diff --git a/scripts/gc-testlink.sh b/scripts/gc-testlink.sh
new file mode 100755
index 0000000..7cd7334
--- /dev/null
+++ b/scripts/gc-testlink.sh
@@ -0,0 +1,143 @@
+#!/usr/bin/env bash
+# Configure a PC ethernet interface for the direct link to the re-bba-rb board.
+#
+# The board's socket-3 UDP test identity and the ARP the on-console test suite
+# sends both assume a point-to-point 10.0.0.0/24 link:
+#
+#     PC    10.0.0.50       <-- this script
+#     board 10.0.0.99       <-- baked into the gateware at synth time
+#
+# The address matters: the GC test app's TX test ARPs for 10.0.0.50, and the
+# kernel answers ARP for an address it actually holds -- no listener process is
+# needed.  That reply is what makes the suite's RX test pass, so without this
+# configuration RX fails legitimately for lack of traffic.
+#
+# Run this ON THE PC (not in the devcontainer).
+#
+#   sudo ./scripts/gc-testlink.sh up          # configure (auto-detects the NIC)
+#   sudo ./scripts/gc-testlink.sh up eth0     # ...or name it explicitly
+#   sudo ./scripts/gc-testlink.sh status
+#   sudo ./scripts/gc-testlink.sh down        # undo, hand the NIC back
+#   sudo ./scripts/gc-testlink.sh watch       # configure, then tcpdump the link
+set -euo pipefail
+
+PC_IP=10.0.0.50
+PREFIX=24
+BOARD_IP=10.0.0.99
+STATE_DIR=/run/gc-testlink
+
+die()  { echo "error: $*" >&2; exit 1; }
+need_root() { [ "$(id -u)" -eq 0 ] || die "must run as root (use sudo)"; }
+command -v ip >/dev/null 2>&1 || die "'ip' not found (iproute2). Run this on the PC, not in the devcontainer."
+
+# Pick a wired interface: prefer one with a carrier, else the first en*/eth*.
+detect_iface() {
+    local candidates=() i
+    for i in /sys/class/net/*; do
+        i=$(basename "$i")
+        case "$i" in
+            lo|docker*|veth*|br-*|virbr*|tap*|wl*|tun*) continue ;;
+        esac
+        [ -e "/sys/class/net/$i/device" ] || continue
+        candidates+=("$i")
+    done
+    [ ${#candidates[@]} -gt 0 ] || die "no wired interface found; pass one explicitly"
+    for i in "${candidates[@]}"; do
+        if [ "$(cat "/sys/class/net/$i/carrier" 2>/dev/null || echo 0)" = "1" ]; then
+            echo "$i"; return
+        fi
+    done
+    echo "${candidates[0]}"
+}
+
+nm_present() { command -v nmcli >/dev/null 2>&1 && systemctl is-active --quiet NetworkManager 2>/dev/null; }
+
+do_up() {
+    local IF="$1"
+    echo "==> interface: $IF"
+
+    # NetworkManager will strip a hand-added address (often within seconds, and
+    # silently).  Take the device out of its control first and remember that we
+    # did, so `down` can hand it back.
+    mkdir -p "$STATE_DIR"
+    if nm_present && [ "$(nmcli -g GENERAL.STATE device show "$IF" 2>/dev/null | head -1)" != "" ] \
+       && nmcli -g GENERAL.STATE device show "$IF" 2>/dev/null | grep -qv unmanaged; then
+        echo "==> NetworkManager manages $IF -- setting it unmanaged"
+        echo "    NOTE: this drops any existing connection on $IF (DHCP lease,"
+        echo "          routes, DNS).  If $IF is your only network interface you"
+        echo "          will lose internet until '$0 down $IF'."
+        nmcli device set "$IF" managed no
+        touch "$STATE_DIR/$IF.was-managed"
+    fi
+
+    ip link set "$IF" up
+    if ip -4 addr show dev "$IF" | grep -q "inet $PC_IP/$PREFIX"; then
+        echo "==> $PC_IP/$PREFIX already present"
+    else
+        ip addr add "$PC_IP/$PREFIX" dev "$IF"
+        touch "$STATE_DIR/$IF.addr-added"
+        echo "==> added $PC_IP/$PREFIX"
+    fi
+
+    echo
+    do_status "$IF"
+    cat <<TXT
+
+Ready.  On the console, run gc_bba_test.dol from Swiss and press START.
+The TX test ARPs for $PC_IP; this machine answers it, which is what the
+RX test waits for.
+
+To see the traffic:   sudo tcpdump -ni $IF -e
+To undo:              sudo $0 down $IF
+TXT
+}
+
+do_down() {
+    local IF="$1"
+    echo "==> interface: $IF"
+    if ip -4 addr show dev "$IF" | grep -q "inet $PC_IP/$PREFIX"; then
+        ip addr del "$PC_IP/$PREFIX" dev "$IF" && echo "==> removed $PC_IP/$PREFIX"
+    else
+        echo "==> $PC_IP/$PREFIX not present, nothing to remove"
+    fi
+    rm -f "$STATE_DIR/$IF.addr-added"
+    if [ -f "$STATE_DIR/$IF.was-managed" ]; then
+        echo "==> handing $IF back to NetworkManager"
+        nmcli device set "$IF" managed yes || true
+        rm -f "$STATE_DIR/$IF.was-managed"
+    fi
+}
+
+do_status() {
+    local IF="$1" carrier speed
+    carrier=$(cat "/sys/class/net/$IF/carrier" 2>/dev/null || echo "?")
+    speed=$(cat "/sys/class/net/$IF/speed" 2>/dev/null || echo "?")
+    echo "    link carrier : $carrier   (1 = cable up)"
+    echo "    link speed   : ${speed} Mb/s   (expect 100 - the W5100S is 100BASE-TX)"
+    echo "    addresses    : $(ip -4 -br addr show dev "$IF" | tr -s ' ')"
+    if [ "$carrier" != "1" ]; then
+        echo "    ** no carrier -- check the cable, and that the console is powered"
+        echo "       (the W5100S rail is gated off GC_ON, so it is dead with the GC off)"
+    fi
+    if ip -4 addr show dev "$IF" | grep -q "inet $PC_IP/$PREFIX"; then
+        echo "    config       : OK, $PC_IP/$PREFIX is set"
+    else
+        echo "    config       : NOT configured -- run '$0 up $IF'"
+    fi
+    echo "    board should be reachable at $BOARD_IP once its socket-3 test runs"
+}
+
+ACTION="${1:-up}"
+IFACE="${2:-}"
+[ -n "$IFACE" ] || IFACE=$(detect_iface)
+[ -e "/sys/class/net/$IFACE" ] || die "no such interface: $IFACE"
+
+case "$ACTION" in
+    up)     need_root; do_up "$IFACE" ;;
+    down)   need_root; do_down "$IFACE" ;;
+    status) do_status "$IFACE" ;;
+    watch)  need_root; do_up "$IFACE"
+            echo; echo "==> tcpdump on $IFACE (ctrl-C to stop)"; echo
+            exec tcpdump -ni "$IFACE" -e ;;
+    *)      die "usage: $0 {up|down|status|watch} [interface]" ;;
+esac