T5533: Fix VRRP IPv6 group enters in FAULT state

Checks if an IPv6 address on a specific network interface is
in the tentative state. IPv6 tentative addresses are not fully configured
and are undergoing Duplicate Address Detection (DAD) to ensure they are
unique on the network.

inet6 2001:db8::3/125 scope global tentative

It tentative state the group enters in FAULT state. Fix it

1 files changed, 31 insertions, 0 deletions

diff --git a/python/vyos/utils/network.py b/python/vyos/utils/network.py
index 2f181d8d9..bc6899e45 100644
--- a/python/vyos/utils/network.py
+++ b/python/vyos/utils/network.py
@@ -117,6 +117,37 @@ def get_interface_namespace(iface):
             if iface == tmp["ifname"]:
                 return netns
 
+def is_ipv6_tentative(iface: str, ipv6_address: str) -> bool:
+    """Check if IPv6 address is in tentative state.
+
+    This function checks if an IPv6 address on a specific network interface is
+    in the tentative state. IPv6 tentative addresses are not fully configured
+    and are undergoing Duplicate Address Detection (DAD) to ensure they are
+    unique on the network.
+
+    Args:
+        iface (str): The name of the network interface.
+        ipv6_address (str): The IPv6 address to check.
+
+    Returns:
+        bool: True if the IPv6 address is tentative, False otherwise.
+    """
+    import json
+    from vyos.utils.process import rc_cmd
+
+    rc, out = rc_cmd(f'ip -6 --json address show dev {iface} scope global')
+    if rc:
+        return False
+
+    data = json.loads(out)
+    for addr_info in data[0]['addr_info']:
+        if (
+            addr_info.get('local') == ipv6_address and
+            addr_info.get('tentative', False)
+        ):
+            return True
+    return False
+
 def is_wwan_connected(interface):
     """ Determine if a given WWAN interface, e.g. wwan0 is connected to the
     carrier network or not """