add max_msg_body_bytes limit and validation for message body size

rrcd/cli.py

@@ -151,8 +151,15 @@ include_joined_member_list = false
 nick_max_chars = 32
 
 # Limits.
+# These limits help mitigate abuse and resource exhaustion, but can be adjusted
+# based on your use case.
+#
+# N.B. max_msg_body_bytes should not allow messages so large that they cannot
+# fit within the link MTU after UTF-8 encoding and envelope overhead. The
+# default of 350 bytes is a safe choice for the default Reticulum MTU of 500.
 max_rooms_per_session = 32
 max_room_name_len = 64
+max_msg_body_bytes = 350
 rate_limit_msgs_per_minute = 240
 
 # Hub-initiated liveness checks (0 disables).
@@ -332,6 +339,12 @@ def _build_arg_parser() -> argparse.ArgumentParser:
         default=None,
         help="Per-link message rate limit",
     )
+    p.add_argument(
+        "--max-msg-body-bytes",
+        type=int,
+        default=None,
+        help="Maximum message body size in UTF-8 bytes",
+    )
 
     p.add_argument(
         "--ping-interval",
@@ -420,6 +433,8 @@ def main(argv: list[str] | None = None) -> None:
         cfg = replace(
             cfg, rate_limit_msgs_per_minute=int(args.rate_limit_msgs_per_minute)
         )
+    if args.max_msg_body_bytes is not None:
+        cfg = replace(cfg, max_msg_body_bytes=int(args.max_msg_body_bytes))
 
     if args.ping_interval is not None:
         cfg = replace(cfg, ping_interval_s=float(args.ping_interval))

rrcd/config.py

@@ -28,6 +28,7 @@ class HubRuntimeConfig:
     nick_max_chars: int = 32
     max_rooms_per_session: int = 32
     max_room_name_len: int = 64
+    max_msg_body_bytes: int = 350
     rate_limit_msgs_per_minute: int = 240
     ping_interval_s: float = 0.0
     ping_timeout_s: float = 0.0

rrcd/router.py

@@ -686,6 +686,26 @@ class MessageRouter:
                         text="message requires room name",
                     )
                 return
+            
+            # Validate message body size (UTF-8 bytes)
+            if isinstance(body, str):
+                body_bytes = len(body.encode('utf-8', errors='replace'))
+                if body_bytes > self.hub.config.max_msg_body_bytes:
+                    if self.hub.identity is not None:
+                        self.hub.message_helper.emit_error(
+                            outgoing,
+                            link,
+                            src=self.hub.identity.hash,
+                            text=f"message too large: {body_bytes} bytes > {self.hub.config.max_msg_body_bytes} bytes",
+                        )
+                    self.log.info(
+                        "Rejected oversized message peer=%s nick=%r body_bytes=%s limit=%s",
+                        self.hub._fmt_hash(peer_hash),
+                        sess.get("nick"),
+                        body_bytes,
+                        self.hub.config.max_msg_body_bytes,
+                    )
+                    return
         elif t == T_NOTICE:
             if not isinstance(room, str) or not room:
                 return