fix: Add security and reliability improvements

1. File locking for config operations
   - add_server_to_config() and remove_server_from_config() now use
     exclusive file locking (fcntl.LOCK_EX) to prevent race conditions
   - Prevents data loss from concurrent modifications

2. Bulk server limit
   - Add MAX_BULK_SERVERS = 10 constant
   - haproxy_add_servers() now rejects requests exceeding limit
   - Prevents potential DoS via large payloads

3. HAProxy command response validation
   - Add haproxy_cmd_checked() helper function
   - Validates responses for error indicators (No such, not found, etc.)
   - State-modifying commands now properly detect and report failures
   - Read-only commands continue using haproxy_cmd()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mcp/server.py

@@ -82,6 +82,7 @@ STARTUP_RETRY_COUNT = 10  # HAProxy ready check retries
 STATE_MIN_COLUMNS = 19  # Minimum columns in HAProxy server state output
 SOCKET_TIMEOUT = 5  # seconds for HAProxy socket connection
 SOCKET_RECV_TIMEOUT = 30  # seconds for HAProxy socket recv loop
+MAX_BULK_SERVERS = 10  # Max servers per bulk add call
 
 
 class HaproxyError(Exception):
@@ -169,6 +170,29 @@ def haproxy_cmd(command: str) -> str:
         raise HaproxyError(str(e)) from e
 
 
+def haproxy_cmd_checked(command: str) -> str:
+    """Send command to HAProxy and raise on error response.
+
+    Args:
+        command: HAProxy command to execute
+
+    Returns:
+        Command response
+
+    Raises:
+        HaproxyError: If HAProxy returns an error message
+    """
+    result = haproxy_cmd(command)
+    # HAProxy returns empty string on success, error messages on failure
+    error_indicators = ["No such", "not found", "error", "failed", "invalid", "unknown"]
+    if result:
+        result_lower = result.lower()
+        for indicator in error_indicators:
+            if indicator.lower() in result_lower:
+                raise HaproxyError(f"HAProxy command failed: {result.strip()}")
+    return result
+
+
 def reload_haproxy() -> Tuple[bool, str]:
     """Validate and reload HAProxy configuration.
 
@@ -485,7 +509,7 @@ def save_servers_config(config: Dict[str, Any]) -> None:
 
 
 def add_server_to_config(domain: str, slot: int, ip: str, http_port: int) -> None:
-    """Add server configuration to persistent storage.
+    """Add server configuration to persistent storage with file locking.
 
     Args:
         domain: Domain name
@@ -493,29 +517,38 @@ def add_server_to_config(domain: str, slot: int, ip: str, http_port: int) -> Non
         ip: Server IP address
         http_port: HTTP port
     """
+    lock_path = f"{SERVERS_FILE}.lock"
+    with open(lock_path, 'w') as lock_file:
+        fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX)
+        try:
             config = load_servers_config()
             if domain not in config:
                 config[domain] = {}
-    config[domain][str(slot)] = {
+            config[domain][str(slot)] = {"ip": ip, "http_port": http_port}
-        "ip": ip,
-        "http_port": http_port
-    }
             save_servers_config(config)
+        finally:
+            fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
 
 
 def remove_server_from_config(domain: str, slot: int) -> None:
-    """Remove server configuration from persistent storage.
+    """Remove server configuration from persistent storage with file locking.
 
     Args:
         domain: Domain name
         slot: Server slot to remove
     """
+    lock_path = f"{SERVERS_FILE}.lock"
+    with open(lock_path, 'w') as lock_file:
+        fcntl.flock(lock_file.fileno(), fcntl.LOCK_EX)
+        try:
             config = load_servers_config()
             if domain in config and str(slot) in config[domain]:
                 del config[domain][str(slot)]
                 if not config[domain]:
                     del config[domain]
                 save_servers_config(config)
+        finally:
+            fcntl.flock(lock_file.fileno(), fcntl.LOCK_UN)
 
 
 def remove_domain_from_config(domain: str) -> None:
@@ -891,8 +924,8 @@ def haproxy_add_server(domain: str, slot: int, ip: str, http_port: int = 80) ->
         results = []
         for suffix, port in get_server_suffixes(http_port):
             server = f"{server_prefix}{suffix}_{slot}"
-            haproxy_cmd(f"set server {backend}/{server} addr {ip} port {port}")
+            haproxy_cmd_checked(f"set server {backend}/{server} addr {ip} port {port}")
-            haproxy_cmd(f"set server {backend}/{server} state ready")
+            haproxy_cmd_checked(f"set server {backend}/{server} state ready")
             results.append(f"{server} → {ip}:{port}")
 
         # Save to persistent config
@@ -942,6 +975,9 @@ def haproxy_add_servers(domain: str, servers: str) -> str:
     if not server_list:
         return "Error: servers array is empty"
 
+    if len(server_list) > MAX_BULK_SERVERS:
+        return f"Error: Cannot add more than {MAX_BULK_SERVERS} servers at once"
+
     # Validate all servers first before adding any
     validated_servers = []
     validation_errors = []
@@ -1014,8 +1050,8 @@ def haproxy_add_servers(domain: str, servers: str) -> str:
         try:
             for suffix, port in get_server_suffixes(http_port):
                 server = f"{server_prefix}{suffix}_{slot}"
-                haproxy_cmd(f"set server {backend}/{server} addr {ip} port {port}")
+                haproxy_cmd_checked(f"set server {backend}/{server} addr {ip} port {port}")
-                haproxy_cmd(f"set server {backend}/{server} state ready")
+                haproxy_cmd_checked(f"set server {backend}/{server} state ready")
 
             # Save to persistent config
             add_server_to_config(domain, slot, ip, http_port)
@@ -1063,8 +1099,8 @@ def haproxy_remove_server(domain: str, slot: int) -> str:
 
         # HTTP only - single server per slot
         server = f"{server_prefix}_{slot}"
-        haproxy_cmd(f"set server {backend}/{server} state maint")
+        haproxy_cmd_checked(f"set server {backend}/{server} state maint")
-        haproxy_cmd(f"set server {backend}/{server} addr 0.0.0.0 port 0")
+        haproxy_cmd_checked(f"set server {backend}/{server} addr 0.0.0.0 port 0")
 
         # Remove from persistent config
         remove_server_from_config(domain, slot)
@@ -1323,8 +1359,8 @@ def haproxy_set_server_state(backend: str, server: str, state: str) -> str:
     if state not in ["ready", "drain", "maint"]:
         return "Error: state must be 'ready', 'drain', or 'maint'"
     try:
-        result = haproxy_cmd(f"set server {backend}/{server} state {state}")
+        haproxy_cmd_checked(f"set server {backend}/{server} state {state}")
-        return result if result else f"Server {backend}/{server} set to {state}"
+        return f"Server {backend}/{server} set to {state}"
     except HaproxyError as e:
         return f"Error: {e}"
 
@@ -1392,8 +1428,8 @@ def haproxy_set_server_weight(backend: str, server: str, weight: int) -> str:
     if not (0 <= weight <= 256):
         return "Error: weight must be between 0 and 256"
     try:
-        result = haproxy_cmd(f"set server {backend}/{server} weight {weight}")
+        haproxy_cmd_checked(f"set server {backend}/{server} weight {weight}")
-        return result if result else f"Server {backend}/{server} weight set to {weight}"
+        return f"Server {backend}/{server} weight set to {weight}"
     except HaproxyError as e:
         return f"Error: {e}"