I wrote a blog about blocking Thunder in tranmission-daemon 2.94.

The patch does not apply cleanly on Transmission 3.00, so I update it here:

--- a/libtransmission/peer-mgr.c    2021-08-31 03:10:20.435836776 +0800
+++ b/libtransmission/peer-mgr.c    2021-08-31 03:14:31.268153266 +0800
@@ -2007,6 +2007,21 @@
     tr_peerMsgsUpdateActive(msgs, TR_DOWN);
 }

+static bool isUserAgentBad(const char* ua)
+{
+    static const char* blacklist[] = {"Thunder", "Xunlei"};
+    int i;
+
+    for (i = 0; i != sizeof(blacklist) / sizeof(blacklist[0]); ++i) {
+        // Performs bad, honestly.
+        if (!strncmp(blacklist[i], ua, strlen(blacklist[i]))) {
+            return true;
+        }
+    }
+    return false;
+}
+
+
 /* FIXME: this is kind of a mess. */
 static bool myHandshakeDoneCB(tr_handshake* handshake, tr_peerIo* io, bool readAnythingFromPeer, bool isConnected,
     uint8_t const* peer_id, void* vmanager)
@@ -2100,21 +2115,29 @@
                 tr_quark client;
                 tr_peerIo* io;
                 char buf[128];
+                const char* ua = "";

                 if (peer_id != NULL)
                 {
-                    client = tr_quark_new(tr_clientForId(buf, sizeof(buf), peer_id), TR_BAD_SIZE);
+                    client = tr_quark_new((ua = tr_clientForId(buf, sizeof(buf), peer_id)), TR_BAD_SIZE);
                 }
                 else
                 {
                     client = TR_KEY_NONE;
                 }

-                io = tr_handshakeStealIO(handshake); /* this steals its refcount too, which is balanced by our unref in peerDelete() */
-                tr_peerIoSetParent(io, &s->tor->bandwidth);
-                createBitTorrentPeer(s->tor, io, atom, client);
+                if (!isUserAgentBad(ua))
+                {
+                    io = tr_handshakeStealIO(handshake); /* this steals its refcount too, which is balanced by our unref in peerDelete() */
+                    tr_peerIoSetParent(io, &s->tor->bandwidth);
+                    createBitTorrentPeer(s->tor, io, atom, client);

-                success = true;
+                    success = true;
+                }
+                else
+                {
+                    tr_logAddDebug("Bad user agent \"%s\" (peer %s) tried to connect to us", ua, tr_atomAddrStr(atom));
+                }
             }
         }
     }

The rest instructions are the same as the previous post.