diff --git a/linux_voice_assistant/util.py b/linux_voice_assistant/util.py
index 31e184b..a55f0a3 100644
--- a/linux_voice_assistant/util.py
+++ b/linux_voice_assistant/util.py
@@ -1,15 +1,76 @@
 """Utility methods."""
 
+import logging
 import uuid
 from collections.abc import Callable
+from pathlib import Path
 from typing import Optional
 
+_LOGGER = logging.getLogger(__name__)
+_CACHED_MAC: Optional[str] = None
 
-def get_mac() -> str:
-    """Get MAC address as a string."""
-    return ":".join(
-        f"{((uuid.getnode() >> i) & 0xFF):02x}" for i in range(0, 8 * 6, 8)
-    )[::-1]
+
+def get_mac_address() -> str:
+    """
+    Get the MAC address as a hex string (lowercase, no colons).
+    Example: "b827eb123456"
+    
+    This method prioritizes physical Linux interfaces to ensure stability 
+    across reboots, avoiding uuid.getnode()'s random fallback.
+    """
+    global _CACHED_MAC
+    if _CACHED_MAC:
+        return _CACHED_MAC
+
+    # 1. Try reading from sysfs (Linux specific, most stable)
+    # We look for common interface names first.
+    interfaces = ["wlan0", "eth0", "end0", "enp", "wlp"]
+    sys_net = Path("/sys/class/net")
+    
+    if sys_net.exists():
+        # Sort to prefer 'wlan0' or 'eth0' over others
+        found_devs = sorted([p.name for p in sys_net.iterdir()])
+        
+        # Reorder to prioritize preferred interfaces
+        target_devs = []
+        for pref in interfaces:
+            for dev in found_devs:
+                if dev.startswith(pref):
+                    target_devs.append(dev)
+        
+        # Add any remaining devices (excluding loopback)
+        for dev in found_devs:
+            if dev not in target_devs and dev != "lo":
+                target_devs.append(dev)
+
+        for dev_name in target_devs:
+            try:
+                addr_path = sys_net / dev_name / "address"
+                if addr_path.exists():
+                    mac_str = addr_path.read_text().strip().replace(":", "").lower()
+                    if len(mac_str) == 12:
+                        _LOGGER.debug(f"Found MAC address from {dev_name}: {mac_str}")
+                        _CACHED_MAC = mac_str
+                        return _CACHED_MAC
+            except Exception:
+                continue
+
+    # 2. Fallback to uuid.getnode()
+    _LOGGER.warning("Could not find MAC from sysfs, falling back to uuid.getnode()")
+    node = uuid.getnode()
+    
+    # Check if the multicast bit is set (indicates a random MAC)
+    if (node >> 40) & 1:
+        _LOGGER.warning("Generated MAC address is RANDOM/MULTICAST. Discovery identity will change on reboot.")
+
+    mac_hex = f"{node:012x}"
+    _CACHED_MAC = mac_hex
+    return _CACHED_MAC
+
+
+def format_mac(mac: str) -> str:
+    """Format a hex MAC string with colons (e.g., aa:bb:cc...)."""
+    return ":".join(mac[i : i + 2] for i in range(0, 12, 2))
 
 
 def call_all(*callables: Optional[Callable[[], None]]) -> None: