/sys/net/xdp-forwarder/include/parsing_helpers.h b/vm/sys/net/xdp-forwarder/include/parsing_helpers.h
new file mode 100644
index 0000000..2446e56
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/include/parsing_helpers.h
@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-clause) */
+/* SPDX-FileCopyrightText: 2021 The xdp-tutorial Authors */
+/* SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev> */
+/* Based on https://github.com/xdp-project/xdp-tutorial/blob/main/common/parsing_helpers.h */
+
+#ifndef __PARSING_HELPERS_H
+#define __PARSING_HELPERS_H
+
+#include <bpf/bpf_endian.h>
+
+//#include <linux/if_ether.h>
+#define ETH_P_8021Q 0x8100 /* 802.1Q VLAN Extended Header */
+#define ETH_P_8021AD 0x88A8 /* 802.1ad Service VLAN */
+
+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));
+}
+
+static __always_inline int parse_ethhdr(struct xdp_md *ctx,
+ struct ethhdr **ethhdr)
+{
+ void *data_end = (void *)(long)ctx->data_end;
+ struct ethhdr *eth = (void *) (long) ctx->data;
+
+ /* Byte-count bounds check; check if current pointer + size of header
+ * is after data_end.
+ */
+ if ((void *) (eth + 1) > data_end)
+ return -1;
+
+ *ethhdr = eth;
+
+ return eth->h_proto; /* network-byte-order */
+}
+
+#endif
diff --git a/vm/sys/net/xdp-forwarder/include/rewrite_helpers.h b/vm/sys/net/xdp-forwarder/include/rewrite_helpers.h
new file mode 100644
index 0000000..7fa6e2c
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/include/rewrite_helpers.h
@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-clause) */
+/* SPDX-FileCopyrightText: 2019 The xdp-tutorial Authors */
+/* SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev> */
+/* Based on: https://github.com/xdp-project/xdp-tutorial/blob/main/common/rewrite_helpers.h */
+
+#ifndef __REWRITE_HELPERS_H
+#define __REWRITE_HELPERS_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 ((void *) (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 ((void *) (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 ((void *) (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 ((void *) (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;
+}
+
+#endif
diff --git a/vm/sys/net/xdp-forwarder/load_physical b/vm/sys/net/xdp-forwarder/load_physical
new file mode 100755
index 0000000..13473e4
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/load_physical
@@ -0,0 +1,4 @@
+#!/bin/execlineb -S1
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+xdp-loader load ${1} prog_physical.o -m skb -p /sys/fs/bpf
diff --git a/vm/sys/net/xdp-forwarder/load_router b/vm/sys/net/xdp-forwarder/load_router
new file mode 100755
index 0000000..d16c26d
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/load_router
@@ -0,0 +1,6 @@
+#!/bin/execlineb -S1
+# SPDX-License-Identifier: EUPL-1.2+
+# SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+if { xdp-loader load ${1} prog_router.o -m skb -p /sys/fs/bpf }
+if { ip link set ${1} promisc on }
+set_router_iface ${1}
diff --git a/vm/sys/net/xdp-forwarder/prog_physical.c b/vm/sys/net/xdp-forwarder/prog_physical.c
new file mode 100644
index 0000000..d547bdd
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/prog_physical.c
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.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) {
+ struct ethhdr *eth;
+ if (parse_ethhdr(ctx, &eth) < 0)
+ return XDP_DROP;
+
+ if (ctx->ingress_ifindex < 1 || ctx->ingress_ifindex > 4096)
+ return XDP_DROP;
+
+ vlan_tag_push(ctx, eth, ctx->ingress_ifindex);
+ return bpf_redirect_map(&router_iface, 0, 0);
+}
diff --git a/vm/sys/net/xdp-forwarder/prog_router.c b/vm/sys/net/xdp-forwarder/prog_router.c
new file mode 100644
index 0000000..18e97cc
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/prog_router.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.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) {
+ struct ethhdr *eth;
+ int r;
+ if (r = parse_ethhdr(ctx, &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/vm/sys/net/xdp-forwarder/set_router_iface.c b/vm/sys/net/xdp-forwarder/set_router_iface.c
new file mode 100644
index 0000000..c828ee3
--- /dev/null
+++ b/vm/sys/net/xdp-forwarder/set_router_iface.c
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: EUPL-1.2+
+// SPDX-FileCopyrightText: 2025 Yureka Lilian <yureka@cyberchaos.dev>
+
+#include <stdio.h>
+#include <net/if.h>
+#include <bpf/bpf.h>
+
+int main(int argc, char **argv) {
+ if (argc < 2) {
+ fprintf(stderr, "missing interface name\n");
+ return 1;
+ }
+
+ int router_idx = if_nametoindex(argv[1]);
+ if (router_idx <= 0) {
+ perror("error getting router interface");
+ return 1;
+ }
+
+ int map_fd = bpf_obj_get("/sys/fs/bpf/router_iface");
+ if (map_fd < 0) {
+ perror("failed to open bpf map");
+ return 1;
+ }
+
+ int id = 0;
+ if (bpf_map_update_elem(map_fd, &id, &router_idx, 0) < 0) {
+ perror("failed to update bpf map");
+ return 1;
+ }
+}
--
2.50.1