jellyfin-qbittorrent-monitor: fix hairpin handling

services/jellyfin/jellyfin-qbittorrent-monitor.py

@@ -38,6 +38,7 @@ class JellyfinQBittorrentMonitor:
         stream_bitrate_headroom=1.1,
         webhook_port=0,
         webhook_bind="127.0.0.1",
+        gateway_ip=None,
     ):
         self.jellyfin_url = jellyfin_url
         self.qbittorrent_url = qbittorrent_url
@@ -77,6 +78,15 @@ class JellyfinQBittorrentMonitor:
             ipaddress.ip_network("fe80::/10"),  # IPv6 link-local
         ]
 
+        # Hairpin marker. When a LAN client reaches Jellyfin via the public
+        # hostname, the router NAT-loopbacks the packet and SNATs the source
+        # to itself — the session arrives looking local but still costs WAN
+        # bandwidth. Sessions whose source equals the gateway must therefore
+        # NOT be skipped. None disables the check (pre-hairpin-aware behavior).
+        if gateway_ip is None:
+            gateway_ip = self._discover_default_gateway()
+        self.gateway_ip = gateway_ip
+
     def is_local_ip(self, ip_address: str) -> bool:
         """Check if an IP address is from a local network"""
         try:
@@ -86,6 +96,39 @@ class JellyfinQBittorrentMonitor:
             logger.warning(f"Invalid IP address format: {ip_address}")
             return True  # Treat invalid IPs as local for safety
 
+    def _discover_default_gateway(self) -> str | None:
+        """Read the IPv4 default gateway from /proc/net/route, or None."""
+        try:
+            with open("/proc/net/route") as f:
+                next(f)  # skip header
+                for line in f:
+                    fields = line.split()
+                    if len(fields) < 8 or fields[1] != "00000000":
+                        continue
+                    flags = int(fields[3], 16)
+                    if not flags & 0x2:  # RTF_GATEWAY
+                        continue
+                    gw_bytes = bytes.fromhex(fields[2])[::-1]  # little-endian
+                    if len(gw_bytes) != 4:
+                        continue
+                    return ".".join(str(b) for b in gw_bytes)
+        except (OSError, ValueError) as e:
+            logger.warning(f"Could not autodetect default gateway: {e}")
+        return None
+
+    def is_skippable(self, ip_address: str) -> bool:
+        """True iff this source IP can be ignored when deciding to throttle.
+
+        Truly LAN-direct sessions are skippable (no WAN cost). Hairpin-NAT'd
+        LAN sessions arrive with the LAN gateway as their source — those still
+        cost WAN bandwidth and must NOT be skipped.
+        """
+        if not self.is_local_ip(ip_address):
+            return False
+        if self.gateway_ip and ip_address == self.gateway_ip:
+            return False
+        return True
+
     def signal_handler(self, signum, frame):
         logger.info("Received shutdown signal, cleaning up...")
         self.running = False
@@ -164,7 +207,7 @@ class JellyfinQBittorrentMonitor:
             if (
                 "NowPlayingItem" in session
                 and not session.get("PlayState", {}).get("IsPaused", True)
-                and not self.is_local_ip(session.get("RemoteEndPoint", ""))
+                and not self.is_skippable(session.get("RemoteEndPoint", ""))
             ):
                 item = session["NowPlayingItem"]
                 item_type = item.get("Type", "").lower()
@@ -354,6 +397,9 @@ class JellyfinQBittorrentMonitor:
         logger.info(f"Default stream bitrate: {self.default_stream_bitrate} bps")
         logger.info(f"Minimum torrent speed: {self.min_torrent_speed} KB/s")
         logger.info(f"Stream bitrate headroom: {self.stream_bitrate_headroom}x")
+        logger.info(
+            f"LAN gateway (hairpin marker): {self.gateway_ip or 'none / autodetect failed'}"
+        )
         if self.webhook_port:
             logger.info(f"Webhook receiver: {self.webhook_bind}:{self.webhook_port}")
 
@@ -484,6 +530,7 @@ if __name__ == "__main__":
     stream_bitrate_headroom = float(os.getenv("STREAM_BITRATE_HEADROOM", "1.1"))
     webhook_port = int(os.getenv("WEBHOOK_PORT", "0"))
     webhook_bind = os.getenv("WEBHOOK_BIND", "127.0.0.1")
+    gateway_ip = os.getenv("LAN_GATEWAY_IP") or None
 
     monitor = JellyfinQBittorrentMonitor(
         jellyfin_url=jellyfin_url,
@@ -499,6 +546,7 @@ if __name__ == "__main__":
         stream_bitrate_headroom=stream_bitrate_headroom,
         webhook_port=webhook_port,
         webhook_bind=webhook_bind,
+        gateway_ip=gateway_ip,
     )
 
     monitor.run()