Fix http transport rejecting remote clients with 421 (#4)

* Fix http transport rejecting remote clients with 421

FastMCP is constructed at import time with the default host 127.0.0.1,
which auto-locks DNS-rebinding protection to a localhost-only Host
allowlist. Rebinding the host to 0.0.0.0 in main() does not update that
frozen allowlist, so remote clients get "421 Invalid Host header".

Set transport_security explicitly in http mode: derive allowed hosts from
VYMCP_PUBLIC_URL (overridable via VYMCP_ALLOWED_HOSTS, or "*" to disable
the check behind a trusted proxy).

* Wrap origins comprehension to satisfy E501

Author: xTITUSMAXIMUSX

.env.example

@@ -22,6 +22,14 @@ VYMCP_TRANSPORT=stdio
 VYMANAGER_API_TOKEN=vym_xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
 
 # http only: bind address, port, and the externally reachable URL.
+# To accept remote clients, bind all interfaces (VYMCP_HOST=0.0.0.0) AND set
+# VYMCP_PUBLIC_URL to the address clients actually use — the server only accepts
+# requests whose Host header matches it (DNS-rebinding protection).
 VYMCP_HOST=127.0.0.1
 VYMCP_PORT=8080
 # VYMCP_PUBLIC_URL=https://vymcp.example.com
+
+# Optional: override the allowed Host header values (comma-separated, e.g.
+# "192.168.9.103:8080,vymcp.example.com"). Defaults to the VYMCP_PUBLIC_URL host.
+# Set to "*" to disable the Host check entirely (only behind a trusted proxy).
+# VYMCP_ALLOWED_HOSTS=192.168.9.103:8080

src/vymcp/config.py

@@ -3,7 +3,8 @@
 from __future__ import annotations
 
 import os
-from dataclasses import dataclass
+from dataclasses import dataclass, field
+from urllib.parse import urlparse
 
 _FALSEY = {"0", "false", "no", "off"}
 _TRUTHY = {"1", "true", "yes", "on"}
@@ -52,6 +53,9 @@ class ServerConfig:
     port: int = 8080
     public_url: str = "http://127.0.0.1:8080"  # externally reachable URL (http mode)
     api_token: str | None = None
+    # Host header values the http server accepts (DNS-rebinding protection). Empty
+    # tuple means allow any host (protection disabled).
+    allowed_hosts: tuple[str, ...] = field(default_factory=tuple)
 
     @classmethod
     def from_env(cls) -> ServerConfig:
@@ -77,6 +81,20 @@ def from_env(cls) -> ServerConfig:
 
         host = os.environ.get("VYMCP_HOST", "127.0.0.1")
         public_url = os.environ.get("VYMCP_PUBLIC_URL") or f"http://{host}:{port}"
+        public_url = public_url.rstrip("/")
+
+        # Which Host headers the http server accepts. Explicit override wins;
+        # otherwise default to the public URL's host:port so external clients that
+        # reach the server at its advertised address are allowed. "*" disables the
+        # DNS-rebinding check entirely (use only behind a trusted proxy).
+        allowed_env = os.environ.get("VYMCP_ALLOWED_HOSTS", "").strip()
+        if allowed_env == "*":
+            allowed_hosts: tuple[str, ...] = ()
+        elif allowed_env:
+            allowed_hosts = tuple(h.strip() for h in allowed_env.split(",") if h.strip())
+        else:
+            netloc = urlparse(public_url).netloc
+            allowed_hosts = (netloc,) if netloc else ()
 
         return cls(
             base_url=base_url.rstrip("/"),
@@ -85,8 +103,9 @@ def from_env(cls) -> ServerConfig:
             transport=transport,
             host=host,
             port=port,
-            public_url=public_url.rstrip("/"),
+            public_url=public_url,
             api_token=api_token,
+            allowed_hosts=allowed_hosts,
         )
 
     def client_config(self, token: str) -> Config:

src/vymcp/server.py

@@ -104,11 +104,29 @@ def main() -> None:
     config = server_config()
     if config.transport == "http":
         from mcp.server.auth.settings import AuthSettings
+        from mcp.server.transport_security import TransportSecuritySettings
 
         from .auth import VyManagerTokenVerifier
 
         mcp.settings.host = config.host
         mcp.settings.port = config.port
+        # FastMCP auto-configures DNS-rebinding protection for localhost at
+        # construction time (allowing only 127.0.0.1/localhost). Since we bind and
+        # advertise a different address in http mode, replace it with an allowlist
+        # derived from config, or disable it when no hosts are configured.
+        if config.allowed_hosts:
+            origins = [
+                f"{scheme}://{h}" for h in config.allowed_hosts for scheme in ("http", "https")
+            ]
+            mcp.settings.transport_security = TransportSecuritySettings(
+                enable_dns_rebinding_protection=True,
+                allowed_hosts=list(config.allowed_hosts),
+                allowed_origins=origins,
+            )
+        else:
+            mcp.settings.transport_security = TransportSecuritySettings(
+                enable_dns_rebinding_protection=False,
+            )
         mcp.settings.auth = AuthSettings(
             issuer_url=config.base_url,  # type: ignore[arg-type]  # pydantic coerces str->AnyHttpUrl
             resource_server_url=config.public_url,  # type: ignore[arg-type]