744 dns resolution normalization fix (#745)

* Fix #744: Preserve domain names while preventing localhost duplicates

The normalize_url method was resolving ALL domain names to IP addresses,
which breaks for services behind CDNs, load balancers, or reverse
proxies. This fix:

- Preserves domain names for external services (CDN/load balancer support)
- Only normalizes 127.0.0.1 to localhost to prevent local duplicates
- Maintains fix for issue #649 for localhost/127.0.0.1 duplicates
- Keeps compatibility with PR #712's duplicate prevention for local services

Changes:
- Removed socket.gethostbyname() for external domains
- Added special case to convert 127.0.0.1 to localhost
- Updated tests to verify both behaviors
- Added regression test for duplicate prevention

This balances the needs of both issues:
- #649: Prevent localhost/127.0.0.1 duplicates (still fixed)
- #744: Support services behind CDNs/load balancers (now fixed)

Signed-off-by: Mihai Criveti <[email protected]>

* Fix localhost resolution

Signed-off-by: Mihai Criveti <[email protected]>

* fix pylint

Signed-off-by: RAKHI DUTTA <[email protected]>

---------

Signed-off-by: Mihai Criveti <[email protected]>
Signed-off-by: RAKHI DUTTA <[email protected]>
Co-authored-by: RAKHI DUTTA <[email protected]>

Author: crivetimihai

mcpgateway/services/gateway_service.py

@@ -42,7 +42,6 @@
 from datetime import datetime, timezone
 import logging
 import os
-import socket
 import tempfile
 import time
 from typing import Any, AsyncGenerator, Dict, List, Optional, Set, TYPE_CHECKING
@@ -257,29 +256,40 @@ def __init__(self) -> None:
     @staticmethod
     def normalize_url(url: str) -> str:
         """
-        Normalize a URL by resolving the hostname to its IP address.
+        Normalize a URL by ensuring it's properly formatted.
+
+        Special handling for localhost to prevent duplicates:
+        - Converts 127.0.0.1 to localhost for consistency
+        - Preserves all other domain names as-is for CDN/load balancer support
 
         Args:
             url (str): The URL to normalize.
 
         Returns:
-            str: The normalized URL with the hostname replaced by its IP address.
+            str: The normalized URL.
 
         Examples:
             >>> GatewayService.normalize_url('http://localhost:8080/path')
-            'http://127.0.0.1:8080/path'
+            'http://localhost:8080/path'
+            >>> GatewayService.normalize_url('http://127.0.0.1:8080/path')
+            'http://localhost:8080/path'
+            >>> GatewayService.normalize_url('https://example.com/api')
+            'https://example.com/api'
         """
         parsed = urlparse(url)
         hostname = parsed.hostname
-        try:
-            ip = socket.gethostbyname(hostname)
-        except Exception:
-            ip = hostname
-        netloc = ip
-        if parsed.port:
-            netloc += f":{parsed.port}"
-        normalized = parsed._replace(netloc=netloc)
-        return urlunparse(normalized)
+
+        # Special case: normalize 127.0.0.1 to localhost to prevent duplicates
+        # but preserve all other domains as-is for CDN/load balancer support
+        if hostname == "127.0.0.1":
+            netloc = "localhost"
+            if parsed.port:
+                netloc += f":{parsed.port}"
+            normalized = parsed._replace(netloc=netloc)
+            return urlunparse(normalized)
+
+        # For all other URLs, preserve the domain name
+        return url
 
     async def _validate_gateway_url(self, url: str, headers: dict, transport_type: str, timeout: Optional[int] = None):
         """

tests/unit/mcpgateway/services/test_gateway_service.py

@@ -1134,6 +1134,47 @@ async def test_update_gateway_integrity_error(self, gateway_service, mock_gatewa
         with pytest.raises(SQLIntegrityError):
             await gateway_service.update_gateway(test_db, 1, gateway_update)
 
+    def test_normalize_url_preserves_domain(self):
+        """Test that normalize_url preserves domain names but normalizes localhost."""
+        # Test with various domain formats
+        test_cases = [
+            # Regular domains should be preserved as-is
+            ("http://example.com", "http://example.com"),
+            ("https://api.example.com:8080/path", "https://api.example.com:8080/path"),
+            ("https://my-app.cloud-provider.region.example.com/sse",
+             "https://my-app.cloud-provider.region.example.com/sse"),
+            ("https://cdn.service.com/api/v1", "https://cdn.service.com/api/v1"),
+
+            # localhost should remain localhost
+            ("http://localhost:8000", "http://localhost:8000"),
+            ("https://localhost/api", "https://localhost/api"),
+
+            # 127.0.0.1 should be normalized to localhost to prevent duplicates
+            ("http://127.0.0.1:8080/path", "http://localhost:8080/path"),
+            ("https://127.0.0.1/sse", "https://localhost/sse"),
+        ]
+
+        for input_url, expected in test_cases:
+            result = GatewayService.normalize_url(input_url)
+            assert result == expected, f"normalize_url({input_url}) should return {expected}, got {result}"
+
+    def test_normalize_url_prevents_localhost_duplicates(self):
+        """Test that normalization prevents localhost/127.0.0.1 duplicates."""
+        # These URLs should all normalize to the same value
+        equivalent_urls = [
+            "http://127.0.0.1:8080/sse",
+            "http://localhost:8080/sse",
+        ]
+
+        normalized = [GatewayService.normalize_url(url) for url in equivalent_urls]
+
+        # All should normalize to localhost version
+        assert all(n == "http://localhost:8080/sse" for n in normalized), \
+            f"All localhost variants should normalize to same URL, got: {normalized}"
+
+        # They should all be the same (no duplicates possible)
+        assert len(set(normalized)) == 1, "All localhost variants should produce identical normalized URLs"
+
     @pytest.mark.asyncio
     async def test_update_gateway_with_transport_change(self, gateway_service, mock_gateway, test_db):
         """Test updating gateway transport type."""