From a5ed0a4321e5fe1348f91b5bb3f99986ca555281 Mon Sep 17 00:00:00 2001
From: Mark Qvist <bc7291552be7a58f361522990465165c>
Date: Sat, 18 Jul 2026 21:26:57 +0200
Subject: [PATCH] Improved HDLC handling

---
 RNS/Interfaces/BackboneInterface.py | 18 ++++++++++++++++--
 RNS/Interfaces/TCPInterface.py      | 17 +++++++++++++++--
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/RNS/Interfaces/BackboneInterface.py b/RNS/Interfaces/BackboneInterface.py
index 123559b4..8c3cba9e 100644
--- a/RNS/Interfaces/BackboneInterface.py
+++ b/RNS/Interfaces/BackboneInterface.py
@@ -759,6 +759,14 @@ class BackboneClientInterface(Interface):
                 RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)
                 self.teardown()
 
+    def check_frame_len(self, frame_len):
+        if   frame_len <= RNS.Reticulum.HEADER_MINSIZE:        return False
+        elif frame_len >  self.HW_MTU + (self.ifac_size or 0): return False
+        else:                                                  return True
+
+    def invalid_frame(self, frame_len):
+        RNS.log(f"Invalid HDLC frame of {RNS.prettysize(frame_len)} received on {self}, dropping frame", RNS.LOG_DEBUG) if RNS.sl(RNS.LOG_DEBUG) else None
+
     def receive(self, data_in):
         try:
             if len(data_in) > 0:
@@ -772,12 +780,18 @@ class BackboneClientInterface(Interface):
                             frame = self.frame_buffer[frame_start+1:frame_end]
                             frame = frame.replace(bytes([HDLC.ESC, HDLC.FLAG ^ HDLC.ESC_MASK]), bytes([HDLC.FLAG]))
                             frame = frame.replace(bytes([HDLC.ESC, HDLC.ESC  ^ HDLC.ESC_MASK]), bytes([HDLC.ESC]))
-                            if len(frame) > RNS.Reticulum.HEADER_MINSIZE:
-                                self.process_incoming(frame)
+                            frame_len = len(frame)
+                            if frame_len != 0:
+                                if self.check_frame_len(frame_len): self.process_incoming(frame)
+                                else:                               self.invalid_frame(len(frame))
+
                             self.frame_buffer = self.frame_buffer[frame_end:]
+
                         else:
+                            if len(self.frame_buffer) > self.HW_MTU*2: self.frame_buffer = b""
                             flags_remaining = False
                     else:
+                        self.frame_buffer = b""
                         flags_remaining = False
 
             else:
diff --git a/RNS/Interfaces/TCPInterface.py b/RNS/Interfaces/TCPInterface.py
index a5566e31..5559732f 100644
--- a/RNS/Interfaces/TCPInterface.py
+++ b/RNS/Interfaces/TCPInterface.py
@@ -333,6 +333,13 @@ class TCPClientInterface(Interface):
                 RNS.log("The contained exception was: "+str(e), RNS.LOG_ERROR)
                 self.teardown()
 
+    def check_frame_len(self, frame_len):
+        if   frame_len <= RNS.Reticulum.HEADER_MINSIZE:        return False
+        elif frame_len >  self.HW_MTU + (self.ifac_size or 0): return False
+        else:                                                  return True
+
+    def invalid_frame(self, frame_len):
+        RNS.log(f"Invalid HDLC frame of {RNS.prettysize(frame_len)} received on {self}, dropping frame", RNS.LOG_DEBUG) if RNS.sl(RNS.LOG_DEBUG) else None
 
     def read_loop(self):
         try:
@@ -389,12 +396,18 @@ class TCPClientInterface(Interface):
                                     frame = frame_buffer[frame_start+1:frame_end]
                                     frame = frame.replace(bytes([HDLC.ESC, HDLC.FLAG ^ HDLC.ESC_MASK]), bytes([HDLC.FLAG]))
                                     frame = frame.replace(bytes([HDLC.ESC, HDLC.ESC  ^ HDLC.ESC_MASK]), bytes([HDLC.ESC]))
-                                    if len(frame) > RNS.Reticulum.HEADER_MINSIZE:
-                                        self.process_incoming(frame)
+                                    frame_len = len(frame)
+                                    if frame_len != 0:
+                                        if self.check_frame_len(frame_len): self.process_incoming(frame)
+                                        else:                               self.invalid_frame(len(frame))
+
                                     frame_buffer = frame_buffer[frame_end:]
+
                                 else:
+                                    if len(frame_buffer) > self.HW_MTU*2: frame_buffer = b""
                                     flags_remaining = False
                             else:
+                                frame_buffer = b""
                                 flags_remaining = False
 
                 else: