fix: thread wg-easy v15's preshared key into the box's wg-quick config

furtka-gateway's wg-easy v15 client (wgeasy_v15.py) generates a
PresharedKey per WireGuard client by default, which the server enforces
even if the box's own conf omits it — dropping it here breaks the
handshake silently. Include it in [Peer] when present; v14 has no
preshared_key field at all, so this stays a no-op for that path.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R3jVwAqESRjPdW9P9nvfem

furtka/wireguard.py

@@ -39,7 +39,16 @@ def _conf_content(wg: dict) -> str:
     subnet only), which this module passes through verbatim rather than
     widening it, so a compromised gateway can't pull the box's other
     traffic through it.
+
+    `preshared_key` is only present when the gateway's wg-easy is v15
+    (which generates one per client by default and enforces it
+    server-side) — omitted (None/missing) for v14, which has none.
+    Dropping a present preshared key here would break the handshake
+    silently, since the server side would still expect it.
     """
+    preshared_key_line = ""
+    if wg.get("preshared_key"):
+        preshared_key_line = f"PresharedKey = {wg['preshared_key']}\n"
     return (
         "[Interface]\n"
         f"PrivateKey = {wg['private_key']}\n"
@@ -47,6 +56,7 @@ def _conf_content(wg: dict) -> str:
         "\n"
         "[Peer]\n"
         f"PublicKey = {wg['server_public_key']}\n"
+        f"{preshared_key_line}"
         f"Endpoint = {wg['endpoint']}\n"
         f"AllowedIPs = {wg['allowed_ips']}\n"
         "PersistentKeepalive = 25\n"

tests/test_wireguard.py

@@ -32,6 +32,26 @@ def test_conf_content_shape():
     assert "PersistentKeepalive = 25" in content
 
 
+def test_conf_content_omits_preshared_key_when_absent():
+    # WG has no "preshared_key" key at all — the v14-gateway shape.
+    content = wireguard._conf_content(WG)
+    assert "PresharedKey" not in content
+
+
+def test_conf_content_includes_preshared_key_when_present():
+    # v15-gateway shape (see furtka-gateway's wgeasy_v15.py) — the server
+    # enforces this even if the client's own conf omits it, so dropping it
+    # here would break the handshake silently.
+    wg = {**WG, "preshared_key": "cHJlc2hhcmVkLWtleQ=="}
+    content = wireguard._conf_content(wg)
+    assert "PresharedKey = cHJlc2hhcmVkLWtleQ==" in content
+    # Must sit between PublicKey and Endpoint, inside [Peer].
+    peer_section = content.split("[Peer]\n", 1)[1]
+    assert peer_section.index("PublicKey") < peer_section.index(
+        "PresharedKey"
+    ) < peer_section.index("Endpoint")
+
+
 def test_enable_requires_wg_config(tmp_path):
     with pytest.raises(ValueError, match="wg config is required"):
         wireguard.set_tunnel_enabled(True, conf_path=tmp_path / "furtka-gw.conf")