20184 FIX proxmox_ve: Fix crash when monitoring an IPv6-only host via IP

Previously, monitoring a Proxmox VE host configured to connect _via IP_
crashed the special agent if the host was IPv6-only (no dual-stack), with

```InvalidURL: Failed to parse: [https://<ipv6-address>:8006/api2/json/access/ticket]```

The IPv6 address was not enclosed in square brackets as required by
RFC 3986, so it couldn't be told apart from the port number. This has been
fixed.

CMK-37108

Change-Id: Iafe5428f6803b9d66cbbb34b0243798e8ccbe490

Author: racicLuka

.werks/20184.md

@@ -0,0 +1,22 @@
+[//]: # (werk v3)
+# proxmox_ve: Fix crash when monitoring an IPv6-only host via IP
+
+key        | value
+---------- | ---
+date       | 2026-08-09T13:49:40.182743+00:00
+version    | 2.5.0p12
+class      | fix
+edition    | community
+component  | checks
+level      | 1
+compatible | yes
+
+Previously, monitoring a Proxmox VE host configured to connect _via IP_
+crashed the special agent if the host was IPv6-only (no dual-stack), with
+
+```InvalidURL: Failed to parse: [https://<ipv6-address>:8006/api2/json/access/ticket]```
+
+The IPv6 address was not enclosed in square brackets as required by
+RFC 3986, so it couldn't be told apart from the port number. This has been
+fixed.
+

packages/cmk-plugins/cmk/plugins/proxmox_ve/special_agent/libproxmox.py

@@ -5,6 +5,7 @@
 
 # mypy: disable-error-code="no-untyped-call"
 
+import ipaddress
 import logging
 from collections.abc import Iterable, Mapping, Sequence
 from json import JSONDecodeError
@@ -22,6 +23,15 @@
 class CannotRecover(RuntimeError): ...
 
 
+def _host_port_for_url(host: str, port: int) -> str:
+    """Render host:port for use in a URL, bracketing IPv6 literals per RFC 3986."""
+    try:
+        is_ipv6 = ipaddress.ip_address(host).version == 6
+    except ValueError:
+        is_ipv6 = False
+    return f"[{host}]:{port}" if is_ipv6 else f"{host}:{port}"
+
+
 class _ProxmoxVeSession:
     """Session"""
 
@@ -84,7 +94,7 @@ def create_session() -> requests.Session:
 
         self._timeout = timeout
         self._verify_ssl = verify_ssl
-        self._base_url = "https://%s:%d/" % endpoint
+        self._base_url = f"https://{_host_port_for_url(*endpoint)}/"
         self._session = create_session()
 
     def __enter__(self) -> Any:

packages/cmk-plugins/tests/cmk/plugins/proxmox_ve/special_agent/test_libproxmox.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+# Copyright (C) 2026 Checkmk GmbH - License: GNU General Public License v2
+# This file is part of Checkmk (https://checkmk.com). It is subject to the terms and
+# conditions defined in the file COPYING, which is part of this source code package.
+
+import pytest
+
+from cmk.plugins.proxmox_ve.special_agent.libproxmox import _host_port_for_url
+
+
+@pytest.mark.parametrize(
+    "host, port, expected",
+    [
+        pytest.param("2a01:4f9:2b:1c86::2", 8006, "[2a01:4f9:2b:1c86::2]:8006", id="ipv6"),
+        pytest.param("::1", 8006, "[::1]:8006", id="ipv6_loopback"),
+        pytest.param("192.168.1.10", 8006, "192.168.1.10:8006", id="ipv4"),
+        pytest.param("pve1.example.com", 8006, "pve1.example.com:8006", id="hostname"),
+    ],
+)
+def test_host_port_for_url(host: str, port: int, expected: str) -> None:
+    assert _host_port_for_url(host, port) == expected